A powder mixture consisting of NiO and graphite was heated by microwave (MW) irradiation, and the reduction reaction kinetics was studied. The heating was performed in a single-mode MW applicator operated at 2.45 GHz. The specimen was placed in the waveguide at the maximum positions of either the electric (E-) field or the magnetic (H-) field. The reaction kinetics was investigated mostly at the E-field with variation of the reaction time, temperature, and grain sizes of NiO and graphite. It was shown that the kinetics was enhanced at the higher temperature and using smaller sized powder particles. The difference of the kinetics in the E-field and the H-field was investigated. A tendency was observed that the reduction rate was larger in the H-field than in the E-field at 600°C. Considering the fact that NiO was heated well in the Efield, while graphite was heated in both fields, graphite particles in the powder mixture might be selectively heated in the H-field. The obtained results were discussed considering the thermodynamics and the gas-solid reaction mechanisms.

Fig. 7.Schematic illustration of models of the enhanced reaction kinetics and formation of metal pieces.

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Microwave (MW) was utilized for radar tele-communication during the World War II. In 1946, Percy Spencer found a candy bar in his pocket was melting during his experiments on the MW generation tube, thus he discovered MW heating.1) Since then, MW heating has been applied to various fields.2,3) At first, it was used in food production areas, such as drying of potato, roasting of coffee beans. It was in '70s, when the MW oven has become popular in house kitchens for cooking. And it was also in '70s when the oil shock or natural gas crisis occurred, which promoted the researches on MW heating in the western countries, because of their political necessity to be dependent on electric heating methods. 3,4) Application of MW heating to the energy and the environmental fields has also been attempted and realized. Their application extends to broad areas, 4) such as incineration of medical wastes, devulcanization of rubber tire, treatment of sewage sludges, regeneration of spent activated carbon and chemical residues of petrol industries. In the nuclear engineering, MW de-nitration for producing MOX (Mixed OXide) nuclear fuels by re-disposal of the used plutonium.5) Vitrification of radioactive wastes by MW heating had been proposed in early '90. 6) In the area of materials processing, MW application to polymer has started earlier. There are bunch of research reports, curing of thermosets 7) is one of the examples. During MW drying of Al 2 O 3 castable, it was recognized that they can be heated well by MW. 8) MW heating of the ceramic powders above 1 400°C made it possible to perform the sintering of ceramics. 9) Later, it was reported that MW sintering enhances the diffusion rate, and so-called non-thermal effect 10) has been pointed out, though its origin has been a long debate. In this field, milli-wave techniques have been developed and applied to new processing of ceramics.2) On the other hand, studies on MW heating of metals are far behind comparing with them. Generally, it is known that a bulk metal reflects MW, however, the metal particles can be heated, and also known that ferro-magnetic metals can be heated more, 11) indicating magnetism is related to the heating mechanism. Roy reported full sintering of metallic materials in 1999.12) MW heating of metals is a new technique and could be applied to synthesis of chemical regents 13) and new functional materials, MW rapid heating of catalysis 14) for the cold start of motor vehicles and so on. The MW heating research history is schematically shown in Fig. 1. Classification of Microwave Heating Application and MW CharacteristicsAmong the broad applications of MW heating in various fields, it is possible to classify their application into two major classes. 1) Graduate School of Environmental Studies, Tohoku University, 6-6-02, Aza Aoba, Aramaki, Aoba-ku, Sendai 980-8579 Japan.2) Formerly Graduate Student, Tohoku University. Now at Sharp Co. Ltd.3) Graduate Student, Tohoku University, 6-6-02, Aza Aoba, Aramaki, Aoba-ku, Sendai 980-8579 Japan. 4) Formerly Graduate S...

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