Joel D. Katz

Microwave sintering of nanocrystalline γ-Al2O3

Abstract The densification and phase transformation behavior of gas condensation synthesized nanocrystalline γ-A1 2 O 3 sintered with microwave radiation has been studied. The polymorphic nucleation and growth phase transformations which occurred as the material was heated through the temperature range of 800–1300°C present significant obstacles in the achievement of specimens which possess high bulk densities. These phase transformations are accompanied by a change in particle morphology, crystallite size, and surface area. Alumina derived from a chemically synthesized boehmite precursor has been shown to exhibit the same nucleation and growth phase transformation behavior when conventionally heated. It is concluded that nanocrystalline γ or δ alumina will not be a viable starting material for the production of dense bodies with grain sizes of less than 100 nm.

On the possibility of a significant temperature gradient in supported metal catalysts subjected to microwave heating

A simple model is presented which estimates the temperature gradients in microwave-irradiated 1 and 100 nm metallic particles which are typically found in a supported metal catalyst structure. The two particle sizes allow limiting case calculations of the temperature rise over the range of typical particle sizes. The model, based on a simple steady-state energy balance, uses the assumption that the particles only lose heat to the gas-phase, and not the support matrix. This represents a best-case scenario for a temperature gradient relative to the surroundings. The model indicates that the temperature gradient is insignificant and this conclusion is supported by an experiment in which the microwave-driven carbon monoxide reaction acts as an in situ temperature probe.

Microwave sintering of boron carbide

A method for forming boron carbide into a particular shape and densifying the green boron carbide shape. Boron carbide in powder form is pressed into a green shape and then sintered, using a microwave oven, to obtain a dense boron carbide body. Densities of greater than 95% of theoretical density have been obtained. 1 tab.

DEVELOPMENT OF NOVEL MICROSTRUCTURES IN ZIRCONIA-TOUGHENED ALUMINA USING RAPID SOLIDIFICATION AND SHOCK COMPACTION

A rapidly solidified alumina-zirconia eutectic material containing nanocrystalline t-ZrO{sub 2} has been synthesized. When heated, the microstructure contained a mixture of t-ZrO{sub 2} and m-ZrO{sub 2}, each of which can facilitate toughening of the composite. Dynamic shock compaction was used to accelerate densification of the material, producing crack-free specimens with high green densities. After sintering to densities measuring {approximately}95{percent} of theoretical, the shock-compacted specimens fabricated with unstabilized alumina-zirconia were extensively microcracked due to an overabundance of the m-ZrO{sub 2} phase. Experiments employing Y{sub 2}O{sub 3} as a chemical stabilizer have shown that the extent of the phase transformation can be controlled, and the microstructure which developed in the stabilized material contained an acceptable level of the microcrack generating m-ZrO{sub 2} phase. {copyright} {ital 1996 Materials Research Society.}

Phase Transformation and Densification Behavior of Microwave Sintered γ-A12O3

The phase transformation and densification behavior of gas condensation synthesized γA1 2 O 3 sintered with microwave radiation has been studied. Nucleation and growth phase transformations, which produce α-A1 2 O 3 occurred as the material was heated through the temperature range of 800–1300°C. These phase transformations resulted in anomalous grain growth with a distinct change in particle morphology, crystallite size and surface area. A1 2 O 3 derived from a chemically synthesized boehmite precursor has been shown to exhibit the same nucleation and growth phase transformation behavior when conventionally heated. It is concluded that nanocrystalline γ or β alumina will not be a viable starting material for the production of dense bodies with grain sizes of less than 100 nm.

Microwave joining of SiC ceramics and composites

Potential applications of SiC include components for advanced turbine engines, tube assemblies for radiant burners and petrochemical processing and heat exchangers for high efficiency electric power generation systems. Reliable methods for joining SiC are required in order to cost-effectively fabricate components for these applications from commercially available shapes and sizes. This manuscript reports the results of microwave joining experiments performed using two different types of SiC materials. The first were on reaction bonded SiC, and produced joints with fracture toughness equal to or greater than that of the base material over an extended range of joining temperatures. The second were on continuous fiber-reinforced SiC/SiC composite materials, which were successfully joined with a commercial active brazing alloy, as well as by using a polymer precursor.

Temperature distribution in microwave sintering of alumina cylinders

Small cylinders of high-purity alumina were encased in a `casket` of low-density zirconia insulation and heated to sintering temperature in a large multi-mode microwave oven. Optical fiber sensors were used to monitor the temperature at several locations in the system. It was found that the alumina samples heat faster than the zirconia insulation at temperatures above 1000 C, and that the temperature distribution in the sample is essentially uniform during the heating process. A two-dimensional mathematical model of the heat transfer process was developed which reproduces the essential features of the observed phenomena. Literature data for all temperature-dependent properties were incorporated into the model. The model suggests that the alumina samples absorb a significant fraction of the microwave energy.

Detoxification of hazardous waste streams using microwave-assisted fluid-bed oxidation

Microwave-assisted oxidation of trichloroethane (TCE) performed at 500-580{degree}C has been found to be significantly more efficient than conventional oxidation methods. Experiments were conducted using a 6 kilowatt, 2.45 gigohertz power supply and a 6 inch bed of silicon carbide granules in a 1 inch diameter quartz reactor tube which in turn was placed in a microwave cavity. After heating the reactor to a given temperature a TCE-air stream was passed through the silicon carbide bed. TCE was almost completely detoxified (98--99%) in a single pass through the silicon carbide bed at 500--580{degree}C. The oxidation products are HCl, CO{sub 2} and CO. By comparison the corresponding single-pass detoxification using conventional thermal methods results in only partial conversion. The principal products being dichloroethylene (C{sub 2}H{sub 2}Cl{sub 2}) and HCl. 5 refs., 1 tab.

Kinetics of the carbon monoxide oxidation reaction under microwave heating

915 MHz microwave heating has been used to drive the CO oxidation reaction over Pd/Al{sub 2}O{sub 3} with out significantly affecting the reaction kinetics. As compared to an identical conventionally heated system, the activation energy, pre-exponential factor, and reaction order with respect to CO were unchanged. Temperature was measured using a thermocouple extrapolation technique. Microwave-induced thermal gradients were found to play a significant role in kinetic observations. The authors chose the CO oxidation reaction over a supported metal catalyst because the reaction kinetics are well known, and because of the diverse dielectric properties of the various elements in the system: CO is a polar molecule, O{sub 2} and CO{sub 2} are non-polar, Al{sub 2}O{sub 3} is a dielectric, and Pt and Pd are conductors.

Theory of sublinear power dependence for microwave heating in some ceramic materials

A sublinear dependence of the microwave heating rate on the incident microwave power implying a saturation effect appears to have been observed recently. We present simple model calculations to address this observation on the basis of an idea of spatial confinement of the absorbing charges in grain boundary regions. Two natural lengths exist in this model: the spatial extent of the confining region, and the maximum distance an absorbing charge travels under the combined action of damping and of the oscillating microwave field. We suggest that a mismatch of these lengths results in the observed saturation, more generally, in the observed decrease in absorption efficiency.