Direct experimental demonstration of non-thermal microwave effects during sintering of ceramics

Summary form only given. During the last decades microwave technology gathered growing industrial interest for thermal processing of various kinds of materials. Many industrial processes using microwave technology, such as for example food processing, drying, rubber vulcanization or polymer curing, were already successfully realized by use of microwave technology. Here the benefit of microwave volumetric heating can be exploited to significantly shorten processing times, especially if large volumes of materials with low thermal conductivity have to be processed. The use of microwaves for sintering of ceramics has been proposed and investigated by several research groups, because a direct energy transfer into the material's volume allows much higher heating rates compared to conventional heating. Furthermore, the densification process of ceramic bodies has been reported to be enhanced by sintering in a microwave field due to non-thermal microwave effects. Non-thermal effects are the usual explanation for phenomenological observations like reduced sintering temperature and soaking time or the evolution of more fine-grained microstructures in case of microwave sintering compared to conventional sintering. Since such non-thermal effects are always superimposed to thermal effects especially in case of ceramics sintering, a direct access for experimental verification of existing theoretical models on such non-thermal microwave effects is difficult. Based on the theory of the so called ponderomotive driving forces which specifies an enhanced diffusion in ionic solids under the influence of the microwave field, the influence, of the microwave field orientation on the diffusion in a faceted pore has been described by Boosky et al. Because of this, a non-isotropic pore closure can be expected during sintering of ceramics in a linearly polarized microwave field. Therefore, systematic investigations of the pore structure evolution in yttria-stabilized zirconia have been started in a single mode 2.45 GHz waveguide applicator. For the first time, strong experimental evidence for the existence of a non-isotropic pore closure due to a non-thermal microwave-effect was found with an adequate statistic evaluation of the pore aspect ratios after sintering