Articles you may be interested in Symmetry determination on Pb-free piezoceramic 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 using convergent beam electron diffraction method

In this investigation we have studied how the presence of a liquid phase affects the grain morphology and grain growth kinetics in Alz03 at 1800°C using the growth of both matrix grains and large spherical single-crystal seeds growing into the matrix. The growth rates of the matrix grains were found to decrease in the following order: undoped AI2O3, A1203 with anorthite, Al2o3 with anorthite and MgO, and AI2O3 with MgO. Except for the samples doped with MgO alone, the matrix grains were faceted and appeared tabular in polished sections. In samples containing anorthite both with and without MgO, the single-crystal seeds exhibit basal facets with continuous liquid films and slow growth in the (0001) relative to all other crystallographic directions. When only MgO is added, the growth of the single-crystal seeds was not isotropic; however, no faceting was observed. We discuss how anisotropic growth rates caused by the anorthite additions can stimulate discontinuous grain growth in &03.

Barium titanate precursors with Ba/Ti ratios ranging from 2: 1 to 1 : 9 were prepared by controlled hydrolysis of mixed barium and titanium species in an alcohol medium. Details of the synthesis and characterization of the resultant products are given. Amorphous powders precipitated by hydrolysis from ethanol solutions of barium and titanium alkoxides crystallize to singleor two-phase 1 : 2 and 1 : 5 compounds at =700°C. These compounds transform at higher temperatures to other known crystalline phases, the 1 : 5 phase being maintained metastably to =lloooc.

The stress due to thermal expansion anisotropy in polycrystalline Al2O3 was measured. The broadening of spectroscopic R lines (692 and 693 nm, due to Cr" impurities) was used to measure the stresses (at 77 K) in samples with grain sizes of 50 to 150 p m that had been cooled, from 2150 K, at constant rates from 0.1 to 100 Wmin. The maximum stress was found to vary from 80 to 100 MPa, depending on the thermal history of the sample. The results are compared to the predictions of a model based on stress relaxation by diffusional creep and are in good agreement for the dependence on cooling rate. No effect due to grain-size changes was observed due to the limited range of grain sizes accessible in this study.

The equilibrium shape of internal cavities in sapphire wasIn principle, there are two ways to form equilibrium shaped determined through the study of submicrometer internal crystals. ‡ One is to heat small crystals in a closed system at cavities in single crystals. Cavities formed from indentation high temperatures so that transport processes are rapid. This cracks during annealing at 1600؇C. Equilibrium could be technique is difficult because of the necessity of controlling the reached only for cavities that were smaller than 100 nm.external environment to prevent a volume change of the crystal Excessive times were required to achieve equilibrium for by evaporation, corrosion, or condensation. The second method cavities larger than 1 m. Five equilibrium facet planes is to use internal cavities, which are, in effect, "negative" cryswere observed to bound the cavities: the basal (C) {0001}, tals that, if lattice diffusion is very slow, are shapes of constant rhombohedral (R) {1012}, prismatic (A) {1210}, pyramivolume in contact with a fixed atmosphere. For the latter case, dal (P) {1123}, and structural rhombohedral (S) {1011}.the conditions of fixed volume in contact with its vapor are The surface energies for these planes relative to the surface easier to maintain than for the former case. The deviation of a energy of the basal plane were ␥ R ‫؍‬ 1.05, ␥ A ‫؍‬ 1.12, ␥ P ‫؍‬ crystal's shape from its Wulff shape at any time depends on the 1.06, ␥ S ‫؍‬ 1.07. These energies were compared with the starting shape and size of the crystal and the rates of kinetic most recent theoretical calculations of the surface energy processes that change its shape. If we want annealing times of sapphire. The comparison was not within experimental to be on the order of days, not years, crystals/cavities must be scatter for any of the surfaces, with the measured relative small, less and 1 m for many materials and annealing condisurface energies being lower than the calculated energies.tions, in order to minimize diffusion distances. The relationship Although the prismatic (M) {1010} planes are predicted between initial shape and size of the cavity and the kinetics of to be a low-energy surface, facets of this orientation were shape change has been discussed in Refs. 4 and 5 and is disnot observed.cussed in detail below. Internal cavities produced during sintering have been used to determine relative surface energies by Nelson et al. 6 for copper I. Introduction and aluminum and by Müller 7 for NaCl. Powell-Dogan and Heuer 8 noted the presence of facetted cavities in commercial A T EQUILIBRIUM, facetted crystals are bounded by flat, relagrades of vitreous bonded aluminum oxide and attempted to tively low energy crystallographic planes. For a given voldetermine the relative surface energies of the cavities using the ume, the total surface free energy of the crystal, ⌫, is given by result that ␥ i /ᐉ i ϭ constant. As a fivefold variation in relative the sum of the specific surface energies of the facets, weighted surface energy was obs...

The mechanisms controlling normal and exaggerated grain growth in lead magnesium niobate-lead titanate (PMN-PT) ceramics have been investigated by varying the PbO-based liquid-phase volume fraction from 0.03 to 0.6 and sintering temperature from 900°to 1100°C. There is a transition in matrix grain growth rate and matrix grain shape with liquid fraction; samples with liquid volume fractions less than ϳ0.15 show relatively small equiaxed grains resulting from grain-tograin impingement. Samples with higher liquid fractions show significantly larger, facetted, cube-shaped grains, whose size is independent of liquid fraction, indicating that a surface nucleation rate mechanism controls growth in this regime. Exaggerated grains were found in the high liquid fraction samples. Electron backscatter diffraction showed that all of the exaggerated grains contained 60°͗111͘ twins but none of the normal matrix grains contained twins. The reentrant angles in the twinned grains give them a growth advantage over untwinned grains, resulting in a population of exaggerated grains.

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