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A promising field of applicating microwaves to heating are vacuum and high pressure processes. Such conditions are relevant for drying, adsorption, desorption and and catalysis. A further application is the extraction with supercritical gases.

Conventional heating transfer heat to the product by convection, conduction or radiation. With microwaves heat is generated in the product itself. This is very advantageously in vacuum processing when free convection is chocked and thermal equilibrium is reached after hours instead of minutes.

Fig.1 shows an example of a resonant microwave cavity employed to heat adsorbents and catalysts at pressures between 0.1 – 100 bar.
The cavity is a TM 010 resonator possessing a stationary time – averaged field distribution. The quality factor Q determines the ratio of energy stored to energy loss per cycle of oscillation. Cavities with high qualities can be used to heat low loss materials (dielectric loss between 10-4 and 10-3). The shown resonator has radial symmetric electromagnetic field with a maximum field intensity in the center of the cavity (Fig. 2). Due to a touchless temperature measurement with an IR-sensor a time controlled heating of the samples is possible (Fig. 3)

The electromagnetic field in the cavity is induced by a special antenna. A main prerequisition is a high pressure stable coaxial connector. Another feature is a power amplifier with a narrow bandwith performing powers till to kW.

GMBU e.V. treats this topic in a special R&D project, which has resulted in a furnace for thermogravimetric analysis.