Microwave heating ceramics industry

Nowadays, many advanced techniques are available in the ceramic industry to coat a solid layer onto a solid surface or to make ceramic materials with special properties [99-1 IS], such as spin-coating [99], chemical vapor deposition [100-106], and chemical vapor infiltration [106-109], thermal spray [110-112], plastic spray [113], and spray-coating [114]. The deposition can be caused by conventional heating, by laser beam, or by microwave heating. 

Of the 700 000 tons SiC produced per year, about 33% is used in metallurgy as a deoxidizing plus alloying agent, and about 50% in the abrasive industry [257]. The remainder is used in the refractory and structural ceramics industries and to a small extent also in electric and electronic industries as heating elements, thermistors, varistors, light-emitting diodes, and attenuator material for microwave devices. 

It heis long been established that a dielectric material, such as many types of ceramics, can be heated with energy in the form of high frequency electromagnetic waves. The frequency range used for microwave heating lies between 400 MHz and 40 GHz, however the allowed frequencies are restricted to distinct bands which have been allocated for Industrial, Scientific and Medical (ISM) use, as shown in Table 1. The principal frequencies are centred at 433 MHz, 915 MHz (896 MHz in the UK) and 2450 MHz since specific industrial equipment can be readily purchased. 

The data shown in Table 2 have been extracted from experimental results of e and as a function of the frequency with typical variations shown in Figures 8 and 9 for various ceramics. In Figure 8 pronounced dipolar relaxations occur for the titanate ceramics such as barium or barium/strontium titanate in the frequency range 10 MHz - 100 GHz. The effective loss factors of these two ceramics at 2.45 GHz are 0.2 and 0.3 respectively indicating that both these materials will readily absorb microwave radiation at this industrial allocated frequency. Lime alumina silicate, steatite and calcium titanate on the other hand have loss factors below 0.02 and as such are not obvious candidates for microwave heating. Figure 10 shows the material properties of a family of ferrites, some types can have extremely high loss factors. 

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