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Hybride microwave heating processes combine sample volume heating with surface sample heating.

The advantage of this method is the levelling of temperature gradients in the sample volume and the acceleration of the heating process versus heating over the contact surfaces. The latter depends mainly on physical parameters like heat conduction, heat capacity and heat transfer coefficient. If one has decided to use microwave heating he can use the dielectric loss number (e´´) as an additional freedom parameter for power input P using the approximate relation.

This delivers new possibilities for creating time-temperature-curves, which were not allowed using classical surface heating. However the temperature dependence of the loss number makes necessary the precise temperature measurement. The precise temperature control is often difficult in practical applications and actually a servere knock out criterion for microwave heating in many cases.

Combined heating concepts are more and more used for e.g. manufacturing special glasses and functional ceramic. This materials are usually low loss microwave materials. Exceptional heating with microwaves is time and energy intensive. But when the sample is raising a higher temperature, microwave absorption is increased. This results often in a characteristic overshooting of the temperature-field intensity curves (S-shaped curve). Therefore sophisticated concepts for power control in combination with a in-time temperature measurement are an urgent need for heating this materials.

The GMBU e.V. Halle treats with hybrid microwave heating for a couple of years and has in addition experiences in heating silicon wafer and fused silica materials.