on millimeter-wave processing of ceramics
at the Research Center Karlsruhe
*G. Linka, L. Fehera, M. Thummb
aForschungszentrum Karlsruhe GmbH, Institut für Hochleistungsimpuls- und Mikrowellentechnik, Germany
balso University of Karlsruhe, Insitut für Höchstfrequenztechnik und Elektronik, Germany
At the Forschungszentrum Karlsruhe, Germany, a compact gyrotron system has been established since 1994 in order to investigate technological applications in the field of high temperature materials processing by means of millimeter wave (mm-wave) radiation. Besides the improvement of the system design, research activities are mainly engaged in studies on debindering and sintering of various types of advanced structural and functional ceramics. Due to volumetric heating and enhanced sintering kinetics the application of microwaves allows to shorten the processing time and therefore a reduction of energy consumption. Besides these effects microwave technology gives the unique possibility to influence microstructure and physical properties of ceramic materials.
The sintering process is one of the most crucial steps in the processing of high-performance ceramics. Due to the low penetration depth of infrared radiation in a standard resistance heated or gas fired sintering furnace, temperature gradients are induced in the ceramic parts leading to inhomogeneous shrinkage and thermal stresses if not an optimized temperature-time program with low heating rates is applied.
The use of microwaves allows to transfer energy directly into the materials volume, where it is converted to heat. This allows to apply high heat-up rates, that means markedly shortening the processing time. Furthermore, the densification process of ceramics was found to be enhanced by sintering in a microwave field allowing a reduction of the sintering temperatures and dwelling times compared to the process in a standard sintering furnace. In addition microwave technology gives the unique possibility to influence microstructure (e.g. grain size) and physical properties of ceramics.
This was the motivation to install a compact 15 kW gyrotron system operating at 30 GHz which corresponds to a wavelength of 10 mm, why it called mm-wave as well. Since the absorbed power is proportional to frequency and to the dielectric loss, which itself is a function of frequency, the use of a frequency higher than the standard industrial frequency of 2.45 GHz allows to heat a larger variety of materials, especially low loss materials. The number of modes, which can be excited in an applicator of fixed size, is increasing with the third power of frequency. Therefore the distribution of electromagnetic fields and that means the heating profile of materials can be much more homogeneous if higher frequencies and an optimized applicator geometry are used .
In order to overcome the problem of inverse temperature profiles , which are the more severe the higher the temperatures are, a combination of conventional and mm-wave heating technique, so called hybrid heating has been established . Here thermal heating allows to compensate gradients caused by volumetric microwave heating. So temperature gradients within a large sample or a batch of samples can be actively controlled. At the same time it is a versatile tool to investigate the influence of different heating methods on the sintering process in a single system. For more detailed investigations of the sintering process a dilatometer-setup was adapted to the mm-wave applicator which allows in situ measurement of the linear shrinkage of a sample during the sintering process .
Numerous experiments on debindering and sintering of various functional and structural ceramics have been performed with pure mm-wave heating, demonstrating the benefits of this technology with respect to processing time and microstructure .
During recent investingations with the new dilatometer setup the linear shrinkage of ceramic powder compacts was measured in parallel to the existing temperature gradients within the sample holder of the dilatometer system (Fig. 1). Those results clearly demonstrate that deriving any nonthermal microwave effects from process parameters only is difficult if there is no information on existing temperature gradients. For example the dilatometer curves obtained with yttria stabilized zirconia demonstrate that the onset of shinkage is shifted to lower temperature values with mm-wave heating if the heating rate is increased. This might indicate an enhancement of sintering under the influence of mm-waves. But at the same time the inverse temperature gradients in the dilatometers sample holder are increasing indicating that the temperature measured at the sample surface underlies a systematic error. That means the temperature measured at the sample surface is less than the effective temperature of the sample volume. With conventional heating this error in temperature measurement is in opposite direction. But this means that differences in sintering temperatures observed by comparing conventional, mm-wave and hybrid sintering might be rather due to systematic errors in temperature measurements than to a nonthermal microwave effect. To get more confidence for such an effect one has to do further investigations with respect to materials properties as for examples investigations on grain growth as a function of sintered density, since such a correlation excludes the temperature parameter.
More examples and details will be shown and discussed.
Fig. 1: Linear shrinkage (left) and temperature gradients in dilatometer system (right) during mm-wave (MWS), conventional (CS) and hybrid sintering (HS) of Toso YSZ at a heating rate of 20 °C/min.
 L. Feher, G. Link, M. Thumm.; Proc. 6th Int. Conf o Microwave and High Frequency Heating, Fermo Italy, Sept. 9-13, 1997, pp. 443-446.
 L. Feher, G. Link, M. Thumm; 24th Int. Conference on Infrared and Millimeter Waves, Monterey, California, Sept. 6-10, 1999, F-B5
 G. Link, L. Feher, M. Thumm; Proc. Int. Conf. on Microwave and High Frequency Heating, Valencia; ed. J.M. Catalá-Civera et al, Valencia, Sept. 13-17, 1999, 165-168.
 G. Link, S. Rhee, M. Thumm; Proc. 36th Annual Microwave Symposium of the IMPI, San Francisco, April 18-21, 2001, pp. 23-26.
 G. Link et al; IEEE Transactions on Plasma Science, Vol. 27, No. 2, April 1999, pp. 547-554.