Combustion Synthesis Novel Routes to Novel Materials

Combustion Synthesis Novel Routes to Novel Materials Maximilian Lackner (Editor)

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Combustion Synthesis Novel Routes to Novel Materials

Cover pictures: TEM micrographs of the combustion-synthesized ceria samples. Pictures provided by Weifan Chen, School of Materials Science & Engineering, Nanchang University, Nanchang 330031, P.R. China

CONTENTS Foreword

i

Preface

v

Contributors

vi

CHAPTERS 1

Introduction M. Lackner

01

2

Combustion synthesis involving thermite reactions C.-L. Yeh

11

3

Emulsion Combustion Synthesis J.Chandradass, M.Balasubramanian, K.H. Kim

25

4

Combustion Synthesis of Nitrogen Ceramics and Nanosized Ceramic Powders J.-T. Li

33

5

Gel Combustion Synthesis S. Mentus

55

6

Cellulose-assisted combustion synthesis of functional materials for energy storage or conversion R. Cai, W. Zhou, Z. Shao

72

7

Related processes to Combustion Synthesis Y. Ando

83

8

Combustion synthesis melt-casting Q. Bi, L. Fu, J. Yang, W. Liu, Q. Xue

98

9

Combustion synthesis of carbon-encapsulated nanoparticles J. Borysiuk, M. Szala, A. Grabias, J. Szczytko

108

10

Low temperature combustion synthesis of α- Fe2O3 and Ni(1-x)ZnxFe2O4 Nanopowders P. P. Sarangi, N. N. Ghosh

123

11

Combustion synthesis of alloys J. Yang, W. Liu, L. Fu, Q. Bi, Q. Xue

132

12

Salt-assisted Combustion Synthesis W. Chen, F. Li, Y. Tong

141

13

Microwave assisted combustion synthesis of mixed oxide electro-ceramic nanopowders R.V. Mangalaraja, S. Ananthakumar

159

14

Spark Plasma Sintering T. B. Holland, A. Muhkerjee

175

15

Combustion in Porous Inert Media M. A. Mujeebu, M. Z. Abdullah, M. Z. Abu Bakar, A.A. Mohamad

195

16

Synthesis through the solution combustion route S.T. Aruna

206

Outlook

222

Glossary

224

i

FOREWORD This book describes new results in combustion synthesis of inorganic substances and materials. In such processes power-consuming systems (chemically active mixtures of simple substances or separate substances of a complicated composition) are used. Their reaction occurring in the combustion mode is followed by huge heat evolution and results in formation of a valuable product (chemical compounds as powders or compact materials). It is well known that all high-temperature processes should be realized in the combustion mode because after the reaction local initiation they can occur spontaneously without using an external heat source. It is always favorable if the synthesized product’s characteristics meet the initial requirements. But in organization of such synthesis there can be some difficulties connected with choosing a proper heat-evolving reaction or with a high price of raw materials. The economic efficiency of a combustion process application for solving a definite synthetic task can be realized before the work organization due to simple calculations but it is difficult to evaluate the quality of combustion products a priori. There are no scientific methods of preliminary evaluation of combustion product quality; a lot of things depend on the specialist in synthesis, his knowledge, skills and experience. There are two ways of synthesis organization by means of combustion processes. Let’s call them chemical-analytical and macrokinetic ones. The chemical-analytical method has been known for ages. It is rather simple from the viewpoint of its background. The combustion process is not considered as a heat process. It is “a black box” with some reagents at its input and a valuable product at the outlet. But the specialist does not know about the processes inside “the box”. He obtains a dependence of the final characteristics of the product on the initial mixture composition which gives him an opportunity to determine the terms of the process organization. It is a very convenient method of an express-synthesis. If the result of such synthesis is positive and it is necessary to develop a technology, they meet great difficulties connected with the production scale. The combustion process in technological installations can occur in a different way in comparison with the experiment. In order to find optimum conditions for obtaining the required product, similar experiments should be carried out in bigger installations with considerable consumption of the reagents. It slurs over the technological importance of this approach. However, the work using the “black box” method can be of great interest due to the possibility of synthesizing new products and studying their properties. Another approach was developed in the Scientific Center of the USSR Academy of Sciences at the end of the 60-s. One of the main directions developed in the Center, which is

ii

situated in the town of Chernogolovka not far from Moscow, was investigation of combustion of heat-evolving power-consuming systems. In 1967 the scientists of the Center discovered some unusual combustion processes in which all the substances (reagents and products) were solid. Such solid-flame combustion had not been known by the time and its existence was considered as a scientific discovery. The products of the combustion were well-known refractory compounds (borides, carbides, silicides, nitrides, etc.) used in industry. It was clear that the solid-flame combustion could become the method for obtaining such compounds. As a result the method of selfpropagating high-temperature synthesis (SHS) was developed. It connected combustion processes with synthesis of inorganic substances and materials. This method had its own peculiarities which set it apart from other synthetic approaches. The main difference implied is that very many factors influence the final product formation. It means that it is necessary to find optimum combinations of parameters for obtaining high-quality products. The combustion and synthesis processes appeared to be closely connected with each other and it was impossible to study them separately. In order to understand them, the specialists in combustion (they were physicists) began studying chemistry and materials science simultaneously with the traditional organization of their investigation. The result of that work is well-known now. SHS processes and products are studied and used in many countries (more than 50); the World Society of SHS specialists has been created; a new field of science – structural macrokinetics connecting combustion science with macrokinetics has been developed. Principles of the alternative technology are being actively developed. There are some good achievements in organization of industrial production. In some countries there are special schools educating SHS specialists. Having overcome doubts of some specialists and criticism of ill-wishers, SHS advances along a wide road as one of the leaders of the scientific-and-technical progress. In the field of SHS there are different directions. The main one is connected with synthesis. But the notion of the word “synthesis” has become much wider. Starting with preliminary synthesis of refractory compounds, the scientists moved to obtaining other materials by the SHS method (thermally unstable hydrides and chalcogenides, complex oxides, etc). Now we have the well-developed conception of the combustion mechanism of various systems and it is not necessary to carry out thorough investigation of combustion regularities of a definite system. The specialists can find the shortest ways to obtain the final product of a very high quality. SHS can be also used for direct synthesis of materials when specialists produce definite requirements both to material structure and item size, shape and operation characteristics. Probably, SHS will be used for direct synthesis of complicated constructions.

iii

SHS is rather promising for production of nano-materials. Earlier some specialists thought that SHS could not be applied for material production because of its high temperature. But now when the structure formation mechanism in SHS processes is clear enough, we can imagine that in future nano-industry will be based on SHS. Nowadays SHS is aimed at low-exothermic reactions. SHS specialists do not like it when somebody says that the SHS method can’t be used in the case of low-exothermic reactions: there are some good results in donor-acceptor interactions between various SHS processes. In this problem both methods are valuable and each of them has found its niche in the investigation. It is obvious that if you wish to get a prior approximate result, you will start the work using the “black box” method. But if you desire to obtain more thorough information and you have enough time, it is better to use SHS. Also it is possible to imagine the interaction of both approaches. The “black box” method will be the first stage of the integrated process of investigation. How should our field of knowledge be called properly? It is necessary to clarify this question. But in our field there is a double terminology. Many years ago such processes were called ignition reactions but then this definition disappeared because the problems were not thoroughly studied in those days. At first the authors of the discovery wanted to name this process as a combustion synthesis. But it was clear that the combustion process occurred in two stages: flame propagation and structure formation in final products. The latter appeared to be the main stage in many processes since it defines the final product properties. Then the authors called the process self-propagating high-temperature synthesis (SHS) as it properly reflected the main scientific idea of the process. In the 70-s it was the only definition. At the beginning of the 80-s J. Crider, a staff member of the Information Center of the USA Army, published an article “Self-propagating high-temperature synthesis – a Soviet method of ceramic material production”. It started an active development of SHS in the world. In the papers of American scientists, who were the first to join us in our investigation, the process was called “combustion synthesis”. Two definitions were in use, that is why we can speak about the double terminology. Then we realized how to divide these notions. There is an opinion that the processes which consider the combustion wave propagation mechanism and regularities should be called SHS but the processes mainly dealing with investigation of combustion products without studying the combustion process itself should be called combustion synthesis. But this question is still being discussed.

v

PREFACE The field of materials science is a very dynamic one. Old and new applications ranging from household appliances over industry to space exploration – all of them are in constant demand for new, advanced materials. Combustion Synthesis is a technology that can be used to produce speciality materials with unique properties, particularly metals and ceramics. These two material classes can be used for workpieces that have to withstand extreme conditions –high mechanical loads and/or extreme temperatures, to name but the most important ones. In this Bentham E-book, 15 chapters on various techniques for combustion synthesis are compiled. They were written by leading experts from industry and the forefront of academic research. There is a balance between fundamentals and applied knowledge, both covering established techniques and new processes that yet need to be scaled-up for more widespread use. The format of an E-book was chosen to enable fast dissemination of new research, and to give easy access to readers. The chapters can be studied individually. The editor trusts that this book contains a wealth of useful information for the material specialist and anyone who is interested in unconventional synthesis methods of metallic and ceramic materials, and high temperature reactions and processes in general.

Maximilian Lackner, Editor Vienna, December 2009

vi

CONTRIBUTORS S.Ananthakumar Materials and Mineral Division National Institute for Interdisciplinary Science and Technology CSIR, Trivandrum-695019, Kerala India E-mail: [email protected]

M. Zulkifly Abdullah School of Mechanical Engineering Universiti Sains Malaysia 14300 Nibong Tebal Penang Malaysia Tel: 006045996310(O) Fax: 006045941025

Yasutaka Ando Associate Professor Faculty of Engineering, Ashikaga Institute of Technology 268-1 Omae Ashikaga Tochigi 326-8558 Japan E-mail: [email protected]

S.T. Aruna Scientist E1 Surface Engineering Division Council of Scientific and Industrial Research-National Aerospace Laboratories Post Bag No. 1779 Bangalore 560 017 India E-mails: [email protected], [email protected]

M. Zailani Abu Bakar School of Mechanical Engineering Universiti Sains Malaysia 14300 Nibong Tebal Penang, Malaysia Tel: 006045996310(O) Fax: 006045941025

M. Balasubramanian Department of Metallurgical and Materials Engineering Indian Institute of Technology-Madras Chennai-600036 India

Qinling Bi Associate Professor State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000 P.R. China Tel: +86-931-4968193 Fax: +86-931-4968193 E-mail: [email protected]

vii

Jolanta Borysiuk Assistant Professor Institute of Electronic Materials Technology Department of Physics Warsaw University Hoza 69 00-681 Warsaw Poland E-mail: [email protected]

Rui Cai State Key Laboratory of Materials-Oriented Chemical Engineering College of Chemistry & Chemical Engineering Nanjing University of Technology No.5 Xin Mofan Road Nanjing 210009 P.R. China

J.Chandradass Assistant Professor Department of Physics Yeungnam University Gyeongsan, Gyeongsangbuk-do-712-749 South Korea Tel: +82-53-810-2346 Fax: +82-53-810-2334 E-mail: [email protected]

Weifan Chen Associate Professor School of Materials Science & Engineering Nanchang University 999 Xue Fu Road, Honggutan New District, Nanchang 330031 P.R. China E-mail: [email protected]

Licai Fu Assistant Professor State Key Laboratory of Solid Lubrication Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences Lanzhou 730000 P.R. China Tel: +86-931-4968269 Fax: +86-931-4968193.

Narendra Nath Ghosh Assistant Professor Chemistry Group Birla Institute of Technology and Science – Pilani Goa Campus, Zuarinagar – 403726 India E-mail: [email protected]

Agnieszka Grabias Assistant Professor Institute of Electronic Materials Technology Wolczynska 133

viii

01-919 Warsaw Poland E-mail: [email protected]

Troy B. Holland Postdoctoral Researcher Dept. of Chemical Engineering and Materials Science University of California Davis 3118 Bainer Ave Davis, CA 95616 USA Tel: (530)752-6290 Fax: (530)752-9554 E-mail: [email protected]

Ki Hyeon Kim Department of Physics Yeungnam University Gyeongsan Gyeongsangbuk-do-712-749 South Korea E-mail: [email protected]

Maximilian Lackner Founder & Owner ProcessEng Engineering GmbH Joergerstrasse 56-58 11070 Wien Austria E-mails: [email protected], [email protected]

Fengsheng Li Professor, Director National Special Superfine Powder Engineering Research Center Nanjing University of Science & Technology 200 Xiao Ling Wei Street Nanjing 210094 P.R. China E-mail: [email protected]

Jiangtao Li Professor, Head of the Laboratory of Functional Ceramic Materials Technical Institute of Physics and Chemistry Chinese Academy of Sciences Beijing 100190 P.R. China E-mails: [email protected], [email protected]

Weimin Liu Professor State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics Chinese Academy of Sciences Lanzhou 730000 P.R. China Tel: +86-931-4968166 Fax: +86-931-8277088

Combustion Synthesis: Novel Routes to Novel Materials, 2010, 01-10

1

CHAPTER 1 INTRODUCTION Maximilian Lackner Institute of Chemical Engineering, Vienna University of Technology, Getreidemarkt 9/166, 1060 Wien, Austria; E-mail: [email protected] Abstract: In this chapter, an introduction to combustion synthesis is provided in the first part. Starting with the definition of a combustion process, the characteristics of combustion synthesis, i.e. a redox reaction for the production of inorganic, solid materials, are explained. Combustion synthesis was first used in prehistoric times to produce carbon black for cave paintings, and has further been developed to an advantageous technique for a wide variety of materials. Combustion synthesis processes are classified by the state of matter of the educts, being gaseous, liquid or solid. Combustion synthesis in the gas phase, also known as „flame synthesis“, can be used to produce non-agglomerated nanoparticles and carbon nanotubes (CNT). In the combustion synthesis in the solid phase, also known as „gasless combustion“ or “solid flame”, one can distinguish between self-propagating combustion synthesis (self-propagating high-temperature synthesis, SHS) and volume combustion synthesis (VCS). In the prominent SHS mode, combustion travels through the reaction mixture. VCS is also termed “simultaneous combustion” mode or “thermal explosion”. Combustion synthesis in the liquid phase can be carried out as solution combustion synthesis (SCS) from metal salts in aqueous solution in combination with organic fuels. Molecular mixing ensures excellent product homogeneity, also for multi-component mixtures. In general, synthesis can start from the elements, or from compounds. The most common type of combustion synthesis is the thermite-type reaction (Goldschmidt process), vis. the reduction of an ore using a metal such as Mg or Al. More than 700 compounds have been produced using combustion synthesis. Combustion synthesis is carried out in simple reactors rather than furnaces. This chapter also lists typical products and their applications. General advantages of combustion synthesis over traditional techniques are presented. In the second part of this chapter, all chapters of this E-book are briefly outlined.

COMBUSTION AND COMBUSTION SYNTHESIS Combustion is an ubiquitious technology, supplying 80-90% of the primary energy worldwide [1]. A fuel, fossil or renewable in origin, is burnt with an oxidizer, usually air, to generate heat, which can be deployed in various ways, ranging from heating to propulsion and energy production. By contrast, a fire is an unwanted combustion. Other purposes of combustion are to destroy harmful pollutants and to get rid of waste or biomass. Combustion can be defined as a redox reaction (reduction/oxidation) or electron transfer process, in which the fuel is oxidized and the oxidizer is reduced in an exothermic reaction. Oxidation is associated with an increase in oxidation state, whereas reduction is linked to a decrease in oxidation state, see equation 1 as example: 2 H2[0] + O2[0] Æ 2H2[1+]O[2-]

(eq. 1)

The small supercripts in [brackets] give the oxidation state (oxidation number). The oxidation state of hydrogen (fuel) changes from [0] to [1+], that of oxygen (oxidizer) from [0] to [2-]. Fuels can be solid, liquid or gaseous. As heat from the reaction is the primary interest of combustion, the combustion products are generally of low interest and even undesirable, for instance pollutants or carbon dioxide. Tremendous efforts are undertaken to get rid of harmful combustion products. Technologies range from pollutant formation avoidance (CO, soot) over pollutant reduction (CO, NOx, SOx, dust) to CO2 capture and storage attempts. Combustion processes are characterized by high temperatures, frequently coupled with high pressures [2], and transient conditions in a multi-phase environment. In contrast, combustion synthesis [3], [4] is primarily concerned with the products of a combustion process. The combustion reaction itself is also used to liberate heat, but this heat is used to produce various types of advanced materials such as a metallic or ceramic power or body. Most interesting in recent years have been various advanced ceramics, catalysts and nano-powders [42]. An example of a combustion synthesis process is the classic thermite reaction as shown in equation 2 [8]: Fe2O3 + 2Al → 2Fe + Al2O3 + heat

(eq. 2)

Maximilian Lackner (ED) All rights reserved - © 2010 Bentham Science Publishers Ltd.

2 Combustion Synthesis: Novel Routes to Novel Materials

Maximilian Lackner

In this reaction, iron oxide is used to oxidize aluminium in a highly exothermic reaction that yields liquid iron at the bottom of the reaction vessel. It can be used to weld railroad tracks, for example. The reaction proceeds because the enthalphy of formation (binding energy) of Al2O3 is lower than that of Fe2O3. CHARACTERISTICS OF COMBUSTION SYNTHESIS Combustion synthesis is a process that uses a combustion reaction to produce inorganic, solid materials in a redox reaction. Combustion synthesis can be a pure redox reaction between two educts, but it is also possible to add fuels or oxidizers, e.g. nitrate as oxidizer and hydrazin, urea or glycin as fuel in solution combustion synthesis. The process of combustion synthesis is generally characterized by high activation energies and highly exothermic reactions, with the reaction temperature being on the order of 500 to 4000K [20]. The heat release is typically on the order of 1012-1014 W/m³ [12] in self-propagating combustion synthesis, see below. Typical products are metallics, intermetallics, metal-matrix composites, cermets [21], solid solutions [22], carbides, nitrides, borides and ceramics, oxide- and non-oxide based. The products of combustion synthesis can be functional or structual. They often have unique properties [3]. Recently, nano-particles obtained by combustion synthesis have seen increased interest. The earliest combustion synthesis reaction dates back to prehistoric times, when man produced carbon black [5] for cave painting and other uses. The ancient Chinese produced carbon black by combustion, too [6]. Although the thermite reaction dates back to 1893 [8], the Pechini process [9] is often seen as the first real implementation of combustion synthesis. The Pechini process is also called “liquid mix” or “resin intermediate” method [10]. Pioneering work on combustion synthesis was done by A.G. Merzhanov at ISMAN in Russia (Institute of Structural Macrokinetics, Russian Academy of Sciences) in the early seventies [19]. Combustion synthesis can be regarded as a fairly simple process, yet complex mechanisms are involved [26]. TYPES OF COMBUSTION SYNTHESIS PROCESSES In combustion synthesis, the educts can be gaseous, liquid or solid. There are several variants which will be described below. Combustion synthesis has to be kept separate from other high temperature processes that are deployed, too, to yield materials, such as plasma synthesis [28], CVD (chemical vapour deposition) or PVD (physical vapour deposition). Some of these work at even higher temperatures than combustion synthesis does; For instance, ICP (inductively coupled plasma) nano particle synthesis reaches temperatures of up to 10,000 K [6]. Combustion Synthesis in the Gas Phase From gaseous educts or solids or liquids injected into a flame, non-agglomerated nano-particles can be produced in flames [5]. One also speaks about „flame synthesis“ in this context. These flame-synthesized nano-particles can be spherical metal oxides [6], carbon nanotubes (CNT) [7], or films [11], to give three examples. By „flame spray synthesis“ using one or two nozzles [14], catalysts can be produced [15]. The term „aerosol flame reactor“ is also frequently used [16]. Often, an oxygen-hydrogen or an oxygen-propane flame is used to inject metal-containing salts, chlorides or hydrides, but also pure metals in the shape of fine dust [5], for nano-particles of metal oxides. If salts are used, a typical temperature is 2000K. With pure metals, 3000K can be obtained. Interestingly, the nano particle size is pretty similar in both cases with 10-100 nm [5]. Combustion Synthesis in the Solid Phase The solid educts are first ground to a fine powder. The powders are then intimately mixed and compacted. One can distinguish between self-propagating combustion synthesis (self-propagating high-temperature synthesis, SHS) and volume combustion synthesis (VCS) as shown below. Self-Propagating High Temperature Synthesis (SHS) In the case of a highly exothermic reaction, the powder mixture is ignited [32] on a spot by the use of an electrical coil, a laser [30], shock waves [31] or another suitable energy source. The reaction will travel through the powder mixture as a combustion wave in a self-sustaining way. Therefore, the term self-propagating hightemperature synthesis (SHS) is frequently used. As the reaction proceeds, one can see a combustion wave that travels through the mixture, compare Fig. 1.

Introduction

Combustion Synthesis: Novel Routes to Novel Materials 3

Fig. 1: Combustion wave in SHS (reproduced from [3] with permission). Note that in several cases, the reaction mixture has to be pre-heated to a certain temperature in order to maintain a self-propagating combustion wave. The combustion wave is up to 25 cm/s fast. Reaction times are on the order of seconds, which is fast compared to conventional solid-state reactions. Temperature gradients are very high, up to 105K/cm [12]. Most of the reaction energy is supplied by the reaction itself. The process, because it is fast, is quasi-adiabatic. After combustion, a post-treatment can be done [41]. Volume Combustion Synthesis (VCS) In the case of a weakly exothermic reaction, the powder mixture will not burn by itself if ignited in one spot (compare SHS). Instead, the entire sample must be brought to a sufficiently high temperature. VCS is also termed simultaneous combustion mode or thermal explosion. Combustion synthesis in the solid phase is also termed „gasless combustion“, as in the thermite reaction introduced above. One also speaks about a „solid flame“. Combustion Synthesis in the Liquid Phase For the production of metal oxides, the solution combustion synthesis (SCS) route has proven to be suitable. Metal salts, for instance nitrates or carboxylates, dissolved in a saturated aqueous solutions in combination with organic fuels (urea, citric acid, glycine and others) can be transformed into useful materials. The nitrate serves as oxidizer, the fuel provides the necessary heat. The fuels also help to keep the metal ions apart by complexing them. The amount of fuel needed (stoichiometry) can be derived from the valency of the ions [27]. The aqueous mixture is continuously heated up. First, gelation occurs, then combustion starts. The evolving gases can form a flame reaching 1000°C [27]. Molecular mixing ensures excellent product homogeneity, also for multi-component mixtures. The resulting oxide powders have a high surface area and are well suited for sintering applications [27]. A wide variety of technically useful oxides such as alumina or zirconia with special magnetic, dielectric, electrical, mechanical, luminescent, optical and catalytic properties was obtained by SCS [42]. Doped oxides are also accessible [42]. Classification of Combustion Synthesis Processes by Type of Educts Another way of classifying combustion synthesis processes other than by the phase in which they occur is to group them by type of educts. Synthesis from the Elements: Carbides, silicides, borides, nitrides, oxides and hydrides can be obtained by this type of reaction. Equations 3a and 3b provide an example: Ti + C Æ TiC Ni + Al Æ NiAl

(eq. 3a) (eq. 3b)


11

CHAPTER 2 Combustion Synthesis Involving Thermite Reactions Chun-Liang Yeh Feng Chia University, E-mail: [email protected] Abstract: The self-propagating high-temperature synthesis (SHS) process involving thermite reactions was introduced as a promising alternative to prepare various Al2O3-reinforced materials, including borides, carbides, and aluminides. The influence of different thermite mixtures on the SHS process is explored in terms of the combustion sustainability, propagation rate of the reaction front, combustion temperature, and phase composition of the synthesized products. On formation of the TiB2-Al2O3 composite, the thermite mixture of Al-TiO2-B2O3 was found to improve the phase conversion when compared with that of Al and TiO2. The addition of the Al-TiO2 thermite mixture to the Ti-Si-C reaction system was demonstrated to produce in situ Ti3SiC2-Al2O3 composites. The thermite reaction of Al and TiO2 can also be applied to prepare the TiAl-Al2O3 composite through combustion synthesis. Two niobium aluminides Nb3Al and Nb2Al were obtained in almost pure form from a thermite-based SHS process with the powder compacts of Al:Nb2O5 = 12:3 and 13:3, respectively. As the thermite reagent of Al-Nb2O5 or Al-Nb2O5-B2O3 was added to the Nb-B combustion system, products were the composites of niobium borides and Al2O3.

INTRODUCTION The thermite reaction is defined as an exothermic reaction which involves a metal reacting with a metallic or a non-metallic oxide to form a more stable oxide and the corresponding metal or non-metal of the reactant oxide. More recently, thermite reactions have become important in the synthesis of refractory ceramic and composite materials. A large number of oxides (e.g., Fe2O3, B2O3, NiO, V2O5, MoO3, and Nb2O5) can be reduced by aluminum up to relatively high temperatures. These Al-based thermite reactions result in the production of Al2O3 and elemental components. Therefore, the integration of combustion synthesis with thermite reactions represents an in situ processing method of producing composite materials with phases uniformly distributed in the materials [1-3]. In this chapter, the preparation of the Al2O3-reinforced composites such as TiB2-Al2O3, NbB2-Al2O3, Ti3SiC2-Al2O3, and TiAl-Al2O3 by self-propagating high-temperature synthesis (SHS) involving thermite reactions is presented. The influence of different thermite mixtures on the SHS process is explored in terms of the combustion sustainability, propagation rate of the reaction front, combustion temperature, and phase composition of the synthesized products. In addition, this chapter presents a complete discussion on formation of niobium aluminides (Nb3Al, Nb2Al, and NbAl3) through a thermite-based SHS process using the compressed samples of the Al and Nb2O5 powder mixtures under a wide range of stoichiometries. METHODS The SHS experiments were conducted in a stainless-steel windowed combustion chamber under an atmosphere of high purity argon (99.99%) [4]. The sample holder was equipped with a 600 W cartridge heater used to preheat the test sample prior to ignition. The ignition was accomplished by a heated tungsten coil with a voltage of 60V and a current of 1.5A. The propagation rate of combustion wave was measured by recording the whole combustion event with a color CCD video camera (Pulnix TMC-7) at 30 frames per second. The exposure time of each recorded image was set at 0.1 ms. To facilitate the accurate measurement of instantaneous locations of the combustion front, a beam splitter (Rolyn Optics), with a mirror characteristic of 75% transmission and 25% reflection, was used to optically superimpose a scale onto the image of the test sample. The combustion temperature of powder compact was measured by a fine-wire (125 μm) Pt/Pt-13%Rh thermocouple (Omega Inc.) attached on the sample surface. After the SHS process, the density of the recovered sample was measured. In addition, the phase composition of combustion products was identified by an X-ray diffractometer with CuKα radiation. FORMATION OF TIB2-AL2O3 AND NBB2-AL2O3 COMPOSITES The starting materials used to produce TiB2-Al2O3 and NbB2-Al2O3 composites included four elemental powders: Ti, Nb, Al, and amorphous boron. Additionally, three metallic oxides, TiO2, Nb2O5, and B2O3, were employed as Maximilian Lackner (ED) All rights reserved - © 2010 Bentham Science Publishers Ltd.


Chun-Liang Yeh

the thermite reagents. The initial stoichiometry of the powder blend for the synthesis of the TiB2-Al2O3 composite was prepared according to two different thermite mixtures involved in the SHS process and described in Reactions (1) and (2) [5,6]. (1 −

5 3 m)Ti + 2(1 − m) B + 2mAl + mTiO2 → (1 − m)TiB2 + mAl2O3 2 2

3 3 8 8 (1 − n)Ti + 2(1 − n) B + 2nAl + nTiO2 + nB2O3 → (1 − n)TiB2 + nAl2O3 5 5 5 5

(1)

(2)

where the stoichiometric parameters m and n represent the mole fraction of Al2O3 formed in the TiB2-Al2O3 composite. The maximum value of m adopted in Reaction (1) was 0.35, because the reaction ceased to be self-propagating after ignition in the sample of m = 0.4, which signifies no elemental Ti included in the reactant mixture. The parameter n was varied from 0.2 to the upper limit of 0.625, under which the sample was composed of three thermite reagents Al, TiO2, and B2O3. The reactant powders were dry mixed in a ball mill and then cold-pressed into cylindrical test specimens with a diameter of 7 mm, a height of 12 mm, and a compaction density of 50% relative to the theoretical maximum density (TMD). When preparing the composite of NbB2-Al2O3, the starting stoichiometry of the powder mixture was formulated as Reactions (3) and (4).

(1 −

11 3 p) Nb + 2(1 − p) B + 2 pAl + pNb2O5 → (1 − p) NbB2 + pAl2O3 5 5

(3)

(1 −

17 17 3 6 q) Nb + 2(1 − q) B + 2qAl + qNb2O5 + qB2O3 → (1 − q) NbB2 + qAl2O3 11 11 11 11

(4)

where p and q stand for the mole fraction of Al2O3 formed in the NbB2-Al2O3 composite. Samples of Reaction (3) were conducted with initial compositions up to the greatest extent of p = 5/11 (about 0.455), indicative of a test specimen without any elemental Nb. For the comparison purpose, the parameter q adopted in Reaction (4) ranged between 0.2 and 0.5. Fig. 1(a) and (b) illustrate typical SHS sequences associated with formation of the TiB2-Al2O3 and NbB2-Al2O3 in situ composites, respectively. As observed from Fig. 1, a distinct combustion front forms upon ignition and propagates along the sample in a self-sustaining manner. It was also evident that the powder compact was subjected to significant elongation as the combustion wave progressed, resulting in the synthesized product with a porous structure. Formation of porous products is inherent in the SHS process. The pores or crevices could be produced by unbalanced diffusion between the reactant particles or by vaporization and expulsion of the volatile impurities due to high temperatures [5]. Fig. 2 plots the flame-front velocity of the SHS process as a function of the Al2O3 content in the TiB2-Al2O3 composites synthesized from the samples with two different thermite mixtures. When compared with those (over 40 mm/s) of the elemental SHS reaction producing monolithic TiB2, the combustion wave velocity in the synthesis of TiB2-Al2O3 composites is considerably lower and decreases with increasing Al2O3 content formed in the products. The decrease of the flame-front velocity is believed to be caused by reduced exothermicity of the overall synthesis reaction, in view of the fact that both the Al-TiO2 and Al-TiO2-B2O3 thermite systems yet releasing heat are less exothermic than the elemental reaction between Ti and B [5]. Moreover, due to lack of sufficient thermal energy to sustain the synthesis reaction, combustion was found quenched in the sample based upon Reaction (1) with m = 0.4, under which no elemental Ti is employed and the powder compact contains B, Al, and TiO2 only. For the samples adopting the thermite mixture of Al-TiO2-B2O3, Fig. 2 reveals a slightly greater speed of the combustion wave and a broader range of the composition that indicates better sustainability of the reaction. This might be due to formation of a molten B2O3 phase that improves contact between the reactant particles and thus facilitates the ignition and increases the reaction rate.

Combustion Synthesis Involving Thermite Reactions


(a)

(b)

Figure 1: Recorded images illustrating self-propagating combustion fronts along (a) a sample based on Reaction (2) with n = 0.45 and (b) a sample based on Reaction (4) with q = 0.3.

Figure 2: Variation of flame-front velocity of SHS processes involving two different thermite mixtures with Al2O3 content formed in TiB2-Al2O3 composites. The flame-front propagation velocities measured from the samples for the preparation of NbB2-Al2O3 composites are presented in Fig. 3. In contrast to formation of the TiB2-Al2O3 composite, addition of either Al-Nb2O5 or Al-Nb2O5-B2O3 thermite mixture to the Nb-B elemental reaction accelerates the reaction front velocity significantly. As indicated in Fig. 3, the highest reaction front velocity up to 33 mm/s is observed in the sample containing the thermite mixture of Al-Nb2O5 with p = 0.455, within which the metallic oxide Nb2O5 serves as the only source of Nb. This implies that the thermite reaction of Al with Nb2O5 is sufficiently exothermic and has a great tendency to proceed.


25

CHAPTER 3 Emulsion Combustion Synthesis 1

J.Chandradass*, 2M.Balasubramanian and 1Ki Hyeon Kim*

1

Department of Physics, Yeungnam University, Gyeongsan, Gyeongsangbuk-do-712-749, South Korea and Department of Metallurgical and Materials Engineering, Indian Institute of Technology-Madras, Chennai600036, India; *Corresponding authors; Tel. +82-53-810-2334; Fax. +82-53-810-4616; E-mails: [email protected] (J.Chandradass); [email protected] (Ki Hyeon Kim) 2

Abstract: The emulsion combustion method (ECM) is a novel powder production process developed to synthesize nanostructured metal-oxide powders. In this process, metal ions present in the aqueous droplets are rapidly oxidized by the combustion of the surrounding flammable liquid. The small reaction field and short reaction period lead to the formation of nano-size ceramic particles. A variety of extremely high surface area materials could be synthesized by the ECM process. For example, hollow spheres of Al2O3 TiO2, ZrO2, and Y2O3 can be prepared by this process using aqueous solutions of aluminum nitrate, TiCl4, zirconium oxynitrate, and yttrium nitrate, respectively. In contrast, solid particles of ZrO2-CeO2, ZnO, Fe2O3, CeO2, MgO and BaTiO3 from aqueous solution of their corresponding nitrates and ZnO/SiO2 from zinc acetate, and silica sol or hexamethyldisiloxane can also be prepared. The powder formation mechanism is discussed in detail. The main aim of this chapter is to introduce emulsion combustion synthesis to the reader.

INTRODUCTION Most of the powders are dispersed in polymers or liquids for their practical use in magnetic tapes, fillers for electronic devices, paints, etc. [1]. Along with the revolutionary development of electronics in the second half of the twentieth century, the huge potential of ceramic materials has been unfolded and introduced into a fascinatingly wide spectrum of electrical and microelectronic devices and applications [2]. The phenomena associated with ceramic processing begin with preparation of the starting powder, and new techniques for synthesizing high purity, non-agglomerated, well-characterized, submicron powders having a narrow size distribution are constantly being sought [3]. The strength of chemistry in materials science is its versatility in designing and synthesizing new materials, which can be processed and fabricated into final component. Chemical synthesis permits the manipulation of matter at the molecular level. Because of mixing at the molecular level, good chemical homogeneity can be achieved. Also, by understanding the relationship between the assembly of matter on atomic and molecular levels, and the material macroscopic properties, molecular synthetic chemistry can be tailored to prepare novel starting compounds. Better control of the particle size, shape, and size distribution can be achieved during particle synthesis [4]. The properties of the ceramic powders depend on their methods of production. Mechanical synthesis of ceramic nanopowder requires extensive mechanical milling, and that easily introduces impurities. Vapor phase reaction for the preparation of nanopowder from a gas phase precursor demands high temperature above 1200◦C. The precipitation method suffers from its complexity and time consumption (long washing times and aging time). The hydrothermal method needs high temperature and pressure. The sol–gel method based on molecular precursors usually makes use of metal alkoxide as raw material. However, the high price of alkoxides and long gelation periods limit the application of this method [5]. Combustion synthesis (CS) [6] has emerged as an important technique for the synthesis, and processing of advanced ceramics (structural, and functional), catalysts, composites, alloys, intermetallics and nanomaterials. In CS, the exothermicity of the redox (reduction-oxidation or electron transfer) chemical reaction is used to produce useful materials [7]. Combustion reactions are characterized by high-temperature, fast heating rates, and short reaction times [8]. These features make CS an attractive method for the manufacture of technologically useful materials at lower costs compared to conventional ceramic processes. Some other advantages of CS are [8]: (i)

use of relatively simple equipment

(ii)

high-purity product

(iii)

Stabilization of metastable phases and

(iv)

Formation of virtually any size and shape products

DIFFERENT TYPES OF COMBUSTION METHOD Depending upon the nature of reactants (elements or compounds in the form of solid, liquid or gas), and the exothermicity (adiabatic temperature, Tad), CS can be classified as self-propagating sol-gel combustion (SSGC), Maximilian Lackner (ED) All rights reserved - © 2010 Bentham Science Publishers Ltd.


Chandradass et al.

volume combustion synthesis (VCS), impregnated inert support combustion (IISC), impregnated active layer combustion (IALC), and emulsion combustion (ECM) [9]. SELF PROPAGATING SOL-GEL COMBUSTION (SSGC) The desired amount of metal nitrate–fuel solution is dried to make a sol-gel-like heterogeneous medium, which is then preheated to 30–80°C, followed by local reaction initiation by a heated tungsten wire. As a result, a selfsustained reaction wave steadily propagates along the medium, forming a nanopowder of the desired composition [9]. VOLUME COMBUSTION SYNTHESIS (VCS) Volume combustion synthesis involves reaction in homogeneous aqueous oxidizer–fuel solution uniformly heated by an external energy source (e.g., hot plate, electrical or microwave oven). Specifically, after water evaporation, the temperature of the formed sol-gel viscous media rapidly increases, achieving ignition temperature (Tig) at which reaction spontaneously initiates over the entire volume (VSC mode), leading to the formation of solid product with the desired phase composition [9]. IMPREGNATED INERT SUPPORT COMBUSTION (IISC) The solution impregnation step consists of dipping porous pellets (oxides of Al2O3, ZrO2, etc. suitable for catalyst support) into the desired reactive solution under vacuum conditions, which leads to the infiltration of the solution into the bulk of the material. A further drying step is followed by initiation of the combustion reaction. The reaction front propagates along the heterogeneous media of impregnated solids similar to that in the SSGC mode. The final product consists of pellets loaded by a thin surface layer of the desired catalyst [9]. IMPREGNATED ACTIVE LAYER COMBUSTION (IALC) The principle behind this method is that a reactive solution is impregnated into a reactive porous medium that can assist the propagation of the main reaction. The desired aqueous solution of fuels, and oxidizers is impregnated into a thin layer of porous media (e.g., cellulose paper, carbon nanotubes, etc.) followed by drying of the thus-formed complex reaction system to decrease the amount of water. Ignition can be initiated by slow smoldering mode. The slowly propagating (0.1–1 mm/s) smoldering mode leads to a higher product yield and its larger surface area. The smoldering mode can be initiated by an electrically heated metal wire with relatively low temperature (~500 K). During IALC, the final product forms as agglomerates of nanoparticles [9]. EMULSION COMBUSTION (ECM) Emulsion combustion, a novel processing method originally developed by Takatori [10] for making a fine powder comprises preparing an aqueous solution of precursor compounds of the desired powder product and forming an emulsion of the aqueous solution in a water-immiscible organic fluid. Burning the oil in the atomized emulsion droplet and oxidizing the metal salts in the aqueous solution leaving only the particles in the form of a powder. The emulsion process [11, 12], and the combustion technology are combined to have the reaction fields of submicron size, and reaction times of a fraction of second. CHARACTERISTICS OF ECM PROCESS [1] The ECM process is characterized by; (1) an isolated small reaction field in which constituent metal ions are mixed homogeneously in the aqueous phase, (2) a short reaction period achieved by the combustion of the thin kerosene film surrounding each aqueous droplet, (3) a continuous fabrication procedure which contributes to lower production costs. COMPARISON BETWEEN ECM AND SPRAY PYROLYSIS PROCESS The ECM process is classified as a chemical synthesis process like, co-precipitation, sol– gel process, and flame spray pyrolysis (FSP) [1]. It differs from the standard organic solution-fed FSP, because the higher amount of aqueous phase (e.g. 65 vol.%) in the precursor emulsion decreases the flame temperature, favoring the precipitation of precursor in the liquid phase rather than precursor evaporation, and oxidation at the gas phase for particle formation during spray pyrolysis [13-18]. The significant differences between ECM and the

Emulsion Combustion Synthesis


conventional spray pyrolysis stem from the size of reaction field, and the reaction period. An isolated small reaction field of about 1 μm is easily prepared using the emulsion process. The reaction period of the conventional spray pyrolysis is longer than that of the ECM. A high temperature furnace, a burner flame and/or a microwave are used to gradually dry up the sprayed particles and transform them to metal-oxide powder in the conventional process. In the ECM process, each dispersed droplet of emulsion is heated by the combustion of a kerosene film surrounding the droplet, which substantially reduces the reaction period of ECM [1]. ADVANTAGE AND APPLICATION OF ECM PROCESS The ECM process is capable of producing a wide variety of fine powders. It is particularly well suited for making metal oxides for which water-soluble precursors are widely available. The aqueous droplets in the emulsion contain precursor compounds in the ratios needed to produce the desired composition of the powder product. Thus, it is possible to achieve precise control of the final powder composition, an advantage particularly important for making ceramic powders that are suitable for electronic applications. In addition, the precursors are dissolved and very intimately mixed in the aqueous solution. Thus, not only is the control of chemical composition (e.g., ceramic stoichiometry) more exactly effected but the method is capable providing a purer product of unsurpassed compositional homogeneity. The powders made by this process are inherently spherical solid and/or hollow particles. Spherical solid particles having a narrow size distribution are considered to be ideal for obtaining high packing densities and are desired for making dense ceramic and metal films in the electronics industry. Hollow particles can be attractive as insulating, and lightweight filler materials as well as catalyst carriers because of their small size (submicrometer), very thin shell (~10 nm), and high specific area (~50 m2/g). Therefore, the ECM process is a promising method for the synthesis of hollow particle of oxides used as fillers (e.g., ZnO), and catalysts (e.g., TiO2 and ZrO2) [13]. SYNTHESIS OF NANOPOWDERS BY ECM PROCESS: GENERAL PROCEDURES [1] The ECM process illustrated in Fig. (1) comprises three steps: (1) preparation of an emulsion, (2) combustion of the atomized emulsion, and (3) collection of the ceramic powder produced. An aqueous solution of the metal salts is stirred with kerosene and a small amount of emulsifier to obtain water-in-oil (W/O) type emulsion. Several metal salts can be mixed homogeneously in the aqueous solution. In many cases, the aqueous phase is approximately 65 vol% of the emulsion and the dispersed droplets are about 1-2 μm in diameter. The diameter of the atomized emulsion could be controlled by the atomization condition and it is around 60 μm. The constituent metal ions in the aqueous droplets are rapidly oxidized at high temperature by the combustion of the surrounding kerosene. The temperature is measured by inserting a thermocouple into the flame, and it is controlled by the atomization rate of the emulsion. Most of the syntheses are carried out between 700°, and 1000°C. Air is introduced into the flame so as to achieve complete combustion. The residence time of the powder at high temperatures is estimated to be less than half a second. The powder produced is collected by a conventional dry filtration process. It is advantageous to prevent the powder from external contamination by avoiding further drying, calcination and milling. Although when the combustion was carried out at temperature less than 800°C, the powder contained small amounts of organic residue because of incomplete combustion. The powders were calcined at 600°C in air before characterization.

Figure 1: Schematic diagram of ECM powder production process [19].


33

CHAPTER 4 Combustion Synthesis of Nitrogen Ceramics and Nanosized Ceramic Powders Jiangtao Li Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China; E-mail: [email protected] Abstract: The nitriding combustion synthesis of nitrogen ceramics has long been an important research area and has found many industrial applications in regarding to the synthesis of a wide range of nitride powders in the past two decades. The energy saving and especially time-saving feature makes the infiltration combustion synthesis technique very attractive in the cost effective fabrication of nitride ceramics. That is why there are continuous efforts devoted to the understanding of the reaction mechanism as well as to the optimizing of the processing parameters on the synthesis of nitride ceramics. In this chapter, the progress on the combustion synthesis of nitride ceramic powders was summarized. It was found that the remarkable achievements obtained in this area could be classified as two aspects. The first aspect is the development of up-scalable cost-effective fabrication process, which follows a direction of decreasing the synthesis pressure, that is, the minimum nitrogen pressure required for implementing the combustion synthesis of nitride ceramics has been decreased from high pressure to normal pressure, and up to now, the nitride ceramic powders could even be synthesized in air. The other aspect is the development of high performance nano-sized nitride powders by combustion synthesis method which showed integrated functional and structural properties. For instance, the Re-doped SiAlON powders could be used as new phosphors for LED lighting and the nano and/or sub-micrometer sized α/β-SiAlON could be densified directly without using any sintering additives. Finally, the regularities for the large-scale synthesis of nitride powders were discussed, and the properties of the as-synthesized nitride powders were systematically characterized.

INTRODUCTION With the evolution of basic theories and development of practical techniques, combustion synthesis has shown an increasing importance in the fabrication of advanced ceramic materials. An important topic involved in combustion synthesis is to prepare nitride ceramics by infiltration combustion reactions in gas-solid systems. In this field, one of the most extensively-studied materials is silicon nitride (Si3N4) and its solid solution SiAlON. This chapter reviews major achievements in the following aspects: effects of processing parameters on reaction kinetics and product properties in combustion synthesis of Si3N4, reaction mechanisms in gas-solid systems, mechanical-activation-assisted combustion synthesis, synthesis of nitride powders in air by selective combustion reaction, and recently-developed preparation of SiC nanopowders by combustion reaction in N2. By this review, the progress in both laboratory research and industrial applications of combustion synthesis of nitride ceramics is outlined, and possible directions for further studies in this field are discussed. PROCESSING PARAMETERS IN THE INFILTRATION COMBUSTION SYNTHESIS OF SI3N4 Critical N2 Pressure Combustion synthesis of nitride ceramic materials has been widely reported in many publications [1, 2]. A conventional approach is direct combustion of solid metallic powders in N2, where continuous supply of N2 to the reaction front is necessary. In this combustion mode, three processes are involved:(1) transportation of N2 to the reaction front by infiltration in a porous sample to guarantee N2 supply at the solid-gas interface;(2) nitridation reaction;(3) diffusion of N atoms through the as-formed nitride layer. The latter two processes have the common kinetics as most multiphase reactions, and the first process usually determines the characters of the whole combustion reaction. According to the ratio between the actually-applied N2 pressure and the critical pressure, nitridation combustion can be classified into infiltration combustion and combustion with infiltration not necessary. The dependence of conversion fraction (or yield) on critical N2 pressure is plotted in Figure 1 (PN2~η relation) for several reaction systems [1]. If the applied N2 pressure is higher than the critical value, combustion reaction can continue without the infiltration of N2. Otherwise, the transportation of N2 by infiltration from the outside to the reaction front is necessary, and the infiltration rate should exceed or equal the consumption rate of N2 by reaction in order to achieve complete nitridation (η=100%). For complete nitirdation of a Si powder compact with 70% porosity, the critical N2 pressure is 3×103 atm, as shown in Figure 1. In this way, early studies on combustion synthesis of Si3N4 were generally performed under very high N2 pressures. Maximilian Lackner (ED) All rights reserved - © 2010 Bentham Science Publishers Ltd.


Jiangtao Li

Figure 1: The dependence of the degree of conversion to nitrides on PN2 for selected elements. (T=1500 K, porosity = 0.7) Effects of Diluents, Compact Porosity, and Additives Combustion synthesis under high pressures requires expensive equipment, gives a small batch, and can bring safety problems. Therefore, it is important to carry out combustion synthesis of nitride ceramics by gas-solid reactions under relatively low pressures, especially for industrial applications [3]. In 1987, Miyamoto reported combustion synthesis of Si3N4 under N2 pressures of 3~10MPa by adding diluents in the starting Si powder [4]. The purpose of adding diluents is to decrease reaction temperature and reduce the agglomeration of Si melt, enhancing the infiltration of N2 with an infiltration rate higher than or equal to the consumption rate of N2. This method is simple and effective for combustion synthesis of Si3N4 under relatively low N2 pressures, and hence widely applied afterwards [5-7]. After the crucial role of diluents was recognized, Cano et al. systematically investigated the effect of diluent content (D) on combustion synthesis of Si3N4 [7]. They found that, the nitridation of Si powder showed different kinetic characters with a diluent content of D=20~60 wt%. For Si powder compacts with 80% porosity, higher propagation velocities of combustion wave were observed with increasing diluent content in the range of D=20~40 wt%. The addition of diluent was thought to prevent the agglomeration of Si melt and keep a constant gas-solid reaction area at the wave front. In the range of D=40-60 wt%, however, the propagation velocity of combustion wave decreased with increasing diluent content. It was proposed that the addition of diluent reduced the amount of heat created in a unit time and accordingly the propagation velocity of combustion wave slowed down. From this investigation, the constant propagation velocity of combustion wave despite the change of diluent content in the range of D=30-50 wt%, which was observed in some other studies, can be well explained as a balance of the two effects of diluent, viz. preserving the reaction area and reducing the production of heat. Cano et al. also studied the influence of porosity and found that with increasing porosity the propagation velocity of combustion wave increased. In combustion synthesis of Si3N4, the porosity has two effects. At first, with less porosity, the reduction of reaction area due to the agglomeration of Si melt is remarkable, and consequently the propagation velocity of combustion wave decreases. On the other hand, less porosity means a higher compacting density and a smaller thermal diffusion coefficient, which results in a decreased propagation velocity. In a Si powder compact with 60% porosity, the propagation velocity of combustion wave is only half that in a compact with 80% porosity. With decreasing porosity, the stability of combustion reaction is reduced, and thus the maximum content of diluent that can be added is lowered. In fact, stable combustion can hardly take place for Si powder compacts with D=50-60 wt%, and self-sustained combustion reaction has never been observed for the samples with 60% porosity and 50~60 wt% diluent.

Combustion Synthesis of Nitrogen Ceramics..


Figure 2: Combustion velocities as a functional of diluents content for specimens with 80% and 60% porosity Recently, combustion synthesis of Si3N4 under a lower N2 pressure of 2MPa has been reported by using (Si+NH4Cl) as reactants and with the assistance of mechanical activation [8]. This reveals that it is possible to realize complete nitridation of Si powder even without the addition of Si3N4 diluent. The additive NH4Cl has similar effect as Si3N4 diluent, which can lowers reaction temperature and improve the nitridation of Si powder. With the addition of NH4Cl, Si3N4 powder with 90.6 wt% α-phase can be prepared by combustion synthesis, where the maximum reaction temperature is close to the melting point of Si (Figure 3). This is a breakthrough beyond the conventional knowledge that the diluent is indispensable for successful combustion synthesis of Si3N4.

Figure 3: The effects of NH4Cl content on the combustion temperature, Tc and velocity, u. Effect of Particle Size and Activity of Si Powder Zakorzhevskii investigated the effects of particle size of Si powder on reaction temperature, phase composition, and specific area of the product in combustion synthesis of Si3N4 [9]. This investigation revealed the smoldering phenomenon of submicron Si powders, where the self-sustained nitridation reaction happened below the melting point of Si. A typical temperature curve is shown in Figure 4. The smoldering mechanism plays a key role in combustion synthesis of high α-content Si3N4 powders. The drawback of this method is the low yield and long reaction period owing to the high diluent content of >66 wt%.


55

CHAPTER 5 Gel-Combustion Synthesis Slavko Mentus University of Belgrade, Faculty of Physical Chemistry, Studentski trg 12, 11158 Belgrade, Serbia; Tel/Fax +381 11 2187133; E-mail: [email protected] Abstract: Recently gel combustion synthesis was widely used to synthesize various forms of simple or complex metal oxides, having a wide range of applications. The chapter, after introductory remarks, begins with the ways of preparation of gelled oxide precursors, their physicochemical characteristics and their ability to burn in terms of chemical composition. Then follows a survey of the synthesis of various ceramics materials (electronic superconductors, ionic conductors, dielectrics, ferroelectrics), catalysts (non-supported and supported) electrode materials for chemical power sources (anode and cathode materials for ion-lithium cells), in a powdery form. Finally, the methods to transform the powdery gel combustion products to other valuable materials, for example to bulk oxide materials, or to films and powders of metals and alloys, will be described.

GEL-COMBUSTION SYNTHESES : GENERAL REMARKS The gel combustion method of synthesis is a simple and short way to produce pure powders, in most cases nanopowders, and in some cases films spreaded along a suitable support, of a great number of simple and complex metal oxides or their mixtures, in both small and large quantities. Combustion synthesis is an easy and convenient method for the preparation of a variety of advanced ceramics, catalysts and nanomaterials. In the shortest words, this method consists in dissolving metal salts together with a suitable organic complexing agent (multifunctional carboxilic acid, aminoacids, amines etc.), evaporation of solvent to obtain homogeneous gel, called oxide precursor gel, and ignition of the gel in order to remove organic matter. The complexing agent helps to obtain homogeneous liquid solutions, even if one deals with metal salts able to hydrolyse. After gelatinization, it serves to hold metal cations distanced mutually and distributed uniformly throughout the gel, in order to prevent either the formation of coarse oxide particles of monocomponent oxides, or the segregation of different phases in the case of multicomponent systems. In the course of combustion, complexing agent burns, leaving finely dispersed metal oxides as main product. The use of nitrate salts provides the presence of internal oxidizer: nitric acid. One may also add independently oxidizers which are completely degradable to gases, such as ammonium nitrate or ammonium perchlorate. The combustion temperature may be adjusted by the choice of both type of complexing agent („fuel“) and the concentration ratio fuel/oxidizer. To burn the gels which contain either insufficient amount of internal oxidizer, or no oxidizer at all, the availability of external oxygen source (for example atmospheric air) is necessary. Namely, there are the examples that metal acetates, carbonates or other thermodegradable salts with no oxidizing function were used to prepare gel. The thermally induced redox reaction between an oxidant and a fuel may be described in terms of propellant chemistry [1]. A lot of gaseous products that develop during combustion (water vapor, CO2, N2, NOx) helps to obtain very dispersed solid product, with the mean particle dimensions mostly in the nanometer range. Low combustion temperature favors low particle dimensions and reduces phase separation, but allows more impurities (carbon particles, hydroxides, carbonates etc.) to reside in the product. An additional thermal treatment in air enables a balance between particle dimensions and purity, through the choice of temperature and time of treatment. Depending on the authors, the variances of this method bear various titles. The earliest one, called often polymerizable complex method (PC), is based on Pechini patent [2], with ehylene glycol - citrate acid ester as both solvent and complexing agent. Some years later, metal nitrates with either glicine or citric acid in aqueous solution were used to obtain gel. These gels were able to autoignite thanks to the presence of nitric acid, and the methods were called glycine–nitrate auto-combustion (GNA), and the citrate–nitrate auto-combustion (CNA) syntheses. More recent common name is aqueous combustion synthesis (ACS). Gel combustion methods compete succesively to the formerly widely used sol-gel methods based on the hydrolysis of metal alkoxides, since use less expensive raw materials, and much less are sensitive to the variations in process parameters, providing thus much higher security in the synthesis of the desired product. This is also low temperature alternative to the solid state reaction syntheses, and a time sparing alternative to the Maximilian Lackner (ED) All rights reserved - © 2010 Bentham Science Publishers Ltd.


Slavko Mentus

mechanochemical („ball-milling“) syntheses. This explains why it experiences an explosive development in last several decades. The gel combustion methods are similar to the so called self propagating reaction methods, developed with the compounds containing both oxidizing and reducing groups, like (NH4)2Cr2O7, which decomposes with deliberation of a lot of gaseous product (N2, H2O) and leaves a voluminous finely granulated Cr2O3. The iron containing complexes, Fe(N2H3COO)2 (N2H4)2 and N2H3Fe(N2H3COO)3.H2O and their solid solutions, thanks to the combustible part of their molecules (the precursor already contains the organic fuel in the molecule), may be ignited at nearly 120 oC by an open flame and combust in air yielding nanosized Fe2O3, as reviewed by Patil et al.[1]. However, fixed stoichiometry of these compounds limits the possibility to vary the quality of final product by variation of reaction conditions. Thanks to a possibility to allow variation in stoichiometric relations metal/complexing agent/internal oxidizer inside the wide limits, the chemistry of synthesis is not always well known and documented. This is not a significant disadvantage for practical use of this metod, having in mind that the quality of the products varies relatively slowly with the variations in reaction parameters. The gel combustion method was widely used to produce complex and multicomponent oxides, suitable for various types of further finishing procedures. Namely, modern functional oxide materials are rarely simple oxides or monophase systems. Gel combustion method allows to scientist practically unlimited possibities to synthesize any simple, doped monophase or complex and multiphase oxide systems. Researcher may easily apply its creativity in design the materials of controlled morphology, homogeneity and desired physicochemical behavior. By use of gel-combustion methods, design and tailoring of multicomponent oxide materials of high homogeneity is not more so complex task as it was before two decades only. Much examples of succesively solved tasks will be outlined in this chapter. The purpose was not to cite all published examples, which are too numerous, but to display characteristic examples which illustrate the possibilities of the gel combustion methods. A SURVEY OF THE WAYS TO PREPARE OXIDE PRECURSORS GELS Citric Acid-Ethylene Glycol Based Gels In an original Pechini patent [2], related to the fabrication of thin film capacitors, a multifunctional organic acid: citric acid (CA), capable to bond the metal ions into stable chelate complex compounds, and a diol: ethylene glycol (EG), which serves as both solvent and reactant in the reaction of polyesterification, form threedimensional polymer network, in which metal ions were distributed homogeneously to the molecular scale. Citric acid may be dissolved in ethylene glycol in a wide range of CA:EG molar ratios. The esterification reaction between citric acid and ethylene glycol takes place already at room temperature, and may be accelerated by heating. Many metal salts: nitrates, acetates, chlorides, carbonates, isopropoxides or other suitable metal compounds may be easily dissolved in the CA-EG liquid mixture to form initial solution. Usually CA is added in a stoichiometric excess, to enable full transformation of dissolved metal salt to the metal-citrate complex.

CO 2 H HO 2 C

CH 2

C

CH 2

CO 2 H

OH Figure 1: Structural formula of citric acid To conduct the procedure, the temperature of the solution should be increased to 100-130 oC to accelerate the reaction between free citric acid and ethylene glycol. This leads to an rapid increase of viscosity ("gelation") primarily due to the polyesterification reaction, resulting in the formation of transparent flexible gel. Inorganic acids deliberated in the course of esterification leave the systems by evaporation. The temperature rise is continued up to 450 oC in air, in order to remove the excess of solvent and to oxidize all organic matter. Only atmospheric oxygen participates in the combustion as the oxidizer. The main volatile products of combustion are water and carbon dioxide. The solid combustion product should be the metal oxide, but often the traces of impurities such as carbon, carbonates and hydoroxide remain in the product, what requires an additional purification, mostly by heating in air.

Gel-Combustion Synthesis


According to the experience, [3,4] CA concentration in the range 50 - 60% provides the highest viscosity of the gel. The rate at which the viscosity increases during solution heating, depends on the nature of the cations. This means that metal ions participate in the cross-inking processes inside the solution. The formed polymer preserves homogeneity of solution close to its initial level, by reducing the mobility of metal cations along the polymer network. A paper by Tai et al.[3] describes in detail the behavior of CA—EG system during thermal treatment involving solvent removal, gelation and thermal decomposition. The behavior of multifunctional organic acid, in this case citric acid, is of high importance for the functionality of this as well as of other further derived methods. As usual for multifunctional acids, the dissociation constants of various carboxilic groups differ mutually. The middle carboxylic group is the most acidic one. In water solution its dissociation constants amounts to pK =2.91 [5], therefore, pH of an aqueous solution is in the range of 0-2, depending on concentration. The terminal carboxylic groups are much less acidic, pK = 4.36 and 5.74 [5], and their dissociation becomes substantial just at high pH values. The hydroxy group of citric acid may also become deprotonated (pK = 10.96) at very high pH values [6]. The OH group participates in the formation of hydrogen bonds with carboxylic groups, and this contributes to the overal stability of gel. Since esterification is a reversible process, the equilibrium is shifted toward polyester formation by removal of water from the reaction medium during heating to obtain gel. Although the boiling point of EG is the lowest among diols, EG evaporates, too. The complete removal of the excess of unreacted EG rarely occurs during very process of polyesterification. The variation in CA/EG ratio helped to achieve desired quality of the final product. In the Pechini patent polymer pyrolysis must be completely avoided in this case. Rosario et al. [7] found that CA/EG ratio of 1:4 is also suitable for fabrication of dense electrochromic films of Nb2O3, although the ratio of CA/EG of even 1:16 was needed for preparation of flat films free of agglomerates. On the other hand, Liu et al [8] stated that the quality of thin films depends primarily from the metal ion/citric acid ratio. CA/EG = 1: 4 was used to produce BaTiO3 in a form od dense film, for capacitor purposes. Foaming during the Strong foaming may prevents segregation during thermal decomposition of the polymer. Mild burning at relatively low temperature provides relatively slow grain growth. Foaming helps to produce fine powders. When more than one salt were dissolved in order to synthesize complex oxide, no each cation react with the complexing agent in the same way. According to Raman, IR and I3C-NMR spectroscopic data, in the synthesis of barium titanate, no any interaction of citric acid with barium ion may be detected [9]. This is expected since alkaline earth citrate complexes are quite unstable. Generally, transition metals build more stable complexes with citric acid. At high pH, when -OH group is deprotonated, transition metals may form binuclear complexes. Thus, the inluence of pH is a new field of investigation in this type of synthesis. The addition of titanium isopropoxide to CA solution caused the shift of carboxylic groups peaks in C-NMR spectra, which was described as the formation of citrate complex [9]. Simultaneous presence of barium and titanium in the solution produced the effect of interactions which may not be explained in terms of barium citrate and titanium citrate separately [9]. Such interactions may change the solubility of the salts, generally increasing it. The problems experienced during the practical use of this method are listed below. They are common with the problems in the use of similar methods derived later [10]: • Formation of precipitate before and during gelation. This is caused by hydrolysis particularly when one deals with the salts of very oxofilic cations like Ti, Nb, Ta, etc. • Reduction of metal ions. Etylene glycol may reduce some electropositive metals, particularly on heating. For instance, the reaction of coper salts with ethylene glycol results in the formation of Cu2O and even metallic copper. The use of compositions that does not require much EG excess, does not require high temperature to evaporate EG, and thus the reduction of metal ions by ethylene glycol may be significantly eliminated. • Evaporation or decomposition of one of the components. During the synthesis of Hg-based hightemperature superconductors, for example, pyrolysis of the polymer starts at 400-450 °C, which is much above the decomposition temperature of mercury oxide and evaporation of mercury.

72


CHAPTER 6 Cellulose-Assisted Combustion Synthesis of Functional Materials for Energy Storage or Conversion Rui Cai, Wei Zhou and Zongping Shao State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry & Chemical Engineering, Nanjing University of Technology, No.5 Xin Mofan Road, Nanjing 210009, P.R. China Abstract: Combustion synthesis has attracted considerable attention recently for its advantages of low processing cost, high energy efficiency, and high production rate. This chapter presents the preparation of functional oxide materials for energy storage or conversion by cellulose-assisted combustion synthesis. In traditional solid-solution-phase combustion synthesis, e.g., the glycine-nitrate process (GNP), a great quantity of gas is evolved during the synthesis, which can create large amounts of ash by blowing away the products. Natural cotton fibers with a hierarchical pore structure were used as a micro-reactor for the GNP in this study. This novel process is environmentally friendly. Furthermore, the resulting particle size was smaller, which was attributed to the blocking effect of cellulose on inter-particle contact during the synthesis. The method was applied for the synthesis of samaria-doped ceria (SDC) as an electrolyte for solid-oxide fuel cells (SOFCs). SDC powder with a particle size as small as 10 nm was obtained, which was easily sintered to form a dense electrolyte at 1350 oC, several hundred degrees lower than that prepared from the traditional solid-state reaction. La0.6Sr0.4Co0.2Fe0.8O3 perovskite oxide was also prepared and showed higher purity and better cathode performance in SOFCs than that prepared by a sol-gel process. By adopting the same method, phase-pure spinel Li4Ti5O12 could be synthesized at 700 oC. The resulting powder had an excellent rate performance in secondary lithium-ion batteries, with a capacity of 140 mAh g-1 even at a 10 C discharge rate. More importantly, solid TiO2 oxides can also be utilized as the raw materials for this synthesis, making the process highly cost-attractive.

INTRODUCTION It is well known that the performance of many functional materials is closely related to their physical properties, e.g., surface area, particle size, particle-size distribution, surface morphology, porosity, and purity; such properties are highly dependent on the synthesis technique by which they are produced. There is considerable interest in solid-oxide fuel cells (SOFCs) and secondary lithium-ion batteries for high-efficiency energy conversion and storage [1, 2]. SOFCs are typically composed of an oxide cathode, an oxide electrolyte, and a nickel-oxide cermet anode. The performance of an SOFC is closely related to the electrode morphology. Nano-sized electrodes can greatly increase the electrochemical reaction area, resulting in increased electrode performance. With the ever-diminishing fossil fuel situation and the low energy-conversion efficiency of the combustion engine, there is currently great interest in the development of electric vehicles. Secondary lithium-ion batteries are believed to be one of the most promising types of batteries for such applications due to their high power and energy density, lack memory effect, and non-toxic components [3]. However, for such applications, high charge and discharge rates are frequently required. For traditional electrodes constructed from coarse powder, poor rate performance has been observed. It has been found that the performance of some electrode materials, such as Li4Ti5O12 anodes and LiFePO4 cathodes, can be improved significantly by decreasing their particle size to the nano-range [4,5]. Traditionally, these composite oxides were prepared by a standard ceramic process, in which a metal oxide, a metal hydroxide, and a metal carbonate were typically employed as the raw materials. As the solid-phase reaction first occurs at the two-phase boundary, the intermediate products created block further reaction between the reactants. Therefore, intermittent grinding in combination with a very high calcination temperature is needed to overcome the diffusion block. This synthesis process is time-consuming and energy-intensive. Furthermore, the resulting powders are severely aggregated with poor surface area. For the synthesis of more complicated oxides containing several metal elements, the products frequently contain impurity phases and suffer from inhomogeneous phase compositions. Furthermore, the high calcination temperature can also lead to the evaporation of high-boiling elements such as barium, resulting in a cation non-stoichiometry in the synthesized oxides. Combustion synthesis has received considerable attention recently [6-8]. In a conventional combustion synthesis, the initial reaction medium is a mixture of particles with size scales in the range of 1-100 μm. Such a process has the advantages of low processing cost, high energy efficiency, and high production rate. After reaction initialization, this mixture burns at relatively high temperatures in a self-sustained manner, forming the material of a desired composition [9]. The above feature makes it difficult to obtain nano-sized products. Recently, a Maximilian Lackner (ED) All rights reserved - © 2010 Bentham Science Publishers Ltd.

Cellulose-Assisted Combustion Synthesis


so-called solution combustion synthesis has received considerable attention [10]. This process typically involves a reaction in a solution of metal nitrates and different fuels, such as citric acid, glycine, or urea. It has proved to be a novel, extremely facile, time-saving, and energy-efficient route for the synthesis of ultrafine powders. One difficulty associated with solution combustion synthesis is the management of the reaction products during the synthesis. The combustion typically produces a huge amount of gas, which can easily blow away some of the reactants. It then leads not only to the loss of products, but also has a serious impact on the environment. The purpose of this study was to introduce a novel method for the low-temperature combustion synthesis of nano-sized functional materials for efficient energy conversion and storage applications, i.e., a cellulose-assisted combustion synthesis of functional composite oxides. The combustion reactions were constrained within the pores of cotton fibers to make the combustion process more manageable. The porous walls of the cellulose also prevented aggregation of the in situ-synthesized powder, as a result facilitating the synthesis of nano-sized powder. The as-synthesized powders showed reduced sintering temperature for densifying the SDC electrolyte, improved the cathode performance of La0.6Sr0.4Co0.2Fe0.8O3 for electrochemical oxygen reduction, and increased the rate performance of Li4Ti5O12 spinel oxide in secondary lithium-ion batteries. BASIC PROPERTIES OF CELLULOSE Cellulose is the structural component of the primary cell wall of green plants, and the most common organic compound on earth; about 33 % of all plant matter is cellulose. For industrial use, cellulose is mainly obtained from wood pulp and cotton. The cellulose content of cotton is 90 % and that of wood is 50 %. It is mainly used to produce cardboard and paper; to a smaller extent it is converted into a wide variety of derivative products such as cellophane and rayon. Converting cellulose from energy crops into biofuels such as cellulosic ethanol is under investigation as an alternative fuel source.

Figure 1: The hierarchical structures of cotton cellulose. C, Cuticula layer; L, lumen; P, primary cell-wall layer; S1, secondary cell-wall layer 1; S2, secondary cell-wall layer 2.


Rui Cai et al.

Recently, owing to the unique hierarchical wall structure, cellulose has also been widely applied in the chemical synthesis process. Fig. (1) shows the typical structures of cellulose from microscopic to macroscopic. Cellulose is an organic compound with the formula (C6H10O5)n, a polysaccharide consisting of a linear chain of several hundred to over ten thousand β (1→4) linked D-glucose units [11, 12]. The polysaccharide chains are assembled to constitute a fibril. The morphological hierarchy is defined by elementary fibrils (between 1.5 and 3.5 nm), micro-fibrils (between 10 and 30 nm), and microfibrillar bands (on the order of 100 nm) [13], which are connected with each other three-dimensionally. The cell walls, composed of these interlaced hierarchical fibrils, are replete with hierarchical holes or apertures. These walls are enclosed to form the lumens, which are open one side, with a diameter on the micrometer level. The reported specific area of cotton fibers is in the range of 30-55 m2 g-1 [14]. Various products ranging from metal to oxides with a hierarchical structure have been prepared by using cellulose as the template due to its distinct fiber morphology [15–18].

Figure 2: SEM images for naked cellulose (a); cellulose–GN precursor (b); as-prepared fresh metal oxide (c). THE ROLE AND FUNCTION OF CELLULOSE IN THE COMBUSTION PROCESS In the cellulose-assisted combustion process, cellulose served as a micro-reactor for the combustion. The first and the most important consideration for the reaction to proceed is whether the metal-organic precursor can be well absorbed into the pores and lumens of the cellulose. Fig. (2a) shows a typical SEM image of the pure cellulose. The dry activated cotton fiber was in a twisted flat tube shape. After impregnating the cellulose with the glycine-nitrate (GN) precursor, the cotton fiber became round and robust, suggesting that the GN was successfully absorbed into the pores and lumens of the cotton fiber, Fig. (2b).


83

CHAPTER 7 Related Processes to Combustion Synthesis Yasutaka Ando Ashikaga Institute of Technology, Japan; E-mail: [email protected] Abstract: Since combustion chemical reaction generates not only thermal energy but also chemically activated particles such as ions and radicals, thermal plasma vapor deposition and combustion flame diamond deposition process are introduced as related process to combustion synthesis in this chapter. This chapter consists of two sections. In the former section, characteristics of thermal plasma are explained. In the latter section, researches on thermal plasma vapor deposition and/or combustion flame diamond deposition process are explained. As the thermal plasma process, thermal plasma CVD (including DC arcjet CVD and solution precursor thermal spray (SPPS)), thermal plasma PVD (including arc ion plating) are introduced. For example, synthesis mechanisms of the films and/or particles and effects of experimental conditions on the growth of the films (or particles) are discussed in cases of the diamond particle synthesis by combustion flame method, SiC and diamond films deposition by thermal plasma CVD and anatase rich TiO2 film deposition by SPPS.

INTRODUCTION In combustion synthesis method, chemical reaction is promoted by thermal energy generated during combustion chemical reaction. However, in some combustion synthesis processes, chemically activated species, such as ions and radicals, as well as thermal energy are utilized in order to promote chemical reactions like thermal plasma vapor deposition processes. As the related process, thermal plasma vapor deposition processes and combustion flame diamond deposition process are introduced in this chapter. Thermal plasma is a plasma with high temperature heavy particles such as atoms, molecules and radicals. Therefore, since it is very difficult to apply thermal plasma to plasma source for promotion of chemical reaction in the industrial field of device processing because of thermal damages of the substrate during operation, this plasma has been mainly used as heat source of welding, thermal spray, garbage furnace and so on. However, since reactive thermal spray was developed early in the 1970s, reactivity of thermal plasma started to be taken into account as plasma source. Recently, thermal plasma is utilized as not only heat source but also plasma source for promotion of chemical reaction in case of chemical vapor deposition process (CVD) and physical vapor deposition process (PVD) and so on. Especially, arc ion plating has been successfully used in the field of coating process for cutting tools. This chapter consists of two sections. In the former section, characteristics of thermal plasma are explained. In the latter section, researches on thermal plasma vapor deposition and/or combustion flame diamond deposition process are explained. THERMAL PLASMA Preface Plasma is defined as a partially ionized gas which includes a certain proportion of electrons and ions as well as atoms and molecules. There are two types of plasmas called thermally non-equilibrium plasma and thermally equilibrium plasma. Thermally non-equilibrium plasma is a plasma whose electron temperature Te is much higher than those of ion Ti, atom Ta and molecule Tm (Te>>Ti, Ta, Tm). Thermally equilibrium plasma is a plasma in which the temperatures of all particles are almost the same. Since electron temperatures are over 10000 K in any cases, thermally non-equilibrium plasma and thermally equilibrium plasma are called “low temperature plasma” and “thermal plasma”, respectively. Conventionally, in semiconductor device fabrication process, the low temperature plasma has been mainly used and thermal plasma has been used only as heat source of welding and thermal spraying. However, since thermal plasma CVD, mentioned in sub-section 8.3.1, was developed in the end of 1980s, the thermal plasma has been regarded as plasma source for promoting chemical reaction rate. Fig. (1) shows temperatures of electron and heavy particles (ion, atom and molecule) as a function of ambient pressure. In both cases of arc discharge and high frequency plasma, mentioned in sub-section 8.2.2, since only electrons are accelerated because of its light weight at the beginning of discharge, the plasma is generated as low temperature plasma even on the condition of atmospheric pressure. However, when the duration time is long, since collisions among particles occur, finally, the plasma becomes thermal plasma. With increasing ambient pressure, the transition time from low temperature plasma to thermal plasma become short. Therefore, the thermal plasma is stable plasma compared to the low temperature plasma in atmospheric pressure. Thermal plasma can be regarded ionization equilibrium plasma as well as thermally equilibrium plasma. Hence, fractions of ions and electrons can be indicated as degree of ionization by calculation using Saha’s equation (Eq. Maximilian Lackner (ED) All rights reserved - © 2010 Bentham Science Publishers Ltd.


Yasutaka Ando

8.1) [1], electroneutrality of the plasma (Eq. 8.2) and equation of state (Eq. 8.3). The degree of ionization is 1percent at 10000K and 50 percent in 15000 K, respectively [2].

Temperature [K]

105 Te (Electron temperature) 104

103! Tg (Heavy particle temperature） 102! 10-4

10-2

1 102 Pressure [Torr]

104

106

Figure 1: Relationship between plasma temperature and ambient pressure.

(pAr+)(pe-) (pAr)

2Тme = 2! ! !2 !

h

3/2

!

Z(Ar +) - EI (kT)5/2 ! ! ! Ø exp Z(Ar) RT

(Eq. 8.1)

Where, Z(Ar), Z(Ar+): Distribution functions of Ar, Ar+, k: Boltzmann constant, h: Plank constant, EI: Ionization energy.

Ne

n

(Eq. 8.2)

≖ Z NZ!

Z=1

Where, Ne: Number of electrons, Z: Charge number, Nz: Number of particles with Z in charge number. n

PV

≖ NZ

Z=1

Ne

(Eq. 8.3)

kT

Where, P: Pressure of the plasma, V: Volume of the plasma. Plasma Generation Method As thermal plasma generation method, there are various types of methods. In the cases of DC and AC power source, thermo electrons generated by heating of cathode are accelerated by electric field between cathode and anode, and finally, plasma (arc discharge) was generated by collision between the electron and the heavy particles. In both cases, there are two types of plasma torches called transfer type and non-transfer type, shown in Fig. (2). In welding, transfer type has been mainly used and non-transfer type has been used for thermal spray. Fig. (3) shows the temperature profiles of these thermal plasmas generated by transferred arc and non-transferred arc plasma torches [3][4]. In both cases, plasma temperature was raised to 20000-30000 K near the electrode points.

Cathode

Cathode －

Power supply ＋

Working gas Nozzle (insulator) Arc

－

Power Supply ＋

Working gas Nozzle (Anode)

Arc Plasma Jet

Plasma jet Target (anode)

a) Transferred arc plasma torch

b) Non-transferred arc plasma torch

Figure 2: Schematic diagrams of the transferred arc and non-transferred arc plasma torches.

Related Processes to Combustion Synthesis

2

Radial distance from the center (mm) 4 6 8 10 12

14

8000 6000 9000 7000

Axial distance from tip of the cathode (mm)

0


5000 Plasma jet 4000 3000

10000 2000 [K]

0

20

40

60

80

100

120 [mm]

Plasma torch

a) Transferred arc plasma

b) Non-transferred arc

Figure 3: Temperature profiles of the transferred arc and non-transferred arc. Fig. (4) shows schematic diagrams of the high frequency plasma torches. In cases of the radio frequency (RF, 13.56MHz) plasma and the microwave (MW, 2.4GHz-) plasma, plasma was generated by following procedure. * Energy of gas atom or molecule is raised because of vibration of electrons in the gas atom or molecule when electric power is applied. * So, ionization of the atom or molecule occurs due to the above mentioned vibration. * By the collision between electron generated by the ionization and heavy particle, the gas is ionized, that is, plasma is generated. MC* Electrode

High frequency coil

Working gas Substrate

Working gas

Power supply

MC* Substrate

Electrode

Power supply

Exhaust a) Capacitively coupled plasma

b) Inductively coupled plasma

Figure 4: Schematic diagrams of the high frequency plasma torches. (*MC: Matching circuit) As for the feature of these cases, with increasing the frequency of electric current, since length of the path of electron becomes short and collision frequency between electron and heavy particles decreases, transition time from low temperature to thermal plasma becomes long even in atmospheric pressure. In these cases, two types of torches called “Capacitively coupled plasma”, “Inductively coupled plasma” have been practically used. In the capacitively coupled plasma, the plasma is generated by two plane electrodes which are located in parallel. On the other hand, in the inductively coupled plasma, the plasma is generated by coil shaped electrode. Since the inductively coupled plasma has the following advantages, this type of torch has been used for plasma source as film deposition process. * Since the plasma is generated without electrode spots in inductively coupled plasma, lifetime of the torch is long compared to the other types of electrodes and film deposition can be conducted without pollution of the film by elements of the electrodes.

98


CHAPTER 8 Combustion Synthesis Melt Casting Qinling Bi, Licai Fu, Jun Yang, Weimin Liu, Qunji Xue State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China; Tel: +86-931-4968193; fax: +86-931-8277088; E-mail address: [email protected] (Q. Bi) Abstract: The combustion temperature of a highly exothermic reaction can be above the melting point of the end products, which results in the formation of melt-casting products. Combustion synthesis melt casting technique possesses remarkable advantages for the low-cost production of structural and function materials with unique properties and characteristics. In this chapter, some combustion synthesis melt-casting reaction systems developed in recent years, such as refractory compounds, intermetallics, as well as advanced ceramics, are introduced, and the solidification mechanisms are discussed.

INTRODUCTION The melt-casting process is rather common to fabricate near-net shape materials for metals, alloys, or composites. However, most of the ceramics have not been fabricated using melt-casting. The main obstacles for the application of melt-casting on ceramics lie in three aspects: First, the temperature required for melting ceramic powders is usually too high to be achieved by conventional techniques. Second, the melt-casting is difficult to eliminate pores, which decreases their mechanical properties. Third, the slow cooling rate means long duration at elevated temperature after crystallization of the melt, which leads to coarse grains with poor toughness or low hardness [1-3]. The method of self-propagating high temperature synthesis (SHS) was founded by Merzhanov and coworkers in the 1960s [4]. SHS process evolves large sums of heat and forms a solid product in the chemically active systems, and shows some excellences such as self-sustaining reaction, high purity, high productivity and so on [5-8]. One of the unique SHS process is combustion synthesis melt casting. When the combustion temperature of the combustion reaction is above the melting point of the end products, melt-casting bulk materials can be obtained. The combustion synthesis melt-casting process combine the advantages of SHS and melt casting techniques and develops a convenient and economic approach to obtain dense, near-net shape components, especially ceramics.

Figure 1: Processing routes in combustion synthesis melt casting technology As can be seen in Fig. 1, the combustion synthesis melt casting products, including bulk materials, coatings and functionally graded materials (FGMs), can be obtained by combustion synthesis melt casting or by direct SHS. Some of the combustion synthesis melt casting materials and techniques are listed in Tab. 1. It shows most of the interests are fastening on the research of new approaches of syntheses of hard alloys, multifunctional materials, ceramics, and various composites. The attractive direct synthesis consists of centrifugal-casting and pressureassisted casting. Maximilian Lackner (ED) All rights reserved - © 2010 Bentham Science Publishers Ltd.

Combustion Synthesis Melt Casting


Table 1: Combustion synthesis melt casting techniques and prepared materials. types

Bulk

Coating

FGMs

Compositions

Techniques

Refs

Mo5Si3

Microwave activated SHS

[9]

TiB2-TiC

High-pressure SHS

[10]

Fe-Cr-Ni/Al2O3

Thermite reaction

[11]

Ni-Ti-C/B4C/B

SHS-arc melting-suction casting

[12]

Ti base/TiB

SHS- non-consumable arc-melting

[13]

Cu-MoSi2

SHS-casting

[14]

Ti-Si

SHS-casting

[15]

Zr–Si

SHS-casting

[16]

Sn-Pb

High-pressure centrifugal infiltration

[17]

TiC-TiB2-MexOy

pressure assisted-SHS

[18]

MgB2

SHS-casting

[19]

NiSiCr

Applied gas pressure assisted SHS

[20]

MoSi2-SiC


[21]

Ni3Al


[22]

FeAl

Field activated pressure assisted synthesis

[23]

Al2O3

SHS-ultrahigh gravity

[24]

C-C

CS

[25]

Cu-TiB2

SHS-quasi-static consolidation

[26]

TiC/Ni3Al

SHS-casting

[27]

IrAl

SHS-casting

[28]

NiAl

SHS-high concentrated solar energy

[29]

TiC-TiB2/Fe

SHS-argon arc cladding

[30]

NiAl

Centrifugal thermite

[31]

Al2O3TiO2TiC/ AlFe-AlCrFe-NiFe

SHS-centrifugal casting

[32]

MoSi2-MoS2


[33]

Ni3Al-Cr7C3


[34]

Ni-Cr-TiC

SHS- plasma densification

[35]

FeCr-TiC

SHS-laser glaze

[36]

TiC-Al2O3

SHS-hot pressing

[37]

MoSi2-TiB2

SHS- vapor deposition

[38]

Fe-TiC

SHS-centrifugal casting

[39]

Ti-B

SHS-punching

[40]

MoSi2-SiC

SHS-hot pressing

[37]

MoSi2/Al2O3

SHS-tape casting

[41]

Al2O3/YAG/YSZ

SHS-high gravity

[42]

In this chapter, some combustion synthesis melt-casting reaction systems developed in recent years, such as refractory compounds, intermetallics, as well as advanced ceramics, are introduced, and the solidification mechanisms are discussed. COMBUSTION SYNTHESIS MELT CASTING TECHNIQUES Combustion Synthesis Melt Casting Bulk Materials Large numbers of the bulk materials have been prepared by combustion synthesis melt casting in recent years. The bulk materials contain metal matrix composites, intermetallics, ceramics, and so on. These combustion synthesis melt casting materials possess unique microstructures and excellent mechanical and chemical properties, such as high strength, oxidation resistance, corrosion resistance and wear resistance, etc.

100 Combusttion Synthesis: Noovel Routes to Novvel Materials

Qinling Bi et al.

Starting frrom powder mixture m of MoO M 3–CuO–All–Si system, Cu–MoSi2 coomposite has been fabricatted successfullly by the SHS and casting [14]. Four densse ceramic parrticulate reinfoorced nickel matrix m composittes have been fabricated froom Ni–Ti–C, Ni–Ti–B, Ni––Ti–B4C and Ni–Ti–C–B syystems, respecctively, by SH HS a melting andd suction castinng [12]. reactions, arc Compared with tradition nal casting matterials, the com mbustion synth hesis melt castting bulk prodducts show good e Przybylski et properties, such as high strength, oxidattion resistance,, corrosion resiistance and weear resistance, etc. M 2, which has h been fabriccated by this technique, is a hard, type II al [19] haave shown thaat the bulk MgB superconduuctor. The superconducting transition t of thhe MgB2 is verry sharp and vvery sensitive to t the ac and dc magnetic fields. fi The critiical current den nsity is extractted from the ab bsorption susceeptibility via a superconductinng critical staate model. Thee bulk Ni3Si intermetallics i with 20, 30, 40 wt% Cr hhave been fabrricated by usinng combustion n synthesis meelt casting undeer 7 MPa applied gas pressurre [20]. All of tthe NiSiCr allooys consist of β1Ni3Si, β3-N Ni3Si and euteectic (β1-Ni3Si+ + Ni5Si2) (Figg. 2). The amoount of β3-Ni3Si phase decrreases as the Cr content deccreases. The micro-hardness m s of the alloys increases with h Cr content and a the alloy with w 30 wt% Cr possesses the t highest compressive streength (1.8 GPaa). The wear rate r of the NiSiCr alloys deecreases with Cr content (Fiig. 3). It indicaates that the opptimal propertties can be obttained by moddifying the com mpositions of the t combustion n synthesis.

Figure 2: Microstructure M e of the Ni3Si alloy with 20, 330, 40 wt% Cr[[20]

a

18

pure Ni3Si Ni3Si+20%Crr

16 Wear rate (10 mm /m)

3

1.5

-4

Compressive strength/GPa

1.8

1.2

Ni3Si+30%Crr Ni3Si+40%Crr

14 12 10 8 6

0.9

4 0

20

Cr content wt%

40

30

40

50

60

70

80

Load (N)

Figure 3: (a) micro-hard dness of the NiSiCr N alloys, (b) wear ratess of the NiSiC Cr alloys with applied loads at sliding speed of 0.05m/s[[20] operties of thee bulk MoSi2–SiC – compositees, La et al [221] also studieed the relation of the compoositions and pro which weree also prepared d under 7 MPaa applied gas prressure. The to oughnesses of the t compositess with 10, 15 annd 20 wt.% SiiC are investiggated comparattively. Silicon carbide phase presented in thhe composites is in the form of large particcles or short fibbers dependingg on the amouunt of SiC (Fig g. 4). Cracks exxist near interffaces of the larrge SiC particlles and the mattrix in the com mposites with 10 1 and 15 wt.% % SiC, whereaas not appears in i the composite with 20 wtt.% SiC. The higher h of the SiC content is, the finer the particles p and fibbers become, and a the more the t areas of innterface exist. The T areas of interface i increaase with the inncreasing of thhe SiC contennt. Therefore, the t

108


CHAPTER 9 Combustion Synthesis of Carbon-Encapsulated Nanoparticles Jolanta Borysiuk*, Mateusz Szala. Agnieszka Grabias and Jacek Szczytko *Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland, Department of Physics, Warsaw University, Hoza 69, 00-681 Warsaw, Poland; Fax: +48 22 864 54 96; E-mail: [email protected]; Institute of Chemistry, Military University of Technology, Kaliskiego 2, 00-908 Warsaw, Poland; Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland and Department of Physics, Warsaw University, Hoza 69, 00-681 Warsaw, Poland Abstract: Synthesis of carbon encapsulated metal nanoparticles by rapid, thermally enhanced synthesis reactions (combustion) is reviewed. Both technical apparatus, the methods, and analysis of the fundamental issues are discussed. In technical issues, the sample preparation procedures with the particular emphasis on their structure and chemical composition is given. Various methods of the reaction ignition and control are assessed. The reaction post processing procedures with particular emphasis on the removal of the by-products is discussed. Different metals that can be encapsulated by the carbon graphite-like layers are mentioned and the reaction parameters are listed. Discussion of the fundamentals of combustion methods includes the principles of the processes, such as their thermodynamic basis, involving the temperature and pressure dependence resulting in different efficiency of the synthesis. The reaction products are characterized including grain size and structure, the crystallographic properties such different phases, change of chemical composition due to segregation, etc. The characterization techniques include the chemical analysis, Transmission Electron Microscopy (TEM), SQUID (Superconducting Quantum Interference Device magnetometer) magnetic measurements and Mössbauer Spectroscopy (in the case of Fe-containing samples). The combustion synthesis is critically compared with other methods. Possible applications in various branches of technology, medicine, environment protection and other areas are listed.

INTRODUCTION For very long time carbon was known as the fundamental component of a plethora of different chemical species. These compounds create foundation of the living organism, including plants and animals. It is therefore understandable that these compounds were subject of intensive research, creating an immense branch of science known as organic chemistry. The investigations in these areas were very intense, resulting in many discoveries which affected not only scientific understanding of fundamentals of chemistry, physics and biology, but also transformed many areas of industrial activity, and deeply changed our everyday life. It is therefore surprising, that the subject was not exhausted, on the contrary, recent discoveries channeled interest of many researchers in this direction. Among significant achievements in the area, the most prominent was the discovery of nanosize carbon structures, such as fullerenes [1,2], carbon nanotubes (CNT) [3] or graphene [4-6]. These discoveries heralded advent of new branch of science and technology – nanotechnology. The importance of these discoveries was underlined by the Nobel Prize in chemistry awarded in 1987 to H. W. Kroto, R.E Smalley and R.F. Curl for the discovery of fullerenes. The discovery of carbon nanotubes by S. Ijima enhanced scientific interests in this area. The CNT structures have interesting optical and electrical properties, which can be modified by introduction of foreign atoms [7]. It is also important that this development was connected with the progress in the most important characterization tool – Transmission Electron Microscopy (TEM) that allowed to study these structures with atomic precision. Yet another discovery of self standing graphene attracted more attention to this area [4-6]. This discovery was even more surprising, as graphene is intimately related to graphite, the material known for centuries. The electric properties of graphene made this material a focal point of the semiconductor research, promising new very attractive applications in high speed electronics [8,9]. Application of graphene-based devices requires deposition of graphene on mechanically strong support. The optimal choice would be fabrication of the graphene on carbon based material, such as silicon carbide. It was therefore fortunate that graphitic films can be grown on SiC surfaces [10-12]. It was recognized only recently that few atomic carbon layer has the electronic transport properties of graphene [13]. It is evident that most important is the structure of SiC-graphene interface. Therefore the atomistic structure was intensively investigated [14,15]. Despite some progress, the relations between the atomistic structure of carbon layers deposited on SiC surfaces are not well understood. Another aspect of the interaction of graphite-like carbons layers with solid support is related to carbon-metal structures. In contrast to graphene-SiC structures which are relatively easy to obtain, the carbon-metal structures Maximilian Lackner (ED) All rights reserved - © 2010 Bentham Science Publishers Ltd.

Combustion Synthesis of Carbon-Encapsulated Nanoparticles


are more difficult to synthesize. This is related to the fact that carbon is easily dissolved in liquid metals, creating solid solutions or in higher concentrations, metal carbides [16]. Therefore in order to synthesize such structures, a rapid high temperature methods have to be used. Fast, high temperature stage of the synthesis of carbon layers on metal surfaces should be followed by rapid cooling down of the system in order to prevent dissolution of carbon layer in the metal interior. Among the processes that are able to fulfill such criteria, the most effective are: Huffman-Krätschmer arc process [17] and combustion synthesis [18,19]. Therefore combustion synthesis is important technologically process which will be discussed in detail in the review. In particular application of the combustion process to creation of Me-C structures will be analyzed. It has to be noted that carbon encapsulation of metal nanoparticles changes their properties drastically. These particles create new type material, combining magnetic and other physical properties of metals with chemical resistance of carbon. They create a new type of structural nano-size materials, which demonstrate the potential of nanotechnology. As such their properties are extremely interesting from the point of materials science and also for their potential applications. Therefore their properties are intensively investigated [20]. The results of the characterization of such systems by many methods will also be reviewed. Their properties open routes to many applications in industry and medicine and other branches of human activity [21]. Therefore such possibilities of potential applications will also be discussed in this review. THERMODYNAMICS OF COMBUSTION In order to evaluate the possibility of spontaneous reaction in given substrates setup, the most convenient o parameter is the standard free energy of reaction ( Δ r G ) [22]. Based on standard parameters of formation for assumed (thermodynamically stable) products and given substrates, the standard entropy (S) and enthalpy (H) of reaction can be calculated:

Δr X o =

∑ν Δ j

products

f

X oj −

∑ν Δ

substrates

i

f

X io

(1)

where: Δ r X – value of standard thermodynamic reaction parameters - entropy (S), enthalpy (H); ν j , ν i – o stoichiometric coefficients for reaction products and substrates, respectively; and Δ f X – standard parameter of o formation of the product. Using standard enthalpy and entropy of reaction, for ambient temperature, Δ r G can be calculated [23]: o

Δ r G o = Δ r H o − TΔ r S o where: Δ r G – standard Gibbs free energy of reaction, entropy of reaction and T – absolute temperature. o

(2)

Δ r H o – standard enthalpy of reaction; Δ r S o – standard

Employing relations presented above, the values of standard Gibbs free energy for any temperature can be calculated, provided that the temperature dependence of enthalpy and entropy is known. From thermodynamic point of view, pyrotechnic mixtures, which are used in production of materials via combustion process, are relatively unstable setups [24]. According to that the main thermodynamic reaction parameters like standard free energy and standard enthalpy of the decomposition reactions of these substrates have high negative values. Therefore a strong thermodynamic stimulus favors the progress of the reaction that leads to decrease of these parameter levels in the more stable final state. It is also known that the entropy (So) in most of the cases have a high positive value. Again this is consistent with the principle of maximum entropy production. Thermodynamic analysis of combustion reactions occurring in the condensed phases is relatively simple because, in most cases, complicated intermediate compounds have not appeared. For example the reaction of titanium with carbon results in the synthesis of titanium carbide, at which a considerable heat is released (230 kJ/mol) [25]. Combustion synthesis of carbonaceous nanomaterials via reductive dehalogenation usually runs with transport of reactive species through gas phase [26]. Pyrolysis products (radicals, carbenes, substituted olefins etc.) generated in the front of combustion wave are ejected as a result of pressure increase. Highly reactive species, mentioned above, are transported through gas phase to colder zone, and can react with each other. The direct precursor of final product usually has completely different chemical structure from simple high temperature pyrolysis products. For this reason, to thermodynamic analysis of processes at which the carbon nanostructures are formed spontaneously, are investigated using ‘black box’ assumption in which the reaction substrates and final products are only explicitly considered [27].


Borysiuk et al.

Combustion reactions run under non-equilibrium conditions and this fact is a main cause of the problems occurring in the thermodynamic modeling of combustion synthesis reactions. It has to be noted that the same thermodynamic state is responsible for many useful features of reactions proceeding via transition of the combustion wave, such as very fast heating and cooling, which leads to formation of nanostructures and valuable metastable phases of materials. A few examples of different setups of the substrates of the reactions with values of reactions thermodynamic parameters and measured heats of reactions (Qr) are presented in table 1. Hexachlorobenzene (C6Cl6) in reaction with sodium azide generates sodium chloride, free carbon (in form of soot when catalyst is absent) and gaseous molecular nitrogen. From stoichiometric equation of reaction, the reactant composition can be calculated. Standard entropy has a high positive value which indicates that assumed directions of reaction run are thermodynamically possible. Highly negative values of standard enthalpy show that written reaction is exothermic and highly negative values of standard free enthalpy evidence that point of equilibrium is shifted into products side. For comparison in last column of the table 1, the experimental measured values of heats of reaction (in constant volume) are shown. They correlate with calculated standard enthalpy of reactions. Short analysis (and later research) shows that assumed equations of reactions are correct [28]. Table 1: The Assumed Reactions, Composition of the Mixtures, their Standard Thermodynamic Parameters and Measured Heats of Reactions. Assumed reactions C2Cl6 + 6NaN3 = 2C + 6NaCl + 9N2 C6Cl6 + 6NaN3 = 6NaCl + 6C + 9N2 2CF + 2NaN3 = 2NaF + 2C + 3N2 C2F4 + 4NaN3 = 4NaF + 2C + 6N2

Composition, wt.% C2Cl6/NaN3 (37,8/62,2) C6Cl6/NaN3 (42.2/57.8) CF/NaN3 (67.8/32.2) C2F4/NaN3 (72.2/27.8)

ΔrSº, J/kg·K

ΔrHº, kJ/kg

ΔrGº, kJ/kg

Tad, K

Qr , kJ/kg

2392

– 3750

– 4463

2300

2388

2007

– 3513

– 4111

2175

3100

947

– 4005

– 4288

3125

3700

2626

– 4203

– 4986

3325

3500

Other important parameter, which characterizes the reaction zone in Self-propagation High-temperature Synthesis (SHS) mode, is adiabatic temperature of combustion (Tad). Suppose that the reaction runs under adiabatic conditions at normal conditions and the heat of combustion is used to heat up the products, then the highest temperature which products can reach is the adiabatic temperature. The Tad can be calculated from the formula [29]: Tad

− ΔH Too = ∫ CmR dT 298

(3)

where: ΔH – enthalpy of reaction in the initial temperature; Tad – adiabatic temperature (absolute scale) and CmR – thermal capacity of products. o To

Usually the adiabatic combustion temperature is higher than the temperature measured in combusted sample (e.g. with thermocouple), especially in solid phase [30]. This is related to the fact that the assumption of perfect thermal insulation is not fulfilled (adiabatic), especially for the slower rates of the reaction. Nevertheless, the adiabatic temperature is an important parameter as it indicates on the upper temperature limit which can be attained by the combustion, which affects the structure of the encapsulated nanoparticles. High adiabatic temperatures, especially these in excess of 2000K, listed in the latter rows in Table 1, make the combustion method as perspective as carbon arc plasma methods, also capable to attaine extremely high temperatures, above 3000K [17,20]. Heat effects of combustion reactions can be simply measured when synthesis is accomplished in a calorimetric bomb placed in a water calorimeter. Closed reactor (autoclave) allows one to keep isochoric and quasi-isobaric conditions during reaction time. Based in these assumptions heat effect measured in calorimeter can be treated as equal to the standard enthalpy of reaction. When product is synthesized from elements, the heat of this reaction is simultaneously standard enthalpy of formation for the product. This technique is useful when combustion is initiated with an electrically heated resistance wire. When impulse from wire is not enough to start chemical reaction in SHS regime, a small portion of incendiary mixture (with


123

CHAPTER 10 Low Temperature Combustion Synthesis of α-Fe2O3 and Ni(1-x)ZnxFe2O4 Nanopowders Prita Pant Sarangi and Narendra Nath Ghosh* Chemistry Group, Birla Institute of Technology and Science – Pilani, Goa Campus, Zuarinagar – 403726, India; *Corresponding author: [email protected] Abstract: Nanoparticles of iron oxide and ferrites are being currently explored for their diverse range of applications such as magnetic storage media, environment protection, sensors, catalysis, clinical diagnosis and treatment etc. The attention which is being focused on their synthesis and characterization is well-deserved as they have the capability to exhibit certain superior properties as compared to bulk. In the pursuit to prepare nanoparticles of iron oxide and ferrites, a variety of synthesis routes like precursor, precipitation, sol-gel, hydrothermal, combustion, solvent evaporation etc. have been reported. However, most of these methods are associated with some limitations. In order to overcome the limitations of the existing methods, we have developed a technically simple but cost effective chemical synthetic route for the preparation of single phase α-Fe2O3 and Ni(1-x)ZnxFe2O4 nanopowders. In this method, precursor powders were synthesized by reacting aqueous solutions of metal nitrates with EDTA (ethylene dimmine tetraacetic acid). Pure α-Fe2O3 and Ni(1-x)ZnxFe2O4 nanopowders were obtained by calcining the precursor powders at different temperatures ranging from 250 to 4500C in air. Precursors and calcined powders were characterized by using TG (thermogravimetric) - DSC (Differential Scanning Calorimetry) analysis, XRD (X-Ray Diffraction), TEM (Transmission Electron Microscopy) and SEM (Scanning Electron Microscopy). DC electrical resistivity of the samples was measured from room-temperature to 2250C. Room temperature magnetization measurement was performed by using VSM (Vibrating Sample Magnetometer).

INTRODUCTION The miniaturization of technology has stimulated us to explore various options in terms of materials, their synthesis and establishment of characteristic properties. The field of nanotechnology is currently being explored to look for exotic properties in materials that are different from bulk and which can help us in enhancing and upgrading the existing technology. Iron oxide (in its different phases) and ferrites are materials that have long been in use for various purposes such as sensors, catalysts, pigments, read write heads, transformer cores, high frequency applications etc [1, 2]. Recently, there has been much interest in the synthesis and characterization of their nanoparticles as they have been shown to exhibit some remarkably different properties as compared to bulk. Therefore, in addition to the above applications, they are now being used in fields such as biomedicine and biotechnology, as contrast agents in magnetic resonance imaging (MRI), as drug carriers for magnetically guided drug delivery systems and ultrahigh density magnetic devices [3, 4]. Moreover, it is possible to tailor the physical properties (magnetic, electrical, optical, mechanical etc.) of nanocomposites by controlling the particle size of the dispersed nanoparticles and their weight ratio with respect to the matrix. Nanocomposites are therefore, expected to demonstrate multifunctionality in terms of performance and applications based on the properties of the individual constituents [5-7]. The quest to prepare nanoparticles of iron oxide and ferrites has led to the use of a variety of synthesis routes like precursor, precipitation, sol-gel, hydrothermal, combustion, solvent evaporation etc. Laurent et al [8] and Ghosh et al [9] have reviewed different synthetic methods used for preparation of magnetic iron oxide and ferrite nanoparticles respectively. However, most of these methods are associated with some limitations such as use of (i) expensive and delicate metal complex compounds as starting materials, which are difficult to handle (ii) strong acids, and/or bases as precipitating agent (iii) frequent use of organic solvents, (iv) higher calcination temperature, (v) formation of mixed or undesired crystalline phases. Therefore, development of a simple synthetic methodology for preparation of pure and homogeneous α-Fe2O3 and Ni(1-x)ZnxFe2O4 nanopowder is a major challenge till now. In order to overcome the limitations of the existing methods, we have developed a technically simple but cost effective chemical synthetic route to synthesize nanoparticles of α-Fe2O3 and Ni-Zn ferrites.In this chapter, we shall describe the intricacies of the developed synthetic route and also the characterization details and properties of the as-synthesized nanopowders. Maximilian Lackner (ED) All rights reserved - © 2010 Bentham Science Publishers Ltd.


Sarangi and Ghosh

EXPERIMENTAL DETAILS: Chemical Synthesis: The chemicals used were Fe(NO3)3.9H2O (99.9%, Merck, India), Ni(NO3)2.6H2O (99.9%, Merck, India), Zn dust (99.9%, Merck, India) and EDTA(99.9%, Merck, India) without further purification. Zn(NO3)2 was prepared by dissolving Zn dust in aqueous nitric acid. (a) Synthesis of α-Fe2O3 nanopowder [10-13]: 0.1 mole ferric nitrate was dissolved in 150 ml of distilled water and mixed with 100 ml aqueous solution of 0.1 mole EDTA. The resulting mixture was stirred for 1hr at room temperature using a magnetic stirrer. A dark brown fluffy precursor was formed when this mixture was evaporated on a hot plate. Precursor powder of iron oxide shall henceforth be referred to as P1. (b) Synthesis of Ni-Zn ferrite nanopowders [14]: Stoichiometric amounts of metal nitrates were dissolved in distilled water according to the molar compositions as shown in Table 1: Table 1: Molar ratio of starting compounds. Target Composition

Fe(NO3)3.9H2O

Ni(NO3)2.6H2O

Zn

EDTA

Ni0.80Zn0.20Fe2O4

0.084

0.034

0.008

0.126

Ni0.65Zn0.35Fe2O4

0.084

0.027

0.015

0.126

Ni0.50Zn0.50Fe2O4

0.084

0.021

0.021

0.126

Ni0.40Zn0.60Fe2O4

0.084

0.017

0.025

0.126

Aqueous solutions of metal nitrates and EDTA were mixed in a molar ratio of 1:1 and stirred for 1hr at room temperature using a magnetic stirrer. pH of the resulting mixtures was found to be 8000A) and pressures (up to 200kN). (a) The entire apparatus including the power supply and (b) the main pressure chamber outfitted with dual pyrometers. The custom configuration approach allows for the use of several commodity-level components in almost arbitrary arrangements. As shown in Figure 4, the lab or workshop space necessary for this equipment is often much less.

Spark Plasma Sintering


Figure 4: Author’s self-built, DC-current SPS. The equipment is modular with the intent of using the pressure cell on arbitrary load frames and power supplies. To further accommodate lab space concerns, the equipment is on castors to ease mobility. Whether using custom or manufactured equipment, the basic configurations, concerns, and consolidation details are the same. BASIC APPROACHES As discussed in Section 1, the basic constraint in SPS is the need for a continuous current path. This has led to a narrowing of options related to the die materials, construction, loading, and temperature controls. Graphite, in high density/low grain size grades, is largely the material of choice for use as both the die and plunger assemblies. It has the benefits of: low cost, ease of machining, excellent conductivity, good high temperature strength, and extremely high operational temperatures in a vacuum environment. As one would expect, the use of largely monolithic dies leads to catastrophic failures. At high temperatures these failures can be damaging to the equipment itself. If one relies on the effects of heating rates only (or perhaps applied fields) the options for die arrangements are greatly increased. Non-conductive dies within an outer die are often used in the application of high pressures at loads sufficiently low to allow the graphite outer dies to survive. This emulates typical hot-pressed applications to some degree though heat is transferred though conduction rather than radiation. With appropriate arrangement high heating rates are still possible, but it can be tricky to maintain symmetric temperature profiles. Thermal differentials have been found to create uneven stress distributions which can lead to die breakage, sample density fluctuations, or both.[4] The heating rates used in SPS are typically between 100ºC/min and 400ºC/min. However, experiments up to 1750ºC/min have been utilized in sintering non-equilibrium microstructures. The constraint of conductivity has limited most applications to pressures less than 200MPa. The use of smaller inserted die and plunger assemblies into the typically dimensioned outer assembly, along with material changes, have allowed some experimenters to achieve up to 1GPa consolidation pressures. To achieve such large pressures it is necessary to reduce the part dimensions greatly, with the resulting disc shaped part diameters reduced to 5mm and 10mm in many cases. The pressures used in SPS are, almost without exception, uniaxial. It is conceivable that forms of granulated quasiisostatic loads are possible with a conductive medium, but are unreported thus far in the literature as are pressureless sintering configurations. TEMPERATURE MEASUREMENT The determination of the operating temperature of the experiment can be problematic, particularly at higher heating rates. The problem is two-fold in SPS: measurement of the process temperatures taken from the external dies and the internal temperature of the samples themselves. Determination of the external temperatures requires techniques applicable over a wide temperature range, reproducibility, tolerance of vacuum (or reducing) environments, and to a lesser degree, accuracy. The two methods commonly employed for this are pyrometers and thermocouples. The determination of the temperature of the samples themselves requires the use of thermocouples in all but the most exotic configurations. These internal temperature measurements require fabrication of suitable plungers, tolerance of reducing and reactive environments, accuracy, and often tolerance of pressure application. The various problems and solutions of each will be discussed in context of the commonly used Sumitomo equipment/arrangement. Other arrangements will have similar details.


195

CHAPTER 15 Combustion in Porous Inert Media M. Abdul Mujeebu*, M. Zulkifly Abdullah, M. Zailani Abu Bakar and A. A. Mohamad Universiti Sains Malaysia; The University of Calgary; *Corresponding E-mail: [email protected] Abstract: The rapid advances in technology and improved living standard of the society necessitate abundant use of fossil fuels which poses two major challenges to any nation. One is fast depletion of fossil fuel resources; the other is environmental pollution. The Porous Medium Combustion (PMC) has proved to be one of the feasible options to tackle the aforesaid problems to a remarkable extent. PMC has interesting advantages compared with free flame combustion due to the higher burning rates, the increased power dynamic range, the extension of the lean flammability limits, and the low emissions of pollutants. This chapter gives an outline of PMC, right from the history till the current stage, based on a thorough review of the documented investigations in this area.

INTRODUCTION Porous media combustion (PMC), also known as filtration combustion, pertains to the heterogeneous interaction between two different media, usually a solid and a gas. The theory of filtration combustion involves a new type of flame with exothermic chemical transformation during fluid motion in a porous matrix. The term "filtration combustion" was introduced by Russian scientists for combustion phenomena where gas flow through porous media (in Russian - filtration flow) plays principal role. This term doesn't correspond to western scientific terminology tradition, yet it can be found in special literature as a synonym to combustion within porous media. In the PMC terminology a porous medium (PM) is defined as a solid medium with interconnected pores, available as either packed bed of discrete solids (mainly ceramic material) or foam. Fig. (1) shows the tomographic image of ceramic porous foam.

Figure 1: Tomographic image of ceramic porous foam. The stationary and the transient systems are the two major design approaches commonly employed in filtration combustion. The first approach (stationary) is widely used in radiant burners and surface combustor-heaters due to high radiant emissivity of the solid. Here, the combustion zone is stabilized in the finite element of a porous matrix by the imposed boundary conditions. The second approach(transient) to filtration combustion leads to the ‘excess enthalpy’ flame theory where an unsteady reaction zone possesses the freedom to freely propagate as a combustion wave in the downstream or upstream direction due to positive and negative energy fluxes [1]. Porous medium burners exhibit the feature of combusting sub-normal lean mixtures due to the intense heat transfer across the solid to preheat the mixture to the temperatures that sustain chemical reactions. With such energy recirculation, the porous burners have wider flame stability limits and can hold an extended range of firing capacities, a feature that cannot be obtained by conventional burners especially at their lean operation. Therefore, the ultra low CO and NOx emissions, associated with the low gas temperatures at such lean conditions, characterize the performance of the porous burners at significantly higher turn down ratios. The relatively higher thermal emission with the lower exhaust gas emissions and the excellent stability behavior are all mandatory Maximilian Lackner (ED) All rights reserved - © 2010 Bentham Science Publishers Ltd.


Mujeebu et al.

requirements for industrial applications. This testifies to the wide spread of porous media in combustors of boilers, gas turbines, heat recovery units, and incinerators for fuel saving and higher energy utilization [2]. BRIEF HISTORY OF PMC The idea of excess enthalpy flames was proposed more that 40 years ago by Alfred Egerton and reiterated later on by Weinberg [3] and Fateev [4], with the reasoning that combustion temperature in a system can be higher than the theoretical adiabatic temperature if the burned products preheat the reactants. The amount of combustion generated energy circulated into the combustion process is given as [3]:

where, Tf is the final temperature, T0 is the initial temperature, Qc is the heat release by chemical energy conversion, Qa is the energy added, Hf and H0 are the enthalpy at two states. The circulation part of thermal energy from combustion generated products will increase the combustion temperature so that enthalpy of the reaction zone will be above the conventional combustion level. This has resulted in the use of term called “Excess Enthalpy Combustion”. This concept forms the basis for compact, low cost, ultra-low pollution emission reactors of very high yields by using high reactant velocity. Since the media are inert, they are suitable for operation with any type of feedstock. In the modern literature one can meet the term “super-adiabatic” combustion as a synonym to the excess enthalpy combustion although this term is often considered as inappropriate. Subsequently, in a series of studies, Lloyd and Weinberg [5-7] and Hardesty and Weinberg [8] have shown that the burning system based on this concept successfully extends the ranges of flammability. The burning of an excess enthalpy flame was achieved by transferring the lost energy from products downstream of a flame to preheat the fresh mixture upstream through external heat recirculation. It was generally concluded that heat recirculation improves the flame stability and extends the flammability limit. Takeno and Sato [9] proposed a way of producing an excess enthalpy flame by inserting a semi-infinite; high-conductivity porous solid into a one-dimensional flame zone to re-circulate heat from the downstream high temperature region towards the upstream low temperature fresh ones. Further developments from 1980’s till 2009 have been outlined in our recent review [1]. However, the key information on combustion modes, basic terminology, PMC research and its widespread applications are summarized in the following sections. COMBUSTION MODES The PMC modes may be broadly classified according to: 1. The location of the flame, as a Matrix-Stabilized Combustion: In this mode the flame is entrapped completely within the porous matrix and no physical extension of the flame is visible above the surface. b Surface Combustion: In the surface combustion mode, a distinct flame is stabilized at the surface of the porous medium. 1. The velocity of the combustion wave [10], as a)

Low velocity (LVR)

0-10-4m/s

b)

High velocity (HVR)

0.1-10m/s

c)

Rapid combustion (RCR)

10 – 100m/s

d)

Sound velocity (SVR)

100-300m/s

e)

Low velocity detonation(LVD)

500-1000m/s

f)

Normal detonation (ND)

1500-2000 m/s

Dobrego [11], pointed out shortage of physical content of the above classification and proposed more meaningful terms - ‘Strong thermal coupling (between gaseous and solid phase) regime’ for low velocity and ‘Weak thermal coupling regime’ for high velocity.

Combustion in Porous Inert Media


Basic Terminology a Modified Peclet number The modified Peclet number Pe [12] is the deciding factor to specify whether combustion can be stabilized in the PM or not. If the Pe < 65, the flame is unable to propagate and quenching occurs. Alternatively, for Pe ≥ 65 flame propagates. The modified Peclet number is defined as, Pe= (SL dm Cp ρ)/k

(1)

where SL is the laminar flame speed, dm is the equivalent diameter of the average hollow space of the porous material, Cp is the specific heat of the gas mixture, ρ is the density of the gas mixture and k is the thermal conductivity coefficient of the gas mixture. The equation shows that the conditions for the development of the flame are essentially dependent on the equivalent diameter dm of the mean hollow space or on the mean pore diameter of the porous material. b Equivalence Ratio The equivalence ratio [12] is defined as the ratio of the actual fuel/air ratio to the stoichiometric fuel/air ratio. Stoichiometric combustion occurs when all the oxygen is consumed in the reaction, and there is no molecular oxygen (O2) in the products. If the equivalence ratio is equal to one, the combustion is stoichiometric. If it is < 1, the combustion is lean with excess air, and if it is >1, the combustion is rich with incomplete combustion. c Turn down Ratio (TDR) It is the measure of the capability of a burner to modulate through a firing range or it is the measure of flame stability of a burner and is defined as the ratio of maximum firing rate to minimum firing rate of a burner. Typical values of TDR for industrial heating operations are in the range of 3:1 to 6:1 [12] d Optical Thickness (OT) Defined as the product of refractive index (n) and thickness (t), i.e., OT = n t

(2)

Refractive Index (n) of a medium is the ratio of the velocity of propagation of an electromagnetic wave in vacuum to its velocity in the medium. Higher value of n indicates higher optical thickness and a lower speed of propagation of UV radiation. Hence an optically thin medium can permit fast propagation of radiation compared to the thick one [12]. PMC RESEARCH The researches on PMC may be grouped into, but not limited to the following categories: ¾ Premixed and Non-premixed combustion ¾ Reciprocating flow combustion ¾ Staged combustion ¾ Surface combustion ¾ Liquid fuel combustion ¾ Development of innovative burners ¾ PMC materials ¾ Numerical modeling ¾ Applications PREMIXED AND NON-PREMIXED COMBUSTION PMC systems may work with a premixed flow (fuel and air been forced through one end), or, more rarely, with a non-premixed fuel flow that meets a back-diffusing air flow at the exit. Premixed porous burners consist of two sequential zones: the premix fuel/air stream first enters a fine-pore hot solid matrix (below the flame quenching

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CHAPTER 16 Solution Combustion Synthesis-An Overview S.T. Aruna Council of Scientific and Industrial Research-National Aerospace Laboratories; E-mail: [email protected] Abstract: In recent years solution combustion synthesis is the most widely used process by chemists, physicists and materials scientists world-wide for preparing a variety of oxide materials in both nano and micron size regime. The solution combustion process which has its origin in Indian Institute of Science, India has now spread its combustion wave worldwide. The beauty of this process is the ease and simplicity with which nanosize oxides can be prepared. It involves rapidly heating a solution containing stoichiometric amounts of respective metal nitrates (oxidizer) and a fuel like urea, glycine or hydrazide. All kinds of nanosize oxides have been prepared in nano-size ranging from catalysts, dielectric materials, piezoelectric materials, phosphors, Solid oxide fuel cell (SOFC) materials, pigments, etc. The solution combustion synthesized nanosize oxide catalysts have shown greater promise towards environmental remediation. Among the publications related to synthesis of nanosize oxides, solution combustion synthesis occupies the lion share. In this chapter, the history of solution combustion process, a brief summary on materials prepared along with the recent advances, trends and future prospects is presented.

INTRODUCTION Nanosize oxide materials exhibit a variety of properties that are different and considerably improved in comparison to the conventional coarse grained materials [1]. One of the challenges faced by materials scientists today is the synthesis of nanosize materials with the desired composition, structure and properties for specific applications [2]. Oxide materials both simple and complex are usually prepared on both laboratory and industrial scale by solid-state (ceramic method) and co-precipitation methods [3,4]. The solid state method requires high temperatures, longer duration and yields inhomogeneous, impure and coarse products and thus obviates the suitability of this process for the preparation of nanosize oxides. In case of co-precipitation method it is difficult to precipitate all the relevant metal ions if they do not form insoluble precipitates and hence it is difficult to control the stoichiometry. Thus, there is an increasing demand for alternate routes to the synthesis of oxide material that exhibits superior properties when compared to those available from conventional routes. There are two approaches to the synthesis of nanomaterials (i) building up (soft process) and (ii) breaking down (brute force method) processes. The soft processes are preferred over the brute force methods in order to assert a better control of stoichiometry, structure and phase purity of metal oxides. Soft chemical routes like co-precipitation, hydrothermal, electrochemical methods, spray pyrolysis, ion exchange method, solvothermal, reverse micelle, sol-gel, solution combustion synthesis (SCS) etc., are now increasingly becoming important to prepare a variety of nanocrystalline oxide materials [5]. Among these methods, solution combustion or fire synthesis is quite simple, fast and economical process. SCS is a low temperature initiated self-propagating high-temperature combustion method [6, 7]. This method makes use of a highly exothermic reaction between oxidizers and fuels to produce high temperature (flaming or smouldering type) due to spontaneous combustion. The exothermicity of the reaction is utilized in the crystallization of the desired product. Metal nitrates act as oxidizers and compounds like urea (NH2-CO-NH2), glycine (NH2-CH2-COOH), metal acetates (Mx)(CH3COO)x and hydrazides (Rx-COy-(N2H3)z) act as reducing agents (fuels). Each of these fuels differ in the number of C,H,N and O atoms and the amount of gases they produce during combustion also varies. SCS has emerged as a viable technique for the preparation of advanced ceramics, catalysts, alloys, composites, intermetallics and nanomaterials [8]. The solution combustion synthesis of nanocrystalline oxide materials although appears to be breaking-down process is in fact an integrated approach as the desired oxide products nucleate and grow from the combustion residue [9]. This combustion process is different from the well known Pechini and citrate gel processes that uses further calcinations of the products at high temperatures to burn away the extra carbon [10,11]. SCS product is devoid of carbon and the product is typically crystalline. Today, solution combustion synthesis of oxide materials has become popular and is practised world-wide by several materials scientists, physicists and chemists [12,13]. The process is particularly useful to prepare nanocrystalline oxide materials. More details and ready recipes for the preparation of all types of oxide materials have been covered in a recent book [9]. The latest developments in nano-combustion synthesis with special emphasis on recent trends in combustion science, as well as on materials applications have been discussed recently [14]. Maximilian Lackner (ED) All rights reserved - © 2010 Bentham Science Publishers Ltd.

Solution Combustion Synthesis-An Overview


In this chapter, the history of solution combustion process, process details, reaction parameters, types of fuels, a brief summary on materials prepared along with the recent advances, trends, issues and future prospects will be discussed. HISTORY Of SOLUTION COMBUSTION SYNTHESIS, PROCESS DETAILS AND REACTION PARAMETERS India is the origin of solution combustion method. SCS is relatively a new process and it was accidentally discovered two decades ago by Kingsley and Patil during the reaction between aluminium nitrate and urea at the Indian Institute of Science, Bangalore, India [15]. A mixture of Al(NO3)3.9H2O (20 g) and urea (8g) when rapidly heated around 500 °C in a preheated muffle furnace, the solution foamed and ignited to burn with an incandescence flame with the evolution of large amounts of gases yielding voluminous white product which was identified as α-Al2O3 [15]. Based on this serendipitous experiment, a large number of oxides were prepared and alternate fuels for urea were explored. The SCS method employs the concepts of propellant chemistry. In order to calculate the stoichiometry of the oxidizer and fuel, valence of the oxidizing elements was modified to be considered as negative, and the reducing elements as positive similar to the oxidation number concept familiar to chemists. The stoichiometry of an oxidizer and a fuel mixture is expressed in terms of the elemental stoichiometric coefficient ( Φ e ).

Φe =

∑ (Coefficient of oxidizing elements in specific formula) x (Valency) (−1)∑ (Coefficient of reducing elements in specific formula) x (Valency)

The mixture is stoichiometric when Φe = 1, fuel lean when Φe > 1 and fuel rich when Φe