The use of microwaves for sintering has been proposed and investigated by several research groups, because it allows a direct energy transfer into the material's volume and therefore allows an essential acceleration of the heating process. Furthermore several publications in this field conclude an enhanced densification from a possible reduction of sintering temperatures and/or soak time if microwaves are used for heating instead of gas fired or resistance heated furnaces. Such phenomenological observations are usually explained by so called non-thermal microwave effects. But quite often possible errors in temperature measurement resulting from intrinsically different temperature gradients were not taken into account, when microwave and conventional heating are compared. Therefore a novel experimental approach has been developed which allows a direct experimental access to non-thermal microwave effects. Based on the theory of the so called ponderomotive driving forces which specifies an enhanced diffusion in ionic solids under the influence microwave fields, the influence of the microwave field orientation onto the diffusion in a faceted pore has been described by Booske et al. [1]. In consequence of this, an anisotropic pore closure can be expected during sintering of ceramics in a linearly polarized microwave field. 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 an anisotropic pore closure due to a non-thermal microwave effect was found with an adequate statistic evaluation of the pore aspect ratios after sintering.
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