A dilatometer is a standard instrument to measure thermal expansion coefficients, phase transformations or the linear shrinkage during sintering processes. This information can be very effectively used to optimize the thermal processing during sintering of advanced ceramics. One of the major drawbacks for precise measurements are thermal gradients within the samples. Such temperature gradients are present during the heating cycle with conventional heating as well as with microwave heating, but with an opposite sign and temperature gradients are the more pronounced the higher the heating rates are.

A possible solution to overcome this problem is the combination of conventional and millimeter-wave heating technique. This allows to control temperature gradients during the sintering process. The compact 30 GHz, 15 kW gyrotron installation of the Forschungszentrum Karlsruhe has been equipped with such a dilatometer system for detailed investigations in sintering of various functional and structural ceramics by means of millimeter wave radiation [1].

This equipment allows a direct comparison of conventional, mm-wave and hybrid heating in a single system. Even the effect of changing heating conditions during the sintering process can be investigated which gives interesting additional information about the effect of mm-waves in the sintering process. While the onset of sintering is shifted to lower temperature values with increasing heating rates during mm-wave processing, it is shifted in the opposite direction with conventional heating. This gives the impression of an enhancement of the sintering process with increased heating rates in case of mm-wave heating. Careful analysis of these measurements show that temperature gradients have to be taken into account in order to avoid misinterpretations. Therefore temperature is not an adequate parameter to compare different heating techniques unless there is no additional information about the existing temperature gradients.

1. G. Link, S. Rhee, M. Thumm; Proc. of the 36^th Annual Microwave Symposium of the IMPI, San Francisco, April 18-21, 2001, pp. 23-26