Eberhard Müller

Synthesis of nanocrystalline silicon carbide powder by carbothermal reduction

A solution of sugar in silica sol was used as parent material for the manufacture of nanocrystalline SiC powders. After mixing and freeze drying of the components the conversion was performed in two steps. First the sugar was converted to carbon particles and secondly the silica carbon mixture was heated up to the approved reaction temperatures between 1550 and 1800 °C under an argon pressure of 180 kPa. When the synthesis temperature was reached the pressure was reduced to 0.02 kPa what leads to a vigorous reaction resulting in extremely fine particles. The obtained powders were characterized with regard to the particle and crystallite size, the BET surface and their chemical composition. The oxygen content was < 0.5 wt% and the particles were < 0.5 μm with crystallites < 100 nm and specific surface areas in the range of 20–30 m2g−1. Conclusions concerning the synthesis parameters to the resulting powder properties are given in this paper.

Silicon Carbide — A Survey of Synthetic Approaches, Properties and Applications

The challenge to develop tailor-made precursor molecules for silicon carbide has significantly intensified the progress in synthesis of organosilicon polymers. A critical overview is presented regarding useful synthesis routes towards such molecules with appropriate chemical composition as well as controllable architecture (metal condensation of halogenosilanes or silahalogenocarbons, disproportionation of disilanes, dehydrocoupling of hydrosilanes, hydrosilylation of olefins).

Properties of nanocrystalline ceramic powders prepared by laser evaporation and recondensation

Nanocrystalline ceramic powders (ZrO 2 , Al 2 O 3 , Si 3 N 4 , AlN) are prepared by evaporation of solid materials (sintered rods or coarse powder) in the-foeus of a CO 2 laser and the following recondensation in a carrier gas stream. The production rates are in the range of 40-100 g h -1 (cw mode, oxides) or more than 200 g h -1 (cw mode, nitrides), respectively. The particle size distribution can be influenced by different parameters of the formation process like laser power, area of the laserfocus, pulse length in case of pw mode, streaming velocity and type of the carrier gas. The powders consist of nearly spherical particles with diameters in the range of 10-100 nanometers. Features of the produced alumina and zirconia powders are reported concerning phase transformation, behaviour in suspensions and processing properties.

Diamond-like carbon: alteration of the biological acceptance due to Ca-O incorporation

Abstract Diamond-like carbon (DLC) coatings are deposited on glass substrates by plasma decomposition of gaseous carbon precursors in a direct current discharge. In addition to the hydrocarbon gas, CaO–H 2 O vapor is supplied to the continuously evacuated vacuum chamber via two mass flow controllers and a needle valve for the CaO–H 2 O inlet. While maintaining the same deposition time and relative partial pressure of the hydrocarbon, the relative partial pressure of the CaO–H 2 O vapor was modified. The first information about the biocompatibility was gained by cell experiments with L929 mouse fibroblasts, sessile drop tests, scanning electron and atomic force microscopy, and correlated to the microstructure of the coatings. While mouse fibroblasts of the type L929 attach and grow on unmodified DLC coatings synthesized by the decomposition of hydrocarbon, the addition of CaO–H 2 O into the precursor gas improves the coatings biological acceptance by the cells.

Crystallisation behaviour and polytype transformation of polymer-derived silicon carbide

Abstract This paper describes the crystallisation behaviour of polysilane-derived amorphous silicon carbide. The polytype formation and the transformation from β-SiC into hexagonal α-polytypes have been more specifically investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), 29 Si Solid State Magic Angle Spinning Nuclear Magnetic Resonance ( 29 Si MAS-NMR) and Raman spectroscopy. The crystallisation of β-SiC starts around 1200 °C. The different polytypes like 3C, 2H, 4H, 6H and 15R have been identified by the various investigation techniques at temperatures higher than 1400 °C. Crystallisation occurs with a high density of stacking faults that cover large areas in the crystallites as found by TEM observation. They promote the formation of temporary existing α-polytypes which are finally reconverted into β-SiC as found by X-ray and electron diffraction.

Crystallization of polymer derived silicon carbide materials

Abstract Silicon carbide fibres are produced by melt spinning of chlorine containing polysilanes under argon and cross linked by application of ammonia as curing agent and finally pyrolyzed. Their thermal behaviour is investigated. The crystallization process with increasing pyrolysis temperature is characterized by X-ray diffraction (XRD) and small angle X-ray scattering (SAXS). Oxygen, nitrogen and free carbon contents are determined and combined with density measurements in order to obtain information about the microstructure of the system. The comparison of non-cured systems (Si–C) and cured materials (Si–N–C) allows to give an insight into the high-temperature processes. Investigations concerning the nucleation mechanism are further carried out and were described in detail.

Investigation of Electrical Double Layers on SIO2 Surfaces by Means of Force vs. Distance Measurements

The formation of electrical charges at solid/liquid interfaces results in a diffuse electrical double layer close to the solid surface. This layer determines, for example, the behaviour of ceramic particles in aqueous media and the effective pore radius of membranes acting in liquids. The SiO2/SiO2 system is used to demonstrate that linear measurements using atomic force microscopy are suited for characterizing double layers at real surfaces. It has to be taken into consideration that the tip itself, which is acting as the probe, is also surrounded by a double layer and responds to the composition of the electrolyte in a material-specific way. For this reason an oxidized silicon tip was used for the present investigations, so that tip and sample consisted the same material. In this way it is possible to obtain results that can be evaluated easily and compared with the DLVO theory.© 1997 John Wiley & Sons, Ltd.

Conversion process of chlorine containing polysilanes into silicon carbide: Part I Synthesis and crosslinking of poly(chloromethyl)silanes–carbosilanes and their transformation into inorganic amorphous silicon carbide

Synthesis of chlorine containing polysilanes by catalytic redistribution is described and the obtained polymers are characterized by infrared (IR)-, magic angle spinning nuclear magnetic resonance (MAS-NMR)-spectroscopy and elemental analysis. The pyrolysis of such polysilanes was investigated by thermoanalysis, IR-, MAS NMR- and electron spin resonance (ESR)-spectroscopy. The conversion process from polysilane into polycarbosilane is described in terms of the results of the performed investigations and a model of the process is developed. Differences between this conversion process and those for chlorine free polysilanes or polycarbosilanes previously described by other authors are discussed.

Submicro structure of silicon carbide derived from poly(methylchlorosilane)

Abstract Poly(methylchlorosilane) characterized by silicon domains in the macromolecules is pyrolysed at 165 °C. The resulting structure is investigated by Raman-spectroscopy. Silicon carbide, silicon, and carbon areas of nanoscale dimensions are detected. The silicon carbide and carbon particles are strong disordered and small in particle size which was proved by spreaded signals and an additional Roman-band in the spectra. Additionally, a remarkable amount of silicon was found in the material. The silicon and carbon domains are extremely fine dispersed and properly originated from the molecule structure of the polysilane. An estimation of the particle dimensions is performed. Furthermore a description of the relation between molecular structure of the poly(methylchlorosilane) and the silicon carbide is given in the paper.

Crystallization kinetics of amorphous silicon carbide derived from polymeric precursors

Phase separation processes like nucleation, crystallization and degradation of a polymer-derived amorphous silicon carbide precursor were quantified by means of thermoanalytical methods (DTA/TG). It is shown that the crystal size of the nanostructure could be controlled and limited by nuclei-inducing heat treatments. Furthermore, the justification for application of the JMAK theory was partially given. A comparison to amorphous SiC processed by other means (ion implanted) was drawn and reveals surprising similarities.