Fig1.6: (left) interdigitated capacitor, (right) SEM picture of fingers. ... sometimes, e.g., titanium, hafnium, niobium, tantalum, tungsten, vanadium, and zirconium.
B.Sc. Graduation Project 2014
Synthesis and Characterization of Nanoporous Alumina Memebranes
Presented by
Alaa Yahia Faid Alyaa Ali Bakr Hadeer Abdel Hameed
Under the supervision of
Prof. Dr. Saad El-Raghy Prof. Dr. Randa Abdel-Karim
ACKNOWLEDGMENTS We have worked with a great number of people whose contribution in assorted ways to the research and the making of the thesis deserved special mention. It is a pleasure to thank those who made this thesis possible. First and foremost we offer our sincerest gratitude to Prof Dr. Saad El-Raghy, Metallurgical Engineering Dept, Faculty of Engineering, Cairo University for his guidance, patience and support. we consider ourselves very fortunate for being able to work with a very considerate and encouraging professor like him. And also we would like to thank him for his detailed and constructive comments, and for his important support throughout this work. In addition, he was always accessible and willing to help his students with their research. We owe our deepest gratitude to Prof. Dr. Randa Abdel Karim, Metallurgical Engineering Dept, Faculty of Engineering, Cairo University who has supported me throughout our thesis with her patience and knowledge . We attribute our B.sc thesis to her encouragement and effort and without her this thesis, too, would not have been completed or written. We are grateful for Eng. Mohamed Abdel Hameed for his help and patience with us.
Abstract Membranes with nanometer-scale features have many applications, such as in optics, electronics, catalysis, selective molecule separation, filtration and purification, bio sensing, and single molecule detection. The aim of this project is to use the anodizing technique in synthesizing nanoporous alumina membranes. two sets of samples are investigated , first category at room temperature (20 ± 2 o c)we examined effect of anodizing electrolyte type (sulphuric and oxalic acids ) at the same concentration (0.3M ) and the same pretreatment (mechanical polishing ) for different voltages and different times, Second set of samples investigated at low temperatures ( 4 ± 1 oc ) for the same electrolyte ( sulphuric acid ) ,the same concentration and the same voltage (25 volt ) with different anodizing time and different pretreatments. The characterization of nanoporous alumina is carried out using Scanning Electron Microscope (SEM) analysis, Energy Dispersive X-Ray analysis (EDX). We investigated structural characteristics of nanoporous alumina and we found that it has pore diameter of (23 ± 2) nm and interpore distance of (36-60 nm ) . Theoretical calculations of thickness, oxygen consumed through anodizing process and efficiency of the process are done. .The experimental results indicated that anodizing at room temperature in oxalic acid at low voltages (15, 20, 25) will give a barrier oxide film .while sulphuric acid can produce porous structure at 25 voltage but it’s not regular or ordered. We found that the bet condition to produce nanoporous alumina is anodizing at sulphuric acid (0.3M) at temperature (3 ± 1) oc, 25 volt, 60 min and chemical polishing as a surface pretreatment. Theoretical calculations of efficiency indicate that porous oxide has low efficiency compared to barrier type because of aluminium dissolution in electrolyte ,Theoretical calculations of thickness of oxide and oxygen consumed in the process indicates that as voltage and time increase , their value increases.
Contents Chapter one: Introduction 1.1. 1.2. 1.3. 1.4.
Porous anodic oxides General Structure of Anodic Alumina Terminologies and Definitions Applications of Porous Anodic Alumina
Chapter two: Literature Survey 2.1. 2.2. 2.3. 2.4. 2.5. 2.6. 2.7. 2.8. 2.9. 2.10.
Anodization Types of anodization process Types of anodic oxides of aluminum Composition and density of Inner and Outer Oxide Thermodynamics of aluminum anodizing process Kinetics of aluminum anodizing process Electrochemistry of aluminum anodizing process Formation and growth mechanisms of anodic porous alumina Influence of electrochemical conditions on porous anodized alumina Morphologies of porous anodic alumina
Chapter three: Experimental Techniques 3.1. 3.2. 3.3. 3.4. 3.5. 3.6. 3.7. 3.8.
Introduction Test specimens Preparation of test specimen Electrolytic cell and anodization process Test devices and tools used Scanning Electron Microscope (SEM) Analysis Energy Dispersive X-ray Spectroscopy (EDX) Theoretical calculations
Chapter four: Results and Discussion 4.1. Group (1): Room temperature anodizing 4.2. Group (2): Low Temperature Anodizing
Chapter five: Conclusions and Future Work 5.1. 5.2.
Conclusions Future Work
chapter six: References
1 2 5 6 7 13 14 15 16 18 19 21 24 25 29 33 36 37 37 38 39 41 43 44 45
46 47 53
68 69 69
70
List of Figures Fig 1.1: Schematic diagram of (left) barrier type alumina and (right) porous type alumina. Fig 1.2: Idealized structure of anodic porous alumina (A) and a cross-sectional view of the anodized layer (B). Fig 1.3: Ideal Hexagonal Pore Array. Fig 1.4: (a) Fabrication Process (b) SEM of CNT using PAA as a template. Fig 1.5: Schematic representation of the process to making nanowires. Fig1.6: (left) interdigitated capacitor, (right) SEM picture of fingers. Fig 1.7: Schematic of circuit with AAO layer. Fig 1.8: SEM cross-section micrograph of multilevel aluminum interconnections with the interlevel porous alumina insulator. Fig 1.9: schematic figure of nanowires in batteries. Fig 1.10: plastic solar cell. Fig 1.11: mechanism of desalination. Figure 2.1: Schematic of general anodization cell. Fig.2.2: current and voltage- time transients during a) barrier and b) porous type oxide growth. Fig 2.3 inner and outer oxide. Fig 2.4: X-Ray analysis of porous alumina. Fig 2.5: Pourbaix diagram of Al-H2O at 298 k. Fig 2.6: Voltage and Current – Time graphs and porous alumina formation steps during anodizing. Fig 2.7: Summary of the interfacial reactions at the anode (Al). Fig 2.8: Basic anodization cell. Fig 2 .9: field assisted dissolution model steps. Fig 2.10: mechanism of Al dissolution. Fig 2.11: Schematic figure of dissolution of Al2O3. Fig 2.12: Al+3 ejection model. Fig 2.13: Interpore- Distance relationship for various electrolytes. Fig 2.14: A) empirical Interpore Distance and pore distance - anodizing voltage relationship B) SEM micrographs show effect of increasing voltage on pore size and interpore distance.
Fig 2.15: SEM micrographs show effect of increasing temperature on pore size and interpore distance. Fig 2.16: pore distance - anodizing time relationship Fig 3.1: Experimental work plan. Fig 3.2: anodizing cell used for synthesis of nanoporous membranes. Fig 3.3: A 30 volt / 6 Amperes power supply Fig 3.4: 4 digits electric balance Fig 3.5: magnetic stirrer device Fig 3.6: pH meter device Fig 3.9. The (SEM) Unit. Fig 4.1: SEM Micrographs and corresponding anodizing current-time relationship for sulphuric acid anodizing Fig 4.2: Efficiency – voltage relation for sulphuric acid anodizing at room temperature. Fig 4.3 :( A and B) Thickness of oxide layer by faraday's law and experimental measured for each sample. Fig 4.4: voltage time relationship for pits initiation in sulphuric acid anodizing Figure 4.5: Anodizing Current-time relationship curves for Oxalic group. Fig 4.6: efficiency – voltage relation for oxalic acid anodizing at room temperature Fig 4.7 :( A and B) Thickness of oxide layer by faraday's law and experimental measured for each sample in oxalic acid. Fig 4.8: Effect of temperature of anodizing bath on current at different voltages. Fig 4.9: Current – voltage relationship for different types of acids Fig 4.10. SEM micrographs and Current-time experimental curves for nanoporous alumina membranes. Fig 4.11. EDX analysis of nanoporous alumina membranes Fig 4.12: pore diameter measurements of nanoporous alumina membranes. Fig 4.13: pore size distributions of nanoporous alumina membranes. Fig 4.14: Interpore distance distributions of nanoporous alumina membranes. Fig 4.15: thickness calculated from measured weight and faraday’s calculations for nanoporous alumina membrane
List of Tables Table 1.1: Different anodic metal oxides and their SEM images. Table 1.2: Parameters of porous alumina and calculation formula. Table 2.1: Comparison between mild anodization versus hard anodization in 0.3M H2C2O4 (1 ◦C). Table 2.2: Comparison between barrier and porous oxides. Table 2.3: AAO templates with different pore arrangements achieved under different selfordering. Table 2.4: Effect of electrochemical conditions on the film physical and chemical properties. Table 2.4 Contd: effect of electrochemical conditions on the film physical and chemical properties. Table 2.5: Different Morphologies of porous anodic alumina. Table 3.1: The chemical composition of test materials. Table 3.2: Working conditions for anodizing process. Table 4.1: Efficiency calculations of room temperature sulphuric acid at different voltages. Table 4.2: Efficiency calculations of room temperature oxalic acid at different voltages. Table 4.3: Chemical composition of nanoporous alumina membranes. Table 4.4: Calculated structural characteristics of nanoporous alumina membranes Table 4.5: Efficiency calculations of nanoporous alumina sample.
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Chapter one Introduction
N
anotechnology represents one of the most exciting frontiers in modern science and
engineering research. In nanotechnology research, fabrication of functional nanoscale structures and devices in well-controlled ways remains one of the most important challenges facing today’s researchers and engineers. [1]
1.1. Porous anodic oxides Porous anodic oxide films have also been achieved on surfaces of many other metals, sometimes, e.g., titanium, hafnium, niobium, tantalum, tungsten, vanadium, and zirconium. The surfaces of these so-called valve metals could be immediately covered with a native oxide film of a few nanometers when these metals are exposed to oxygen containing surroundings. While these oxides retard the rate of reaction on the metal surface inherently, for each of these valve metals there are some process conditions that may promote growth of a thin, dense, barrier oxide of uniform thickness. The thickness and the properties of such a barrier layer vary greatly among different methods. [2] As in Table 1.1 different anodic metal oxides and their SEM images [2] Anodic metal oxide SEM image for oxide Anodic titanium oxide: The most significant difference between typical anodic titanium oxide (ATO) and anodic aluminum oxide (AAO) : is that the latter is a continuous film with a pore array while the former consists of separated nanotubes as demonstrated Applications : photo catalysis , gas sensors , photo electrolysis and photovoltaics Anodic Hafnium oxide: has many interesting properties, e.g. its high chemical and thermal stabilities, high refractive index and relatively high dielectric constant. Anodization potential was found to be a key factor affecting the morphology and the structure of the porous oxide. The pore diameter was found to increase with increasing potential. Applications: used as a protective coating, optical coating, gas sensor or capacitor 2|P ag e
Anodic Tungsten oxide: was obtained by galvanostatic anodization in oxalic acid Applications: It’s used in gas sensing, electro chromic and photochromic processes.
Anodic Zirconium oxide A unique feature in comparison with other anodic metal oxides mentioned above is that the growth of the compact ZrO2 layer at room temperature directly leads to a crystalline film rather than an amorphous film as observed from other anodic metal oxides Applications: An important functional material that plays a key role as an industrial catalyst and catalyst support. Anodic aluminum oxide It has been known from as early as 193258 that the porous anodic oxide film on aluminium consists of two regions: an outer region of thick porous-type oxide and a thin, compact inner region lying adjacent to the metal. From the many electronoptical investigations, the structure of the anodic alumina films formed in acidic electrolyte turned to be the close packed near hexagonal pores array with a narrow distribution of pore size Porous anodic aluminum oxide (AAO), also known as porous alumina, is a self-ordered nanostructured material well-suited for use in electronic, magnetic, optical and biological applications due to its small pore size (4-200nm) and spacing (10-500nm). Under slightly acidic conditions, both oxidation and dissolution of aluminum leads to the formation of aligned pores that have close packed (hexagonal) order at short range AAO pores over certain ranges of anodic potential and ph. [3] P
A self-assembly system - nanoporous alumina that forms self-ordered pores during anodic oxidation of aluminum. While the use of anodized aluminum for protective and decorative applications has a long history, basic research on controlling the self-assembly and ordering of pores during porous alumina formation started only in the late 1990s. [3]
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The concept that the formation of porous anodic metal oxides is based on two continuous processes, one being oxide dissolution at the electrolyte/oxide interface and the other being oxidation of metal at the oxide/metal interface, is widely accepted by former researchers. However, the formation mechanism of these pores, often hexagonally ordered, is much more complicated than people normally predicted. [3] There remain a number of questions from the pore initiation at the very beginning to the formation of highly ordered patterned pore arrays at the last. Although many efforts have been made in fabrication, application and understanding the formation mechanism of nanoporous anodic metal oxides, there is still much work to do in order to understand fully the electrochemical process during the anodization. Then it will be possible to optimize the anodization conditions to manage precise control of the growth of the anodic metal oxide, finally improving and widening their scientific and industrial applications. [3] In recent years, there has been a renewed interest in AAO layers have diverse applications in the prevention of corrosion of metal substrates from their service environment, forming capacitor dielectrics, templating nanomaterials and in many other fields such as catalysis, optics and electronics. as electronic devices, magnetic storage disks, sensors in hydrogen detection, adsorption of volatile organic compounds; bio devices; and in drug delivery The best known porous anodic oxide, anodic aluminum oxide (AAO), was first reported 50 years ago and is now commercially available. [3] It can be used as an ideal template for preparing various nanoparticles, nanowires and nanotubes. This is due to the highly controllable pore diameter and cylindrical shape, their periodicity and their density distribution. Using the conventional anodization process the arrangement of the pores is quite disordered, however Masuda et al. in 1998, using a twostep anodization process was able to produce a highly ordered hexagonal pore structure from a set of pre-arranged macroscopic parameters. These controllable macroscopic parameters dictated the resulting nano scaled structure that is formed in the AAO layer, thus producing a nano array that can be used in a variety of nanotechnology applications. [3] The advantages of using the AAO membrane as a template are: 1- That it allows the diameter of the nanowires, nanorods and nanotube to be tailored to the respective pore size in the membrane. 2- It ensures that the growth of the nanocrystal (nanowire, nanorod, nanotube) is aligned within the high aspect ratio nano-channel which is also perpendicular to the substrate surface at the base of the membrane. 3- A wide variety of materials that include metals, oxides, conductive polymers and semiconductors can then be deposited into the pores of the membrane. [3]
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1.2. General Structure of Anodic Alumina
Fig 1.1: Schematic diagram of (left) barrier type alumina and (right) porous type alumina. [4]
Detailed structure of porous alumina:
Fig 1.2 : Idealized structure of anodic porous alumina (A) and a cross-sectional view of the anodized layer (B).
[4]
Table 1.2 parameters of porous alumina and calculation formula Parameter Pore Diameter (Dp)
Inter Pore Distance (Dc)
Wall Thickness (W) thickness of the barrier layer (B) porous oxide layer porosity (α) pore density (n)
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Calculation formula Dp = λp U Where λp is the pore proportionality constant = 0.9 nm per vol, U denotes an anodizing potential (V). Dc = λc U λc is inter pore proportionality constant = 2.5 nm per volt W = ½ (Dc − Dp) H 2 SO 4 → B = 1.33 W H 2 C2O 4 → B = 1.12 W
𝛼𝛼 =
𝜋𝜋
2√3
𝐷𝐷𝐷𝐷
( )2 𝐷𝐷𝐷𝐷
𝑛𝑛 =
2.10^14 √3 𝐷𝐷𝐷𝐷 2
1.3. Terminologies and Definitions Interpore Distance (Dc) The interpore distance is the distance between the centres of two adjacent pores. Cell Width The cell width is the distance between two opposite vertices of a cell. For perfect hexagonal pore arrays, the interpore distance is also equal to the cell width. However, in anodic porous alumina, the distance between two opposite vertices of a cell (i.e. cell width) varies due to imperfect structures, and it was found that the interpore distance is more regular and predictable. Hence for this project, the interpore distance was used to estimate the cell width.
Interpore distance
Fig 1.3: Ideal Hexagonal Pore Array
[5]
Pore Diameter (Dp) The pore diameter denotes the size of the pore opening. Pore Depth The pore depth is the distance from the top of the pore to the bottom, when the pore meets the barrier layer. Anodizing Voltage The anodizing voltage is the voltage that is applied across the anode and cathode during anodization. Anodizing Current The anodizing current is the current that flows across the anode and cathode during Anodization. [5]
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1.4. Applications of Porous Anodic Alumina Anodic porous alumina which exhibits a characteristic nano-honeycomb structure has received increasing attention both experimentally and theoretically. Due to the quasiperiodic arrangement of the nanopore channels, narrow distribution of pore sizes and interpore distances, relative ease to control the porous scales and self-ordering qualities by anodization conditions, excellent thermal stability, and very low cost, anodic porous alumina has been extensively used as templates for fabrication of various nanostructured materials such as nanodots, nanowires, nanotubes, and many other types, especially to realize the collective functioning of arrays of nano-elements which may not be realized by individual nano-elements, for applications in high density magnetic media,
photonic crystals,
semiconductor devices, lithium- ion batteries, solar cells, nanocapacitor, biosensors, and so on. [6]
1.4.1 Nanotechnology applications Recently, the research on self-organization in nanotechnology has gained momentum with the use of porous anodic alumina. 1D materials such as nanowires or carbon nanotubes have already shown great promise in applications for quantum devices. By using PAA, researchers have been able to fabricate an inexpensive, high throughput and easily tunable template. Anodized Porous Alumina has several advantages in the effort to produce Carbon Nanotubes (CNTs). Primarily, they offer consistently parallel pore channels, the ability to engineer varying pore diameters, are optically transparent in the visible spectrum and are resistant to most chemicals except for strong bases and acids. CNTs in PAA have been explored by several groups. The potential applications in electrochemical devices, quantum wires and electrodes for rechargeable Li-batteries are just some of the numerous areas that have been explored. [6]
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Most importantly CNTs in PAA can withstand high temperatures, up to a 1000°C; this is more than sufficient to handle
the
Chemical
high
Vapor
temperatures
of
Deposition (CVD),
which is the most common method for synthesizing CNTs. As shown in Figure 1, CVD offers control over the length and diameter of the CNT. Once the deposition is completed, the PAA template is removed, releasing the tubes. [7]
. Fig 1.4: (a) Fabrication Process (b) SEM of CNT using PAA as a template
In addition to CNTs, magnetic nanowires are attracting a growing interest for applications in magnetic storage. By using PAA, one can control the height and diameter of the magnetic wires. The ability to create a dense array of magnetic wires will be a promising candidate for magnetic hard disks with a recording density of up to 1 terabit/in2. [7]
Several types of metals and alloys have been
successfully
deposited
or
electroplated into nanowire structures as seen in Figure 2.
Fig 1.5: Schematic representation of the process to making nanowires
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[7]
[7]
The optical properties of PAA have been well documented and the material has been utilized in both polarizers and electroluminescent devices. The photoluminescent (PL) properties of PAA are further enhanced when they are filled with semiconductor composites such as CdS or ZnO. These optical nanowires are being utilized in polymer light emitting diodes (PLEDs) and optical displays. [7]
1.4.2. Humidity sensors and biosensors Due to its hydrophilic properties, PAA is currently used in micro humidity sensors that show good response and are easily fabricated. These devices are based on interdigitated electrodes that take advantage of the sensitive capacitance-humidity relationship. [8]
Another interesting characteristic of PAA is its biological properties, and for years, PAA has been used in dental and bone implants due to its biocompatibility and ease of integration with medical implant. [8] Furthermore, these PAA membranes are now employed as electrochemical biosensors. The membrane acts as a support for enzymes and other biological materials. The sensors shown in Figure 3 have been used to monitor blood glucose levels and for DNA detection. [8]
Fig 1.6: (left) interdigitated capacitor, (right) SEM picture of fingers [8]
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1.4.3. MEMS and RF applications To date, for RF and microwave applications, PAA has been used mainly as an isolation layer in multilevel circuits. In PAA was formed on a glass substrate to produce Multichip Module Deposited (MCM-D) substrates. In this process, they were able to fabricate several interconnecting layers of porous and barrier layers of PAA as shown in Figure 4. The measured resistance of the layer insulation dielectric layer was on the order of 109 Ω·cm much, higher than porous Si at 106 Ω·[9]
Fig 1.7: Schematic of circuit with AAO layer [9]
To improve interconnect delays; PAA was used as a low dielectric material for isolation . Here they introduced a CMOS compatible process where a multilayer system of Niobium and aluminum was used. By selectively anodizing areas, an interlevel alumina insulator was fabricated as shown in Figure 5. A dielectric constant of 4.4 and a breakdown voltage of more than 400 V were reported.
Fig 1.8: SEM cross-section micrograph of multilevel aluminum interconnections with the interlevel porous alumina insulator
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1.4.4. Applications in energy storage and conversion Electric energy can be stored as surface charges on conducting electrodes in electrostatic capacitors, or in electrochemical double layers in ultra-capacitors. Due to the large surface area of nanostructured materials, the energy storage density is much larger than those with conventional configurations, thus the total system size and weight can be largely reduced under the same energy capacity. Metal-insulator-metal (MIM) electrostatic capacitors which were fabricated by anodic porous alumina templates have shown profound increase in capacitance of 100 times, or more over planer Structured devices. By refining the anodic porous alumina topography, t h e performance of the fabricated nanocapacitor can be further improved. 1.4.4.1. Lithium-ion micro-batteries Lithium batteries Composed of nanowire arrays as anode show much higher energy capacity compared with conventional thin film batteries, due to the large surface area and a reduced Li-ion diffusion length. With the length of nanowire increasing, the capacity can be further increased. However, due to the agglomerate of the high aspectratio nanowires, the total surface area decreases, and significant degradation of performance was found. To overcome the above drawback, Wang et al. fabricated Ni/TiO2 nanowires within a 3D network by using a novel 3-D anodic porous alumina template for micro-battery applications. [10]
Fig1.9: schematic figure of nanowires in batteries
1.4.4.2 Organic photovoltaic cells Organic photovoltaic cells based on conjugated polymers are also promising solar energy harvesting devices due to the low fabrication cost and mechanical flexibility. However, the highest energy conversion efficiency reported was about 8.3% for plastic solar cells, which is not high enough. In the organic photovoltaic cells, the dissociated free charges are generated at the interface between the e-donor and eaccepter phases, and then transport to their respective electrodes, so that an external circuit is formed. By increasing the surface area with nanostructured donor-accepter interfaces, the energy conversion efficiency can be extremely increased, especially if the e-donors are arranged in an ordered network. [10] 11 | P a g e
Fig1.10 plastic solar cell
[11]
[10]
1.4.5. Sea-water desalination Selectively remove ions from water by the manipulation of electric-double-layer (EDL) inside nanochannels through unipolar electrical static charges. Based on electrical static working principle, this device is more efficient in power consumption than reversal osmosis (RO) and even electrodialysis (ED) methods for water purification. It has been successfully
demonstrated
the
EDL
controlling technique can effectively reduce the concentration of NaCl from 0.1514M to 0.01M. [11]
11 Fig 1.11: mechanism of desalination [ ]
The operation principle is shown in Fig.2.15. The surface of the nanochannel array of the nano Desalinator was coated with a conductive metal layer and an insulation layer. When high Enough voltages applied on the nano channel wall, the thickness of the EDL will increase to Overlap owning to the increment of the charge density on the channel wall. Co-ion can Thus be rejected from the nano desalinator due to electrostatic expelling while the counter- Ion and water can go through. [11]
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Chapter two Literature research 2.1. Anodization Anodic oxidation (or anodization) is the process by which an electrochemically active species is oxidized by the passage of current or an applied voltage in appropriate electrolytes" Anodization of metals, especially aluminum, has received a lot of attention due to their wide variety of applications, including protective and decorative coatings, dielectrics and more recently, nanoscience and technology. When a fresh oxide-free aluminum surface is exposed to air at room temperature, an oxide initially forms at a rapid rate, Metals form an oxide layer in air, aluminum, Al, forms alumina, A1 2 0 3 but slows over time to form a stable oxide film ('native oxide') of a few nanometers. [12] Anodic oxidation is one of the techniques used to form a thicker oxide on aluminum to provide corrosion resistance and other desirable properties. During anodic oxidation of aluminum, as in figure (2.1), the aluminum serves as the anode and a chemically stable metal such as platinum, carbon etc serves as the cathode. Various electrolytes include phosphoric, chromic, oxalic, malonic, citric and sulfuric acid and the anodization: is carried out at constant temperatures. This technique was developed in the early 1900s as a method to protect aluminum from corrosion as well for dying or coloring the surface of aluminum. [12] Fig 2.1: Schemat ic of general anodization cell [12]
Anodization (anodic-oxidation) is a process similar to electrolysis in that it involves the use of two electrodes and an acid as an electrolyte, as shown in figure (2.1) The difference is that when current is passed, the aluminum anode does not dissolve away, and oxygen is not evolved in significant amounts. Instead, much of the oxygen liberated combines with the aluminum to form a layer of porous aluminum oxide (Al2 O 3 ). Hydrogen is liberated at the cathode. The amount of aluminum oxide formed is directly proportional to the current used. The progress of formation of the alumina film depends on the conditions of electrolysis and the chemical composition of the electrolyte used. If the electrolyte does not have a solvent action on the oxide coating, then the anodization process will cease quickly, leaving a thin film of oxide referred to as the barrier layer. If the electrolyte has some solvent action, as in the case of sulphuric acid (H 2 SO 4 ) phosphoric acid (H 3 PO 4 ) or oxalic acid (C 2 H 2 O 4 ), then a porous film is formed and the oxidation process continues. [13] 14 | P a g e
2.2. Types of anodization process 2.2.1. Mild anodization (MA) The fabrication of self-ordered Al2 O 3 pore arrays, under conventional so-called ‘mild anodization’ (MA) conditions, requires several days of processing time and the self-ordering phenomenon occurs only in narrow process windows, known as ‘self-ordering regimes’ With specific values of the interpore distance (Dint), such as 25 V in0.3 M H 2 SO 4 at 0 oC with Dint = 63 nm, Owing to the slow oxide growth rates (for example, 2–6 μmh-1), MA processes based on Masada’s approach have not been used in industrial processes so far. For practical applications, simple and fast fabrication of highly ordered AAO with a wide range of pore sizes and Interpore distances would be highly desirable. [14] Self-ordered anodic porous alumina with < 15 nm pore diameter and large aspect ratio of pore channels has recently been fabricated by anodization in an ethylene-glycol-containing sulfuric acid electrolyte. However, due to the slow oxide growth rate (e.g. 2-6 μm h-1) prolonged anodization time of typically more than two days (e.g. 160 h) was required under MA, and this may be too slow for batch production in industrial-scale applications. [14]
2.2.2. Hard anodization (HA) Hard anodization (HA), which was conducted under similarly low temperatures as MA but high anodization voltages or current densities, was regarded as a promising approach to replace the MA for fast (e.g. 50-100 μm h-1) and self-ordered anodic porous alumina fabrication. However, because of the high anodization voltages of HA (generally 2 to 3 times higher than that of MA, e.g. 40-80 V for H 2 SO 4 electrolyte). The very rapid heat generation caused by the high electric field across the oxide barrier layer was difficult to be dissipated quickly and as a result, many macroscopic burns or cracks, observable even by the naked eye, may form on the alumina surface. Furthermore, direct anodization under HA may result in disordered pore arrangements. [14] To solve these problems, Lee et al. and Schwirn et al. [15]first conducted anodization under MA for several minutes to generate a protective oxide layer and then gradually increase the voltage to the target high voltage of HA, and as a result macroscopic burns or cracks were eliminated. However, under HA with H 2 SO 4 electrolyte, macroscopic corrugations were found to extend across the entire surface of the alumina sample, indicating the possibility of plastic deformations in the porous alumina or Al substrate. Thus, fast and mechanically stable fabrication of anodic porous alumina with self-ordered pore arrangement is still a challenge Table (2.1) shows comparison between mild anodization versus hard anodization in 0.3M H 2 C 2 O 4 (1 ◦C). [14]
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Table (2.1) comparison between mild anodization versus hard anodization in 0.3M H 2 C 2 O 4 (1 ◦C). [10]
Voltage (V) Current density (mA cm−2)
Mild anodization 40 5
Hard anodization 110–150 30–250
Film growth rate (μmh−1)
2.0 (linear)
50–70 (nonlinear)
Porosity (P ; %)
10
3.3–3.4
Interpore distance (Dint; nm)
100
220–300
Pore diameter (Dp; nm)
40
49–59
Pore density (ρ; pores cm−2)
1.0×1010
1.3–1.9×109
ζ (nmV−1)†
2.5
2.0
Density (g cm−3)
2.8
3.1
2.3. Types of anodic oxides of aluminum The morphology, physical and structural properties of the anodic oxide films as well as the kinetics of the oxide growth depends on the applied voltage or current, temperature and most importantly, the nature and type of the electrolyte. [16]
2.3.1 Barrier anodic oxides of aluminum "Barrier-type films" are formed in weak or basic electrolytes in which almost all of the aluminum that electrochemically reacts is converted to aluminum oxide with very little or no dissolution into the electrolyte, e.g. neutral boric acid, ammonium borate, tartrate, ammonium tetra borate in ethylene glycol and organic electrolytes such as glycolic or malic acid. A characteristic of barrier-type oxide films is that the thickness of the oxide is not affected by the electrolyzing time or temperature of the electrolyte, but only affected by the applied voltage (1.4nm/V). If the voltage is fixed, the total current for barrier oxide formation decreases exponentially due to the increasing resistance to migration and diffusion of anions and cations through the oxide. The total current or current density1 saturates at a very low steady state value corresponding to the leakage current (Fig. 2-2). The maximum thickness of the barrier oxide is restricted by the oxide breakdown at high voltages, typically around 500-700V. [16]
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2.3.2. Porous anodic oxides of aluminum "Porous-type films" are formed in mild acidic solutions (phosphoric, chromic, oxalic, sulfuric acid etc) in which both dissolution of aluminum into solution and oxidation of aluminum to its oxide occur. Dphosphoric > Doxalic > Dsulfuric A schematic of the barrier and porous type films is shown in Fig. 2-2. [16]
In the case of porous type films, pores grow at a steady finite current or voltage (Fig. 2-2). The thickness of the porous layer is dependent on the anodization time, current density, electric field and temperature. While the maximum thickness of a barrier type oxide is "'0.7-1um, porous aluminum oxide can be grown to 100um thickness and higher. At low temperatures (0-2C), the porous anodic films are compact and hard; however, at high temperatures (>60C), thin and soft oxide films are formed due to high dissolution rates of aluminum ions/oxide into solution, sometimes, even leading to electro polishing or complete dissolution of oxide films. Table 2.2 elucidates the differences between barrier and porous type films. [16] Table 2.2 comparison between barrier and porous oxides [16] Property
Barrier-type oxide
Porous-type oxide
Structure
Thin, compact, non-porous
Inner layer- thin, compact barrier-type
Thickness
1.4 nm/V
Inner layer - l nm/V Varies with current density/voltage, pH and electrolyte
Anionic impurity content
-1-2%
Up to 17 % ; varies with pH, electrolyte, temperature and current density/voltage
Water content
-2.5%
Up to 15 % ; varies with pH and electrolyte
Current efficiency for oxide formation
>90% < 10%
< 70-80% > 20-30 %
Current efficiency for dissolution
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2.4. Composition and density of Inner and Outer Oxide Several previous studies on the anodic alumina film properties(for e.g., crystallinity, water content, anionic impurities and the oxide phase of both barrier-type and porous-type films) have concluded that the both the barrier and porous type oxide films consists of an amorphous layer adjacent to the oxide-electrolyte interface which is contaminated by anionic species from the acid electrolyte ("outer layer") with a thin relativepure and crystalline oxide near the metal-oxide interface ("inner-layer"). [17] P
Fig 2.3 inner and outer oxide [17]
The oxides consist of nanocrystallites, hydrated alumina, anionic impurities as well as water molecules, although there is a major difference in their contents between barrier and porous type oxides. Barrier-type oxide films consist of microcrystalline gama Al203 or gama''ɤAl203, an intermediate between amorphous and ɤgama'' Al203. Oxides of barrier-type films are generally considered to be anhydrous although a small amount of water content (2.5%) is reported to be present in the form of boehmite (AIO(OH)). In the case of porous oxides, water content ranging from 1% - 15% as well as hydrated aluminum oxide phases have been observed The content of anions in the oxide is substantially higher (up to 17%) for porous type oxides than barrier- type oxides (less than 1%) The density of anodic alumina films varies significantly with anodizing conditions. For constant current anodizing conducted for 30 min in sulfuric acid, the density of oxide film was found to be 2.78 g/cm3,Found that increasing the anodizing temperature, and increasing the electrolyte concentration as well as increasing the forming potential leads to a slight decrease in oxide density. In addition to this a hardness of nine on the Mohs hardness scale making it the second hardest mineral, behind only diamond. Alumina has high abrasion resistance, high melting point, non-reflective, insulator and dielectric and non-bio active. [18]
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Fig 2.4: X-Ray analysis of porous alumina [18]
2.5. Thermodynamics of aluminum anodizing process It is well-known to a vacuum scientist that formation of aluminum oxide from aluminum is thermodynamically favorable in an oxygen ambient, even at room temperature. The spontaneous reaction of oxidation of Al is driven by a large negative Gibb's free energy change during oxidation. 𝟐𝟐 𝑨𝑨𝑨𝑨(𝐬𝐬) + 𝟑𝟑𝑯𝑯𝟐𝟐 𝑶𝑶 → 𝑨𝑨𝑨𝑨𝟐𝟐 𝑶𝑶𝟑𝟑 (𝐬𝐬) + 𝟑𝟑 𝑯𝑯𝟐𝟐 𝟑𝟑
2Al(s) + O 2 → Al2O3 (s) 𝟐𝟐
ΔGo = - 1582 KJ/mol
(A)
o
ΔG = - 871 KJ/mol
(B)
Aluminum also readily reacts with water in aqueous environments, but yields various stable byproducts including Al2O3 and Al2O3.3H 2 0, aluminum ions (A13+), and aluminate ions (A1O 2 -). For the Al-water system, 6 reactions are known to occur, assuming the absence of complexing agents with Al. For example, Al forms its oxide with water by, 2Al + 3H2O → Al2O3 + 6H+ +6e−
(1)
From the Nernst equation, the equilibrium equation of the reaction (1) is given as a Function of electrode potential E and pH of the solution by (2)
Where E is the standard reduction potential, R is the universal gas constant, T is the absolute temperature, z is the charge number of the electrode reaction (in this case, z = 3), and F is the Faraday constant (96,500 C/mol). The other 5 reactions are
Each reaction has corresponding equilibrium equations, determined by the Nernst equation if the reaction is electrochemical in nature (the reaction (1), (6), and (7)) and Gibb's free energy of the reactions if the reaction is chemical in nature (the reaction (3), (4), and (5)). [19]
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The resulting equilibrium equations allow construction of domain boundaries in a potential-pH diagram or a Pourbaix diagram of the Al-water system as shown in Figure (2.5) When Al is anodically polarized or anodized, that is E > 0 in Fig (2.5), in neutral, weak acidic, and basic solutions, the growth of the oxide is thermodynamically favored as described by reaction (1). A compact layer of aluminum oxide, called a barrier-type oxide, is known to grow in these conditions. If Al is anodized in a strong acid such as perchloric acid, Al dissolves into the solution, and this process is called electropolishing of Al [2]. [19]
Fig 2.5: Pourbaix diagram of Al-H2O at 298 k . [19]
The Electropolishing of Al proceeds by the reaction (6). In mildly acidic solutions, such as diluted phosphoric, oxalic, and sulfuric acids, it is well-known that porous alumina grows as a result of anodization. It should be noted that formation of porous alumina is not expected from the simple thermodynamic considerations shown in Fig. Generally, it is widely believed that combination of the oxidation reaction (1) and the Al dissolution reaction (6) occurs [1]. However, this argument relies on slow kinetics of Al dissolution, not thermodynamics. Therefore, a more detailed thermodynamic argument for porous alumina formation must be provided to answer whether porous alumina solely the result of kinetic constraints. [19]
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2.6. Kinetics of aluminum anodizing process During the first few seconds of the anodization process, the current density decreases abruptly (in Figure). In this stage, the aluminium substrate is covered by a thin, compact layer of Al2O3. Then current reaches its minimum value due to local instabilities in the electric field across the oxidem barrier layer and subsequently increases to its maximum value During this period of time, pores nucleate on the oxide thin film .Finally, the current density decreases slightly and asymptotically to a constant value at which pore growth is under steady state (i.e. pores grow at a constant rate). [20]
Fig 2.6 : voltage and current – time graphs and porous alumina formation steps during anodizing [20]
2.6.1. Potential and current transients Aluminum oxide, both barrier-type and porous-type can be anodized at constant potential or constant current while the other is monitored over time. The shapes of current-time and potentialtime transient curves are well-established and can provide insight into the growth mechanism and kinetics. Tajima et al classified the voltage-time and current-time transients into 3 different types depending on the physical phenomena that occur during oxide growth. 1. Barrier-type films: At constant current, the voltage increases linearly with time due to a linear growth rate of the oxide at constant field until the potential for breakdown is reached. At constant voltage, the current decreases exponentially with time to low leakage current values at long times. 2. Porous-type films: At constant current, the voltage increases linearly with time until a critical value when transition from barrier to porous type films occurs. The voltage decreases slightly and then reaches a steady state whose characteristics depend on the pH and the applied current density. At constant voltage, the current decreases rapidly for a short period of time due to a sharp increase in barrier layer thickness. After a critical time which is associated with pore formation, the current increases and reaches steady values at long times. The decrease in voltage and the increase in current after pore formation are related to the increase in the active surface area due to the pores. 3. Pitting: At constant current, the voltage increases to a maximum value and decreases gradually over time to low currents. At constant voltage, the current decreases sharply at short times and reaches a minimum and then increases slowly over time. [21] 21 | P a g e
Anodic oxidation of Al proceeds through various kinetic steps associated with different ionic species (Al+3 and O2-). It is generally accepted from marker experiments that the anodic oxide grows simultaneously at both interfaces, i.e. at the metal/oxide (m/o) interface by transport of oxygen ions and at the oxide/electrolyte (o/e) interface by transport of aluminum ions. [21] P
Fig 2.7 Summary of the interfacial reactions at the anode (Al). [21] P
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P
2.6.3. Rates of Oxide Formation and Oxide Dissolution [22] A wide variety of methods were employed to measure the thickness of the oxide layer formed by anodization of aluminum. Recently, optical and microscopic techniques including TEM or SEM have mainly been used to evaluate anodic oxide layer thickness. For the constant current density anodization, the total thickness of the oxide layer can be calculated from the pore-filling method, using the formula produced by Takashi and Nagayama
h=10-7 .B U.V P
- 𝒏𝒏 𝒊𝒊.𝒕𝒕.𝑴𝑴 . f .𝒅𝒅
Al
(Al2O3)
(𝟏𝟏 − 𝑻𝑻)
where, BU is the barrier layer thickness per volt (nm /V), i is the current density (mA/cm2), MAl is the atomic weight of aluminum, n is the number of electrons associated with oxidation of aluminum, F is Faraday constant, k is the weight fraction of aluminum in alumina (0.529), d( Al2O3) is the density of porous alumina(3.2 g cm3), T( Al+3) is the transport number of Al+3ions (about 0.4), and Vp and tp are the voltage and time, respectively measured at the point where two straight parts of the voltage–time transient meet. P
P
the thickness of the oxide layer can be calculated from Faraday’s law. As the efficiency of anodizing is not usually 100%, the recorded current density cannot be used simply for theoretical estimation of the grown oxide layer, and the current efficiency should be considered as follows:
M (Al2O3) = K (Al2O3). j.t.ɳ =
𝐌𝐌 Al2O3 𝒁𝒁.𝑭𝑭
.j.t.ɳ
where, m Al2O3 is a mass of formed oxide, k A2 lO3 is the electrochemical equivalent for aluminum oxide, j is the passing current (A), t is the time (s), h is the current efficiency, M Al2O3 is the molecular weight of aluminum oxide (g/mol), z is the number of electrons associated with oxide formation, and F is Faraday's constant. Taking into account that the oxide mass can be expressed as the product of oxide density (d Al2O3) and oxide volume (V Al2O3 ) or as the product of density, the surface area (S) and oxide height (h):
M(Al2O3) = d (Al2O3). V(Al2O3) = d(Al2O3) .S.h the oxide layer thickness formed at constant current anodizing is:
h=
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𝐌𝐌Al2O3
𝒁𝒁.𝑭𝑭.𝐝𝐝( Al2O3)
𝒋𝒋
. . 𝒕𝒕. ɳ = 𝑺𝑺
𝐌𝐌(𝐀𝐀𝐀𝐀𝐀𝐀𝐀𝐀𝐀𝐀)
𝒁𝒁.𝑭𝑭.𝐝𝐝( 𝐀𝐀𝐀𝐀𝟐𝟐𝐎𝐎𝟑𝟑)
. 𝒊𝒊. 𝒕𝒕. ɳ
2.7. Electrochemistry of aluminum anodizing process 2.7.1 Anodization electrochemical cell Consist of four components: 1234-
Aluminum as anode graphite or platinum as cathode electrolyte of acidic solutions power supply
Fig 2.8: Basic anodization cell [21]
2.7.2 Reactions during anodization process [12] During the formation of the porous oxide layer the anodic, Anodic Reaction (Al Dissolution): Al(s) →Al3+ + 3e− (*2) Acid Decomposition: 𝐻𝐻3 𝑃𝑃𝑃𝑃4 → 𝐻𝐻2 𝑃𝑃𝑃𝑃4− + 𝐻𝐻 + At the oxide/electrolyte interface the water-splitting reaction occurs (Water Decomposition): 𝐻𝐻2 𝑂𝑂 → 𝐻𝐻 + + 𝑂𝑂𝑂𝑂 − 𝑂𝑂𝑂𝑂 − → (𝑂𝑂𝑂𝑂)𝑠𝑠𝑠𝑠𝑠𝑠. + 𝑒𝑒 − (𝑂𝑂𝑂𝑂)𝑠𝑠𝑠𝑠𝑠𝑠. → 𝐻𝐻 + + 𝑂𝑂2− + 𝑒𝑒 − Hydrogen evolution (cathode)
𝐻𝐻2 𝑂𝑂 → 2𝐻𝐻+ + 𝑂𝑂2− + 2𝑒𝑒 − (*3)
6𝐻𝐻 + + 6 𝑒𝑒 − → 3 𝐻𝐻2 (𝑔𝑔) ‘Bubbles at the cathode’ Anode reactions taking place at the metal/oxide boundary (Oxygen anions react with Al) 2Al + 3O2- → Al2O3 + 6e− At the oxide/electrolyte boundary (Al cations react with the water molecules) 2Al3+ + 3H2O → Al2O3 + 6H+ The Overall Reaction: Pore Initiation:
2 𝐴𝐴𝐴𝐴 + 3𝐻𝐻2 𝑂𝑂 → 𝐴𝐴𝐴𝐴2 𝑂𝑂3 + 3 𝐻𝐻2
+ 3+ 𝐴𝐴𝐴𝐴2 𝑂𝑂3(𝑠𝑠) + 6 𝐻𝐻(𝑎𝑎𝑎𝑎) → 2𝐴𝐴𝐴𝐴(𝑎𝑎𝑎𝑎) + 3𝐻𝐻2 𝑂𝑂(𝑙𝑙)
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2.8. Formation and growth mechanisms of anodic porous alumina In contrast to the flourishing picture of applications of anodic porous alumina in various fields, the formation mechanism of anodic porous alumina has been continuously investigated and under debate for more than six decades, i.e. from Edwards &Keller, Anderson, Hoar & Mott in the 194050’s, to O’Sullivan, Wood &Thompson et al., Siejka et al.1 in the 1970-80’s, Parkhutik &Shershulsky, Golovin et al., Jessensky et al., Li et al. in the 1990’s,Patermarakis et al., GarciaVergara et al. in the 2000-10’s, and most recently Hebert & Houser in 2012. [4] However, contradictory viewpoints still exist and no generally accepted theory has been established mechanisms self-organized formation of oxide nanopores. [22]
2.8.1. Pore initiation mechanisms It is well-known that pores are initiated by roughening of the o/e interface during anodization, Pore formation mechanisms at valleys of the o/e interface, the E-field is concentrated, and pores can grow either by field-assisted dissolution or by field-assisted plastic flow of the oxide. However, very few studies have been conducted on what causes the initial roughening of the o/e interface to form a periodic pattern For example, Krishnan proposed that tensile stress from Al vacancy formation causes the initial roughening at the Al/oxide interface due to a strain-induced instability. [4]
2.8.2. Pore Formation Mechanisms In terms of the driving force for nanopore formation, previous theories tend to follow three directions, namely, whether electrostatic energy, Joule heating or mechanical energy can give rise to pore formation as well as self-ordering. I.
Field-Assisted Dissolution Model
In this model it is assumed that the dissolution rate of A1203 is greatly enhanced in presence of an E-field the strength of the E-field is greatly increased at the pore bottom, P, due to the geometry, and therefore, the dissolution rate of the oxide is also increased at the pore base and the dynamic equilibrium between dissolution and oxidation can be establish some local variations in field strength can appear on a surface with defects, impurities or preexisting features including subgrain boundaries, ridges and troughs as remains of pretreatment procedures (e.g., mechanical or electrochemical polishing, etching). This non-uniform current distribution leads consequently to the enhanced field-assisted dissolution of oxide and a local thickening of the film. [22] .
Fig 2 .9 : field assisted dissolution model steps
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[22]
The theory of "field-assisted dissolution" was first proposed by Hoar and Mott in 1959. [22] 1- Qualitatively suggested that under the high electric field on the order of 1 V nm-1, oxygen anions would be pulled from the oxide/electrolyte (o/e) interface to the metal/oxide (m/o) interface to form new oxide at the latter, while aluminum ions would be pushed in the opposite direction across the barrier layer and ejected into the electrolyte. Ion migration in the barrier layer was proposed to take place by means of jumping from one interstitial position to another. 2- they emphasized that no space charge was set up within the oxide, on the basis that the processes of anion and cation migration are comparatively easy. [22] Fig 2.10: mechanism of Al dissolution
O'Sullivan and Wood: [21]
1- O’Sullivan and Wood,95 who proposed that the barrier layer thickness was determined as a result of a competition between oxide formation in the barrier layer and field-assisted dissolution at the pore base. 2- They also suggested that the high electric field could stretch or break the aluminum oxygen bonds, thus aiding the dissolution of oxide and resulting in a faster rate than open-circuit chemical dissolution Fig 2.11: Schematic figure of dissolution of Al2O3[21]
Other experiments revealed that the field-assisted oxide dissolution at the pore base is virtually negligible.
Thamida and Chang predicated a critical pH value of 1.77 for the transition from porous to nonporous alumina. [4]
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II.
Al+3 Direct Ejection Model
Oxygen isotope (18O) studies indicated that the pore formation does not take place through a simple oxide dissolution process, and suggested that pore formation consists of some kind of oxide decomposition through the direct ejection of Al3+ into the solution and the oxide formation at the m/o interface through oxygen transport. This overall reaction is called Al3+ direct ejection. It must be noted that while the direct ejection of Al ions model is not directly related to pore formation, it is generally believed as a necessary condition for porous oxide formation. In other word, any initiated pores would be healed without the Al3+ direct ejection mechanism due to preferential formation of A1203 at the o/e interface because of the high E-field at the initiated pore base. [22]
III.
Fig 2.12: Al+3 ejection model
Field-Assisted Plastic Flow Model
Recent tungsten tracer studies by a group in Manchester According to their explanation, 1- if pores were formed by field-assisted dissolution, W tracer at the pore base would migrate ahead of the W tracer at pore walls, but they found that W tracer at the pore base lagged behind that in the pore walls 2- Indicate that flow of oxide materials has a major role in forming pores, contrary to expectations of a dissolution model of pore development. The flow of the oxide was suggested to arise from the field-assisted plastic flow of oxide materials from pore base toward the cell boundary and the generation of stress due to electrostriction and the oxidation of aluminum. This flow-based pore formation mechanism has been further supported by theoretical study 3- The unexpected Nd and Hf tracer distribution was attributed to the faster migration rate of the tracer atoms compared with that of Al ions, but it should also be noted that researchers such as Oh169 have indicated that a tracer study alone cannot yield sufficient evidence to prove oxide flow or disprove electric-field-assisted dissolution as the mechanism for pore formation. [22]
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IV. Other Models of Porous Alumina formation Shimuzu et al [4] considered 1- the pore initiation process as a transition from a barrier-type film to a porous-type film due to cracking of film under tensile stress as a result of PBR values less than 1. 2- They concluded that the build-up of tensile stress in the oxide contributed to local cracking of the film above pre-existing metal ridges (from electropolishing) on the metal surface. They also suggested that the cracked regions are repaired by oxidation processes; however this leads to non-uniform film growth. [4]
2.8.3. Steady state growth It is generally accepted for steady-state film growth that oxide nanopores are generated as results of a dynamic equilibrium between the rate of field-assisted oxide dissolution at the electrolyte/oxide (e/o) interface and the rate of oxide formation at the metal/oxide (m/o) interface, which keeps the thickness of the barrier layer constant. [17] It is also of interest to note that van Overmeere et al. [4] recently performed an energy-based perturbation analysis for porous structure growth in anodic porous alumina, and they concluded that the electrostatic energy, rather than the mechanical strain energy-induced surface instability, was the main driving force for pore initiation as well as a controlling factor for pore spacing selection. [6]
2.8.4. Pore ordering mechanisms For self-organized growth of AAOs, Jessensky et al. [4]proposed that that repulsive forces between neighboring pores can arise during anodization. These forces were conjectured due to mechanical stress at the metal/oxide interface that is associated with volume expansion during oxidation of aluminum was proposed as a main driving force for the close packed hexagonal arrangement of oxide nanopores. [23] They reported that volume expansion of approximately 1.4 for all of the self-ordered regimes Based on the volume expansion and the porosity, they calculated the stress in alumina to be 4 GPa, compressive. However, an important question is whether it is the moderate electric voltage (related to electrostatic energy) or the moderate volume expansion ratio (related to mechanical stress energy) which really causes the ordering in the porous structure. These two factors cannot separated in their experiments, and so sufficient evidence has not been established to support that the main reason for ordering is due to mechanical stresses. [15]
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2.9. Influence of electrochemical conditions on porous anodized alumina [4] Factors affecting formation of porous alumina :a comprehensive study was released which outlined the four major factors that influence the growth of the pores: electrolyte type and concentration , electrolyte temperature, voltage, and time. 2.9.1. Electrolyte The electrolyte solution used for anodization also has a big influence on the porous structure. Naturally, a more acidic electrolyte will result in faster dissolution and the presence of more ionic conductors will also increase the anodization rate. It can be seen in comparing the oxalic and sulfuric acid graphs that the stronger sulfuric acid shows a much higher ionic activity, resulting in a larger current density than oxalic acid solutions at the same applied voltage. The strength and activity of the acid also seems to influence the size of the pores.
Fig 2.13: Interpore- Distance relationship for various electrolytes Table 2.3 AAO templates with different pore arrangements achieved under different self-ordering conditions
Electrolyte
Interpore distance (nm)
H2 SO4 (25 V, 0.3 M)
66.3
7.2
24
(COOH)2 (40 V, 0.3 M)
105
9.1
31
H3 PO4 (195 V, 0.1 M)
501
54
158.4
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Inner-wall thickness Pore diameter (nm) (nm)
2.9.2 Anodizing voltage The size of the pores in any porous film is linearly dependent on the anodization voltage, sothat larger voltages lead to larger pores.
Fig 2.14 : A) empirical Interpore Distance and pore distance - anodizing voltage relationship B) SEM micrographs show effect of increasing voltage on pore size and interpore distance
2.9.3. Temperature anodization at constant current density, an increase in temperature results in an increase in pore size and interpore spacing.The structural features of anodic porous alumina formed in 2.4M H2SO4 (especially pore diameter) depend on the electrolyte temperature as, with increasing temperature, an enhanced oxide dissolution is observed. Temperature affects the rate of oxide growth. Increase of temperature raises the rate of oxide growth, as well as the pore diameter due to thermally-enhanced dissolution at a given anodization voltage .However, anodizing at high temperature(higher than room temperature) promotes the dissolution of oxide. Therefore the process is usually conducted below room temperature conductivity of electrolyte and retard the process due to diffusion limitation of ions in electrolyte. 30 | P a g e
Fig 2.15 : SEM micrographs show effect of increasing temperature on pore size and interpore distance
2.9.4 Duration of anodization The thickness of aluminium oxide depends on the duration of anodization. The longer the duration of an anodization process, the thicker is the oxide obtained. Oxide thickness increases linearly with time until diffusion of electrolyte solution to the pore base becomes the determining factor and the growth rate of oxide is then reduced . pore diameter increase with increase anodizing time. [4]
Additives
Fig 2.16: pore distance - anodizing time relationship
Additives in the electrolyte are reported to permit the manipulation of pore morphology. The addition of polyethylene-glycol (PEG) to phosphoric acid was reported by Chen and coworkers for the successful anodization at a high anodization voltage (> 200 V) and with suppressed current density, so a wider range of interpore distance (up to 610 nm) was obtained but with the AAO pores having small diameters. A highlyordered hexagonal arrangement of the pores can be observed at the backside of the barrier layer. In general, the use of additives provides a wider window for tuning of pore dimensions. [4] P
Post-treatment Widening is usually carried out to further adjust the pore diameter without changing the interpore distance and the areal number density of pores . It can becarried out in phosphoric acid with different concentrations and temperatures, depending on the final pore diameter required. pore diameter increase with increase widening time. [4]
Stirring speed: As stirring speed decrease, local field assisted temperature increase, rate of oxide dissolution increase rate of oxide formation increase, current density increase and pore diameter decrease. [4]
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The most important parameters that affect the anodic oxidation of aluminum and the oxide film properties are the applied voltage or current, pH and type of the electrolyte and temperature. For brevity, we only discuss the effect of these parameters on the formation of porous-type films Table 2.4: effect of electrochemical conditions on the film physical and chemical properties .[5]
Table 2.4 contd: effect of electrochemical conditions on the film physical and chemical properties. [5]
[5]
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2.10. Morphologies of porous anodic alumina [23] In general, two different methods of fabrication of AAMs can be distinguished, (1) Prepatterned guided anodization and (2) self-organized anodization. Table 2.5 : show different Morphologies of porous anodic alumina
1.Common Morphology of AAMs: The periodic pore arrangements seen in Fig. 4.1a–d with pore diameters of 38, 55, 80, and 200 nm were obtained in sulfuric, oxalic, and phosphoric acid solutions under voltages of 25, 40, 70, and 160 V, respectively. It can be seen in Fig. 4.1a–c that the AAMs have a perfect hexagonal nanopores For disordered pore arrangement in Fig. 4.1(d),it may be of the responsibility of the contraction or the strong volume expansion
2.Cylindrical AAMs The process to obtain cylindrical AAMs is based on the well known method to obtain AAMs. Figure 4.2 shows a schematic view of the two-step anodization process together with SEM images of the microtubular thickness and the cylindrical outer surface of the cylindrical AAMs anodized under oxalic acid. High-resolution scanning electron microscopy (HRSEM) images reveal a great similitude between cylindrical AAMs and conventional planar AAM nanostructures, that is, large parallelism in the nanopore axis and similar interpore distances. It should be noted that the ratio between interpore distance and typical aluminum wire radius, used in these experiments, is about 1:1000.
3. Conformal AAMs
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4-AAMs with Diamond and Hybrid Triangle-Diamond Patterns This Figure FESEM images of diamond (a–c) and hybrid triangle-diamond (d–f) patterns. Post developed PMMA surface of the diamond (a) and hybrid (d) pre-patterning arrangements with 300 nm periodicity. Diamond (b) and hybrid triangle-diamond (e) porous templates following a single-step, 30 min anodization at 120 V in a 0.3M H3PO4 electrolyte (5°C). Partially etched back alumina template showing the relative thicknesses of the inner (bright cell regions) and outer (semi-bright regions) oxides for the diamond (c) and hybrid (f) patterns. An SOG layer (darker region) covers the sidewalls surrounding the perimeter of the pore openings. All scale bars are 500 nm.
5.AAMs with Square and Triangular Nanohole Arrays Masuda et al. successfully fabricated the AAMs with square and triangular nanohole arrays according to the aluminum indentation and then anodization conducted with constant applied voltage in oxalic acid or phosphoric acid solution. The shape of the openings in as-anodized membranes is circular. However, the shape of the openings varied from circular to square through the etching treatment. This was because the dissolution rate of the anion-free inner wall of the cells is lower than that of the anion-incorporated outer wall, and the square openings composed of the remaining inner walls were formed. For triangular cells and openings, the arrangement of the initiation sites of graphite structure lattices was adopted
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6.AAMs with Symmetric Six-Membered Ring Structure Zhao et al. [14] first reported the novel structures of AAMs with a symmetric six-membered ring structure. Figure 4.6a is a sectional image of the AAM sample, confirming that normal AAM nanopores (as shown by the arrows) having an interpore distance of 85 nm are closed, while the smaller pores (as shown by the open arrows) at the triple cell junction are already opened. The small pores belonging to the six-membered ring structure are formed at the bottom surface as observed in Fig. 4.6 but span over the whole thickness (50 μm) of the AAM film, as confirmed by the AAM top-surface SEM image shown in Fig. 4.6b.
7. AAMs with a Checkerboard Pattern
8.AAMs with Inverted Cone Porous Structure According to the multistep anodization and leaching process, Nagaura et al. successfully produced three-dimensional (3D) nanometer-scale structured AAMs. During the leaching process, the AAMs were dipped in phosphoric acid solution for pore widening. Each anodization process was followed by this leaching process. This method produced AAMs with a multistep structure. Meanwhile, with five-step film production, the structure showed an inverted cone structure. The Figure Cross-sectional morphologies of AAMs with unique 3D structures formed with different anodization and pore-widening repetition times. (A) Two-step stair structure: two times. (B) Three-step stair structure: three times. (C) Inverted cone structure: five times. 35 | P a g e
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Chapter 3 Experimental techniques 3.1 Introduction This chapter will include a full description and illustration for the techniques, devices and the tools used to study the factors affecting Nano porous fabrication. The effect of the voltage used, electrolyte type, temperature, and surface pretreatment on the formation of Nano pores via electrochemical anodization of commercial pure aluminum will be explained.
3.2 Test specimens The substrates used in the anodization process for AAO fabrication are: A- Commercial pure aluminum substrate of 99.46% aluminum. This commercial pure aluminum purchased from El sabtia , cairo , Egypt. Chemical composition Analysis was done in central metallurgical research institute ( in Tebbin , Helwan and detailed chemical composition indicated in table 3.1
Table 3.1 the chemical composition of test materials
Sample
Al
Fe
Si
Ti
V
Mg
Zn
Sn
pb
Ni
Co
Mn
Cu
A
99.46
0.439
0.0598
0.0265
0.0117
0.001
0.0009
0.0007
0.0006
0.0004
0.0004
0.0002
0.0001
Aluminum sheet was cut in to small pieces and then make small specimens of anodes The anode was made out of 2×2×0.01 cm for commercial aluminum samples (99.00%).
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3.2.1 The following figure illustrates the experimental work plan: voltage surface pretreatment
anodization
commercial pure aluminum
mechanical polishing chemical polishing
time
post treatment
Temperature Type of elecrolyte Fig 3.1 :Experimental work plan
3.3 Preparation of test specimens Test specimen surface was carefully prepared in order to remove dirt, grease or any other impurities attached to its surface where these dirt, grease and impurities might lead to ineffective bonding between coat layer and the substrate. The specimen were picked into acetone for 15 minutes and rinsed with distilled water, then and order to smooth the surface irregularities and decrease the surface roughness
3.3.1. Surface pretreatment 3.3.1.1Mechanical Polishing The surfaces of commercial aluminium substrates are tarnished and have trenches several micro metres deep. These surface imperfections yield different pore growth rates on the aluminium substrate that modify the pore arrangement and are the source of structural defects on the resulting NAATs. In order to reduce the surface roughness and remove tarnishes from the Al surface mechanical polishing applied before anodizing. Aluminum is a relatively soft metal and its one of the easiest metals to be polished. we used diamond paste MetaDi 6 um and 2 um ,Buehler GmbH,Germany
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We used diamond paste to polish the aluminium surface. Diamond paste is a type of polishing compound made from finely ground or powdered diamond particles, and some kind of liquid, usually water based. Diamond paste is incredibly effective at removing oxidation, tarnish, stains, and water spots. Most diamond paste grinding and polishing is done progressively, starting with the coarser grade, 6 microns, and then finishing with the 1 micron grade. We started hand polishing for about 15 minutes in the same direction until the surface was a little bit shiny and then we cleaned the surface using a piece of cotton and then we used 1 micron diamond paste to polish the surface until the surface was totally shining. Then acetone spray was applied to the specimen before anodization process.
3.3.1.2Chemical polishing: Chemical polishing processes are of more recent origin than electropolishing and have replaced them to a considerable extent. Their primary attraction is the comparatively much smaller outlay in plant required, not only for electrical power, but also for tanks and fixtures. As no electric current or anode rod movement is employed, work can often be treated in baskets or on simple fixtures. Put aluminum specimen in NaoH (1M) for 10 minutes then neutralize it in mixture of ( 75% H 3 PO 4 + 25 % HNO 3 ) at 85 oc for 5 minutes. Chemical polishing make the surface more smooth and free of notches and scratches in little time compared to mechanical polishing
3.4 Electrolytic cell and anodization process The anodization process was carried at the corrosion lab, metallurgy department, faculty of engineering, Cairo University. The working cell consisted of 1 liter glass beaker with a Teflon cover. schematic figure of the anodizing cell indicated in fig 3.2. Both the aluminum anode and the platinum cathode are connected to the source of electricity as positive and negative poles respectively and Distance between electrodes 4 cm.
Fig 3.2: anodizing cell used for synthesis of nanoporous membranes
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3.4.1 Conditions of anodization process:
Table3.2 Working conditions for anodizing process
Sample
1
2
Anodizing Voltage , volt
4
5
6
15,20,25
Time, min Type of Electrolyte
3
30 ,60 Sulphuric acid Oxalic acid
Temperature
Room temperature ( 20 ± 2 ) Low temperature ( 3 ± 1 )
Concentration of electrolyte ,M
0.3 M
pH
0.75
Stirring speed , rpm
300
3.4.2 Electrolytic bath: The solution composition and working conditions are illustrated in the following table (table 3.2).
3.4.3 pore widening (opening) After AAO process and in order to possess a controllable pore diameter therefore pore adjustment was done by exposing the specimens to a chemical solution containing 3 wt.% H3PO4 for 10 minutes to increase the diameter of the pores.
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3.5 Test devices and tools used A 30 volt / 6 Amperes power supply of type GW Instek GPC-3030DQ – Taiwan, was used in the process also a magnetic stirrer was used with speed 300 rpm.
Fig 3.3: A 30 volt / 6 Amperes power supply
we used 4 digits electric balance of type Ohaus PA214 Laboratory Scale,Bradford, MA .USA , as shown in fig 3.4
Fig 3.4: 4 digits electric balance
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we used magnetic stirrer of type Wisestir digital magnetic stirrer with hotplate and square type,witeg,Germany as shown in fig 3.5
Fig 3.5: magnetic stirrer device
we used pH meter of type 8314 pH meter Hanna Insturments, USA.
Fig 3.6: pH meter device
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3.6. Scanning Electron Microscope (SEM) Analysis The SEM was used to demonstrate the microstructure and phases morphology where it was carried out at the National Research Institute -Dokki. Specimens were prepared before SEM analysis through gold sputtering for 2 minutes in order to improve contrast and conductivity. The image is formed in an SEM by scanning an electron beam across a sample and collecting some signal from the beam-sample interaction, which is used to control the intensity of the spot on a television monitor which is scanning in synchronization with the beam on the sample. The nature of the signals collected by an SEM in order to form images is dependent on the detector used to collect them. the SEM column consists of the electron gun and then several magnetic condense lenses, which are used to focus the electron beam into a small spot onto the sample surface
Fig 3.7. the (SEM) Unit
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3.7.Energy Dispersive X-ray Spectroscopy (EDX) Energy dispersive X-ray spectroscopy (EDX or EDS) done at at the National Research Institute -Dokki. , its an analytical tool predominantly used for chemical characterization. EDS allows one to identify what those particular elements are and their relative proportions Being a type of spectroscopy, it relies on the investigation of a sample through interactions between light and matter, analyzing X-rays in this particular case. High energy electron beams (in an SEM or TEM) strike the material to be analysed, and X-rays are emitted. These X-rays can be detected by a SiLi detector, calibrated with respect to cobalt metal emission (6.925 keV), and then used to identify and analyze the elemental composition of the specimen surface. Its characterization capabilities are due in large part to the fundamental principle that each element of the Periodic Table has a unique electronic structure and, thus, a unique response to electromagnetic waves. Essentially, an X-ray photon hits a diode in the detector producing a charge that is converted into a positive voltage pulse via a field effect transistor (FET). The pulse is subsequently converted by an analogue to digital converter, into a numerical value relative to the X-ray’s incoming energy. The signal is then assigned to a particular energy channel and registered as a single count. Counts are compiled to produce an energy dispersive spectrum. The various emission lines associated with X-rays emitted from an atom are named after the shell of the initial vacancy, i.e. K, L, M, etc.
A Greek letter subscript is used to indicate the shell of the electron that fills the gap. For example, Kα radiation refers to radiation resulting from a vacancy in the K shell being filled by an electron from the next highest shell. Kβ denotes a K-shell vacancy filled by an electron from two shells above.
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3.8. Theoretical calculations 3.8.1. Thickness of anodic oxide Calculation of mass of oxide from Faraday's law
2Al +
m
𝟑𝟑 𝟐𝟐
O2
faraday
Al 2 O 3
=
Where M= molecular weight of oxide =102 , I*T= Q total electric charge = area under the anodizing current -time curve. F= Faraday's constant = 96500c , Z=6, the valence number of ions of the substance (electrons transferred per ion) Then we have m exp measured and can calculate as previous the actual thickness and compared with ideal from m faraday
3.8.2. Calculations of efficiency of the process Increase in weight measured after anodizing = m exp Efficiency = m exp / m faraday
3.8.3. Calculations of oxygen consumed during the process From equation above by simple weight ratio of O 2 / Al2 O 3 We can get amount of oxygen consumed from m exp and m
Factors to be studied :-Voltage ,Tme, Electrolyte type and Surface Pretreatment.
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faraday
Chapter Four Results and Discussion
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Chapter 4 Results and Discussion 4.1. Group I (Room temperature anodizing) 4.1.1. Sulphuric acid Aluminium specimens were pretreated mechanically with diamond paste and then were anodized at room temperature in a solution of sulphuric acid of concentration (0.3M). Anodising was carried at different voltages (15,20,25)Volts and for different durations of (15,30,60) minutes. The specimens were then examined under the SEM and Figure 4.1 shows the micrographs obtained and the time current relationship obtained for each specimen. Anodizing Current-time relationship represent kinetics of formation of anodic oxide and its morphology, with it we can know morphology barrier or porous . Specimens were examined under the SEM with low magnification (1.5KX), at low voltage 15 and 20 volt the anodic films exhibit a barrier( non-porous ) surface; the SEM micrograph of samples (anodized at high voltage 25 volt ) showed the existence of pores. And white particles of dry acids can appear in micrographs. As voltage increases current density increases linearly. Anodizing at the same voltage for different duration causes the formation of anodic oxides of different thicknesses. Specimen anodized at 25 voltage show some electric burning in room temperature Specimen anodized at 25 volts was promising and shoed some porosity that’s why another two specimens were anodized at 25 volts to examine their micrographs and to determine whether anodic porous alumina will form or not .
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SEM microstructure at 1500x
Sample
Anodizing(current-time) relationship
Sample (1) 15 volt,30 min
15 volt – 30 min
0.6
CURRENT(A)
0.5 0.4 0.3 0.2 0.1 0
0
500
1000
1500
TIME(SEC)
Sample (2)
Current (A)
15 volt-60min
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0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
15 volt,60min
0
1000 2000 3000 4000 Time(sec)
2000
Sample (3) 20 volt,30 min current)(A)
20volt-30min
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
0
500 1000 1500 2000 Time(sec)
Sample (4) 20 volt-60min
20 volt, 60 min
0.6 Current(A)
0.5 0.4 0.3 0.2 0.1 0
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0
1000 2000 3000 4000 Time(sec)
Sample (5) 0.7
25volt-30min
25 volt,30 min
Current(A)
0.6 0.5 0.4 0.3 0.2 0.1 0
0
500 1000 1500 2000 Time(sec)
Sample (6) 25 volt,60 min
Current(A)
25 volt-60min
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0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
0
1000 2000 3000 TIME(SEC)
4000
Sample (7) 25volt,30min
current(A)
25volt- 30min
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
0
500
1000 1500 time(sec)
2000
Sample(8)
Current(A)
25volt-60min
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
0
1000 2000 3000 4000 Time(sec)
Figure 4.1: SEM Micrographs and corresponding anodizing current-time relationship for sulphuric acid anodizing
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4.1.1.2. Theoretical calculations of sulphuric acid The calculations show that the efficiency increases when the experimental time (anodizing time) and voltage increases. At 25V-60min the efficiency is not high the reason is the formation of pores in this sample and dissolution of alumina. Figure 4.2 shows the voltage efficiency relation and a trend line . Table 4.1 efficiency calculations of room temperature sulphuric acid at different Voltages.
Sample Condition
area(I*T)
1 15volt-30min
M faraday
55.39 0.009758
M(Exp)
F (o2)
0.002071
0.54048 6 1.06973 3 0.93737 3 3.65744
0.000975
7.62329987
1.61796875
21.22399
0.003619
10.72478141
6.0078125
56.01804
0.002024
10.03938633
3.359375
33.46196
0.007953
19.83076425
13.203125
66.579
1.52087 4 11.1287 1
0.0024
12.78784812
3.984375
31.15751
0.009459
34.59180699
15.703125
45.3955
2 15volt-60min
77.925
0.013728
0.00769
3 20volt-30min
72.945
0.01285
0.0043
4 20volt-60min
144.088
0.025383
0.0169
5 25volt-30min
92.915
0.016368
0.0051
6 25volt-60min
251.34
0.044278
0.0201
wt(o2)
thickness(calc) thickness (exp)( μm) ( μm)
70 60 EFFICIENCY,%
50 40 30 20 10 0 0
1
2
3
4
5
6
7
SAMPLE NUMBER
Figure 4.2 Efficiency results for sulphuric acid anodizing at room temperature 52 | P a g e
Efficiency
Thickness of oxide layer The Alumina layer thickness formed during the anodizing process is calculated by faraday's law and experimental measured. This is shown in Figure 4.11. The thickness calculated by faraday's law is higher than that measured by experimental techniques. The oxide layer thickness increases with the increase of the voltage and anodizing time from figure 4.3 we can see that maximum thickness obtained was about 16 micrometer at 25 volt-60min sample . 40 35 30 25 20 15 10 5 0 1
2
3 F(thickness)
4
5
6
wt(thickness)
40 35
Fthickness
30 25 20 15 10 5 0 0
5
10 Wt(thickness)
15
20
Fig 4.3 :( A and B) Thickness of oxide layer by faraday's law and experimental measured for each sample 53 | P a g e
4.1.1.3 Voltage - time relationship for pits initiation The relation between voltage and pits initiation time in sulphuric acid anodizing is explained in Figure 4.4. As the voltage increases, the pits initiation time decreases. According to the figure, at 25 volt the pore initiation time is 12 seconds and at 20 volt the pits initiation time is 25 seconds and at 15 volt the pits initiation time is 60 seconds.
30 25
pits
voltage (V)
20 15 10
No pits
5 0
0
10
20
30 40 time(sec)
50
Fig 4.4 : voltage time relationship for pits initiation in sulphuric acid anodizing
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60
70
4.1.2 Oxalic acid group Oxalic acid group at the same concentration (0.3 M ) at different voltages 15 .20 .25 and different times allow show barrier behavior through anodizing current-time relationship. Oxalic acid anodizing at this low voltages have low current and can reach to 0.01 and 0.00 Ampere at 15 volt.
Sample
Anodizing current-time relationship
Sample (1)
15volt-30min
0.1
current (A)
15 volt – 3omin
0.12 0.08 0.06 0.04 0.02 0 0
Sample (2)
500
1000 time(sec)
1500
2000
0.12 0.1
current (A)
15 volt-60min
15volt-60min
0.08 0.06 0.04 0.02 0 0
Sample (3)
1000
2000
time(sec)
3000
4000
0.2
20 volt-30 min
2o volt-30min
current (A)
0.15 0.1 0.05 0 0
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500
1000
time (sec)
1500
2000
Sample (4)
0.3 0.25
current (A)
20volt-60min
20volt-60min
0.2 0.15 0.1 0.05 0 0
1000
2000
3000
4000
time (sec)
Sample (5) 25 volt-30min
0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 0
Sample (6)
500
1000
1500
2000
0.5 0.45
25volt-60min
25volt-60min
0.4
current (A)
0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 0
1000
2000
3000
time (sec)
Figure 4..5 : Anodizing Current-time relationship curves for Oxalic group.
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4000
4.1.2.1 Theoritical Calaculations of Oxalic acid Oxalic acid specimens showed lowest efficiencies compared to that of sulphuric acid yet it should be considered that we the experiment was not done within the oxalic acid range of voltage. Sample (1) where anodizing was carried at 15 volt for 30 minutes shows the lowest efficiency about 18.7% Sample (6) where anodizing was carried at 25 volts for 60 minutes showed maximum efficiency about 42%. Table 4.2 Efficiency calculations of room temperature oxalic acid at different voltages.
1 2 3 4 5 6
condition
area(I*T)
15volt30min 15volt60min 20volt30min 20volt60min 25volt30min 25volt60min
18.195 36.225 18.325 36.38 54.425 108.355
mAl2o3
F(o2)
M(Exp.value) wt(o2)
0.003205 0.006382 0.003228 0.006409 0.009588 0.019088
0.001508 0.003003 0.001519 0.003016 0.004512 0.008983
0.0006 0.0015 0.0008 0.0016 0.0035 0.008
0.000282 0.000706 0.000376 0.000753 0.001647 0.003765
thickness(calc) thickness(exp) Efficiency μm μm ( μm) 2.504169365 0.46875 18.71878 4.985629858 1.171875 23.50505 2.522061205 0.625 24.78132 5.006962435 1.25 24.96524 7.49048737 2.734375 36.50463 14.91284812 6.25 41.91017
45 40 35 Efficiency
sample
30 25 20 15 10 5 0 0
1
2
3
4
5
6
7
Sample number
Fig 4.6 : Efficiency – voltage relation for oxalic acid anodizing at room temperature
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Thickness of oxide layer thickness of oxide layer obtained from oxalic acid anodizing reach maximum thickness up to 6micrometer at 25volt-60min sample and minimum up to 1 micrometer at 15 volt-30 min sample
16 14 12 10 8 6 4 2 0 1
2
3
4
F(thickness)
5
6
wt(thickness)
16 14
F(thickness)
12 10 8 6 4 2 0 0
1
2
3
4
5
6
7
Wt(thickness)
Fig 4.7. (A and B) Thickness of oxide layer by faraday's law and experimental measured for each sample in oxalic acid . 56 | P a g e
4.1.3. Effect of temperture on current at different voltages The relation between the voltage and the current for stable pore growth at different temperatures is shown below in figure 4.8. At the same range of the voltage of 15, 20,and 25 volt, the current at low temperature is lower than that at room temperature. 30
25
voltage
20
15
10 room temp current-voltage 5
low temp current - voltage
0 0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
current for stable pore growth
Fig 4.8: Effect of temperature of anodizing bath on current at different voltages.
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0.08
4.1.4. Current – voltage relationship for different types of acids As in figure 4.9 At low temperature anodizing, the pore growth is more stable Current at room temperature of anodizing at sulphuric acid anodising is higher than that of oxalic acid. 30 25
volta ge
20 15 oxalic at room temp
10
room temp stable current-voltage
5 0 0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
stable current
Fig 4.9: Current – voltage relationship for different types of acids
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0.08
4.2. Group (2): Low temperature anodizing
From previous results anodizing at 25 volt in 0.3 M sulphuric acid is the most promising for production of porous structure, so we investigate this condition at low temperature with different pretreatment mechanically with diamond paste, electrochemical polishing ( 75 % H3PO4 – 25 % HNO3 at 85 oc for 1 minute then neutralizing at 1M NAOH for 10 minutes ) or combination of electrochemical and chemical polishing. 4.2.1 SEM characterization SEM micrographs for both conditions in figures 4.3 Shows well-ordered porous alumina membranes. The best condition was anodizing 2 hours at 25 volt with combined mechanical and electrochemical polishing. Pore opening for two samples was 10 minutes in 3%wt of H3PO4 Current-time anodizing curve confirmed growth of porous structure through being as much close to theoretical ideal curve.
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Sample
SEM micrograph
25volt- 60 min Temperature (3 ±1)
Sample
Anodizing Current-time relationship
25volt- 60 min
0.09 0.08
Temperature (3±1) current (A)
0.07 0.06 0.05 0.04 0.03 0.02 0.01 0
0
500
1000 1500 2000 2500 3000 3500 4000
time (sec)
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Sample (2) 25-volt – 120 min Temperature (3 ± 1)
Sample
Anodizing Current-time relationship
25-volt – 120 min 1.4
Temperature (3±1) current (A)
1.2 1 0.8 0.6 0.4 0.2 1 72 143 214 285 356 427 498 569 640 711 782 853 924 995 1066 1137 1208 1279 1350 1421
0 time (sec)
Fig 4.10. Shows SEM micrographs and Current-time experimental curves for nanoporous alumina membranes 63 | P a g e
4.2.2. EDX of Nanoporous Alumina Membranes EDX analysis of nanoporous alumina membranes show majority composition of aluminum and oxygen with traces of sulphur and phosphorus.
Fig 4.11. EDX analysis of nanoporous alumina membranes
EDX analysis in table 4.5 shows detailed chemical composition of nanoporous alumina and it contains 56 % Al , 36% O , 6%S and about 1% P.1% P.
Table 4.3. chemical composition of nanoporous alumina membranes Element O K AlK P K S K Total
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Wt %
At %
36.15 56.79 0.99 6.07 100.00
49.28 45.90 0.69 4.13 100.00
K-Ratio 0.1302 0.4509 0.0056 0.0411
Z 1.0443 0.9724 0.9641 0.9852
A 0.3446 0.8154 0.5875 0.6873
F 1.0009 1.0013 1.0026 1.0000
4.2.3. Image Analysis of Nanoporous Aumina Membranes
Fig 4.12 : pore diameter measurements of nanoporous alumina membranes.
using image analysis software ( imageJ ) we make pore size and interpore distance distribution . 4.2.3.1 Pore diameter distribution 120
number of pores
100 80 60 40 20 0 20
21
22
23
24
pore size (nm)
Fig 4.13: Pore size distributions of nanoporous alumina membranes. 65 | P a g e
25
number of pores
4.2.3.2. Interpore distance distribution 100 90 80 70 60 50 40 30 20 10 0 36
42
45
50
60
interpore distance(nm)
Fig 4.14. Interpore distance distributions of nanoporous alumina membranes
pore size in range of 30-25 nm and interpore distance in range of 36-60 nm from knowing pore diameter and interpore distnce we can make calculations as in table 4.2 and get porosity and pore density and wall thickness of nanoporous membrane. Table 4.4: calculated structural characteristics of nanoporous alumina membranes
Parameter
Calculation formula
porous oxide layer porosity (α) pore density (n)
𝛼𝛼 =
Wall Thickness (W)
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𝜋𝜋
2√3
𝐷𝐷𝐷𝐷
( )2 𝐷𝐷𝐷𝐷
𝑛𝑛 =
2.10^14
√3 𝐷𝐷𝐷𝐷 2 𝑊𝑊 = ½ (𝐷𝐷𝐷𝐷 − 𝐷𝐷𝐷𝐷)
Calculation results
α= 9.8 %
n= 1.9 * 10 ^ 12 pore / cm2 W = 20 nm
4.2.4.
Theoritical
Calculations
of
Low temperature anodizing
This calculations for sample (1) which represents the best condition which produces well-ordered nanoporous membranes. and table 4.4 indicate that it has a low efficiency about 20 % Table 4.5. Efficiency calculations of nanoporous alumina specimen sample
25volt60min
area(I*T) mAl2o3
F(o2)
69.69 0.012277 0.005778
M(Exp.value) wt(o2)
thickness(calc) (μm)
0.0025 0.001177
9.591402
12 10 8 6 4 2 0 1 F(thickness)
wt(thickness)
Fig 4.15: thickness calculated from measured weight and faraday’s calculations for nanoporous alumina membrane
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thickness (exp)( μm) 1.953125
Efficiency
20.36329
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Chapter Five Conclusions and Future Work 5.1. Conclusions 1. Anodizing in oxalic acid at voltage range from 15 – 25 volt in room temperature produces only barrier oxide layer. 2. Anodizing in sulphuric acid at room temperature produces micropits 3. A production of nanoporous alumina membranes can be carried out using anodizing process of sulphuric acid at low temperature. 4. EDX analysis shows nanoporous alumina membrane composition of Al , O and sulphur and traces of phosphorus 5. Nanoporous alumina membrane production is a low efficiency process about 20 % . 6. Nanoporous alumina membranes produced has pore diameter of 23 nm and interpore distance of 60 nm
5.2. Future work Recommended future work includes: 1.
Use nanoporous membrane as templates for nanowires and nanorods production.
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Chapter six References
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