The Microwave Heating Behaviour of Metallic-Insulator Composite Systems
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THE MICROWAVE HEATING BEHAVIOUR OF METALLIC-INSULATOR COMPOSITE SYSTEMS CLAUDE P. LORENSON, MEL D. BALL, RALPH HERZIG, HAMISH SHAW Alcan International Limited, Kingston Research & Development Centre, P.O. Box 8400. Kingston, Ontario, Canada K7L-5L9 ABSTRACT The microwave heating behaviour of composite systems is of considerable interest for the processing of ceramics or the creation of materials with interesting dielectric properties. In this investigation, the heating response of different mixtures of metallic and insulating powders is described. The heating rate as a function of metal type, mass fraction and powder types will be presented for pressed pellet samples. From the results, three distinct heating regimes have been identified. At low temperatures. induced electric currents cause the metal particles to heat. As the temperature increases, exothermic oxidation of the metal powder particles increases the heating rate. Eventually, the insulating powder (alumina) begins to absorb microwave energy more efficiently and this dominates the high temperature behaviour. The relative importance of these three effects for different temperatures depends on the particular powders which are used. INTRODUCTION The microwave properties of metal filled composites are of interest for various reasons. Interest has ranged from the effects of metal powder additives in polymers (reference #1 ) to the addition of Fe304 powders to castable ceramics [2]. In addition to developing a better understanding of size-induced metal-insulator transitions in composite materials, potential applications have been identified in the sintering of ceramics and in methods of making EM shielding materials. In this study the systems of interest are mixtures of metallic powders with AI.O3 powder. These materials provide a model system to study percolation and to test the different theories of dielectric properties in mixed systems. In this presentation we focus on the heating behaviour of these metal/AI20= systems at 2.45 GHz. Different concentrations, metal types and shapes were investigatedin an effort to improve the heating rate of AI203. EXPERIMENTAL PROCEDURE The alumina powders and the metallic powders were mixed in a ball mill using hexane to facilitate the mixing and dispersion. For the heating rate experiments, the powders were pressed into 5 gm cylindrical pellets under 10,000 Ib of pressure in a cylindrical die. In order to study the heating rates below 200 C, the pellets were placed in a nominal 650 Watts multimode microwave oven (2.45 GHz) equipped with a mode stirrer and four fluoroptic temperature probes (Luxtron Inc.). For the sintering and the heating rate at higher temperatures, pellets were placed in a nominal 750 watt multi-mode microwave cavity with the mode stirrer disabled. The sintering cavity was equipped with four ACCU-FIBER radiation detectors so that measurements of the temperatures from 400 °C to the highest temperatures reached in these experiments (1800 °C) could be made. In each experiment, the pellets were placed in exa
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