Effect of MgO Solute on the Kinetics of Grain Growth in A12O3

The kinetics of grain growth in fully dense Al₂O₃ with and without MgO solute additions were measured. The MgO was found to retard boundary migration and to rkvelop more uniform microstructures. A grain-growth mechanism involving solute partitioning of segregated ions (calcium and magnesium) between different boundary types is proposed. Implications regarding the sintering of Al₂O₃ are discussed.     

Effects of Addition of Na2O, Sb2O3, CaO, and ZrO2 on the Properties of Lithium Zinc Ferrite

Some effects of CaO, Na₂O, Sb₂O₃, and ZrO₂ additions on Li_0.3Zn_0.4Fe_2.3O₄ ferrite were studied to improve its density and other material properties. The densification behavior of the ferrite depended on the amount and type of additive. A relative density of ∼98.5% was achieved with the addition of CaO. The grain size decreased with the addition of Na₂O, CaO, and Sb₂O₃. The permeability and electrical resistivity increased with additives. CaO remarkably increased resistvity, whereas, ZrO₂ increased permeability. Na₂O and Sb₂O₃ increased the Curie temperature, whereas CaO and ZrO₂ decreased it. These effects were attributed to mainly additive segregation on the grain boundaries, which suppressed grain-size development during the sintering of lithium zinc ferrite.     

Grain Growth in Fe3O4

Sintering and microstructural evolution were studied in Fe₃O₄ as a model system for spinel ferrites. Fe₃O₄ powder, purified by the salt-crystallization method, was sintered to ∼99.5% density in a CO-CO₂ atmosphere. The p _O2 Of the sintering atmosphere drastically affects the microstructure (grain size) of sintered Fe₃O₄ without significantly affecting density. The measured grain-boundary mobilities, M , of Fe₃O₄ fit the equation M=M ₀( T ) p _O2^−1/2 with M ₀( T ) = 2.5×10⁵ exp[-(609kJ·mol-¹/ RT ](m/s)(N/m²)^−l. The grain-boundary migration process appeared to be pore-drag controlled, with lattice diffusion of oxygen as the most likely rate-limiting step.     

Effect of Grain Coalescence on the Abnormal Grain Growth of Pb(Mg1/3Nb2/3)O3-35 mol% PbTiO3 Ceramics

Abnormal grain growth (AGG), which occurred during the heat treatment of Pb(Mg_1/3Nb_2/3)O₃-35 mol% PbTiO₃ (PMN-35PT) with excess PbO, was investigated. AGG has been suggested to be the consequence of grain coalescence that results in the formation of Σ3 coincidence site lattice and low angle grain boundaries. Because of reentrant edges appearing at the ends of these boundaries, the coarsening rate of grains was significantly enhanced and AGG occurred.     

Influence of the Addition of Bi2O3 on the Grain Growth and Magnetic Permeability of MnZn Ferrites

Additions of Bi₂O₃ were used to promote grain growth and to increase magnetic permeability during sintering of MnZn ferrites. The results showed that small additions of Bi₂O₃ of <0.05 wt% remarkably increase the permeability of MnZn ferrites. On the other hand, addition of 0.05 wt% Bi₂O₃ induced the formation of a microstructure composed of giant grains with trapped pores embedded in a normal microstructure. The permeability of these samples showed a pronounced secondary maximum in permeability. At still higher Bi₂O₃ concentrations, above 0.2 wt%, the grain growth was retarded and a normal microstructure appeared; however, the magnetic permeability was strongly reduced.     

Kinetics of Templated Grain Growth of 0.65Pb(Mg1/3Nb2/3)O3·0.35PbTiO3

Single-crystal layers of 0.65Pb(Mg_1/3Nb_2/3)O₃·0.35PbTiO₃ (PMN-35PT) were grown heteroepitaxially on {001}-BaTiO₃ template crystals. A {001}-BaTiO₃ crystal was embedded in a fine-grained matrix of PMN-35PT containing excess PbO and heated between 950° and 1150°C for 0–5 h. The initial growth of the PMN-35PT on the {001} surface and the growth of the matrix grains both displayed a t ^1/3 dependence which is characteristic of diffusion-controlled growth. Growth was limited to ∼100–150 μm due to the significantly reduced driving force at longer times because of matrix coarsening and porosity evolution.     

Grain Growth Control in Sb2O3-Doped Zinc Oxide

Microstructure development in Sb₂O₃-doped ZnO was studied to evaluate the influence of inversion boundaries (IBs) on ZnO grain growth. In general, the addition of Sb₂O₃ is believed to inhibit the ZnO grain growth via the formation of spinels and IBs, but we have shown that even the conditions of exaggerated grain growth can be created in this system. We designed an experiment for diffusional doping of ZnO under slightly increased partial pressure of Sb₂O₃. In the high-concentration regime we observed no spinels, and yet the ZnO grains were small and inhibited in growth, while in the low-concentration regime we found huge grains, several times larger than normal ZnO grains, showing an obvious exaggerated growth. By controlling the number of nuclei with IBs we can design coarse-grained microstructures even with Sb₂O₃ doping, which has far-reaching implications in the production of low-voltage varistor devices.     

Kinetics of Growth of Al2O3 Whiskers

The kinetics of the growth of Al₂O₃ whiskers by the reaction of Al with a trace amount of H₂O in an H₂ atmosphere were studied. The activation energy in the region of constant growth rate was ∼77 kcal mol⁻¹. This activation energy value, in the light of thermodynamic calculations and other theoretical considerations, suggests that the rate-determining step in the growth of these whiskers is the dissociation of H₂O molecules on the surface of Al₂O₃.     

Effects of MoO3, on Phase Separation of Na2O-B2O3-SiO2 Glasses

Immiscibility temperatures of Na₂O-B₂O₃-SiO glasses, with andwithout 1 mol% MoO₃, additions, were determined and the effect of MoO₃ additions on the 65O°C immiscibility isotherms was established. In addition, immiscibility temperature and phase-separation morphology of an Na₂O-B₂O₃-SiO₂ glass with progressive additions of MoO₃, were investigated. It was found that the addition of small amounts of MoO₃ extends the immiscibility boundary of the system and raises the immiscibility temperature by ∼l8°C for each mol % MoO₃, addition. Analysis of phase-separation morphology suggests that the MoO₃, additions do not significantly alter the tie lines of phase separation in the system, although such additions cause a lowering of the viscosities and the glass-transition temperatures of these glasses.     

Kinetics of β-Si3N4 Grain Growth in Si3N4 Ceramics Sintered under High Nitrogen Pressure

The kinetics of anisotropic β-Si₃N₄ grain growth in silicon nitride ceramics were studied. Specimens were sintered at temperatures ranging from 1600° to 1900°C under 10 atm of nitrogen pressure for various lengths of time. The results demonstrate that the grain growth behavior of β-Si₃N₄ grains follows the empirical growth law Dⁿ– D₀ⁿ = kt , with the exponents equaling 3 and 5 for length [001] and width [210] directions, respectively. Activation energies for grain growth were 686 kJ/mol for length and 772 kJ/mol for width. These differences in growth rate constants and exponents for length and width directions are responsible for the anisotropy of β-Si₃N₄ growth during isothermal grain growth. The resultant aspect ratio of these elongated grains increases with sintering temperature and time.     

Effect of Sintering Atmosphere on Grain Boundary Segregation and Grain Growth in Niobium-Doped SrTiO3

To investigate the donor segregation in grain boundary regions and its effect on grain growth in SrTiO₃, SrTiO₃ powder compacts were doped with Nb₂O₅ and sintered in air or in hydrogen. The Nb-doped SrTiO₃ sintered in air did not show any detectable Nb segregation at the grain boundary region while an appreciable segregation was observed in the space-charge region of the sample sintered in H₂. The observed donor segregation in H₂ suggests a negative grain boundary charge and compensating positive space charge, which are the opposite to those in air. The negative grain boundary core was attributed to the segregation of inherently present acceptor impurities and the trapping of electrons at grain boundaries. In the H₂-sintered sample, where the added Nb ions were segregated in the space-charge region, the grain growth was suppressed. This result may indicate that the grain growth suppression in H₂ is due to the Nb solute drag of the boundary motion and the reduction in Ti vacancies.     

Effect of Al(OH)3 Addition on β-LiAlO2 Crystal Growth

Crystal growth of rod-shaped β-LiAlO₂ was previously reported by us, and the rod-shaped β-LiAlO₂ crystals were 1.5 μ in diameter and 10 to 15 μm long. In the present study needle-shaped β-LiAlO₂ crystals which were thinner and had larger aspect ratios (length/diameter) than the rodshaped β-LiAlO₂ crystals were grown by using LiOH–Al₂O₃–Al(OH)₃–NaOH as the raw material. These crystals were 0.7 to 1 μm in diameter, 9 to 13 μm long, and had aspect ratios of about 10 to 13.     

Grain Growth During Sintering of Donor-Doped BaTiO3

Grain growth of donor-doped BaTiO₃ in the presence of KF b'quid phase was studied. The results showed that the composition of the liquid phase present during sintering had a pronounced influence on grain growth and on the formation of semiconducting donor-doped BaTiO₃.     

MoO2 Diffusion in Aluminosilicate Glass

A diffusion couple of an oxidized molybdenum disk and a glass cylinder was used to measure the solubiiity and effective binary diffusion coefficient of MoO₂ in a non-alkali aluminosilicate glass. At 1400°C, the solubility limit was 8.4 mol%; the value of the diffusion coefficient (4.1 × 10⁻¹⁶ m²/s) was significantly lower than that estimated from the Stokes-Einstein relation.     

Formation Mechanism of MoO3-Doped YSr2Cu3Oy Using the Citrate Process

The formation mechanism of the synthesis of MoO₃-doped YSr₂Cu₃O _y powders using the citrate process was investigated. It was shown that the precursor phase (Sr_{1- x} Y _x )₁₄Cu₂₄O₄₁ played a crucial role in forming the superconducting phase. It was found that the precursor phase (Sr_{1- x} Y _x )₁₄Cu₂₄O₄₁ interacted with water and decomposed when it was heavily milled and heated.     

Effect of Praseodymium and Uranium Addition on the Jc of Ba2 YCu3O7

Ba₂YCu₃O₇ ceramics doped with either Pr or U at 0.000 17 to 1.7 wt% levels have been prepared. For each sample J _c (magn) has been measured with a vibrating sample magnetometer. No improvement in J _c was found for either dopant and it is concluded that neither provides the clusters necessary to produce suitable pinning sites.     

Grain Growth in Very Porous Al2O3 Compacts

Extensive grain growth was observed by scanning electron microscopy in very porous Al₂O₃ compacts, even at densities <40% of theoretical. After ∼7% shrinkage at 1700°C, the grain size increased from ∼0.3 to 0.51 μm in a compact having a relative green density of 0.31. During grain growth in highly porous compacts, the grains appear initially to be chainlike, then to be oblong, and finally to be equiaxed. The proposed mechanism of initial grain growth involves the filling of necks between adjacent grains followed by the movement of the grain boundary through the smaller grain. Although grain growth in very porous compacts is quite different from coalescence and ordinary grain growth, the kinetics are similar.     

Habit Changes of Sapphire Grown from PbO-PbF2, and MoO3-PbF2 Fluxes

Single crystals of Al₂O₃ (sapphire) were grown from PbO-PbF₂ and MoO₃-PbF₂ fluxes; they varied from flat plates (PbF₂-rich melts) to equidimensional crystals (PbO- or MoO₃-rich melts). The primary growth planes are basal {0001}, first-order rhombohedral {1011}, and second-order rhombohedral {0112}. The habit change is interpreted on the basis of F^- contamination and Pb²⁺ surface adsorption. Possible ion species in the melts and their relative importance on crystal growth from these systems are discussed.     

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.