Grain Growth in UO2-Al,O3 in the Presence of a Liquid Phase

The theory of grain growth in the presence of a liquid phase is examined using modifications of equations derived for coalescence of solid particles widely dispersed in a liquid. Although the grain diameter-time relation can still be represented by d ³= kt , the absolute growth rates are increased as the amount of liquid is decreased. The grain growth kinetics in UO₂ compacts containing 0.5 wtyo A1₂O₃ were studied for temperatures between 1960° and 2200°C. The interrelation of grain size, temperature, and time is in agreement with that predicted by the theory.     

Transmission Electron Microscopy Microstructure of 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 Ceramics

Microstructural characterizations using transmission electron microscopy on 0.95(Na_0.5K_0.5)NbO₃–0.05BaTiO₃ ceramics sintered at 1030°–1150°C for 2 h were carried out. The liquid phase was found at the triple junction of the grains in all specimens and abnormal grain growth occurred in the presence of the liquid phase. Abnormally grown grains whose shapes were cuboidal were well developed. Anisotropically faceted amorphous liquid phase pockets were observed inside the grain in a specimen sintered at 1060°C for 2 h. The interface between the grain and the liquid matrix was flat and some were identified to be {100} planes of the grains. A certain amount of liquid at the sintering temperature of 1060°C enhanced the abnormal grain growth and contributed to the improvement of the piezoelectric properties.     

Grain Growth in Two-Phase Ceramics: Al2O3 Inclusions in ZrO

The effect of Al₂O₃ inclusions with a greater average size (0.6 μm) than the average particle size of the major phase powder (<0.1 μm) on grain gowth was examined by sintering ZrO₂/Al₂O₃ composites (0,3,5,10, and 20 vol%) at 1400°C and then heat-treating at temperatures up to 1700°C. Normal grain growth was observed for all conditions. The inclusions appeared to have no effect on grain growth until the ZrO₂ grain size was ∼1.5 times the average inclusion size. Grain growth inhibition increased with volume fraction of the Al₂O₃ inclusion phase. At temperatures 1600°C, the inclusions were relatively immobile and most were located within the ZrO₂ grains for volume fractions <0.20; at higher temperatures, the inclusions could move with the grain boundary to coalesce. Grain growth was less inhilited when the inclusions could move with the boundaries, resulting in a larger increase in grain size than observed at lower temperatures. Analogies between mobile voids, entrapped within grain at lower temperature due to abnormal grain growth during the last state of sintering, and the observations concerning the mobile inclusions are made suggesting that grain-boundary movement can "sweep" voids to grain boundaries and eventually of four-grain junctions, where they are more likely to disappear by mass transport.     

Influence of Grain Growth on Dielectric Properties of Nb-Doped BaTiO3

Effects of grain size and grain growth in Nb-doped BaTiO₃ on temperature and frequency dependencies of the dielectric constant were investigated. When 0.65 μm powder is sintered to an average grain size of 1 μm, two dielectric constant peaks indicate the presence of Nb-free BaTiO₃ and of Nb-containing material. Single peaks are observed above room temperature after additional grain growth or when 0.07 μm powder is sintered to an average grain size of 1 μm. The Curie point of pure BaTiO₃ with 1 μm grains is 4 to 6°C lower than that of material with grains >10 μm. Thermodynamically, this behavior is accounted for by a phase inversion stress ∼ the room-temperature stress.     

Grain Growth During Gas-Pressure Sintering of β-Silicon Nitride

The microstructures of gas-pressure-sintered materials from β-Si₃N₄ powder were characterized in terms of the diameter and aspect ratio of the grains. The size distributions of diameters in materials fabricated by heating for 1 h at 1850° to 2000°C were nearly constant when they were normalized by average diameters because of normal grain growth. The rate-determining step in the densification and grain growth was expected to be the diffusion of materials through the liquid phase. The activation energy for grain growth was 372 kJ/mol. The average aspect ratio of the grains was 3 to 4, whereas that of large grains was smaller because of shape accommodation. The fracture toughness was about the same as that of material from α-Si₃N₄ powder despite the smaller aspect ratio of the grains     

Control of Liquid-Phase-Enhanced Discontinuous Grain Growth in Barium Titanate

Barium titanate shows discontinuous grain growth in the presence of small amounts of the liquid eutectic Ba₆Ti₁₇O₄₀-BaTiO₃ at T > 1312°C. The exaggerated grain growth in the presence of the liquid phase can be explained in terms of a solution-segregation process. The grain size distribution of sintered BaTiO₃ ceramics and the average grain size are strongly modified by addition of "seed grains" to BaTiO₃ powder. The addition of seed grains seems to be a general method of controlling the discontinuous grain growth of ceramics.     

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.     

The Effect of MgO Addition on Grain Growth in PMN–35PT

When Pb(Mg_1/2Nb_2/3)O₃–35PbTiO₃ (mol%) (PMN–35PT) is sintered at 1200°C after packing in PbZrO₃ powder, the grains show normal growth with time invariant normalized grain size distributions. If 0.5 wt% MgO is added to PMN–35PT, abnormal grain growth occurs with the large abnormal grains developing nearly cubic shapes. The interfaces between grains and PbO-rich liquid at grain triple junctions are flat, indicating that they are singular. Many central segments of the liquid films and possibly grain boundaries between the abnormal grains and the small neighboring grains are also flat along the {100} planes of the abnormal grains. The abnormal grain growth in the MgO-doped specimens is likely to be caused by the presence of these singular interfaces. Most of the large abnormal grains do not contain any Σ=3 penetration twin boundaries unlike the previous observations in PbO-excess PMN–35PT.     

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.     

Transformation-Toughened ZrO2: Correlations between Grain Size Control and Composition in the System ZrO2-Y2O3

The average grain size of ZrO₂(+Y, o,) materials sintered at 1400°C was observed to depend significantly on the Y₂O₃ content. The average grain size decreased by a factor of 4 to 5 for Y₂O₃ contents between 0.8 and 1.4 mol% and increased at Y₂O₃ contents of 6.6 mol%. Grain growth control by a second phase is the concept used to interpret these data; compositions with a small grain size lie within the two-phase tetragonal + cubic phase field, and the size of the tetragonal grains is believed to be controlled by the cubic grains. This interpretation suggests that the Y₂O₃-rich boundary of the two-phase field lies between 0.8 and 1.4 mol% Y₂O₃. Transformation toughened materials fabricated in this binary system must have a composition that lies within the two-phase field to obtain the small grain size required, in part, to retain the tetragonal toughening agent.     

Effect of Second-Phase Particles on Grain Growth in Calcite

Grain growth in polycrystalline caicite (CaCO₃) at 800°C and 300-MPa confining pressure is inhibited by the addition of Al₂O₃ particles; volume fractions of second phase (f) ranged from 0.02 to 10.0 vol%, and several powders with different average particle size were used. The stable grain size reached during heat treatment was inversely proportional to l/f^m where m varied from 0.3 to 0.55 — agreeing with results from other grain growth experiments in ceramics and metals, and with models developed for particles located at grain boundaries and grain corners, but not agreeing with models developed for random dispersions. The dependence of the stable grain size on second-phase particle size in these experiments was less systematic, possibly because of variations in the particle size distributions used, or because of particle agglomeration effects. In the single-phase aggregates, the growth kinetics were consistent with a normal grain growth equation with n=3, although uncertainties were large.     

Microstructure, Microchemistry, and Flexural Strength of Mullite Ceramics

The microstructure of mullite ceramics hot-pressed and sintered at different temperatures was studied using transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) with EDS, and electron probe microanalysis (EPMA). The specimens, consisting of stoichiometric mullite grains without glassy phase, are obtained by hot-pressing stoichiometric mullite powder at 1575°C for 1 h. Silica-rich glassy phases are observed using TEM at three-grain junctions of mullite grains in specimens heated at and above 1600°C. However, high-resolution transmission electron micrographs show no glassy phase at two-grain boundaries in all specimens. SEM with EDS analyses show that the average value of Al₂O₃ contents of mullite grains increases slightly with increasing temperature. These results are consistent with a published Al₂O₃–SiO₂ phase diagram. The flexural strength of mullite specimens at room temperature depends on their microstructure, such as the grain size and grain size distribution of mullite grains. The strength is high at room temperature and up to 1200°C, and it decreases at and above 1350°C, irrespective of the presence of the glassy phase.     

Experimental Assessment of Plasticity of Nanocrystalline 1.7 mol% Yttria Tetragonal Zirconia Polycrystals

High temperature mechanical behavior of nanocrystalline 1.7 mol% (3 wt%) yttria tetragonal zirconia polycrystals (nc-YTZP) was characterized by compression creep tests. The hot isostatically pressed nc-YTZP with mean grain size of 120 nm was subjected to grain growth to obtain grain sizes in the range of 120–310 nm. Direct measurements of the creep parameters were performed in the temperature range 1150°–1300°C and stress range 5–400 MPa. The strain rates at 1150°C ranged between 2 × 10⁻⁷ and 9 × 10⁻⁵ s⁻¹ when increasing the stress from 15 to 400 MPa. Values of the stress exponent, n =2.0±0.3, and the activation energy, Q =630±40 kJ/mol, were obtained for all test conditions. A value of the grain size exponent, p =1.5±0.3, was obtained at 1150°C in the stress range studied. Detailed microstructural observations revealed the absence of glassy phase at the grain boundaries. The creep parameters were compared with those from the literature, and the results were discussed in terms of the model recently developed by the authors, with a reasonable agreement.     

Microstructural Development in Partially Stabilized ZrO2 in the System CaO-ZrO2

The microstructures of 3 zirconias partially stabilized with CaO were investigated using scanning electron microscopy and qualitative and quantitative X-ray analysis. The structure was closely related to the heat treatments involved in fabrication. A bimodal structure with small grains of pure ZrO₂ dispersed along the grain boundaries of larger cubic solid-solution grains developed during slow cooling from 1850° to 1300°C. The presence of a liquid phase greatly enhances the growth of the pure ZrO₂ phase. An anneal at 1300°C induces precipitation of fine ZrO₂ particles within the solid-solution grains. The relative mechanical strengths of the materials are explained in terms of the weakening of the grain boundaries associated with the transformation of the grain-boundary phase on cooling.     

Kinetics of Mullite Grain Growth in Alumino Silicate Fibers

Grain growth kinetics of mullite in laboratory-produced and commercial (3M-Nextel 720) alumino silicate fibers was analyzed in the temperature range between 1500° and 1700°C. The lab fibers consist of mullite plus traces of α-alumina, while the phase content of the commercial Nextel 720 fibers is about 60 wt% mullite plus 40 wt%α-alumina. The temperature-induced grain coarsening of mullite follows the empirical law D ^{1/ n} − D ₀^{1/ n} = kt . Two different temperature regimes can be distinguished with respect to the grain growth exponents: above 1600°C the growth exponent is ≈1/3, while below 1600°C the growth exponent of ≈1/12 is exceptionally low. Laboratory-produced and commercial fibers show almost identical mullite grain growth kinetics.     

Grain Growth of α-SiAlON in the Calcium-Doped System

Two calcium-doped α-SiAlON compositions (Ca_0.6Si_10.2Al_1.8−O_0.6N_15.4 and Ca_1.8Si_6.6Al_5.4O_1.8N_14.2) were prepared by hot pressing at 1600° and 1500°C, respectively, for complete phase transformation from α-Si₃N₄ to α-SiAlON. Both samples were subsequently fired at different temperatures for different periods of time to study the grain growth of α-SiAlON. Elongated α-SiAlON grains were developed in both samples at high temperatures. The kinetics of grain growth was investigated based on the variations in length and width of the α-SiAlON grains under different sintering conditions. Different growth rates were found between the length and width directions of the α-SiAlON crystals, resulting in anisotropic grain growth in the microstructural development.     

Abnormal Grain Growth of Niobium-Doped Strontium Titanate Ceramics

We have investigated the influence of the Ti/Sr ratio and particle size on abnormal grain growth of Nb-doped SrTiO₃ ceramics. As the Ti/Sr ratio increases, the onset time and temperature of abnormal growth increase, for specimens sintered at temperatures above 1440°C. When the particle size is decreased by milling, the onset and completion temperatures of the abnormal growth and the final grain size are decreased. The abnormal grains growing during isothermal heating are nearly tetrakaidecahedral in shape, whereas those growing during rising temperature have the morphology of an edge-cut cube.     

Grain Growth of Silica-Added Zirconia Annealed in the Cubic/Tetragonal Two-Phase Region

The grain growth in silica-doped 3-mol%-yttria-stabilized tetragonal zirconia polycrystals (SiO₂-doped 3Y-TZP) and undoped 3Y-TZP has been examined in the temperature range of 1400°-1800°C. The presence of a SiO₂ phase inhibits rather than promotes the grain growth in 3Y-TZP, particularly at high temperatures. During the grain growth in 3Y-TZP, yttrium ions are partitioned between grains, and the grain growth mechanism can be understood from Ostwald ripening dominated by lattice diffusion of cations. In SiO₂-doped 3Y-TZP, an amorphous SiO₂-rich phase exists only in the grain-boundary corners or junctions, not in the grain-boundary faces. The grain growth in SiO₂-doped 3Y-TZP is controlled by using different mechanisms below and above the eutectic temperature of the zirconia-silica (ZrO₂-SiO₂) system. The glass phase does not have a major role in grain growth below the eutectic temperature, and the grain growth is dominated by a similar mechanism in undoped 3Y-TZP. The grain growth is more effectively retarded by the presence of a SiO₂ phase above the eutectic temperature and is likely to be controlled by a solution-reprecipitation process in the amorphous phase at the grain-boundary corners or junctions.     

Abnormal Grain Growth and Intergranular Amorphous Film Formation in BaTiO3

The effect of abnormal grain growth on the formation of amorphous films at grains boundaries was studied in a model system BaTiO₃. 0.4 mol% TiO₂-excess BaTiO₃ powder compacts were sintered at 1380°C for various times up to 16 h. During the sintering, abnormal grains formed. With the growth of the abnormal grains, amorphous films formed and eventually thickened up to 19.2 nm at grain boundaries. The film formation is attributed to the accumulation of Ti solutes at grain boundaries with the grain growth, while the film thickening is mostly caused by the redistribution of liquid at triple junctions. Extended annealing of the 16-h-sintered sample at 1350°C for 15 days resulted in a thinning of the film to nearly 1.7 nm without a change in the grain size, showing an equilibrium thickness. This result demonstrates that the film thickness observed during the growth of the grain may not be the equilibrium thickness. The result further suggests that the shape of the abnormal grains, even when equiaxed, can differ from the equilibrium shape.     

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.