High-pressure high-temperature in situ diffraction studies of nanocrystalline ceramic materials at HASYLAB

Abstract

High-pressure in situ diffraction studies were performed up to 8 GPa in a cubic anvil cell MAX80 (Station F2.1) and up to 45 GPa in a Diamond Anvil Cell (DAC-Station F3 at HASYLAB, Hamburg). A series of nanocrystals of SiC with grain sizes ranging from 2 nm to several μm were examined in non-hydrostatic conditions by pressing pure powders. A new method of evaluation of powder diffraction data measured at high pressures is presented. This method is based on quantitative evaluation of asymmetry of Bragg reflections where each peak is described as a combination of two reflections of two similar crystallographic phases having different compressibilities. The measured changes of the lattice parameters calculated for split Bragg reflections were used for determination of the pressure gradient which occurs across the grain boundaries in the compressed materials. A model of the strain induced in compacts of pure powders under high pressures is proposed. The model accounts for the presence of two phases: a volume phase corresponds to cores of individual grains which are surrounded by a surface phase which is formed of free surfaces in loose powders and of grain boundaries in solids. Due to extreme hardening of the boundaries under non-hydrostatic pressure conditions, the effective pressure in the interior of the grains is much lower than the applied external pressure. It is suggested that additional ‘hardening’ of the grain boundaries results from the presence of dislocations which are generated at the surface of the grains. The actual gradient of the pressure depends on the size of the grains, and also on the method of synthesis of the materials.