Microwave (MW) heating behavior of various metal particles was investigated using a single-mode applicator. Considering the distributions of the electromagnetic fields in the wave guide, specimens were placed at four specific positions with respect to the electric and the magnetic fields of MW. They were heated at conditions of constant power input.It was demonstrated that iron particles were heated well in the magnetic field, and that ferro-magnetic metal particles having the higher Curie point was heated the better. It was possible to heat iron bulk particles ($3 mm) in a magnetic field without occurrence of electric discharge. In the range of nickel particle size between 45 and 150 mm, the particles with the smaller size were heated the better.Nickel oxide (NiO) was heated well only in the position of large electric field, which indicates that the heating was caused by the different (dielectric heating) mechanism from the metal particles.From these results, contribution of magnetic field to heating metal particles was discussed, considering the heating mechanisms of the magnetic loss and the eddy current loss. The dependence of the heating rate of metal particles on their size was discussed in terms of the heat transfer rate.

It is known that bulk metallic samples reflect microwaves while powdered samples can absorb such radiation and be heated efficiently. In the present work we studied heating mechanisms of metallic powders in a multimode 2.45 GHz microwave applicator. The present paper shows direct evidence of penetration of a layer of metallic powder by microwave radiation and provides theoretical explanation of such behavior.The most effectively heated powder is Fe because both eddy current loss (in alternating H-field) and magnetic reversal loss (in alternating E-field) mechanisms act in case of such metal. Diamagnetic metals Sn and Cu are heated better than paramagnetic Ti while Au is also only slightly heated. Cu- and Ni-based metallic glassy powders are also moderately heated. Weak heating of Au powder (which is a noble metal) can be explained by the absence on the particles of the oxide layer (shell), which allows eddy currents flowing through larger area compared to other metals, and reflection mechanism works much better in such case.

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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.

As the fundamentals of microwave (MW) interaction with metals, boundary conditions of electromagnetic (EM) field on metal surface are discussed, which consider the EM field in the metal surface layer in terms of surface impedance. Experimental report on heating behavior of separated electric (E-) and magnetic (H-) fields of metal particles and films are shown. Temperature peak formation at the first heating curves was observed in both cases, which are discussed considering the microstructural alteration by MW heating. In the last half section, various reports on MW heating of metal are reviewed. They were classified into the major application for sintering and materials fabrication. And also, its usage as a heating aid of glasses and soils, topics on metal hydride and catalytic metal particles are included.