Papers by Author: Friedrich Raether

Abstract: Binder removal in small and large green compacts of AlN ceramic was investigated by combining different in situ measuring techniques. Dimensional changes, wetting behaviour and weight loss were measured by a thermooptical measuring device (TOM). Thermal diffusivity was measured by the laser-flash technique and evolved gas species were identified by FTIR analysis. The properties of the binder changed with increasing temperature. Wetting properties drastically improved between 150°C and 350°C. The activation energy for the rate determining step of binder decomposition was derived from weight loss data using the kinetic field method. It was 180 kJ/Mol up to a temperature of 350°C, where it strongly increased. The binder enhanced thermal conductivity of the green compacts – especially in the last stage of binder removal - which was explained by its location in the vicinity of the contacts between the AlN particles. With large samples and high heating rate a superposition of weight loss reactions which could be clearly separated in other experiments was observed. This was attributed to the effect of the high partial pressure of evolved gases in the pore system of the green compacts.

Abstract: The fracture toughness increases but the long-term oxidation resistance of carbon short fiber reinforced C/SiC composites decreases if the volume fraction of carbon fibers is high. To understand the oxidation kinetic and their dependence from the C/SiC microstructure the composites and also the used carbon short fiber bundles were investigated by a thermo balance and by scanning electron microscopy. Additionally the oxidation of single carbon short fibers was measured in situ by a hot stage light microscope. To simulate oxidation kinetics a software program using Finite Element methods was developed. Below 800 °C oxidation resistance can be improved by increasing the degree of graphitization of the carbon material within the short fiber bundles. Above 800 °C the number of junctions between the short fiber bundles has to be reduced.

Abstract: Many ceramic materials are composed of various phases, which can differ in their individual thermal, elastic or electrical properties by orders of magnitude. The microstructural arrangement of the phases controls important material properties of the composite. To simulate these macroscopic material properties from the material properties of the constituting phases, a 3-D FEM model is used. The key for an adequate description of real materials is the accurate threedimensional modeling of their microstructure. Basic morphological parameters of many ceramics are reflected by a modified Voronoi model, e.g.: the volume fractions, grain size ratios and contiguity of the phases. By automatically generating thousands of test structures and comparing them to quantitative data derived from image analysis of scanning electron micrographs, structures are selected which closely fit to the microstructure of experimental samples. The model considerations are illustrated on two types of bi-continuous ceramic materials, a porous alumina (Al2O3) and a dense zirconia toughened alumina (ZTA) ceramic. Using different volume fractions of the phases, Voronoi type microstructures and truncated sphere models are exemplified. For these two ceramic systems, elastic moduli and thermal conductivity are calculated and compared to experimental data of samples of the respective microstructure.