Processing of Silicon Carbide

The classic method of manufacturing industrial-grade sihcon carbide is the Acheson process, as patented in 1893 (Acheson, 1893). Today, in excess of 800000 tons of SiC are manufactured worldwide via this process, which is based on the carbothermal reduction of Si02 whereby quartz sand and petroleum coke are mixed and heated electrically to a temperature of about 2000 °C. Consequently, the intermediate product Si combines with carbon to form essentially hexagonal a-SiC in one or more of its polytypic structures according to  

The reaction begins at about 1600 °C, but leads only to a crystalline product at temperatures beyond 1900 °C. 

The main disadvantages of the Acheson process include (i) the need for very high temperatures to achieve a complete reaction (ii) the large blocks of SiC product must be broken up, ground and comminuted to yield the classified granu- 

The problem of CO evolution has been addressed by the modern, environmentally compatible ESK process, which reUes on the basic precursor materials of the classic Acheson process but captures the CO within a canopy covering the reaction mixture and funnels it into a gas collection duct. In addition, any unwanted liquid byproducts are collected into a porous (sand) bed beneath the reacting mass. 

A compilation of the properties of SiC monoHthic shapes produced by a variety of densification methods is provided in Table 11.3. 

---

Laine, R.M., Z.F. Zhang, K.W. Chew, M. Kannisto, and C. Scotto. 1995. Synthesis and processing of silicon carbide fibers state of the art. Pp. 1979 1986 in Ceramic Processing Science and Technology, Vol. 51 in Ceramic Transactions, H. Hausner, G.L. Messing, and S. Hirano (eds ). Westerville, Ohio American Ceramic Society. 

P. Colombo, T. E. Paulson, and C. G. Pantano, Atmosphere effects in the processing of silicon carbide and silicon oxycarbide thin films and coatings, J. Sol-Gel. Sci. Technol. 1994,2,601-604. 

Su, Z. M., Tang, M., Wang, Z. C., Zhang, L. T., Chen, L. F., Processing of silicon carbide fibers from polycarbosilane with polypropylene as the additive. Journal of the American Ceramic Society 2010,93(3), 679-685. 

L. Chen, C. Leonelli, T. Manfredini, C. Siligardi, Processing of Silicon Carbide Whisker-Reinforced Glass-... 

Figure 3.2 Chemical potential diagrams for the transport of silicon carbide by chlorine, showing that the much greater stability of SiCU than CCI4 makes this process very inefficient, while the use of HCl as the transporting gas can be operated under optimum conditions...

Another process for silicon carbide fibers, developed by Verbeek and Winter of Bayer AG [45], also is based on polymeric precursors which contain [SiCH2] units, although linear polysilmethylenes are not involved. The pyrolysis of tetramethylsilane at 700°C, with provision for recycling of unconverted (CHg Si and lower boiling products, gave a polycarbosilane resin, yellow to red-brown in color, which was soluble in aromatic and in chlorinated hydrocarbons. Such resins could be melt-spun but required a cure-step to render them infusible before they were pyrolyzed to ceramic... 

The conventional production method for SiC - the reaction of coke and sand (Acheson process) -does not involve soluble or fusible intermediates. For many applications of silicon carbide this fact is not necessarily a disadvantage, but for the preparation of ceramic composites such intermediates are required. 

A review article on the CVD processes used to form SiC and Si3N4 by one of the pioneers in this area, Erich Fitzer [Fitzer, E., and D. Hegen, Chemical vapor deposition of silicon carbide and silicon nitride—Chemistry s contribution to modem silicon ceramics, Angew. Chem. Int. Ed. Engl, 18, 295 (1979)], describes the reaction kinetics of the gas-phase formation of these two technical ceramics in various reactor arrangements (hot wall, cold... 

Manufacture of P-Silicon Carbide. A commercially utilized application of polysilanes is the conversion of some homopolymers and copolymers to silicon carbide (130). For example, polydimethylsilane is converted to the ceramic in a series of thermal processing steps. Silicon carbide fibers is commercialized by the Nippon Carbon Co. under the trade name Nicalon (see Refractory FIBERS). 

In the following sections some examples are given of the ways in which these principles have been utilized. The first example is the use of these techniques for the low temperature preparation of oxide ceramics such as silica. This process can also be used to produce alumina, titanium oxide, or other metal oxides. The second example describes the conversion of organic polymers to carbon fiber, a process that was probably the inspiration for the later development of routes to a range of non-oxide ceramics. Following this are brief reviews of processes that lead to the formation of silicon carbide, silicon nitride, boron nitride, and aluminum nitride, plus an introduction to the synthesis of other ceramics such as phosphorus nitride, nitrogen-phosphorus-boron materials, and an example of a transition metal-containing ceramic material. 

The traditional synthesis route involves the direct reaction of silicon with nitrogen at temperatures above 1,300 °C, or by heating silica with carbon (coke) in a stream of nitrogen and hydrogen at 1,500 °C.41 However, as in the case of silicon carbide, the high processing and fabrication temperatures focused attention on the need for alternative access routes based on preceramic polymers. 

Manufacturing process for silicon carbide whiskers by VLS from rice husk (reproduced by permission of Chapman Hall)56. 

Slip-casting of technical ceramics has been steadily introduced over the past 60 years or so, and now it is standard practice to cast alumina crucibles and large tubes. The process has been successfully extended to include silica, beryllia, magnesia, zirconia, silicon (to make the preforms for reaction-bonded silicon nitride articles) and mixtures of silicon carbide and carbon (to make the preforms for a variety of self-bonded silicon carbide articles). Many metallics and intermetallics, including tungsten, molybdenum, chromium, WC, ZrC and MoSi2, have also been successfully slip-cast. 

For the preparation of technically important metal carbide and metal nitride materials the application of organosilicon compounds as preceramic precursors is advantageous under certain conditions [1-5]. Compared with the conventional metallurgical powder process, one benefit is the utilization of very low process temperatures for the preparation of individual ceramic materials. Another improvement is the high purity of the ceramics obtained from tailor-made preceramic precursors. Usually, after pyrolysis organosilicon compounds afford silicon-containing ceramic powders Likewise, they can also be used under certain conditions for the production of silicon carbide or silicon nitride fibers. 

It is technically easier to manufacture a molded component from SiC-powder by slip casting or dry pressing, working it mechanically and then sintering pressureless at 1950 to 2000°C. Due to the low sintering activity of silicon carbides, such processes have only been recently successfully carried out with the advent of fine particulate SiC-powders (specific surface area > 5 m /g) with low oxygen-contents (< 0.2%). Boron or aluminum and free carbon or boron carbide are added as sintering aids. 

The practical utilization of silicon carbide and silicon nitride ceramics or of SiAlON in the above-mentioned application sectors has steadily increased in recent years. Ca. 400 t/a of SN-powder is currently consumed in the manufacture of SN-components. The main applications are for cutting tools, roller bearings, dosing and deliver pipes for aluminum processing, as well as a multiplicity of other components which enjoy the advantages of SN-ceramics. 

The formation of silicon carbide whiskers can occur through one of three general methods vapor-solid (VS), chemical vapor deposition (CVD), vapor-liquid-solid (VLS). The processes vary widely in the raw... 

---