Silicon carbide impurities

Pure silicon carbide is colorless, but iron impurities normally impart an almost black color to the crystals. Carborundum is an excellent abrasive because it is very hard, with a diamondlike structure that fractures into pieces with sharp edges (Fig. 14.43). 

In the drying of compound intermediates of refractory and reactive metals, particular attention is given to the environment and to the materials so that the compound does not pick up impurities during the process. A good example is the drying of zirconium hydroxide. After the solvent extraction separation from hafnium, which co-occurs with zirconium in the mineral zircon, the zirconium values are precipitated as zirconium hydroxide. The hydroxide is dried first at 250 °C for 12 h in air in stainless steel trays and then at 850 °C on the silicon carbide hearth of a muffle furnace. 

Resistivity measurements of doped, alpha-silicon carbide single crystals from —195 to 725°C showed a negative coefficient of resistivity below room temperature, which gradually changed to positive above room temperature (45). The temperature at which the changeover occurred increased as the ionization of the donor impurity increased. This is believed to be caused by a change in conduction mechanism. 

Semiconducting Properties. Silicon carbide is a semiconductor it has a conductivity between that of metals and insulators or dielectrics (4,13,46,47). Because of the thermal stability of its electronic structure, silicon carbide has been studied for uses at high (>500° C) temperature. The Hall mobility in silicon carbide is a function of polytype (48,49), temperature (41,42,45—50), impurity, and concentration (49). In n-type crystals, activation energy for ionization of nitrogen impurity varies with polytype (50,51). 

Small changes in impurity content did not affect this rate but the presence of water vapor and changes in partial pressure of oxygen were critical (61,62). Steam and various impurities and binders also affect the oxidation of silicon carbide (63). Differences have been observed in oxygen adsorption on the different SiC crystal faces (64). 

A fresh surface of silicon carbide is thus constandy being exposed to the oxidizing atmosphere. Active oxidation takes place at and below approximately 30 Pa (0.23 mm Hg) oxygen pressure at 1400°C (66). Passive oxidation is determined primarily by the nature and concentration of impurities (67). 

Ferrosilicon - [CARBON - CARBON AND ARTIFICIALGRAPHITE - APPLICATIONS OF BAKED AND GRAPHITIZED CARBON] (Vol 4) - [SILICON AND SILICON ALLOYS - CHEMICAL AND METALLURGICAL] (Vol21) - [METALLURGY - EXTRACTIVE METALLURGY] (Vol 16) - [MINERALS RECOVERY AND PROCESSING] (Vol 16) - [SILICON COMPOUNDS-SILANES] (Vol 22) -as calcium carbide impurity [CARBIDES - CALCIUM CARBIDE] (Vol 4)... 

Aluminum is a constituent of many minerals, including clay (ka-olinite), mica, feldspar, sillimanite, and the zeolites. Some of these minerals are discussed under the chemistry of silicon, in Chapter 31. Aluminum oxide (alumina), occurs in nature as the mineral corundum. Corundum is the hardest of aU naturally occurring substances except diamond it scratches all other minerals, but is itself scratched by diamond, and also by the artificial substances boron carbide, and silicon carbide, SiC. Corundum and impure corundum (emery) are used as abrasives. 

For many traditional ceramics such as structural elements (tiles, bricks, etc.), white-wares, (tableware, sanitaryware, etc.), and common refractories, the raw materials are naturally occurring minerals, and moderate levels of impurities are tolerated. More specialized technical ceramics such as electronic ceramics (substrates, electronic packages, capacitors, inductors, etc.) or high performance structural ceramics (silicon carbide, silicon nitride, etc.) demand low or controlled levels of impurities and make use of higher purity powders often made by more specialized techniques. 

The form that silicon carbide takes depends on many factors including thermal history, impurity type and level, and environment. The p form is generally felt to be the stable phase at low temperatures, whereas the a form is the high-temperature form. There are many exceptions to the rule, as the conversion to a from /3 and the converse have been reported. The stability and transformations of the various polytypes vary among themselves and constitute a subject that is too broad for this effort. The basic a and p descriptors will be used for the remainder of this section. 

Docekal B. and Krivan V. (1992) Direct determination of impurities in powered silicon carbide by electrothermal atomic absorption spectrometry using the slurry sampling technique, J Anal At Spectrom 7 521-528. 

Docekal B., Broekaert J. A. C., Graule T., Tschopel P. and Tolg G. (1992) Determination of impurities in silicon carbide powders, Fresenius J Anal Chem 342 113-117. 

The acceptor impurities have a much weaker dependence of solubility on growth rate than nitrogen [39]. This is consistent with the fact that the diffusion mobility is higher for the acceptors in silicon carbide (see Chapter 7). In contrast to the donors, acceptor impurities tend to increase their solubility at elevated temperatures [10,40,41]. The increase of growth... 

Hartman, J. S., Richardson, M. F., Sherriff, B. J., and Winsborrow, B. G., Magic angle spinning NMR studies of silicon carbide Polytypes, impurities, and highly inefficient spin-lattice relaxation, J. Am. Chem. Soc., 109, 6059 (1987). 

---