Achim Zern

The morphology of silicon carbide in C/C-SiC composites

Abstract In the present investigations C/C–SiC has been studied by means of SEM, X-ray diffraction (XRD) and TEM to reveal the morphology of the silicon carbide areas. It was found that there exist two different areas of SiC, a fine grained β-SiC layer with a high amount of stacking faults at the C–SiC interface, and a zone of coarser β-SiC at the SiC–Si interface. From these observations, reaction mechanisms governing the siliconization of porous C/C preforms are proposed. After an initial reaction of carbon with silicon vapour, liquid silicon has to diffuse through the already formed SiC. A violent reaction far away from equilibrium conditions and a high number of nucleation sites leads to the observed formation of a fine grained SiC with a high density of stacking faults. Thermodynamically this is an instable configuration so that the coarser grained zone emerges by solution and precipitation.

Plasmon energy mapping in energy-filtering transmission electron microscopy

In this paper, we demonstrate the two-dimensional mapping of plasmon energies by energy-filtering transmission electron microscopy. The maps are obtained from a series of energy-filtered images in the plasmon energy region. Examples are shown for a nano-crystalline Si-B-C-N ceramic. This material contains SiC and Si(3)N(4) grains as well as intergranular regions composed of hexagonal BN (h-BN) and turbostratic carbon (t-C). The different phases can be clearly identified by their specific plasmon energies. An energy resolution of < or =0.1eV is achieved. In addition, the plasmon map of an amorphous carbon film is used to visualize the non-isochromaticity of the Corrected Omega filter (90 degrees filter) of the SESAM2. A procedure is proposed for the correction of the non-isochromaticity.

High temperature deformation behavior of crystallized precursor-derived Si-B-C-N ceramics

Annealing of amorphous precursor-derived Si-B-C-N ceramics for 3 h at 1800°C in 15 bar nitrogen yielded a material consisting of SiC with a mean grain size of about 100 nm, embedded in a turbostratic B-C-N phase. The deformation behavior of this material was investigated by compression tests at 1400°C in air. Dependent on the applied stress the strain rate decreased with time. The deformation rate consists of a stress dependent component that is proportional to the applied stress, which indicates that this part of the deformation mechanism is based on viscous flow. Furthermore, the influence of oxidation on the deformation behavior is investigated.

Electron diffraction study of the local atomic arrangement of as-pyrolysed Si-B-C-N ceramics

Abstract The local atomic arrangement of as-pyrolysed Si–B–C–N ceramics has been studied by electron diffraction analysis. The results revealed that in the amorphous matrix of the Si–B–C–N ceramics pyrolysed above 1300 °C, Si atoms are predominantly bonded to N atoms. In addition, the combined effect of other correlations such as C–C and Si–C bond has also been detected.