Processing, properties and applications of silicon carbide - Term Paper Example

?Processing, properties and applications of silicon carbide The unique properties of silicon carbide have made it the material of choice in many industrial as well as everyday applications. These properties are intrinsically linked to the structure of silicon carbide. Results from various research laboratories reveal potential applications, which have led to huge demand for silicon carbide above the amount offered by nature. Thus synthetic approaches have been developed to meet market demand. This term paper investigates the processing, properties and applications of silicon carbide. Table of Content Introduction Processing of Silicon Carbide Properties of Silicon Carbide Applications of Silicon Carbide Conclusion Introduction Silicon carbide, also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. Historically, natural SiC was first found in meteorite in 1893 by Dr. Ferdinand Henri Moissan (Moissan 773), thus natural SiC, an extremely rare mineral, came to be known as moissanite. Although rare on earth, SiC is very common in outer space, where it is found around carbon-rich stars in the form of stardust. The natural SiC found in meteorites and that found in space are the beta-polymorph suggesting the possibility of having the same origin. This is substantiated by analysis of SiC grains found in the Murchison carbonaceous chondrite meteorite that revealed anomalous isotopic ratios of carbon and silicon, indicating an origin from outside the solar system; indeed, 99% of these SiC grains originate around carbon-rich asymptotic giant branch stars (Alexander 715). Due to the extreme rarity of natural SiC, virtually all the SiC sold in the world market is synthetic in origin. The simplest and the commonly used approach for synthesis of SiC is the Acheson process. This approach offers the possibility of large quantity synthesis of SiC of adequate quality that it suitable for electronic application and for use in other applications. When high purity SiC is desired, methods such as the Lely process, modified Lely process, chemical vapor deposition and thermal decomposition of poly(methylsilyne) are employed. The structure and properties of SiC make it suitable for many applications. This term paper investigates the processing, properties and applications of SiC. Processing of silicon carbide Acheson process As mentioned above, natural SiC is an extremely rare mineral. Thus synthetic methods have been developed to meet the demand for SiC. Among these method, Acheson process is the widely used (Knippenberg 161). The Acheson process involves the high temperature (1600 – 2700 °C) heating of a mixture of silica, carbon, sawdust and common salt (e.g. 50% silica, 40% coke, 7% sawdust and 3% common salt) in an Acheson graphite electric resistance furnace (Knippenberg 161). The active ingredients in the mixture are the silica, in the form of silica sand and the coke. By making the mixture porous, the sawdust facilitates the removal of carbon (II) oxide formed during the reaction process. High pressures of gas may locally be built up to form voids and channels to more porous parts of the mixture to eventually find its way out. The common salt serves as a purifier of the mixture. The chlorine will react with impurities to form volatile chlorides which escape. The heating is accomplished by means of graphite resistor placed centrally in the furnace. The mixture is place around the resistor. The purity of the SiC, which depends on the distance from the resistor, is adequate for many applications. Nitrogen and aluminum are common impurity in the SiC obtained by the Acheson process. These impurities affect the electrical conductivity of silicon carbide. Lely process High purity SiC can be obtained by the Lely process, which involves sublimating SiC powder in argon atmosphere at 2500 °C followed by re-depositing into flake-like single crystals, at a slightly colder substrate (Lely 229). This process is similar to the Acheson process in the voids and channels formed by built-up gas pressure. However, the two approaches are fundamentally different. In the Lely process, SiC is lumped between two concentric graphite tubes; thereafter the inner tube is withdrawn leaving a cylinder of SiC lumps inside the outer graphite tube also known as the crucible. A lid made of graphite or silicon is used to close the crucible, which is placed inside an oven maintained at 2500 °C, atmospheric pressure and in an atmosphere of argon. Under these conditions, the SiC sublimes to form single crystals of SiC of high purity. Despite the merits of this method, the method is limited by low yield, irregularity of the size of the crystals, lack of control of the shape of the crystal. Modified Lely process Notwithstanding the high purity and crystalline property of the SiC obtained by the Lely process, it is not considered the ultimate industrial method for the production of SiC given that the yield is low and the size of the crystals is irregular. The modified Lely process, which is a seeded sublimation technique and involves induction heating of SiC in a graphite crucible, eliminate the problem of poor yield though it lowers the degree of crystalline (Tairov 209). The modified Lely process is similar to the Lely process except that the operating pressure is below atmospheric pressure while a temperature gradient in the order of 20 – 40 °C/cm is applied across the length of the crucible. At the high temperature, usually 2200 °C, the volume inside the crucible is filled with vapor of the sublimated SiC to form different molecular species such as Si2C, SiC2, Si2 and Si. With an appropriately selected temperature gradient, the crystals grow in the coldest part of the crucible. The growth rate of the SiC crystal has been reported to be a function of temperature, the pressure, the temperature gradient, and the distance of the seed from the source. As a result of mass transport, diffusion of the species from the source to the seed, specifically determine the growth rate. A low total pressure will enhance diffusion and increase the growth rate. Epitaxial Growth Different epitaxial growth techniques have been use to grow silicon carbide. For instance, the cubic form of silicon carbide has been grown by the expensive process of chemical vapor deposition (CVD) (Byrappa 180). The principle of the CVD, which is carried out in the pressure regime of several mbar to atmospheric pressure, involves the transporting reactive compounds (precursors) by a carrier gas to a hot zone where the precursors will thermally decompose into atoms or radicals of two or more atoms which may diffuse down onto a substrate and produce an epitaxial film. Several parameters must be correctly chosen in order to achieve desirable results. These include the precursors, carrier gas, the flow of the gases, and the material used for the hot parts of the reactor. For instance, the flow of gases must be laminar and the material used for the hot parts of the reactor must not contaminate the system (Byrappa 180). Properties of silicon carbide The properties of SiC cannot be discussed without reference to the different polytypes of silicon carbides. SiC exists in about 250 crystalline forms known as polytypes (Cheung 3). Polytypes are variation of the same chemical compound that are identical in two dimensions but differs in the third dimension. As shown in Figure 1, three major polytypes, namely (?)3C-SiC, 4H-SiC, and (?)6H-SiC, exist for silicon carbide. All polytypes have a hexagonal frame with a carbon atom situated above the center of a triangle of Si atoms and underneath a Si atom belonging to the next layer. Figure 1 a-c. Structure of major SiC polytypes; a, (?)3C-SiC; b 4H-SiC; and c, (?)6H-SiC (Cheung 3). a b c The fundamental difference between these polytypes is the stacking of double layers of Si and C atoms and this difference is responsible for the difference in electronic and optical properties of the polytypes. The bandgaps of the different polytypes differs, ranging from 2.39 eV for 3C-SiC polytype and 3.33 eV for the 2H-SiC polytype at liquid helium temperature. Generally, all SiC polytypes are characterized by high sublimation temperature, high chemical inertness, high thermal conductivity, high electric field breakdown strength and high maximum current density (Bhatnagar 645). In addition, it has very low thermal expansion coefficient and experiences no phase transitions that would cause discontinuities in thermal expansion. Properties such as electric field breakdown strength, the saturated drift velocity and the impurity ionization energies are all specific for the different polytypes. Some of these differences are shown in Table 1. In the case of 6H-SiC for instance, the electric field breakdown strength is an order of magnitude higher than Si and the saturated drift velocity of the electrons is even higher than that of GaA. SiC is a semiconductor. It can be doped n-type by nitrogen or phosphorus and p-type by aluminum, boron, gallium or beryllium. Heavy doping with boron, aluminum or nitrogen results in metallic conductivity. In addition, superconductivity has been detected in 3C-SiC doped with aluminum, 3C-SiC doped with boron and 6H-SiC doped with boron at the same temperature of 1.5 K. Applications of silicon carbide The unique properties of SiC make it the material of choice in many applications (Park 20). For instance, its high sublimation temperature of approximately 2700 °C, its ability to remain stable at any known pressure combined with its chemical inertness makes it suitable for bearings and furnace parts. The possibility of being employed as a semiconductor material in electronics is being explored lately. Indeed, its high thermal conductivity; its high electric field breakdown strength and its high maximum current density make it more promising than silicon for high-powered devices. Table 1. Properties of major silicon carbide polytypes (Park 20) Polytypes (?)3C-SiC 4H-SiC (?)6H-SiC Crystal structure Zinc blende (cubic) Hexagonal Hexagonal Space group T2d-F43m C46v-P63mc C46v-P63mc Pearson symbol cF8 hP8 hP12 Lattice constants (A) 4.3596 3.0730 3.0730 Density (g/cm3) 3.21 3.21 3.21 Bandgap (eV) 2.36 3.23 3.05 Bulk modulus (GPa) 250 220 220 Thermal conductivity (W/(cm·K)) 3.6 3.7 4.9 SiC has also found application as an abrasive and cutting tool due to its durability, low cost and hardness. For instance, particles of SiC are laminated to paper to create sandpapers. Furthermore, SiC is also use in production of structural material, such as bulletproof vest, and automobile parts, such as high performance ceramic brake discs. The voltage dependent resistance of SiC accounts for its use in electric power systems as lightening arresters. In the electronic industry, SiC is used for ultrafast, high-voltage Schottky diodes, MOSFETs and high temperature thyristors for high-power switching. The high sublimation temperature of SiC makes them good-to-excellent heating element. In fact, SiC offers higher operating temperatures than metallic heaters. Thus SiC elements are used today in the melting of non-ferrous metals and glasses, heat treatment of metals, float glass production, production of ceramics and electronics components, and igniters in pilot lights for gas heaters. SiC is also used as a fuel in high temperature gas cooled nuclear reactors and in steel making. Other uses include catalyst support, graphene production, and in jewelery. Conclusion Indeed, SiC offers lots of potentials, which has made it an ideal material for many applications. These potentials are linked to the unique structure of SiC. With advances in research, more uses will be discovered and better synthetic methods will be developed. 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