ORIENTATION OF CERAMIC MICROSTRUCTURES BY HOT-FORMING METHODS.

摘要

Bi(,2)WO(,6), Bi(,2)MoO(,6), PbBi(,2)Nb(,2)O(,9) and Bi(,4)Ti(,3)O(,12) are layered structure compounds which are anisotropic and polar in their properties. Their physical characteristics are suitable for controlled microstructure development. Various techniques were used to study the effects of powder morphology, sample preparation methods, and hot-forming, on the microstructural engineering of grain-oriented polycrystalline ceramics. Materials were fabricated into grain-oriented ceramics, some of which had three dimensional characteristics, similar to pseudo-single-crystal-like properties.;In summary, layered-structure compounds were fabricated into dense, grain-oriented, polycrystalline ceramics with pseudo-single-crystal-like electrical properties. Interrelationships existing between powders, processing techniques, and hot-forming methods for materials, were determined for the fabrication of grain-oriented ceramic microstructures. The culmination of this research was the formation of 99 percent uniaxial grain orientation, and the first reported development of three dimensional orientation in ceramic microstructures.;Flux grown, solid-state calcined and salt synthesized powders were mechanically aligned by hot-pressing, hot-forging and hot-extrusion. Hot-forged samples had the greatest densities and degree of alignment. Densities and orientation factors greater than 99 percent of theoretical were obtained. Powders were also electrically aligned (by a method termed dielectrophoretic poling) and subsequently mechanically aligned (by hot-forging) for the development of three dimensional grain-orientation in polycrystalline ceramic materials. Deformation mechanisms at elevated temperatures were investigated. Grain-orientation factors were determined by x-ray diffraction, and confirmed by electron microscopy. Electrical measurements were made for dielectric constant, dielectric loss factor, a.c. conductivity, d.c. resistivity and piezoelectric coefficient. They were all found to be microstructure dependent.