Microwave heating of electrically conductive materials (original) (raw)
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Chapter 3. Microwave Theory and Background
In conventional thermal demagnetisation heat is applied to a sample which creates lattice vibrations (phonons). These phonons are in a higher energy state than the surrounding magnetic system so they exchange energy with the magnetic system, and spin waves (magnons) are created (Walton et al., 1992; 1993). The generation of spin waves within the magnetic grains enables the individual domain magnetisations to reverse and thus demagnetise in zero field (Walton, 1986) or to realign with an ambient fixed field to produce a TRM. In microwave demagnetisation / remagnetisation, the first steps are bypassed; magnons are directly excited with the use of high frequency microwaves thus eliminating the need to heat the bulk samp le. Some heating of the bulk sample does occur due to the generation of phonons in the relaxation process but to a much lesser extent than in conventional heating.
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Microwave heating processes involve electromagnetic and thermal effects coupled together through the local temperature dependence of the material dielectric properties. This paper presents a one-dimensional model for the coupled electromagnetic-thermal process and demonstrates its solutions for typical problems. The local temperature dependence of the lossy dielectric medium is taken into account in two different time scales. One is the heat-generation time scale due the microwave radiation, and the other is the temperature diffusion time scale. The two timescale approach minimizes the computation time and provides an efficient simulation tool for the analysis of various phenomena. The two-scale model presented in this paper is benchmarked by a comparison of its numerical results with other models published in the literature. Several examples of microwave heating processes in various materials are simulated. Effects of heatwave propagation in matter are predicted by the model. The results show the temporal and spatial evolution of the temperature and power-dissipation profiles. Variations in the (microwave) impedance profile in the medium due to the heating are computed. A further development of this model, including more complicated geometries and various loss mechanisms, may yield useful numerical tools for the synthesis and design of microwave heaters in which the heated material acts as a nonlinear load in the microwave circuit.