Phase Evolution During Formation of SrAl2O4 from SrCO3 and α-Al2O3/AlOOH

Through the execution of experimental investigation, thermogravimetry, X-ray diffractometry, Fourier transform-infrared spectrometry, transmission electron microscopy, and energy-dispersive spectrometry, a variant reaction mechanism model was proposed for the solid-state reaction between SrCO₃ and Al₂O₃/AlOOH for formation of SrAl₂O₄ material. The solid-state reaction is observed to be dependent on the calcination temperature. At temperatures lower than the transformation temperature of SrCO₃ from orthorhombic to hexagonal (920°C), the reaction is attributed to the interfacial reaction between SrCO₃ and alumina. Conversely, at temperatures higher than that, the solid-state reaction is dominated by the diffusion of Al³⁺ ions into the SrCO₃ lattice. In this mechanism, two metastable species, hexagonal SrCO₃ and hexagonal SrAl₂O₄, were observed. The activation energies of SrCO₃ decomposition in the solid-state reaction also support these results. The interfacial reaction at low temperatures is characterized by a high activation energy of ∼130 kJ/mol; whereas, in the reaction at higher temperatures, the activation energy of SrCO₃ decomposition decreases to 34 kJ/mol.     

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.     

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.     

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.     

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.     

Stability of ZrO2 Phases in Ultrafine ZrO2-Al2O3 Mixtures

Mixtures of ultrafine monoclinic zirconia and aluminum hydroxide were prepared by adding NH₄OH to hydrolyzed zirconia sols containing varied amounts of aluminum sulfate. The mixtures were heat-treated at 500° to 1300°C. The relative stability of monoclinic and tetragonal ZrO₂ in these ultrafine particles was studied by X-ray diffractometry. Growth of ZrO₂ crystallites at elevated temperatures was strongly inhibited by Al₂O₃ derived from aluminum hydroxide. The monoclinic-to-tetragonal phase transformation temperature was lowered to ∼500°C in the mixture containing 10 vol% Al₂O₃, and the tetragonal phase was retained on cooling to room temperature. This behavior may be explained on the basis of Garvie's hypothesis that the surface free energy of tetragonal ZrO₂ is lower than that of the monoclinic form. With increasing A1₂O₃ content, however, the transformation temperature gradually increased, although the growth of ZrO₂ particles was inhibited; this was found to be affected by water vapor formed from aluminum hydroxide on heating. The presence of atmospheric water vapor elevates the transformation temperature for ultrafine ZrO₂. The reverse tetragonal-to-monoclinic transformation is promoted by water vapor at lower temperatures. Accordingly, it was concluded that the monoclinic phase in fine ZrO₂ particles was stabilized by the presence of water vapor, which probably decreases the surface energy.     

Electrical Conduction in Sintered α-Sb2O4

Electrical conductivity and thermoelectric power were measured on sintered α-Sb₂O₄ at 250° to 780°C. Oxygen partial pressure dependence of the conductivity and sign of the Seebeck coefficient showed α-Sb₂O₄ to be a p -type semiconductor above 600°C in the oxygen pressure range of lo⁵ to 10² Pa. A hopping conduction was proposed from very small hole mobility with an activation energy of 18 kJ/mol.     

Growth of Tabular α-Al2O3 Grains on Porous Alumina Substrate

Gradient, porous alumina ceramics were prepared with the characteristics of microsized tabular α-Al₂O₃ grains grown on a surface with a fine interlocking feature. The samples were formed by spin-coating diphasic aluminosilicate sol on porous alumina substrates. The sol consisted of nano-sized pseudo-boehmite (AlOOH) and hydrolyzed tetraethyl orthosilicate [Si(OC₂H₅)₄]. After drying and sintering at 1150°–1450°C, the crystallographic and chemical properties of the porous structures were investigated by analytical electron microscopy. The results show that the formation of tabular α-Al₂O₃ grains is controlled by the dissolution of fine Al₂O₃ in the diphasic material at the interface. The nucleation and growth of tabular α-Al₂O₃ grains proceeds heterogeneously at the Al₂O₃/glass interface by ripening nano-sized Al₂O₃ particles.     

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.     

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.     

Novel Low-Temperature Processing Route of Dense Mullite Ceramics by Reaction Sintering of Amorphous SiO2-Coated γ-Al2O3 Particle Nanocomposites

Dense mullite ceramics were successfully produced at temperatures below 1300°C from amorphous SiO₂-coated gamma-Al₂O₃ particle nanocomposites (AS-gammaA). This method reduces processing temperatures by similar/congruent300°C or more with respect to amorphous SiO₂-coated alpha-Al₂O₃ particle microcomposites (AS-alphaA) and to other Al₂O₃-SiO₂ reaction couples. The good densification behavior and the relatively low mullite formation temperature make AS-gammaA nanocomposites an excellent matrix raw material for polycrystalline aluminosilicate fiber-reinforced mullite composites.     

Fabrication of Nano-Scaled α-Al2O3 Crystallites Through Heterogeneous Precipitation of Boehmite in a Well-Dispersed θ-Al2O3-Suspension

The possibility of eliminating finger or vermicular growth of α-Al₂O₃ particles obtained by calcination of boehmite was examined. Heterogeneous precipitation of boehmite in a well-dispersed θ-Al₂O₃ suspension was first prepared, in which the mass ratio of boehmite to θ-crystallite was evaluated to form agglomerates of similar sizes that will form α-Al₂O₃ crystallites of <100 nm in diameter. θ- to α-phase transformation of alumina experiences a nucleation and growth mechanism, with the critical size of nucleation being ∼25 nm for θ-Al₂O₃ and the size for accomplishment of transformation followed by finger growth being ∼100 nm. Hence, fabricating agglomerates that would form α-Al₂O₃ crystallites with sizes <100 nm accompanied with appropriate thermal treatments can be a method for obtaining α-Al₂O₃ crystallites free of finger growth. It is found that proper preparation of the agglomerate with appropriate size may initiate a simultaneous and lower temperature θ- to α-Al₂O₃ phase transformation for such powder systems, substantially limiting the mass transfer among the newly formed α-Al₂O₃ particles. Moreover, α-Al₂O₃ crystallites free of finger growth can be obtained.     

Possible Role of the Oxygen Potential Gradient in Enhancing Diffusion of Foreign Ions on α-Al2O3 Grain Boundaries

Thermally grown, alpha-Al₂O₃ external scales formed on alloys after oxidation in pure oxygen at temperatures between 1000° and 1500°C were analyzed using FEG-STEM/XEDS. Alloy dopants such as Y, Zr, La, Hf, and Ti were found to segregate to the alpha-Al₂O₃ grain boundaries and to the alloy-scale interface. With increasing oxidation time and temperature, the amount of segregant on the oxide grain boundaries near the gas interface increases until a critical level is reached and precipitates begin to nucleate and grow. These observations are a result of the outward transport of dopants from the alloy, through the external alumina scale, to the gas interface. The apparent driving force for the dopant diffusion is the oxygen potential gradient in the growing oxide scale.     

Compressive Creep of SiC-Whisker-Reinforced Al2O3

Compressive creep of SiC-whisker-reinforced Al₂O₃ composites (0, 5, 15, and 25 wt% SiC) was measured in the temperature range of 1300° to 1500°C in air and argon. The creep resistance increased with increasing whisker concentration. The results indicated that the whiskers degraded in air, increasing strain rates compared to those in argon. Stress exponents between 1.0 and 2.0 and an activation energy of 620 ± 100 kJ/mol were measured. Transmission electron microscopy observations indicated that cavitation was minimal and that the deformed composites had the same dislocation structure as did the as-received samples.     

Transformation, Microstructure Development, and Densification in α-Fe2O3-Seeded Boehmite-Derived Alumina

Addition of α-Fe₂O₃ seed particles to alkoxide-derived boehmite sols resulted in a 10-fold increase in isothermal rate constants for the transformation of γ- to α-Al₂O₃. Changes in porosity and surface area with sintering temperature showed no effect of seeding on coarsening of the transition alumina gels, but the 200-fold decrease in surface area associated with transformation to α-Al₂O₃ occurred ∼ 100°C lower in seeded gels compared with unseeded materials. As a result of high nucleation frequency and reduced microstructure coarsening, fully transformed seeded alumina retained specific surface areas >22 m²/g and exhibited narrow pore size distributions, permitting development of fully dense, submicrometer α-Al₂O₃ at ∼ 1200°C.     

Sintering of Nanophase γ-Al2O3 Powder

The sintering of ultrafine γ-Al₂O₃ powder (particle size ∼10–20 nm) prepared by an inert gas condensation technique was investigated in air at a constant heating rate of 10°C/min. Qualitatively, the kinetics followed those of transition aluminas prepared by other methods. Measurable shrinkage commenced at ∼ 1000°C and showed a region of rapid sintering between ∼1125° and 1175°C followed by a transition to a much reduced sintering rate at higher temperatures. Starting from an initial density of ∼0.60 relative to the theoretical value, the powder compact reached a relative density of 0.82 after sintering to 1350°C. Compared to compacts prepared from the as-received powder, dispersion of the powder in water prior to compaction produced a drastic change in the microstructural evolution and a significant reduction in the densification rate during sintering. The incorporation of a step involving the rapid heating of the loose powder to ∼1300°C prior to compaction (which resulted in the transformation to α-Al₂O₃) provided a method for significantly increasing the density during sintering.     

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.     

Formation of Sm2+ lons in Sol-Gel-Derived Glasses of the System Na2 O-Al2O3-SiO2

Samarium ions (Sm²⁺) incorporated into aluminosilicate glasses by a sol-gel process showed persistent spectral hole burning at room temperature. Gels of the system Na₂O-Al₂O₃SiO₂ synthesized by the hydrolysis of Si(OC₂H₅)₄, Al(OC₄H₉)₃, CH₃ COONa, and SmCl₃·6H₂O were heated in air at 500°C, then reacted with H₂ gas to form Sm²⁺ ions. Whereas Al³⁺ ions effectively dispersed the Sm³⁺ ions in the glass structure, Na⁺ ions were not effective. The Al₂O₃-SiO₂ glasses proved appropriate for reacting the Sm³⁺ ions with H₂ gas and exhibited the intense photoluminescence of Sm²⁺ ions. The reaction of Sm³⁺ ions with H₂ in the Al₂O₂-SiO₂ glasses was determined by first-order kinetics, and the activation energy equaled 95 kJ/mol. At 800°C, the maximum photoluminescence of the Sm²⁺ ions was achieved within 20 min.     

Influence of Cr and Fe on Formation of α-Al2O3 from γ-Al2O3

The effect of Cr and Fe in solid solution in γ-Al₂O₃ on its rate of conversion to α-Al₂O₃ at 1100°C was studied by X-ray diffraction. The δ form of Al₂O₃ was the principal intermediate phase produced from both pure γ-Al₂O₃ and that containing Fe³⁺ in solid solution, although addition of Fe greatly reduced crystallinity. Reflectance spectra and magnetic susceptibilities showed that Cr exists as Cr⁶⁺ in γ-Al₂O₃ and as Cr³⁺ in α-Al₂O₃, with θ-Al₂O₃ as the intermediate phase. The intermediates formed rapidly, and the rates of their conversion to α-Al₂O₃ were increased by 2 and 5 wt% additions of Fe and decreased by 2 and 4 wt% additions of Cr. An approximately linear relation observed between α-Al₂O₃ formation and decrease in specific surface area was only slightly affected by the added ions. This relation can be explained by a mechanism in which the sintering of δ- or θ-Al₂O₃, within the aggregates of their crystallites, is closely coupled with conversion of cubic to hexagonal close packing of O^2- ions by synchro-shear.     

Kinetics and Mechanism of Hot Corrosion of γ-Y2Si2O7 in Thin-Film Na2SO4 Molten Salt

γ-Y₂Si₂O₇ is a promising candidate material both for high-temperature structural applications and as an environmental/thermal barrier coating material due to its unique properties such as high melting point, machinability, thermal stability, low linear thermal expansion coefficient (3.9 × 10⁻⁶/K, 200°–1300°C), and low thermal conductivity (<3.0 W/m·K above 300°C). The hot corrosion behavior of γ-Y₂Si₂O₇ in thin-film molten Na₂SO₄ at 850°–1000°C for 20 h in flowing air was investigated using a thermogravimetric analyzer (TGA) and a mass spectrometer (MS). γ-Y₂Si₂O₇ exhibited good resistance against Na₂SO₄ molten salt. The kinetic curves were well fitted by a paralinear equation: the linear part was caused by the evaporation of Na₂SO₄ and the parabolic part came from gas products evolved from the hotcorrosion reaction. A thin silica film formed under the corrosion scale was the key factor for retarding the hot corrosion. The apparent activation energy for the corrosion of γ-Y₂Si₂O₇ in Na₂SO₄ molten salt with flowing air was evaluated to be 255 kJ/mol.