A novel design for a high temperature SOFC, based on lamellar electrode-electrolyte segments obtained by solidification of an oxidic eutectic melt on an electrolyte substrate is presented. Such “composite” electrodes contain NiO or MnO - 8Y-ZrO2 lamellae, which after reduction / oxidation yield electrode-electrolyte lamellae with 1–2 μm width and a vertical dimension of> 100 μm, depending upon the amount of eutectic melt solidified on a polycrystalline substrate. The nucleation of the eutectic on a polycrystalline substrate followed by a semi-directional crystallization of the two phases yields a gradient of 3-phase boundaries over the height of such an electrode, with the number of 3-phase boundaries increasing towards the substrate.

The variety of possible microstructures obtained upon microwave sintering of hardmetals is described with regard to microwave specific heating mechanisms. While the reduction of processing time and temperature is evident, the influence of the microwave sintering process on the microstructure is not fully understood. From microwave sintering experiments at different power levels processes that depends on the local field strength are identified. The mechanical properties of microwave sintered tool bits are compared with commercial WC-Co hardmetals. An incising wear resistance of MW-sintered hardmetals for speed turning of cast iron is found.

Microstructural differences between microwave and conventionally annealed solid solution TiO2-SnO2 and TiO2-SnO2-(Al2O3)0,02-samples demixed by spinodal decomposition are reported. The decomposition is retarded upon microwave annealing as compared to conventional heat treatment. This result is in accordance with theoretical predictions, if a superposition of the microwave field with an internal periodic electrostatic potential resulting from kinetic effects is assumed.

Microwave reaction sintering of tungsten- carbon-, cobalt mixtures is described as a method that combines the liquid phase reaction sintering of tungsten carbide cobalt hardmetals with the enhanced densification behavior of hardmetals in the microwave field. Starting with green parts composed of W-, Co- powder, soot as carbon source and with varying amount of additives a dense and extremely fine grained hardmetal can be obtained in one pressureless microwave reaction sintering step.

By this method hardmetals with improved mechanical properties can be obtained in a drastically simplified processing cycle, without time consuming steps such as carburizing and milling.

Material parameters important for microwave processing are identified in case of powder synthesis from precursor compounds and in case of sintering Al2O3- as well as SiC-matrix ceramics. By varying the spatial distribution of the precursor in microwave transparent materials, different pyrolysis temperatures are obtained, which can be attributed to different heating rates due to selective microwave heating. The microwave sintering behavior of oxide ceramics is strongly influenced by the specific surface area of the powder and by aliovalent dopants. In contrast, for covalent ceramics, like SiC+TiC, no experimental evidence for similar effects was obtained.

Application of paraffin as a MW-transparent matrix for microwave pyrolysis of metalorganic compounds enables synthesis of large quantities of nanosized powders with excellent storability and packing behavior. Fully crystalline, nanosized t-ZrO2 powders with specific surface area > 200 m 2 g -1 are obtained from Zr-alcoholate, and alumina powders with > 500 m2g-1 from Al-alcoholates. Some differences exist between microwave pyrolysis and conventional pyrolysis of such mixtures. Powders from microwave pyrolysis transform at lower calcination temperature to α-A12O3 and exhibit after calcination a more homogeneous morphology as compared to powders from conventional pyrolysis. Furthermore, a different composition of the volatile pyrolysis products is found. t-ZrO2 powder from the microwave processing route exhibits an increased stability towards transformation to monoclinic ZrO2 as well as smaller initial grain sizes than a conventionally synthesized powder.

The spatial distribution and sintering activity of ZrO2 governs the microstructure evolution upon microwave sintering of alumina-zirconia ceramics. Using the same microwave sintering profile thermal runaway occurs, when ZrO2-powders with high specific surface area are employed, yielding ceramics with eutectic Al2O3-ZrO2 inclusions, whereas ZTA-type ceramics are obtained from low specific surface area powders. Microwave sintering yields ceramics with a higher density at a particular sintering temperature as compared to conventional sintering.

A novel approach to synthetic ceramic powders based on microwave heating of metallorganic ceramic precursor compounds or mixtures of such compounds with a appropriate matrix material is described.

Numerous oxides as well as nonoxide powders with high specific surface area were obtained. Pyrolysis as well as enhanced reactivity is observed with microwave heating at lower temperatures than in conventional heating.

Preliminary results are decribed on synthesis of ceramic-metal composites with a functional gradient material character utilizing infiltration of a porous metallic matrix with ceramic precursors and thermal processing by MW-heating. Infiltration of the metal matrix with a ceramic precursor facilitates bonding between metal and ceramic. By repeated infiltration and decomposition, complete filling of the pores with a ceramic phase is achieved.

Using SiC as MW absorbing material in a sandwich-like configuration, heating of the metal-ceramic composite in a strong MW field is possible. Temperatures of apr. 1000°C were achieved with this method.

A method for microstructural modelling of ceramic materials utilising precursor compounds and heating by microwave radiation to introduce a second phase as well as to control grain growth is described. The precursor compounds are introduced into the ceramic material by infiltration of a porous compact or by impregnating a classical powder with the precursor prior to compaction. Microwave radiation is used for decomposition of the precursor and for sintering. Decomposition rate controlled pyrolysis of the precursor is only possible by microwave heating. The resulting microwave processed ceramics display a finer and in some cases more bimodal microstructure as compared to conventionally processed samples with the same density.