J. D. Katz

Theory of microwave interactions in ceramic materials: the phenomenon of thermal runaway

A theory of the phenomenon of thermal runaway in ceramic materials undergoing microwave heating is presented on the basis of a simple temperature-time equation. The non-linear evolution inherent in the equation is shown to arise naturally from physical arguments and it is shown how the parameters of the theory may be calculated from the microscopic absorption processes and information about the material under consideration. The theory is applied to experimental observations reported on several different materials and shown to be in good agreement with the data.

Polymeric precursors to refractory metal borides

Polymeric precursors to zirconium and hafnium diboride are described. Initial studies concentrated on carbothermal/borothermal reduction of metal alkoxides; however, improved results were obtained from oxide free-precursors prepared from the metal borohydride and borazine. The metal borides are obtained in good chemical and ceramic yield upon pyrolysis, and the polymeric precursors obtained through the reaction of borazine with the metal borohydride exhibit viscosities amenable to use as preceramic binders in powder processing.

Microwave and conventional sintering of rapidly solidified Al2O3-ZrO2 powders

The Al2O3-ZrO2 eutectic composition was rapidly solidified, forming amorphous and crystalline structures. The as-quenched material was crushed and pressed into pellets which were sintered conventionally or with microwaves. Conventional and microwave sintering at temperatures up to 1600 °C resulted in a microstructure where 100–200 nm ZrO2 grains were present intergranularly in the α-Al2O3 grains. Larger ZrO2 grains (∼1 μm) were found intergranularly. The as-quenched lamellar structure spheroidized during sintering at high temperatures. Boron contamination of the powders resulted in more homogeneous and dense as-fired samples but promoted the ZrO2 tetragonal-to-monoclinic transformation, which was attributed to increased grain boundary diffusivity. Conventional sintering at low temperatures resulted in the formation of “rods” of an Al2O3-rich phase which grew from a low-melting B2O3-rich liquid.

Compaction of Submicron and Nanocrystalline Al2O3-ZrO2 Ceramics

The eutectic in the Al2O3-ZrO2 system was rapidly solidified by heating with an O2-H2 torch and then quenching the melt with a twin roller device. The as-quenched material, which was amorphous or nanocrystalline, was milled with B4C or Al2O3 media and pressed into pellets which were sintered conventionally or with microwaves. Samples contaminated with boron carbide resulted in compacts with substantially reduced porosity and a lower fraction of tetragonal ZrO2. Conventional sintering at low temperatures resulted in the formation of ‘rods’ of an Al2O3 rich phase which grew from the presence of a low melting B2O3-rich liquid. Conventional and microwave sintering at temperatures up to 1600°C resulted in a microstructure where 100–200 nm ZrO2 grains were present intragranularly in the α—Al2O3 grains. Larger ZrO2 grains (~1 μm) were found intergranularly. As-quenched lamellar microstructures were preserved at low temperatures but spheroidized at sintering temperatures > 1600°C.