universidade federal de santa catarina curso de

UNIVERSIDADE FEDERAL DE SANTA CATARINA CURSO DE GRADUAÇÃO EM ENGENHARIA DE MATERIAIS

KAREN BOLIS

OPTIMIZATION OF PECHINI SYNTHESIS AND CHARACTERIZATION OF CE-TZP AND Y-TZP MIXTURES USING COMMERCIAL AND PECHINI POWDERS

FLORIANÓPOLIS 2015

2


KAREN BOLIS


Thesis presented to the Graduation Course of Materials Engineering from the Federal University of Santa Catarina as partial requirement for the attainment of Materials Engineer degree. Supervisor: Prof. Dr. Marcio Celso Fredel. Co-supervisor: Dr. Joana Mesquita Guimarães.

FLORIANÓPOLIS 2015

3


KAREN BOLIS


This work was assessed adequate to the attainment of the title “Engenheiro de Materiais” and approved by the Graduation Course of Materials Engineering from the Federal University of Santa Catarina. ___________________________________ Professora Sônia Maria Hickel Probst Course Coordinator

________________________________ Professor Dylton do Vale Pereira Filho Subject Professor

EXAMINING COMMITEE _________________________________ Professor Marcio Celso Fredel, Dr. Ing Supervisor _________________________________ Joana Mesquita-Guimarães, Dr. Co-supervisor _________________________________ Rafael Santiago Florianai Pereira, M.Eng. _________________________________ Professor Dylton do Vale Pereira Filho, M.Eng.

Florianópolis, 2015.

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ACKNOWLEDGEMENTS To the Federal University of Santa Catarina and the professors of the Materials Engineering undergraduate course. To Professor Dr. Ing Marcio Celso Fredel, for giving me the opportunity of develop a project in CERMAT and for the supervision. To Dra Joana Mesquita-Guimarães for the trust, for sharing your knowledge, for show me the greatness of scientific world and by worrying about my well-being above all. Your importance in my personal and professional development is immeasurable! I will be grateful forever. To CERMAT colleagues, for their teaching, assistance and companionship. In special to Helena which allowed all tests execution due to its intense support. To Gabriella for always be present and for the language review. To Marcus for the emotional support and for make me happier. To my family for provide the best education possible, for the incentive, trust, support and love. Moacir, Anutoshi, Aline, Adriana and Kátia thank you for being part of my life making it more happy and full of meaning. My sincere thanks, Karen.

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ABSTRACT Tetragonal zirconia polycrystal is a widely used material in dentistry due to its toughening mechanism as well for its esthetic aspect, chemical stability and biocompatibility. This toughening mechanism is achieved by the use of stabilizers such as Y2O3, CeO2, MgO and CaO. 3%molar Y2O3-ZrO2 is extensively used for dental application although it may present low thermal degradation (LTD). LTD triggers zirconia from tetragonal to monoclinic phase, which reduces its mechanical resistance. CeO2-ZrO2 can prevent LDT, however this powder results in a yellowish ceramic, which can difficult color adjustment in dental application. Differently from 3%molar Y2O3-ZrO2, supplies of 12%molar CeO2-ZrO2 powders are limited. Therefore, 12%molar CeO2-ZrO2 powders were synthetized using an optimization of Pechini’s method. This synthesis resulted in completely stabilized powders with small particle size at the tetragonal phase. 12%molar CeO2-ZrO2 powders produced by Pechini method were mixed with commercial 3%molar Y2O3-ZrO2 powders. Then, its properties were compared with mixtures of 12%molar CeO2-ZrO2 and 3%molar Y2O3-ZrO2, both commercial powders. Colorimetric characterization was possible as the samples were homogeneous. XRD analysis showed that commercial raw powders present a high monoclinic content which disappear after sintering. Both mixtures destabilize after sintering as the quantity of monoclinic phase was superior than 5%.

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RESUMO Policristais de zirconia tetragonal são amplamente utilizados em dentística devido ao mecanismo de tenacificação bem como seu aspecto estético, estabilidade química e biocompatibilidade. Esse mecanismo de tenacificação é alcançado através do uso de estabilizantes como Y2O3, CeO2, MgO e CaO. 3%molar Y2O3-ZrO2 é vastamente utilizado em aplicações dentárias, porém apresenta degradação em baixas temperaturas (DBT). A DBT provoca transformação de fase tetragonal para monoclínica o que reduz a resistência mecânica desse material. 12%molar CeO2-ZrO2 pode prevenir a DBT, porém esse pó resulta em cerâmicas amareladas, podendo dificultar o ajuste de cor na aplicação dentária. As ofertas de pós de 12%molar CeO2-ZrO2 são limitadas, diferentemente das ofertas de pós de 3%molar Y2O3-ZrO2. Portanto, pós de 12%molar CeO2-ZrO2 foram produzidos através do método Pechini otimizado. Essa síntese resulta em pós completamente estabilizados na fase tetragonal com tamanho de partícula pequeno. Pós de 12%molar CeO2-ZrO2 produzidos por Pechini foram misturados com pós comerciais de 3%molar Y2O3-ZrO2. Então, suas propriedades foram comparadas com misturas de pós comerciais de 12%molar CeO2-ZrO2 e de 3%molar Y2O3-ZrO2. A caracterização colorimétrica foi possível já que as amostras foram homogêneas. As análises de DRX mostram a presença de fase monoclínica nos pós comerciais iniciais a qual desaparece depois da sinterização. Ambas as misturas desestabilizam após a sinterização já que a quantidade de fase monoclínica foi superior do que 5%.

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LIST OF FIGURES Figure 1 – Examples of bioceramics applications in dentistry (Crystal dental centers) ............ 9 Figure 2 - Zirconia crystallographic phase. (a) Cubic (b) Tetragonal (c) Monoclinic ............ 12 Figure 3 - Representation of the transformation toughening process (Vagkopoulou, Koutayas, Koidis, & Strub, 2009) ......................................................................................................... 13 Figure 4 - ZrO2-Y2O3 phase diagram (Grzebielucka, 2009) .................................................. 14 Figure 5 - ZrO2-CeO2 phase diagram (Quinelato et al., 2000) ............................................... 15 Figure 6 - Scheme of Pechini method ................................................................................... 16 Figure 7 - Sample positioning in the impulse excitation technique. (a) Microphone (b) Pulser (c) Sample (d) Support system (ATCP Engenharia física) .................................................... 20 Figure 8 – Table of clinical significance (Volpato, 2005) ..................................................... 22 Figure 9 – Table of reagents for Pechini method .................................................................. 23 Figure 10 – Table of raw powder characteristics .................................................................. 24 Figure 11 – Table of mixtures’ identification ....................................................................... 25 Figure 12 - Derivative TG of Ce-TZPP powder ...................................................................... 26 Figure 13 - Dry and wet milling at different milling times of Y-TZPP and Ce-TZPP, respectively ............................................................................................................................................ 27 Figure 14 - Dry milling ........................................................................................................ 28 Figure 15 - Wet milling ........................................................................................................ 28 Figure 16 - Diffraction pattern of Ce-TZPP. Large and small scale, respectively .................. 29 Figure 17 - Diffraction pattern of Y-TZPP. Large and small scale, respectively ................... 29 Figure 18 – Table of monoclinic content and crystallite size ................................................ 30 Figure 19 – Ce-TZP, Y-TZP and Ce-TZP P micrography, respectively ................................ 31 Figure 20 - Particle size of raw powders ............................................................................... 32 Figure 21 - Particle size of Mixture A .................................................................................. 32 Figure 22 - Particle size of Mixture B .................................................................................. 32 Figure 23 – Table of particle size measured and expected for Mixture A and Mixture B ...... 33 Figure 24 – Relative densities for all compositions............................................................... 33 Figure 25 - Sintered samples used to colorimetric characterization ....................................... 34 Figure 26 - Reflectance curve for Mixture A and Mixture B ................................................. 34 Figure 27 – Table of ΔS values for Mixture A ...................................................................... 35 Figure 28 – Table of ΔS values for Mixture B ...................................................................... 35 Figure 29 - L*, a*, b* coordinates for Mixture A and Mixture B .......................................... 35 Figure 30 –Table of color difference between Mixture A and Mixture B .............................. 35 Figure 31 - ΔE values for Mixture A using 100Y as pattern ................................................. 36 Figure 32 - ΔE values for Mixture B using 100Y as pattern .................................................. 36 Figure 33 – Porosity of 100Y ............................................................................................... 36 Figure 34 - Porosity of Mixture A and Mixture B. b) 80Y20Ce c)80Y20CeP d) 60Y40Ce e) 60Y40CeP f) 40Y60Ce g) 40Y60CeP h)20Y80Ce i)20Y80CeP j)100Ce k)100CeP ............. 37 Figure 35 - Diffraction pattern, monoclinic content and crystallite size of raw powders ....... 38 Figure 36 - Diffraction pattern of Mixture A ........................................................................ 39 Figure 37 - Monoclinic content and crystallite size results for Mixture A ............................. 39 Figure 38 - Diffraction pattern of Mixture B ........................................................................ 40 Figure 39 - Monoclinic content and crystallite size results for Mixture B ............................. 40 Figure 40 - Graph comparing the elastic modulus theoretical and measured for Mixture A and Mixture B ............................................................................................................................ 41

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SUMARY 1.

INTRODUCTION .......................................................................................................... 9

2.

OBJECTIVES............................................................................................................... 11

2.1

GENERAL OBJECTIVES ........................................................................................ 11

2.2

SPECIFIC OBJECTIVES .......................................................................................... 11

3.

LITERATURE REVIEW.............................................................................................. 12

3.1

ZIRCONIUM OXIDE ............................................................................................... 12

3.2

STABILIZERS.......................................................................................................... 13

3.2.1

YTTRIUM OXIDE ................................................................................................ 14

3.2.2

CERIUM OXIDE .................................................................................................. 15

3.3 4

PECHINI METHOD ................................................................................................. 16 CHARACTERIZATION .............................................................................................. 17

4.1.

LASER DIFFRACTION ........................................................................................... 17

4.2.

SEM .......................................................................................................................... 18

4.3.

XRD.......................................................................................................................... 19

4.4.

IMPULSE EXCITATION TECHNIQUE .................................................................. 20

4.5.

SPECTROPHOTOMETRY ...................................................................................... 21

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MATERIALS AND METHODS .................................................................................. 23

5.1

POWDER PRODUCTION ........................................................................................ 23

5.1.1 5.2 6

PECHINI METHOD.............................................................................................. 23 MIXTURES PRODUCTION .................................................................................... 24

RESULTS AND DISCUSSION .................................................................................... 26

6.1

PECHINI SYNTHESIS - POWDERS PRODUCTION ............................................. 26

6.1.1

Differential thermal analysis to study calcination heating rate ............................... 26

6.1.2

Particle size distribution to study milling conditions .............................................. 27

6.1.3

Scanning Electronic Microscope (SEM)................................................................... 27

6.1.4

X-Ray Diffraction to determine crystalline phases .................................................. 29

6.2

MIXTURES PRODUCTION .................................................................................... 31

6.2.1

CHARACTERIZATION OF RAW POWDERS AND MIXTURES ...................... 31

6.2.2

COLORIMETRIC CHARACTERIZATION ......................................................... 33

6.2.3

GRAIN SIZE ......................................................................................................... 36

6.2.4

XRD ...................................................................................................................... 38

6.3

ELASTIC MODULUS AND POISSON COEFFICIENT .......................................... 41

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CONCLUSIONS .......................................................................................................... 42

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BIBLIOGRAPHY......................................................................................................... 43

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1. INTRODUCTION The development of bioceramics focuses primarily in the areas of orthopedics and dentistry. (Shakelford, J. F) The ceramics composed of alumina, zirconia, leucite, among others, are examples of bioceramics used in the manufacture of implants, crowns, fixed bridges, inlays, onlays, shown in Figure 1. (Anusavice KJ. Phillips).

Figure 1 – Examples of bioceramics applications in dentistry (Crystal dental centers)

The potential of the ceramics as biomaterial arise from its similarity to the physiological environment, due to its basic constitution ions which are also routinely found in the physiological medium (calcium, potassium, magnesium, sodium, etc.) and others whose toxicity is quite limited (zirconium and titanium). (Hench, L. L; Wilson, J.) Zirconium oxide or zirconia (ZrO2) is a promising restorative biomaterial, since its physical and chemical properties are favorable and suitable for medical applications. The combination of its particular properties and the development of computer-aided manufacturing implants stimulated the growth of its use in dental biomaterial. Toughened zirconia represents a promising substructure material. Although, it must be taken into account that these zirconia ceramics may involve some difficulties such as longterm instability in the presence of water, porcelain compatibility issues, and some limitations in case selection due to their opacity, indicating that studies must be taken to enhance quality. (Kelly & Benetti, 2011) For dental application, tetragonal zirconia polycrystals are used due to its toughening mechanism. Y-TZP – tetragonal zirconia polycrystal stabilized with 3%molar of Y2O3 - are widely used in dentistry. To improve the dental application this Y-TZP material is also commonly studied. Ce-TZP - tetragonal zirconia polycrystal stabilized with 12%molar of CeO2 – possess promising features to prevent the LTD – low thermal degradation – issue present in Y-TZP but commercial supply is limited.

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Synthetize Ce-TZP is an alternative to solve the limitations of commercial suppliers. Pechini method, based on polymeric precursor, is a good option due to permit a high control of stoichiometry combined with nano-size particles. Materials used in dental applications have requirements like non-toxicity, biocompatibility, aesthetics, strength and durability. To attempt all these requirements functionally graded materials are grate options since natural biomaterials exhibit this structure. A functionally graded material (FGM) may be characterized by the variation in composition and structure gradually over volume, resulting in corresponding changes in properties of the material. It can be said that these are types of materials that are designed to have innovative properties and perform functions that cannot be achieved by conventional homogeneous materials. (Kawasaki & Watanabe, 1997) The development of FGM concept had its origin in the sophisticated properties which arise from materials in nature, such as shells, bamboo (Tan et al., 2011), teeth (He and Swain, 2009) and bones (Pompe et al, 2003) The development of functionally graded dental implants aimed to improve the implant performance in terms of biocompatibility and stress distribution. (Sadollah and Bahreininejad, 2011; Lin et al., 2009)

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2. OBJECTIVES 2.1 GENERAL OBJECTIVES -

Synthetization and optimization of Pechini synthesis;

-

Characterization of Ce-TZP and Y-TZP mixtures.

2.2 SPECIFIC OBJECTIVES -

Synthetization of Y-TZP by Pechini method to validate and optimize the production;

-

Production of Ce-TZP by an optimized Pechini production;

-

Characterization of Pechini and commercial powders of Ce-TZP and Y-TZP;

-

Preparation of mixtures using commercial and Pechini powders;

-

Characterization of the mixtures.

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3. LITERATURE REVIEW 3.1 ZIRCONIUM OXIDE Zirconia (ZrO2) is a promising biomaterial due to its good chemical and dimensional stability, mechanical strength and toughness.(Lyon, Chevalier, Gremillard, & Cam, 2011) Zirconia in its pure form is classified as a polymorph - Figure 2 - that occurs in three phases: monoclinic (from room temperature to 1170°C), tetragonal (from 1170°C to 2370°C) and cubic (from 2370°C to the melting point, 2680°C).

Figure 2 - Zirconia crystallographic phase. (a) Cubic (b) Tetragonal (c) Monoclinic

While cooling down, transformation from tetragonal to the more stable monoclinic phase occurs followed by a volumetric variation, causing an increase of 3-5% on volume. (Stevens, R.) This volumetric variation is responsible for generating a catastrophic failure as it exceeds the elastic and fracture limits of the material. This martensitic transformation is athermal, diffusionless and occurs with a change in crystal structure, which involves simultaneous and cooperative movement of atoms - over distances less than an atomic diameter-, resulting in a macroscopic change in shape of transformed regions. (Kelly PM, Rose LRF) When in its ‘stabilized’ state zirconia becomes applicable. In some cases, the tetragonal phase can be metastable, which means that there is some energy capable to transform back to the monoclinic state. If sufficient quantities of the metastable tetragonal phase is present, an applied stress - magnified by its concentration at a crack tip - can cause the tetragonal phase to convert to monoclinic with the associated volume expansion. This phase transformation can then put the crack into compression, retarding its growth and enhancing the fracture toughness. This mechanism is known as transformation toughening, and significantly extends the reliability and lifetime of products made with stabilized zirconia (Ramesh, Gangaiah, Harish, Krishnakumar, & Nandakishore, 2012). Figure 3 gives a schematic

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representation of this phenomenon. To obtain this metastable tetragonal phase some additives like CaO, MgO, CeO2 and Y2O3 are used.

Figure 3 - Representation of the transformation toughening process (Vagkopoulou, Koutayas, Koidis, & Strub, 2009)

There are two types of stabilized zirconia: Partially Stabilized Zirconia (PSZ) and Tetragonal Zirconia Polycrystal (TZP). PSZ is a multiple phase material formed by a matrix of cubic phase, with monoclinic and tetragonal zirconia precipitates as the minor phase. Stabilized zirconia is classified as TZP when it contains up to 98% of the metastable tetragonal phase. (Ramesh et al., 2012)

3.2 STABILIZERS Tetragonal zirconia polycrystal stabilized with yttria (Y2O3-ZrO2) has high strength than tetragonal zirconia polycrystal stabilized with ceria (CeO2-ZrO2). However when exposed to humid atmospheres its exhibits low thermal degradation (LDT) - also known as aging -which is a phenomenon that reduces mechanical resistance due to the spontaneous transformation from tetragonal to monoclinic phase. CeO2-ZrO2 shows high toughness than Y2O3-ZrO2 and good aging behavior. A disadvantage of CeO2-ZrO2 is the fact that higher temperatures and/or longer sintering times are needed for densification of the material. This results in a large grain size after sintering, which is a disadvantage for plastic deformation processes and for obtaining high strength. It would be very favorable if good chemical stability (CeO2) and a small grain size (Y2O3) could be combined in the same ceramic material. (G.S.a.M. Theunissen, Winnubst, & Burggraaf, 1992) The positive effects of yttria and ceria might be combined. A work on Y2O3-ZrO2, CeO2ZrO2 ceramics (Bastide, Canale, & Odier, 1989) indeed shows that good mechanical properties

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are obtained in Y2O3-ZrO2, CeO2-ZrO2 ceramics, especially when the cerium concentration is low.(G. S. a. M. Theunissen, Bouma, Winnubst, & Burggraaf, 1992) 3.2.1 YTTRIUM OXIDE To stabilize tetragonal zirconia polycrystals with yttrium oxide or yttria (Y2O3) a quantity of 2-4% molar is commonly used. 3% molar content of yttria forms tetragonal phase only. (G. S. a. M. Theunissen et al., 1992) Figure 4 shows the phase diagram between ZrO2 and Y2O3. Between 5 and 7%mol of Y2O3 there are both cubic and tetragonal phases in solid solution, which are stable above 550°C approximately. Among 2 and 5%mol, tetragonal and cubic phases are found in solid solution with traces of monoclinic in solid solution as well. Below 2%mol there is monoclinic phase in solid solution or monoclinic in solid solution with small quantities of solid solution of tetragonal phase.

Figure 4 - ZrO2-Y2O3 phase diagram (Grzebielucka, 2009)

According to Theunissen et al. to obtain a tetragonal metastable phase at room temperature, the grain size must be less than 0.8 µm and the amount of stabilizing oxide must be no more than 3 mol% (G. S. a. M. Theunissen et al., 1992) . When the grain size is smaller than approximately 0.4 µm, no stress induced phase transformation occurs.(Duran et al., 1989)

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3.2.2 CERIUM OXIDE Cerium oxide, or ceria (CeO2), is a cubic fluorite-type oxide in which each cerium site is surrounded by eight oxygen sites in face-centered cubic arrangement, where each oxygen site has a tetrahedron cerium site. (Goharshadi, Samiee, & Nancarrow, 2011) Figure 5 shows the phase diagram between ZrO2 and CeO2. Between 20 and 90%mol of CeO2 there are cubic and tetragonal phase in solid solution, which are stable above 1000°C approximately. Below this temperature, there is solid solution of cubic and monoclinic phase. Below 20%mol there is tetragonal phase above 1000°C. CeO2-ZrO2 has high fracture toughness and high thermal stability. Cerium atoms replace zirconium positions spreading the crystal lattice, acting as stabilizing agent location and generating a crystal field. Ce-TZP powders have the tetragonal phase as a single phase at a content of 2 mol% of ceria due to the presence of small crystals. With 12 mol% these characteristics are optimized.(Quinelato, Longo, Perazolli, & Varela, 2000)

Figure 5 - ZrO2-CeO2 phase diagram (Quinelato et al., 2000)

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3.3 PECHINI METHOD To obtain advantages, such as synthesis at low temperatures, low contamination, direct and precise control of stoichiometry, homogeneity and the possibility of obtaining nanometric powders; the Pechini method was chosen as the synthesis route. Pechini method is a chemical route to produce powders using polymeric precursors. In this method one alpha-hydroxycarboxylic acid is used to form a chelate of various cations forming a polybasic acid. In the presence of these chelates, polyhydroxy alcohol react to form ester and water as products. When the mixture is heated, polyesterification occurs in the liquid solution and becomes a resin in which the metal ions are uniformly distributed throughout the organic polymer matrix. Oxides in powder form are obtained by firing the resin to remove all organic substances. (Qu & Qu, 2001) In Figure 6 there is a scheme to represent this method.

Figure 6 - Scheme of Pechini method

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4 CHARACTERIZATION The characterization of the samples was made using several techniques analysis. Each of them is focused on the study of a specific aspect, for example: -

Differential

thermal

analysis,

as

Differential

scanning

Calorimetry

and

Thermogravimetric analysis (DTA/TG) was used to identify the thermal events of Pechini resin; - Laser diffraction was used to determine agglomerate size distribution; - Scanning Electronic Microscope (SEM) was used to visualize the morphology and size of the powders; and grain size of the sintered samples; - X-Ray Diffraction (XRD) was used to identify crystalline phases. Toraya and Scherrer equations were used to determine monoclinic phase and crystallite size of the samples; - UV-Vis Spectrophotometry analysis was used to obtain the color by comparison with a reference color sample; - Dilatometry was made in green samples to determine the sintering curve and in sintered samples to determine the coefficient of thermal expansion; - Mechanical properties were evaluated by impulse excitation technique.

4.1.

LASER DIFFRACTION

Laser diffraction method is widely used to determine the particle size distribution. In this technique the spatial distribution of scattered light is a function of the particle size. (Stojanovic & Markovic, n.d.) The equipment utilized was the Mastersizer 2000 (Malvern, UK), capable of measuring sizes ranging from 0.2 to 880 microns, based on a low angle laser light scattering theory (Low Angle Laser light scattering, LALLS). The light source is a laser He - Ne with a wavelength λ = 632.8 nm. The laser beam interacts with the particles in suspension by dispersing at certain angles depending on the particle size. Large particles scatter light at smaller angles relative to the laser beam and small particles scatter light at larger angles. The angular scattering intensity data is analyzed to calculate the size of the particles responsible for creating the scattering pattern. (Malvern products) Knowing the absorption coefficient, the refractive index of the sample and the refractive index of the solvent, the measurement can be performed based on Mie theory, which presents greater accuracy in size estimation. This theory holds that the particles are spherical and are

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dispersed in a homogeneous medium and not absorbed in all spatial directions. The Mie theory postulates that diffraction of light is a phenomenon of resonance. If a light ray with a certain wavelength focus upon a particle, the particle creates electromagnetic oscillations at the same frequency of the reflected light. Therefore, it is established a dependent relationship with the wavelength of light, the particle diameter and the index refraction between the particles and the environment. However, if the material is formed of various compounds and the refractive index or absorption coefficient of the sample are not known, one can perform the measurement using the Fraunhofer theory. According to this theory, the characteristics of light scattering are not dependent on optical properties of the sample. This model can predict the dispersion pattern that is created when a solid and opaque disc of known size is passed through a laser beam. (Queiroz et al., 2012) Absorption coefficient and refractive index were used for pure powders using Mie Theory mode and for mixtures powders was used the Fraunhofer mode.

4.2.

SEM

Scanning Electron Microscope is an important tool in material’s analysis, which can be combined with a chemical microanalysis through Energy Dispersive X-ray Spectroscopy (EDS). The measurements were made in a SEM/EDS Tabletop (TM3030 Tabletop Microscope, Hitachi, Japan) with 30,000x of magnification and 15kV of accelerating voltage. The powders and the sintered samples were coated with thin gold coating to guarantee the best images conditions. The area to be examined is irradiated with a finely focused electron beam. The greatest interest is in signals from secondary and the backscattered electrons, since these vary according to differences in surface topography as the electron beam sweeps across the specimen. Chemical analysis is performed by measuring the energy and intensity distribution of the x-ray signal generated by a focused electron beam. (Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, A. D. Romig Jr., Charles E. Lyman, Charles Fiori, 1974) A micrographic analysis was necessary to evaluate the morphology, particle and grain size of sintered samples, as it is fundamental to verify the differences between the powders and the mixtures. Chemical analysis was used to adapt parameters during the production of Pechini powders.

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4.3.

XRD

The X-ray diffraction is an important technique for the identification of the crystalline phases of the material. This technique uses the Bragg's Law to identify the presence of a crystalline network or the frequency of an atomic arrangement. The equipment used was Philips x’pert (PANalytical, Netherlands). X-ray diffraction analysis was used in order to analyze the crystalline phases, phase content

and

crystallite

size

of

the

powder

and

sintered

samples.

The relative amounts of the monoclinic, tetragonal and cubic phases influence the mechanical properties thus determining the amounts present in the material is important. To estimate the monoclinic quantity the integrated area of three peaks were determined and Equations 1 and 2 were applied. (Toraya, Yoshimura, & Somiya, 1984).

̅11)+𝐼𝑚 (111) 𝐼𝑚 (1 ̅ (1 11)+𝐼 𝑚 𝑚 (111)+ 𝐼𝑡 (101)

𝑋𝑚 = 𝐼

1,311𝑋

𝑚 𝑉𝑚 = 1+ 1,311𝑋

(1)

(2)

𝑚

Xm is the integrated intensity ratio for the system tetragonal-monoclinic ZrO2, Im(1̅11) and Im(111) are the intensity of 28,2° and 31,5°peaks form monoclinic phase and It(101) is the intensity of 30,2° peak form the tetragonal phase. Vm is the volumetric fraction of monoclinic ZrO2. These equations derived from Garvie and Nicholson equation that determine the linearity between volume fraction and intensity relation of monoclinic peaks. According (Toraya, Yoshimura, & Somiya, 1984) this linearity is not strictly correct therefore they determine a nonlinear calibration resulting in 1,311±0,004 for diffraction intensity relation. (Queiroz et al., 2012) The crystallite size of the samples were determined by Scherrer equation, Equation 3. kλ

𝐷 = (βcos(𝜃))

(3)

D is the average crystallite size, λ is the X-ray wavelength (Cu Kα), θ is the diffraction angle and β is the pure full width of the diffraction line at half the maximum intensity. (Alexander & Klug, 1950)

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With crystallite size above the critical value, the t phase transforms spontaneously to monoclinic phase without absorption of energy at the crack tip leading to lower fracture toughness.(Sharma, Gokhale, Dayal, & Lal, 2002) The transformation appears more critical and the tetragonal phase suddenly disappears above ~23 nm.(Djurado, Bouvier, & Lucazeau, 2000)

4.4.

IMPULSE EXCITATION TECHNIQUE

The impulse excitation technique is a non-destructive characterization technique based on ASTM –E1876. It is used to obtain dynamic modulus by measuring the fundamental resonant frequency of flexural vibration of a specimen after it has been excited by a light mechanical impulse (tap). A microphone located beneath the specimen receives the sound waves and tepasses the signals to the electronics box for analysis. The fundamental resonant frequency is identified and then displayed in digital form. (Heritage, Frisby, & Wolfenden, 1988) Figure 7 shows a scheme of impulse excitation technique. The equipment used was Sonelastic PC Based (Atcp Physical Engineering Brazil).

Figure 7 - Sample positioning in the impulse excitation technique. (a) Microphone (b) Pulser (c) Sample (d) Support system (ATCP Engenharia física)

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4.5.

SPECTROPHOTOMETRY

The equipment used was a horizontal spectrophotometer CM-3600A (ICONICA MINOLTA) with a wavelength range from 360 to 740nm, a wavelength of 10nm. This measurement instrument is designed to evaluate the color, relative gloss, and UV characteristics of samples small to large in size. This high accuracy, versatile spectrophotometer is used to evaluate, reproduce, and help control the color and appearance of opaque, transparent, translucent, and fluorescent samples in a more effective, streamlined process. (Konica Minolta) The measurement was made with specular component included (SCI) in a standard illuminant D65 that corresponds to average daylight, including ultraviolet wavelength. International commission of I’Eclairage (CIE) develop a CIELab* system which permit expression color by number and calculate differences between two colors corresponding to visual perception. This system uses three coordinates: L*, a* and b*: -

L* is the degree of lightness of an object, L* values fluctuate from 0 corresponding to absolute black to 100 absolute white

-

a* value is the degree of redness/greenness and the object color tends to red when a* is positive

-

b* value is the degree of yellowness/blueness and the object color tends to yellow when b* is positive. Color difference, ΔE is calculated by equation 4: (Vichi, Louca, Corciolani, & Ferrari,

2011) 𝛥𝐸𝑎𝑏 = [(𝐿1 − 𝐿2 )2 + (𝑎1 − 𝑎2 )2 + (𝑏1 − 𝑏2 )2 ]1/2

(4)

Color fluctuation index, ΔS, measures the superficial color homogeneity in the sample and it is calculated comparing the measures to each other. A ΔS value lower than 2 permit the correct color evaluation. (KUEHNI, R. G.; MARCUS, R. T.) Due to the dental application a clinical significance reported by (Volpato, 2005) will be used during colorimetric analysis. Figure 8 shows the clinical significance.

22

ΔE 0 2 4 6 8 10

Clinical significance Exact color choice, difference is not clinically perceived Very slight color difference Obvious color difference, but acceptable for most patients Acceptability limit Unacceptable for most patients Completely unacceptable Figure 8 – Table of clinical significance (Volpato, 2005)

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5 MATERIALS AND METHODS 5.1 POWDER PRODUCTION Tetragonal zirconia polycrystal, stabilized with ceria and yttria (referred on text by CeTZPP and Y-TZPP), were produced by Pechini method to achieve a synthesis optimization. Since Y-TZP is most known than Ce-TZP, the Pechini production of this powder was made to validate and to optimize the Pechini route. After validation, Ce-TZP powders were produced by an optimized Pechini route and were used to produce mixtures. Optimization in milling, heat treatment and changes on stabilizer’s precursors were made during production to increase the quality and the quantity of produced powders. To validate its SEM, EDS, laser diffraction and DSC/TG analysis were made. 5.1.1 PECHINI METHOD The precursor materials used in the synthesis are zirconium (IV) oxychloride octahydrate, yttrium (III) nitrate hexahydrate and the cerium (III) acetate hydrate. Figure 9 shows the characteristics of the reagents. Reagent

Chemical formula

Purity

Zirconium (IV) oxychloride octahydrate

Cl2OZr*8H2O

99,5%

Yttrium (III) nitrate hexahydrate

(NO3)3Y*6H2O

99,9%

Cerium (III) acetate hydrate

(C2H3O2)3Ce*H2O

99,9%

Ethylene glycol

C2H6O2

99,5%

Citric acid anhydrous

C6H8O7

99,5%

Figure 9 – Table of reagents for Pechini method

Citric acid and distilled water were added under magnetic stirring and heating until 80°C. Then precursor of zirconia and the stabilizer were added and ethylene glycol was dribbled out. Stabilizers oxides were introduced in the proportion of 3mol% for Y2O3 and 12mol% for CeO2. The molar ratio of citric acid to total oxides were 4:1 to obtain chelate structure and the molar ratio of citric acid to ethylene glycol were stablished to 1:5 to guarantee small crystallite size. (Quinelato, Longo, Leite, & Varela, 1999) After 30 minutes, this solution was heated until 140°C during 24 hours to eliminate water and to form a resin.

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To eliminate the organics the resin was heated until 400°C during 90 minutes with a heating rate of 0.1°C/min. After this calcination treatment, the powders were dry milled during 80 minutes in a high energy mill with 400 rpm. The last heat treatment aimed the tetragonal phase crystallization. This was realized with a heating rate of 4°C/min until 700°C during 300 minutes for Y-TZPP and with a heating rate of 4°C/min until 850°C during 300 minutes for Ce-TZP. (Rebouças, Lara) After more than 40 hours, the synthesis yield is 3 grams of tetragonal zirconia polycrystal. To use Ce-TZPP powders in the mixtures it must to be optimized. To optimize the quality and quantity of synthesis a study of the calcination heat treatment and an increase in the precursors quantity were made. These changes were validate by characterization technique.

5.2 MIXTURES PRODUCTION Two tetragonal zirconia polycrystal commercial powders were mixed in different proportions to study color and structural characteristics. Moreover, due to the small supply of commercial Ce-TZP, the Ce-TZPP was mixed with commercial Y-TZP. These mixtures will be referred for now on by Mixture A, for commercial Ce-TZP mixed with commercial Y-TZP, and by Mixture B, for Ce-TZP mixed with commercial Y-TZP. The main characteristic of the raw powders are listed in Figure 10. Reference Sintered Density (g/cm³) Y-TZP 3 mol% Y2O3-ZrO2 TZ-3YBE, TOSOH 6,06 Ce-TZP 12 mol% CeO2-ZrO2 CEZ 12, DAIICHI 6,26 Ce-TZPP 12 mol% CeO2-ZrO2 PECHINI METHOD 5,08 Figure 10 – Table of raw powder characteristics

The low sintered density of Ce-TZPP powders indicates that mixtures using it as raw material must to have different sintering parameters. Twelve compositions, listed on Figure 11 were prepared by mixing the raw powders during 16 hours in a ball mill (SERVITECH 180 rpm). To estimate the quantity of each powder, the rule of mixture (Yadama & Englund, 2007) was used considering the component fraction in volume.

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Mixture A

Observation

Mixture B

Observation

100Y

100% Y-TZP

100Y

100% Y-TZP

80Y20Ce

80% Y-TZP;20% Ce-TZP

80Y20CeP

80% Y-TZP;20% Ce-TZPP

60Y40Ce


60Y40CeP


40Y60Ce


40Y60CeP


20Y80Ce


20Y80CeP


100% Ce-TZP

100CeP

100% Ce-TZPP

100Ce

Figure 11 – Table of mixtures’ identification

To produce samples the mixtures were pressed in 300 MPa. The sintering conditions were a heating rate of 10°C/min until 1500°C during 2h for the Mixture A and during 4h for Mixture B. Sintered samples of Mixture A and Mixture B were characterized by impulse excitation technique, SEM, dilatometry, XRD and spectrophotometry.

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6 RESULTS AND DISCUSSION 6.1 PECHINI SYNTHESIS - POWDERS PRODUCTION 6.1.1 Differential thermal analysis to study calcination heating rate In a previous work the conditions to calcination treatment were 0.1°C/min of heating rate until 400°C in which it remained for 90 minutes. (Rebouças, Lara) Considering that this rate is quite long for the furnaces available, two heating rates, 1°C/min and 5°C/min, were performed to study the influence of heating rate during the synthesis. The optimization of heating rates and calcination temperatures in Pechini method was evaluated using Differential Scanning Calorimetry/Thermogravimetric analysis (DSC/TG). The derivative TG results of both heating rates are presented in Figure 12.

Figure 12 - Derivative TG of Ce-TZPP powder

The first weight loss peak at ~100°C corresponds to water elimination, the second peak, from 100 to 200°C corresponds to structural water elimination and the third peak, from 220°C to 350°C is related to the resin elimination. The last event around 450 – 500°C is associated to organic material burning. It can be noted that the difference between the heating rate of 1°C/min and 5°C/min is due to the presence of a second peak and the shift of events after ~150°C. This is related to the kinetic reaction that, in the case of 1°C/min, leave more time for reactions to occur (Rebouças, Lara). After 150°C, reactions in the 1°C/min heating rate starts before the reactions in the 5°C/min. This event occurs due to the accumulated heat that promoted the elimination of structural water (second peak) together with additional water in one step.

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To guarantee the complete organic elimination, 1°C/min heating rate was used and a maximum temperature of 550°C was reached, which is 50°C above all the events. 6.1.2 Particle size distribution to study milling conditions As stated in the previous work, the powders were ball milled after calcination in dry conditions. (Rebouças, Lara) Considering that dry conditions are more aggressive than wet conditions to ball wear, a study of wet and dry conditions was performed. In both cases, 10 agate balls of 10 mm diameter were used and 1/3 of volume capacity of the milling cup was filled with powder. In the case of the wet conditions, isopropyl alcohol was used and the milling conditions were run at 400 rpm. To study the optimization of milling efficiency, samples were collected every 40 minutes to analyze the particle size distribution by laser diffraction. The result is shown on Figure 13 presenting D50 and D90 of all the samples.

Figure 13 - Dry and wet milling at different milling times of Y-TZPP and Ce-TZPP, respectively

According to the results, milling in wet conditions present higher efficiency than in dry conditions. After 80 minutes, the reduction of D50 was around 64% for Y-TZPP and for CeTZPP it was 43%, which resulted in a particle size of 2.8 µm for Ce-TZP P and 2.7 µm for Y-TZPP. Therefore, it was concluded that milling conditions with isopropyl alcohol must be run at 400rpm during 80 minutes. 6.1.3 Scanning Electronic Microscope (SEM) SEM was performed to estimate agglomerate size and morphology of the powders produced by Pechini method. Figure 14 and Figure 15 show the micrographs of dry and wet milling conditions, respectively.

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Figure 14 - Dry milling

Figure 15 - Wet milling

Micrographs of the milled powders present less agglomerates and most of the particles are spherical and well disperse, which justify the choice of milling in wet conditions.

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6.1.4 X-Ray Diffraction to determine crystalline phases XRD analysis were made to confirm the correct stabilization of tetragonal phase of powders synthesized by Pechini method in small scale – yield 3g - and large scale production – 5 times bigger resulting in a yield of 18g. In Figure 16 and Figure 17 X-ray spectra of Ce-TZP and Y-TZPP for large and small scale production is shown.

Figure 16 - Diffraction pattern of Ce-TZPP. Large and small scale, respectively

Figure 17 - Diffraction pattern of Y-TZPP. Large and small scale, respectively

JCPDS diffraction patterns were used to identify the crystalline phases. The presence of monoclinic and tetragonal phases can be seen in Figure 16 and Figure 17. To confirm the complete stabilization of the tetragonal phase, the quantity of monoclinic phase and

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crystallite size were determined by using Toroya and Scherrer equations. The results are listed in Figure 18.

Y-TZPP small scale Y-TZPP large scale

Crystallite size (nm)

Monoclinic content (%)

16,03

0

9,67

0

Ce-TZPP small scale Ce-TZPP large scale

Crystallite size (nm)

Monoclinic content (%)

15,72

2

9,67

0

Figure 18 – Table of monoclinic content and crystallite size

Both results present good characteristics for the use of these powders. A crystallite size bellow 20 nm and a reduced content of monoclinic phase validate the large scale production and the use of Ce-TZPP powders to produce the mixtures.

31

6.2 MIXTURES PRODUCTION 6.2.1 CHARACTERIZATION OF RAW POWDERS AND MIXTURES A morphological comparison among raw powders was made once these differences affect the conformability, Figure 19. Ce-TZP

Y-TZP

Ce-TZP P

Figure 19 – Ce-TZP, Y-TZP and Ce-TZP P micrography, respectively

The Y-TZP commercial powder has the best conformability due to its spherical microstructure. Ce-TZP does not have a specific microstructure and Ce-TZP P presents some agglomerates, therefore its conformability is affected. Particle size distribution of raw powders and of mixtures were determined. Figure 20 represent the results of raw powders. The D50 particle size of Ce-TZP P is 5 µm, of Ce-TZP is 34 µm and Y-TZP is 50 µm.

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Figure 20 - Particle size of raw powders

Mixing these raw powders results in different particle sizes. To estimate the expected value of this mixture the rule of mixtures was used. Figure 21 and Figure 22 show the distribution and the cumulative of Mixture A and Mixture B, respectively. Figure 23 correlates the particle sizes measured with the expected values.

Figure 21 - Particle size of Mixture A

Figure 22 - Particle size of Mixture B

33

Mixture A

Particle size D50 (µm)

100Ce

34

Expected value (µm) 34

20Y80Ce 40Y60Ce 60Y40Ce 80Y20Ce 100Y

42 50 59 62 50

37 40 44 47 50

100CeP

Particle size D50 (µm) 5

Expected value (µm) 5

20Y80CeP 40Y60CeP 60Y40CeP 80Y20CeP 100Y

73 6 30 130 50

14 23 32 41 50

Mixture B

Figure 23 – Table of particle size measured and expected for Mixture A and Mixture B

Mixture A show a bimodal distribution increase in particle size with increase in Ce-TZP content, although Mixture B show a multimodal distribution indicating a destabilization of particle size mainly in 20Y80CeP and 80Y20CeP. It can be assumed that some problem occurred in the mixing of these two compositions, since they are the only ones that presented distribution problems, when compared to the others mixtures. The relative densities were determined after sintering by the theoretical value, given by the commercial powders supplier, and determined by Archimedes method for Ce-TZP P

powder, Figure 24.

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Figure 24 – Relative densities for all compositions

Mixture B present low relative density, it indicates that the sintering parameters must to be studied. 6.2.2 COLORIMETRIC CHARACTERIZATION For colorimetric characterization, two samples of each composition were measured three times in absolute black. By comparing the sintered samples, a color difference can be detected without any magnification tool, Figure 25.

Figure 25 - Sintered samples used to colorimetric characterization

To quantify the color difference between the mixtures the ΔE were determined. The clinical significance of ΔE were analyzed using 100Y as a pattern since it material is widely used on dentistry.

35

Analyzing the reflectance curve for the mixtures on Figure 26 it can be concluded that CeO2 causes a displacement towards the yellow wavelength due to the yellow color of Ce-TZP and Ce-TZPP powders.

Figure 26 - Reflectance curve for Mixture A and Mixture B

To validate the colorimetric characterization it is necessary to determine ΔS value which corresponds to compare L*a*b* coordinates measured in different places on the same sample. Figure 27 and Figure 28 show the results for Mixture A and Mixture B.

ΔS

100Y

80Y20Ce

60Y40Ce

40Y60Ce

20Y80Ce

100Ce

0,4

1,1

0,3

0,5

0,8

1,7

Figure 27 – Table of ΔS values for Mixture A

ΔS

100Y

80Y20CeP

60Y40CeP

40Y60CeP

20Y80CeP

100CeP

0,05

0,18

0,13

0,07

0,45

0,15

Figure 28 – Table of ΔS values for Mixture B

Misture A deltaS Misture B deltaS 100Y

0,4

100Y

0,05 (0,1)

80Y20Ce

1,1

80Y20CeP

0,18 (0,2)

60Y40Ce

0,3

60Y40CeP

0,13 (0,1)

40Y60Ce

0,5

40Y60CeP

0,07 (0,1)

20Y80Ce

0,8

20Y80CeP

0,45 (0,5)

100Ce

1,7

100CeP

0,15 (0,2)

36

All ΔS values are smaller than 2 which indicates that the samples present slight color differences and confirms the samples’ homogeneity. L*a*b* coordinates were determined and are shown in Figure 29. This values were used to determine the color difference between Mixture A and Mixture B, Figure 30.

Figure 29 - L*, a*, b* coordinates for Mixture A and Mixture B

ΔE

80Y20Ce/ 80Y20CeP 5,53

60Y40Ce/ 60Y40CeP 10,97

40Y60Ce/ 40Y60CeP 11,39

20Y80Ce/ 20Y80CeP 13,94

100Ce /100CeP 9,11

Figure 30 –Table of color difference between Mixture A and Mixture B

According to the Figure 30 the color difference between Mixture A and Mixture B is high than 5. This values are on the acceptability limit or are completely unacceptable according to clinical significance, Figure 8. Figure 31 and Figure 32 show the color difference using the 100Y sample as a pattern. Analyzing the results can be concluded that the color difference of all compositions are completely unacceptable according to the clinical significance.

ΔE

80Y20Ce/ 100Y

60Y40Ce/ 100Y

40Y60Ce/ 100Y

20Y80Ce/ 100Y

100Ce /100Y

17,2

21,9

23,0

24,8

17,2

Figure 31 - ΔE values for Mixture A using 100Y as pattern

ΔE

80Y20CeP /100Y

60Y40CeP /100Y

40Y60CeP /100Y

20Y80CeP /100Y

100CeP/ 100Y

14,89

12,72

12,85

11,31

14,89

Figure 32 - ΔE values for Mixture B using 100Y as pattern

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6.2.3 GRAIN SIZE Figure 33 and Figure 34 present the grain size of Mixture A and Mixture B. Samples with larger amount of CeO2 on Mixture B present higher porosity, which is confirmed by the smaller relative density of the mixtures with Ce-TZPP powder, as was verified in Figure 24. This indicates that optimizations on sintering parameters are necessary. Higher contents of Y-TZP in both mixtures guarantee a small grain size.

a) 100Y

Figure 33 – Porosity of 100Y

38

e)60Y40CeP

i)20Y80CeP

Figure 34 - Porosity of Mixture A and Mixture B. a) 110Y, b) 80Y20Ce, c) 80Y20CeP, d) 60Y40Ce, e) 60Y40CeP, f) 40Y60Ce, g) 40Y60CeP, h) 20Y80Ce, i) 20Y80CeP, j) 100Ce and k) 100CeP.

39

6.2.4 XRD To confirm the correct stabilization of tetragonal phase of powders mixtures, XRD analysis were made. The diffraction pattern for the raw powders show the presence of tetragonal and monoclinic phases on commercial powders. The quantities were determined by Toroya equation. The results are shown in

Figure 35.

Figure 35 - Diffraction pattern, monoclinic content and crystallite size of raw powders

The crystallite size of commercial Y-TZP powder, given by the manufacture, is 27 nm. For commercial Ce-TZP powder, this size is 32 nm. (Swain, 2007) These sizes check with measurements and validate this technique. Ce-TZP P show a crystallite size and a total stabilization of tetragonal phase, which justifies the applied method for the synthesis. The monoclinic content of Y-TZP and Ce-TZP powders is considered high and indicates a destabilization of the tetragonal phase. If this high value is not reduced during sintering, the destabilization may endure, damaging the results of the sintered samples. The sintered samples were analyzed by XRD to verify if sintering conditions affect the monoclinic content and crystallite size. Mixture A results are shown in Figure 36 and Mixture B in Figure 38.

40

Figure 36 - Diffraction pattern of Mixture A

Analyzing the diffraction pattern of Mixture A and Mixture B, it can be concluded that this mixture destabilizes the tetragonal zirconia. Toroya and Scherrer equation were used to determine the quantity of monoclinic phase and the crystallite size, Figure 37 and Figure 39.

Figure 37 - Monoclinic content and crystallite size results for Mixture A

Pure samples (100Y and 100Ce) show complete stabilization of tetragonal phase indicating that the sintering parameters reduced the quantity of monoclinic phase on raw powders. The quantification of monoclinic content confirm the destabilization of mixtures.

41

Figure 38 - Diffraction pattern of Mixture B

Figure 39 - Monoclinic content and crystallite size results for Mixture B

For Mixture B, Figure 38 and Figure 39, a destabilization does not occur in pure samples and there is no linearity in destabilization. A two-step sintering or a Pechini synthesis, mixing ceria and yttria, may solve the destabilization problem, which leads to further studies.

42

6.3 ELASTIC MODULUS AND POISSON COEFFICIENT Elastic modulus (E) and Poisson coefficient (ѵ) were determined by impulse excitation technique. The dimensions and the weight of each sample were set in the software to determine the density before the measurements. The Poisson coefficient obtained was 0.25±0.01 for Mixture A and 0.24±0.01 for Mixture B. The theoretical Poisson coefficient is 0.3 (Y. Zhang & Kim, 2009)(Y. Zhang, Sun, & Zhang, 2012). In order to determine the theoretical values for the mixtures, the rule of mixtures were used considering the fraction and the parameter of each component. Figure 40 shows the graph comparing the theoretical elastic modulus with the measured values for Mixture A and Mixture B.

Figure 40 - Graph comparing the elastic modulus theoretical and measured for Mixture A and Mixture B

It can be noted that both mixtures present a decrease of elastic modulus with the increase of Ce-TZP content. According to the literature Y-TZP has an elastic modulus of 210 GPa (Z. Zhang et al., 2012) and Ce-TZP modulus is 162 GPa (Lima et al., 2004). In the case of mixtures 80Y20CeP and 60Y40CeP, the relative density values of this samples were around 80% that comparing to others samples, which relative density is higher than 90%, is expectable that present lower elastic modulus.

43

7 CONCLUSIONS Changes on milling and heating treatments of Pechini synthesis were made to optimize the resultant powders and the production process. The Pechini production of Y-TZPP confirm that this synthesis result in good quality powders and validate the production of Ce-TZPP. The complete stabilization of tetragonal phase and the reduction on crystallite size validate the increase in production by Pechini method, as well as the use of this powders to produce the mixtures. Colorimetric study of mixtures was validated due to the homogeneity of the samples. It was concluded that increasing CeO2 content causes a displacement towards the yellow wavelength in reflectance curve. The color difference of both mixtures using 100Y as pattern is significantly high. Therefore, combination turns difficult due to the clinical unacceptability. As for the mechanical properties, both mixtures presented a decrease of elastic modulus with the increasing of Ce-TZP content. The quantification of monoclinic phase content confirms the destabilization of both mixtures. Destabilization does not occur for pure samples and there is no linearity in destabilization for Mixture B. Two-step sintering of the mixtures may solve the destabilization problem, yet to be confirmed by further studies.

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8 BIBLIOGRAPHY Alexander, L., & Klug, H. P. (1950). Determination of crystallite size with the x-ray spectrometer. Journal of Applied Physics, 21(2), 137–142. doi:10.1063/1.1699612 Anusavice KJ. Phillips - Materiais Dentários. 11ª ed. Rio de Janeiro: Elsevier; 2005 ATCP Engenharia física. Available in http://www.atcp.com.br/pt/produtos/caracterizacaomateriais/propriedades-materiais/modulos-elasticos/metodos-caracterizacao-.html Bastide, B., Canale, P., & Odier, P. (1989). Characterization of a new ternary Ce-Y-tetragonal zirconia. Journal of the European Ceramic Society, 5(5), 289–293. doi:10.1016/09552219(89)90023-X Callister, W. D., & Rethwisch, D. G. (2009). Materials Science and Engineering: An Introduction, 992. Ceramic industry. Available in http://www.ceramicindustry.com/articles/87409-innovations-indilatometry Crystal dental center. Available in http://crystaldentalcenters.com/images/General/inlayonlay.png Djurado, E., Bouvier, P., & Lucazeau, G. (2000). Crystallite Size Effect on the TetragonalMonoclinic Transition of Undoped Nanocrystalline Zirconia Studied by XRD and Raman Spectrometry. Journal of Solid State Chemistry, 149(2), 399–407. doi:10.1006/jssc.1999.8565 Duran, P., Recio, P., Jurado, J. R., Pascual, C., Capel, F., & Moure, C. (1989). Y(E)-doped tetragonal zirconia polycrystalline solid electrolyte. Journal of Materials Science, 24(2), 708–716. doi:10.1007/BF01107464 Goharshadi, E. K., Samiee, S., & Nancarrow, P. (2011). Fabrication of cerium oxide nanoparticles: Characterization and optical properties. Journal of Colloid and Interface Science, 356(2), 473–480. doi:10.1016/j.jcis.2011.01.063 Grzebielucka, E. C. (2009). Obtenção e sinterização de nanopartículas de ZrO2-4,5%Y2O3. He LH, Swain MV. Enamel—A functionally graded natural coating. J Dent, 2009; 37: 5 9 6-603 Hench, L. L; Wilson, J. “An introduction to Bioceramics”. World Scientific Publishing Co, 1993

45

Joseph I. Goldstein, Dale E. Newbury, Patrick Echlin, David C. Joy, A. D. Romig Jr., Charles E. Lyman, Charles Fiori, E. L. (1974). Scanning electron microscopy and X-ray microanalysis. Science (New York, N.Y.) (Vol. 183). Kawasaki A, Watanabe R. Concept and P/M Fabrication of Functionally graded materials. Ceramics International, 1997; 23: 73-83. Kelly, J. R., & Benetti, P. (2011). Ceramic materials in dentistry: Historical evolution and current practice. Australian Dental Journal, 56, 84–96. doi:10.1111/j.1834-7819.2010.01299.x Kelly PM, Rose LRF. 2002. The martensitic transformation in ceramics: its role in transformation toughening. Prog. Mater. Sci. 47:463–557 Konica Milnolta. Available in http://sensing.konicaminolta.us/products/cm-3600a/ KUEHNI, R. G.; MARCUS, R. T. An experiment in visual scaling of small color differences. Color Res. Appl., Virginia, v. 4, n. 2, p. 83-91, Summer 1979. McLEAN, J. W. Evaluation of dental ceramic in the twentieth century. J. Prosthet. Dent., St Louis, v. 85, n. 1, p. 61-66, Jan. 2001 Lima, L. F. C. P., Godoy, a. L. E., & Muccillo, E. N. S. (2004). Elastic modulus of porous Ce-TZP ceramics. Materials Letters, 58(1-2), 172–175. doi:10.1016/S0167-577X(03)00439-7 Loehman, Ronald E., Characterization of Ceramics, Momentum Press, 2010) Lyon, D., Chevalier, J., Gremillard, L., & Cam, C. a D. (2011). Zirconia as a Biomaterial. Comprehensive Biomaterials, 20, 95–108. doi:10.1016/B978-0-08-055294-1.00017-9 Malvern products. Available in http://www.malvern.com/en/products/technology/laserdiffraction/ Nature materials. Available http://www.nature.com/nmat/journal/v11/n7/fig_tab/nmat3367_F2.html

in

Qu, D. De, & Qu, I. De. (2001). Synthesis and sintering of ZrO 2 -CeO 2 powder by use of polymeric precursor based on Pechini process, 6, 3825–3830. Queiroz, J., Guimar, D. E. M., Para, E. S., Qu, C., Garc, E. S., & Dra, G. (2012). Comportamiento de recubrimientos multicapa de Mullita / ZrO 2 diseñados para barreras ambientales en condiciones de vapor de agua y alta temperatura, (Icv). Quinelato, A. L., Longo, E., Leite, E. R., & Varela, Â. A. (1999). Synthesis of Nanocrystalline Tetragonal Zirconia by a Polymeric Organometallic Method, 507, 501–507. Quinelato, A. L., Longo, E., Perazolli, L. A., & Varela, J. A. (2000). Effect of ceria content on the sintering of ZrO2 based ceramics synthesized from a polymeric precursor. Journal of the European Ceramic Society, 20(8), 1077–1084. doi:10.1016/S0955-2219(99)00269-1

46

Ramesh, T. R., Gangaiah, M., Harish, P. V, Krishnakumar, U., & Nandakishore, B. (2012). Zirconia Ceramics as a Dental Biomaterial – An Over view, 26(3), 154–160. Rebouças, Lara Relatório de estágio curricular IV, Universidade Federal de Santa Catarina, UFSC Sadollah A, Bahreininejad A. Optimum gradient material for a functionally graded dentalimplant using metaheuristic algorithms. J Mech Behav Biomed Mater, 2011; 4: 13841395 Shakelford, J. F. “Bioceramics – Advanced ceramics; v. 1”. Gordon and Breach Science Publishers, 1999. Sharma, S. C., Gokhale, N. M., Dayal, R., & Lal, R. (2002). Synthesis, microstructure and mechanical properties of ceria stabilized tetragonal zirconia prepared by spray drying technique. Bulletin of Materials Science, 25(1), 15–20. doi:10.1007/BF02704588 Stevens, R. Zirconia and Zirconia Ceramics. Second Edition. Magnesium Elektron Ltd. 1986 Swain, B. S. (2007). EFFECT OF SINTERING ATMOSPHERE ON THE PROPERTY OF CERIA DOPED TETRAGONAL ZIRCONIA POLYCRYSTALS ( Ce-TZP ) EFFECT OF SINTERING ATMOSPERE ON THE PROPERTY OF CERIA DOPED TETRAGONAL, (May), 65. Tan T, Rahbar N, Allameh SM, Kwofie S, Dissmore D, Ghavami K, Soboyejo WO. Mechanical properties of functionally graded hierarchical bamboo structures. Acta Biomater,2011; doi:10.1016/j.actbio.2011.06.008. Theunissen, G. S. a. M., Bouma, J. S., Winnubst, a. J. a., & Burggraaf, a. J. (1992). Mechanical properties of ultra-gine grained zirconia ceramics. Journal of Materials Science, 27, 4429– 4438. doi:10.1007/bf00541576 Theunissen, G. S. a. M., Winnubst, a. J. a., & Burggraaf, a. J. (1992). Effect of dopants on the sintering behaviour and stability of tetragonal zirconia ceramics. Journal of the European Ceramic Society, 9, 251–263. doi:10.1016/0955-2219(92)90060-Q Toraya, H., Yoshimura, M., & Somiya, S. (1984). Calibration Curve for Quantitative Analysis of the Monoclinic-Tetragonal ZrO2 System by X-Ray Diffraction. Communications of the American Ceramic Society. doi:10.1111/j.1151-2916.1984.tb19715.x Vagkopoulou, T., Koutayas, S. O., Koidis, P., & Strub, J. R. (2009). Zirconia in dentistry: Part 1. Discovering the nature of an upcoming bioceramic. The European Journal of Esthetic Dentistry : Official Journal of the European Academy of Esthetic Dentistry, 4, 130–151.

47

Vichi, A., Louca, C., Corciolani, G., & Ferrari, M. (2011). Color related to ceramic and zirconia restorations: A review. Dental Materials, 27(1), 97–108. doi:10.1016/j.dental.2010.10.018 Volpato, C. A. M. (2005). INFLUÊNCIA DO TIPO DE SUBSTRATO E DA ESPESSURA DOS MATERIAIS CERÂMICOS (IPS-EMPRESS e IPS-EMPRESS 2) QUANTO AO COMPORTAMENTO ÓPTICO. Yadama, V., & Englund, K. (2007). Rule of Mixtures. Yet-Ming Chiang, Dunbar P. Birnie, W. D. K. (1997). Physical Ceramics - Principles for Ceramic Science and Engineering. John Wiley & Sons, Inc. Zhang, Y., & Kim, J. W. (2009). Graded structures for damage resistant and aesthetic allceramic restorations. Dental Materials, 25, 781–790. doi:10.1016/j.dental.2009.01.002 Zhang, Y., Sun, M. J., & Zhang, D. (2012). Designing functionally graded materials with superior load-bearing properties. Acta Biomaterialia, 8(3), 1101–1108. doi:10.1016/j.actbio.2011.11.033 Zhang, Z., Zhou, S., Li, Q., Li, W., & Swain, M. V. (2012). Sensitivity analysis of bi-layered ceramic dental restorations. Dental Materials, 28(2), e6–e14. doi:10.1016/j.dental.2011.11.012