Grain Growth of ZnO in ZnO-Bl2O3 Ceramics with Ai2O3 Additions

Grain growth of ZnO during liquid-phase sintering of a ZnO-6 wt% Bi₂O₃ ceramic was investigated for A1₂O₃ additions from 0.10 to 0.80 wt%. Sintering in air for 0.5 to 4 h at 900° to 1400°C was studied. The AI₂O₃ reacted with the ZnO to form ZnAl₂O₄ spinel, which reduced the rate of ZnO grain growth. The ZnO grain-growth exponent was determined to be 4 and the activation energy for ZnO grain growth was estimated to be 400 kJ/mol. These values were compared with the activation parameters for ZnO grain growth in other ceramic systems. It was confirmed that the reduced ZnO grain growth was a result of ZnAl₂O₄ spinel particles pinning the ZnO grain boundaries and reducing their mobility, which explained the grain-growth exponent of 4. It was concluded that the 400 kJ/mol activation energy was related to the transport of the ZnAl₂O₄ spinel particles, most probably controlled by the diffusion of O^2- in the ZnAl₂O₄ spinel structure.     

Microstructures and Properties of Three Composites of Alumina, Mullite, and Monoclinic SrAl2Si2O8

Three composites that were 96% alumina were mixed and uniaxially dry-pressed into bars and pellets; all had monoclinic SrAl₂Si₂O₈ as an intergranular phase. The diffraction patterns, microstructure, density, dielectric properties, strength, and toughness were measured. The first composition, which contained crystalline SrCO₃, Al₂O₃, and SiO₂, in a 1:1:2 molar ratio, as the 4% component, densified but was generally inferior to the second and third compositions, which contained strontium aluminosilicate (SrAl _x Si _y O _z , SAS) glass as the 4% component, in terms of mechanical properties, defects, and monoclinic SrAl₂Si₂O₈ transformation. The second composition, which lacked B₂O₃, was very tough and was comparable to commercial alumina, in terms of the dielectric constant. The third, which contained 0.068% of B₂O₃ that was dissolved in the SAS glass as a sintering aid, had high strength and toughness and no macroscopically visible defects. Mullite formed, in addition to monoclinic SrAl₂Si₂O₈ in all three composites. Alumina–monoclinic SrAl₂Si₂O₈ composites of the third composition had room-temperature properties that were comparable to commercial aluminas that contained 96% alumina and also had potential for mechanical and refractory applications.     

Effect of Seeding and Water Vapor on the Nucleation and Growth of α-Al2O3 from γ-Al2O3

Isothermal transformation kinetics and coarsening rates were studied in unseeded and alpha-Al₂O₃-seeded γ-Al₂O₃ powders heated in dry air and water vapor. Unseeded samples heated in dry air transformed to alpha-Al₂O₃ with an activation energy of 567 kJ/mol. Seeding with alpha-Al₂O₃ increased the transformation rates and reduced incubation times by providing low-energy sites for nucleation/growth of the alpha-Al₂O₃ transformation. The activation energy for the transformation was reduced to 350 kJ/mol in seeded samples heated in dry air. Seeded samples completely transformed to alpha-Al₂O₃ after 1 h at 1050°C when heated in dry air compared to 1 h at 925°C when heated in saturated water vapor. The combined effects of a lower nucleation barrier due to seeding and the increased diffusion due to water vapor reduced the activation energy for the transformation by 390 kJ/mol and the transformation temperature by ∼225°C compared to the unseeded samples heated in dry air. The accelerated kinetics is believed to be due to increased surface diffusion.     

Grain Growth of ZnO during Bi2O3 Liquid-Phase Sintering

Grain growth of ZnO during the liquid-phase sintering of binary ZnO–Bi₂O₃ ceramics has been studied for Bi₂O₃ contents from 3 to 12 wt% and sintering from 900° to 1400°C. The results are considered in combination with previously published studies of ZnO grain growth in the ZnO–Bi₂O₃ system. For the Bi₂O₃ contents of the present study, the rate of ZnO grain growth is found to decrease with increasing Bi₂O₃. Activation analysis, when combined with the results of similar analyses of the previous studies, reveals a change in the rate-controlling mechanism for ZnO grain growth. Following a low-Bi₂O₃-content region of nearly constant activation energy values of about 150 kJ/mol, further Bi₂O₃ additions cause an increase of the activation energy to about 270 kJ/mol. consistent with accepted models of liquid-phase sintering, it is concluded that the rate-controlling mechanism of ZnO grain growth during liquid-phase sintering in the presence of Bi₂O₃ changes from one of a phase-boundary reaction at low Bi₂O₃ levels to one of diffusion through the liquid phase at about the 5 to 6 wt% Bi₂O₃ level and above.     

Grain Growth in Sintered ZnO and ZnO-Bi2O3 Ceramics

Grain growth in a high-purity ZnO and for the same ZnO with Bi₂O₃ additions from 0.5 to 4 wt% was studied for sintering from 900° to 1400°C in air. The results are discussed and compared with previous studies in terms of the phenomenological kinetic grain growth expression: G ⁿ— G ⁿ₀= K ₀ t exp(— Q/RT ). For the pure ZnO, the grain growth exponent or n value was observed to be 3 while the apparent activation energy was 224 ± 16 kJ/mol. These parameters substantiate the Gupta and Coble conclusion of a Zn²⁺ lattice diffusion mechanism. Additions of Bi₂O₃ to promote liquidphase sintering increased the ZnO grain size and the grain growth exponent to about 5, but reduced the apparent activation energy to about 150 kJ/mol, independent of Bi₂O₃ content. The preexponential term K ₀ was also independent of Bi₂O₃ content. It is concluded that the grain growth of ZnO in liquid-phase-sintered ZnO-Bi₂O₃ ceramics is controlled by the phase boundary reaction of the solid ZnO grains and the Bi₂O₃-rich liquid phase.     

Transformation Kinetics of Ba2Ti9O20 with ZrO2 Additive

Pure Ba₂Ti₉O₂₀ (BT29) was synthesized by a solid-state reaction in one step with various amounts of ZrO₂ powder additive. The transformation kinetics of BT29 were investigated by quantitative X-ray diffractometry (XRD). The results show that stoichiometric powder mixtures transform to the BT29 phase by nucleation and growth mechanism between 1200° and 1300°C with 1.0 mol% ZrO_2. The activation energy of the transformation was found to be 620±60 kJ/mol, but decreases to 515±30 kJ/mol when doped with 1.0 mol% ZrO₂. The addition of ZrO₂ possibly changes the phase transformation mechanism of BT29 from diffusion controlled to interface controlled.     

Kinetics of Solid-State Reaction of Bi2O3 and Fe2O3

Solid-state reactions of equimolar mixtures of Bi₂O₃ and Fe₂O₃ from 625° to 830°C and their kinetics were investigated. The reaction rates were determined from the integrated X-ray diffraction intensities of the strongest peaks of the reactants and products. The activation energy for the formation of BiFeO₃ was 96.6±9.0 kcal/mol; that for a second-phase compound, Bi₂Fe₄O₉, which formed above 675°C, was 99.4±9.0 kcal/mol. Specific rate constants for these simultaneous reactions were obtained. The preparation of single-phase BiFeO₃ from the stoichiometric mixture of Bi₂O₃ and Fe₂O₃ is discussed.     

Differential Thermal Analysis on the Crystallization Kinetics of K2O-B2O3-MgO-Al2O3-SiO2-TiO2-F Glass

The crystallization kinetics of a glass based on one type of mica, NaMg₃AlSi₃O₁₀F₂, with the addition of a nucleating agent, TiO₂, has been studied using differential thermal analysis (DTA) under both isothermal and nonisothermal conditions. Two distinct crystallization exotherms in the DTA curve are observed and resolved that correspond to the initial formation of magnesium titanate (MgTi₂O₅) and the later formation of mica. The activation energy for precipitation of each crystalline phase has been evaluated, and the crystallization mechanism has been studied. The results indicate that the growth of mica is a two-dimensional process, controlled by the crystal-glass interface reaction. The average calculated values of activation energies are 256 ± 11 kJ/mol and 275 ± 6 kJ/mol for the precipitation of MgTi₂O₅ and mica from the glass matrix, respectively.     

Crystallization Kinetics and Phase Development of PbO–BaO–SrO–Nb2O5–B2O3–SiO2-Based Glass–Ceramics

The nucleation and crystallization kinetics of PbO–BaO–SrO–Nb₂O₅–B₂O₃–SiO₂-based glass–ceramics have been investigated. Strontium barium niobate (Sr_0.33Ba_0.67Nb₂O₆) with a tetragonal tungsten–bronze structure formed as the major crystalline phase, which nucleates and grows on the surface region of samples. The results of the present study showed an apparent activation energy of 193 kJ/mol for nucleation, which was controlled by the viscous flow of the glass. Quantitative X-ray analysis and differential thermal analysis showed that the rate-limiting mechanism of crystallization appeared to be a three-dimensional interfacial growth, which has an apparent activation energy of 386–430 kJ/mol, a value that is close to the dissociation of Si–O bonds in the glass system.     

Effects of B2O3 on the Phase Stability of Ba2Ti9O20 Microwave Ceramic

Preparation of dense and phase-pure Ba₂Ti₉O₂₀ is generally difficult using solid-state reaction, since there are several thermodynamically stable compounds in the vicinity of the desired composition and a curvature of Ba₂Ti₉O₂₀ equilibrium phase boundary in the BaO–TiO₂ system at high temperatures. In this study, the effects of B₂O₃ on the densification, microstructural evolution, and phase stability of Ba₂Ti₉O₂₀ were investigated. It was found that the densification of Ba₂Ti₉O₂₀ sintered with B₂O₃ was promoted by the transient liquid phase formed at 840°C. At sintering temperatures higher than 1100°C, the solid-state sintering became dominant because of the evaporation of B₂O₃. With the addition of 5 wt% B₂O₃, the ceramic yielded a pure Ba₂Ti₉O₂₀ phase at sintering temperatures as low as 900°C, without any solid solution additive such as SnO₂ or ZrO₂. The facilities of B₂O₃ addition to the stability of Ba₂Ti₉O₂₀ are apparently due to the eutectic liquid phase which accelerates the migration of reactant species.     

Nonisothermal Reaction Kinetics of SrNb2O6 and BaNb2O6 for the Formation of SrxBa1−xNb2O6

The solid-state reaction of SrNb₂O₆ and BaNb₂O₆ to form Sr _x Ba_{1− x} Nb₂O₆(SBN) at different temperatures and heating rates was investigated. The reaction kinetics were analyzed by X-ray diffractometry for quenched samples, and the internal-standard method was applied to quantify the extent of the reaction. A nonisothermal kinetic empirical model was proposed to evaluate the activation energy and rate constant of SBN with different Sr:Ba ratios. It was found that the kinetic form would change above and below a transition at a reaction fraction of ∼60%, which might be due to the change of the frequency factor. It was also verified that the model that was presented was more favorable to describe the nonisothermal reaction kinetics of SBN.     

Formation of ZnAl2O4 in the Presence of Cl2

The formation of ZnAl₂O₄ spinel in diffusion couples of Al₂O₃ and ZnO was investigated between 1000° and 1390°C in air and in air containing 4.8 vol% Cl₂ by X-ray diffraction, electron probe microanalysis, and scanning electron microscopy. The rate of formation of a spinel layer obeyed a parabolic rate law and was accelerated remarkably by the presence of Cl₂. The interdiffusion coefficient, , and the activation energy, E, were calculated to be 10⁻⁸ to 10⁻⁹ cm²/s and 123 kcal/mol (514 kJ/mol) in air and 10⁻⁷ cm²/s and 31 kcal/mol (130 kJ/mol) in air containing 4.8 vol% Cl₂, respectively.     

Para-to-Ortho H2 Conversion on Gd, Y, Gd2O3, and Y2O3

The mechanism of parahydrogen conversion was studied on Gd₂O₃ and Y₂O₃ powders and on Gd and Y evaporated metal films at low and high temperatures (77° to 90°K and 298° to 418°K). Absolute rates of conversion are compared to theoretical values for 3 possible reaction mechanisms, and it is concluded that a paramagnetic vibrational mechanism is operative on Gd₂O₃, Gd, and Y. On Y₂O₃ the reaction rate is enhanced by additional surface paramagnetic sites. The portion of the surface which is active is ∼1 for the metals and ∼0.01 for the oxides.     

Solid Solution and Chromium Oxide Loss in Part of the System MgO-Al2O3-Cr2O3-SiO2

Thermal and X-ray studies show that there is complete solid solution between MgO.Cr₂O₃ and MgO.Al₂O₃ and that the spinel solid solutions are stable with no exsolution down to temperatures as low as 510°C. There is no solid solution of excess Cr₂O₃ in MgO.Cr₂O₃ nor of MgO.Cr₂O₃ in Cr₂O₃. The join MgO.Cr₂O₃–Al₂O₃ is found to be nonbinary; compositions along that join yield mixtures of a chromium oxide-alumina solid solution and a spinel solid solution on firing to temperatures high enough to promote solid-state reaction. Chromium oxide loss by volatilization increases at higher temperature. At a given temperature, chromium oxide loss is found to vary directly with the partial pressure of oxygen in the furnace atmosphere and with the ratio of MgO to SiO₂ in the charges heated.     

Kinetics of Hexacelsian-to-Celsian Phase Transformation in SrAl2Si2O8

The kinetics of hexacelsian-to-celsian phase transformation in SrAl₂Si₂O₈ have been investigated. Phase-pure hexacelsian was prepared by heat treatment of glass flakes at 990°C for 10 h. Hexacelsian flakes were isothermally heat-treated at 1026°, 1050°, 1100°, 1152°, and 1200°C for various times. The amounts of monoclinic celsian formed were determined using quantitative X-ray diffraction. Values of reaction rate constant, k , at various temperatures were evaluated from the Avrami equation. The Avrami parameter was determined to be 1.1, suggesting one-dimensional growth with the interface rather than a diffusion-controlled transformation mechanism. From the temperature dependence of k , the apparent activation energy for this reaction was evaluated to be 527 ± 50 kj/mol (126 ± 12 kcal/mol). This value is consistent with a mechanism involving the transformation of the layered hexacelsian structure to a three-dimensional network celsian structure which necessitates breaking of the strongest bonds, the Si─O bonds.     

Crystallization Kinetics and Mechanism of Low-Dielectric, Low-Temperature, Cofirable CaO-B2O3-SiO2 Glass-Ceramics

The crystallization kinetics and mechanism of low-dielectric, low-temperature, cofirable CaO-B₂O₃-SiO₂ glass-ceramics were investigated. Crystalline phases formed during firing included calcium silicates (CaSiO₃, Ca₃Si₂O₇, Ca₂SiO₄) and calcium borate (CaB₂O₄), with crystalline wollastonite (CaSiO₃) the major phase. The crystallization kinetics of wollastonite followed an Avrami equation. The results of the present study showed an apparent activation energy of 200-260 kJ/mol. Combined with the results of reduced growth rate (growth rate × viscosity) and thermal analysis, the rate-controlling mechanism of crystallization appeared to be a two-dimensional surface nucleation growth. As the amount of crystalline wollastonite increased, the dielectric constant decreased, but the thermal expansion coefficient remained relatively unchanged.     

The System HfO2-Y2O3-Er2O3

A mathematical model of the liquidus surface based on a reduced polynomial method was proposed for the system HfO₂-Y₂O₃-Er₂O₃. The results of calculations according to this model agree fairly well with the experimental data. Phase equilibria in the system HfO₂-Y₂O₃-Er₂O₃ were studied on melted (as-cast) and annealed samples using X-ray diffraction (at room and high temperatures) and micro-structural and petrographic analyses. The crystallization paths in the system HfO₂-Y₂O₃-Er₂O₃ were established. The system HfO₂-Y₂O₃-Er₂O₃ is characterized by the formation of extended solid solutions based on the fluorite-type (F) form of HfO₂ and cubic (C) and hexagonal (H) forms of Y₂O₃ and Er₂O₃. The boundary curves of these solid solutions have the minima at 2370°C (15. 5 mol% HfO₂, 49. 5 mol% Y₂O₃) and 2360°C (10. 5 mol% HfO₂, 45. 5 mol% Y₂O₃). No compounds were found to exist in the system investigated.     

Crystal Chemistry of Some AB2O4 Compounds

Compounds having the formulas BaGa₂O₄, BaCr₂O₄, BaFe₂O₄, PbAL₂O₄, PbGa₂O₄, PbCr₂O⁴, SrAl₂O₄, SrGa₂O₄, and SrCr₂O₄ were synthesized by heating oxide or carbonate mixtures. All were examined by X-ray powder techniques. Single crystals of BaCr₂O₄, SrAl₂O₄, SrGa₂O₄, and BaGa₂O₄ were also examined. SrAl₂O₄, SrGa₂O₄, and BaGa₂O₄ are structurally similar to BaAl₂O₄ and are thus related to tridymite (SO₂). The occurrence of these stuffed tridymite structures, as well as the other principal structures encountered (spinel, CaFe₂O₄, and perovskite types) can be correlated with the ionic radii of the cations. New special structures peculiar to one compound, e.g., α -CaCr₂O₄, occur at radius ratio values representing a transition between two principal structure types.     

Eletrical Properties of Glasses in the System PbO-Al2O3-B2O3-SiO2

Glasses in the system Pb0–Al₂O₃-B₂O₃-SiO₂ are chemically stable over a wide composition range and have very desirable electrical characteristics such as high electrical resistivities and activation energies for conduction. Variations in these electrical properties were studied as a function of composition changes within the system, the object being to identify the role of the constituent oxides in achieving the highest activation energy and resistivity values consistent with moderate preparation temperatures. Measurements were made in the temperature range 25° to 400°C on carefully prepared glass disks in which the individual oxide components or different oxide ratios such as PbO/SiO₂, Al₂O₃/SiO₂, and B_sO₃/SiO₁ were systematically varied. The activation energy and resistivity values obtained ranged from 1.2 to 1.6 ev and 10° to 10¹⁴ ohm-cm, with dielectric constants ranging from 9 to 19 and densities from 4.30 to 4.50 g/cmY. Indications were that, for the composition range studied, the behavior manifested was basically that of the binary PbO-SO₂ glass with additions of Al₂O₃ or B₂O₃, even in small concentrations, sharply increasing the activation energy for conduction while lowering the density.     

Mechanical Activation of Heterogeneous Sol–Gel Precursors for Synthesis of MgAl2O4 Spinel

MgAl₂O₄ spinel precursor was prepared using a heterogeneous sol–gel process. The effect of high-energy milling on the precursor decomposition and spinel formation was investigated. The milling decreased the Al(OH)₃ dehydroxylation temperature from 190° to about 130°C. The activation energy for spinel formation decreased from 688 kJ/mol for the as-prepared precursors to 468 kJ/mol for the precursors milled for 5 h. Milling of the precursor lowered the incipient temperature of spinel formation from 900° to 800°C, and the temperature of complete MgAl₂O₄ spinel formation from >1280° to ∼900°C.