Carbide silicon

Silicon carbide is covalently bonded with a structure similar to that of diamond. There are two basic structures. One is a cubic form, /i-SiC which transforms irreversibly at about 2000 °C to one of a large number of hexagonal polytypes, and the other is a rhombohedral form also with many polytypes. Both the hexagonal and rhombohedral forms are commonly referred to as a-SiC. 

Pure cubic SiC is a semiconductor with a band gap of approximately 2.3 eV and, as expected, is transparent with a pale yellow appearance in transmitted light. The hexagonal form has a band gap of 3eV. The wide band gap of SiC 

There are three principal methods of manufacturing the types of SiC heating element shown in Fig. 4.1  

Method 1, which accounts for approximately 95% of element manufacture, is the straightforward sintering ( recrystallization ) of a-SiC grit which is formed into a rod or tube and sintered in a carbon furnace at approximately 2500 °C. To give the component the necessary low resistance cold-end terminations the pore volume of a predetermined length of the end sections is infiltrated with silicon or a silicon alloy. 

The above outlines the manufacturing route for one-piece elements. In fact most rod elements are of three-piece construction in which the low-resistance cold end sections (silicon or silicon alloy infiltrated) are manufactured separately from the high-resistance hot centre section. The three sections are then joined by a reaction-bonding process. This is the most economic approach to manufacturing 

Silicon Carbide- and Boron Carbide-Based Hard Materials 

Superhard compounds are obviously formed by a combination of the low atomic number elements boron, carbon, silicon, and nitrogen. Carbon-carbon as diamond, boron-nitrogen as cubic boron nitride, boron-carbon as boron carbide, and silicon-carbon as silicon carbide, belong to the hardest materials hitherto known. Because of their extreme properties and the variety of present and potential commercial applications, silicon carbide (SiC) and boron carbide (B4C) are, besides tungsten carbide-based hard metals, considered by many as the most important carbide materials. 

Berzelius [1] first reported the formation of silicon carbide in 1810 and 1821, but it was later rediscovered during various electrochemical experiments, notably by Despretz [2], Schiitzenberger [3], and Moissan [4]. However, it was Acheson [5] who first realized the technical importance of silicon carbide as a hard material and, believing it to be a compound of carbon and corundum, he named the new substance carborundum . By 1891, Acheson had managed to prepare silicon carbide on a large scale such that, today, it has become by far the most widely used nonoxide ceramic material. 

Due to its great hardness, heat resistance, and oxidation resistance, silicon carbide has become firmly established as an abrasive as well as a raw material for producing refractories such as firebricks, setter tiles, and heating elements. Another major use of silicon carbide is as a siliconizing and carburizing agent in iron and steel metallurgy. 

Ceramics Science and Technology Volume 2 Properties. Edited by Raif Riedel and I-Wei Chen Copyright 2010 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31156-9 

Silicon carbide [409-21-2], chemical formula SiC and relative molar mass 40.097, is an important advanced ceramic with a high melting point (2830 C), a high thermal conductivity (135 Wm K ), and extremely high Mohs hardness of 9. Sihcon carbide is also has a wide band gap for a semiconductor (2.3 eV). The preparation of sihcon carbide involves the reaction of silica sand (SiO ) and carbon (C) at a high temperature (between 1600 and 2500 C). 

The first observation of silicon carbide was made in 1824 by Jons Jacob Berzelius. It was first prepared industrially in 1893 by the American chemist Edward Goodrich Acheson, who patented both the batch process and the electric furnace for making synthetic silicon-carbide powder. In 1894 he established the Carborundum Company in Monongahela City, PA, to manufacture bulk synthetic silicon carbide commercialized under the trade name Carborundum . Silicon carbide was initially used to produce grinding wheels, whetstones, knife sharpeners, and powdered abrasives. Despite being extremely rare in nature, when it occurs as a mineral it is called moissanite after the French chemist Henri Moissan who discovered it in a meteorite in 1905. 

Polymorphism and polytypism. Silicon carbide has two polymorphs. At temperatures above 2000°C alpha silicon carbide (a-SiC), with a hexagonal crystal structure, is the more stable polymorph with iridescent and twinned crystals with a metallic luster. At temperatures lower than 2000°C, beta silicon carbide ((3-SiC) exhibits a face-centered cubic (fee) crystal structure. 

The different polytypes exhibit different electronic and optical properties. The bandgaps at 4.2 K of the different polytypes range between 2.39 eV for 3C-SiC and 3.33 eV for the 2H-SiC polytype. The important polytypes 6H-SiC and 4H-SiC have bandgaps of 3.02 eV and 3.27 eV, respectively. All polytypes are extremely hard, chemically inert, and have a high thermal conductivity. Properties such as the breakdown voltage, the saturated drift velocity, and the impurity ionization energies are all specific for the different polytypes. 

Silicon carbide has long been recognized as an ideal ceramic material for applications where high hardness and stiffness, mechanical strength at elevated temperatures, high thermal conductivity, low coefficient of thermal expansion, and resistance to wear and abrasion are of primary importance. Moreover, because of its low density it offers greater advantages compared to other ceramics. 

Silicon carbide exists in the form of a very large number of crystallographic varieties. The most widespread polytypes are the P-SiC (cubic, low temperature) and a-SiC (hexagonal, high temperatrrre) varieties. SiC powder is densified by reaction sintering reaction, natural or presstue sintering [CHE 80]. 

The process consists of compacting a mixture of SiC and carbon, which is then infiltrated with liquid silicon. During the infiltration heat treatment, the following reaction takes place  

However, some free silicon remains, as it is very difficult to achieve a total reaction. 

Industrially, natural sintering is the most widely used. It requires introducing a small quantity of additives, carbon and boron or carbon and aluminum. The mixture is formed, then the compact treated in inert atmosphere between 2,000 and 2,100°C. The chemical reactions and the derrsification mechanisms are complex and vary with the temperatrrre and the natrrre of the sintering additives. 

The use of pressure sintering or hot isostatic pressing is limited to the production of completely dense pieces with very low additive corrtents. 

In 1891, a small amount of siUcon carbide was produced bypassing a strong electric current from a carbon electrode through a mixture of clay and coke contained in an iron bowl that served as the second electrode (1). The abrasive value of the crystals obtained were recognized and The Carbomndum Company was founded that year (2). About 10 years earlier tetratomic radicals of siUcon (Si2C202, Si2C2N) had been reported (3). That work also produced some SiC. 

The properties of siHcon carbide (4—6) depend on purity, polytype, and method of formation. The measurements made on commercial, polycrystalline products should not be interpreted as being representative of single-crystal siHcon carbide. The pressureless-sintered siHcon carbides, being essentially single-phase, fine-grained, and polycrystalline, have properties distinct from both single crystals and direct-bonded siHcon carbide refractories. Table 1 Hsts the properties of the hiUy compacted, high purity material. 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

Sihcon carbide is well known as a hard material occupying a relative position on Mohs scale between alumina at 9 and diamond at 10 (see Hardness). The average values for Knoop hardness under a load of 100 g are 

Because of high thermal conductivity and low thermal expansion, siUcon carbide is very resistant to thermal shock as compared to other refractory materials. 

While the electrochemical synthesis and deposition of various metal carbides was first reported in 1946 (73), the one and only study on the electrocrystallization of SiC (Elwell et al.. Ref.76) was not reported until 1982. After experimenting with a variety of molten salt mixtures containing 

The overall simultaneous cathodic deposition reactions involved in SiC deposition were 

It has been demonstrated that most of the technologically important semiconductors can be synthesized and deposited as thin films by electrolysis techniques. Byfar.thegreatestamount of effort has gone into the development of electrodeposition methods for low cost silicon solar cells. 

Kunnmann, W., Preparation and Properties of Solid State Materials, (R. A. Leferer, ed.), p. 1, Dekker, New York (1971) 

and Bellavance, D., Preparative Methods In Solid State Chemistry, (P.Hagenmiller,ed.),p. 279, Academic Press, New York (1972) 

The only donor characterized spectroscopically in 3C-SiC is nitrogen on a C site (Nc) [170]. The CB minimum of 3C-SiC is located at the X point of the surface of the BZ so that it is only threefold degenerate, compared to sixfold 

A valley-orbit splitting of the Is state of Nc is apparent from this figure as a temperature raise populates the ls(E) state (a normal ordering of the levels is assumed). The transitions from the ls(E) state are clearly broader than those from ls(Ai). Small sharp lines can also be observed in the two spectra of Fig. 6.10, showing no thermalization effect. They are attributed to an unidentified effective-mass donor with no detectable valley-orbit splitting, denoted EMD in the original reference [170]. 

Below the label Level energy Calculated energies (meV) of the EM donor states in 3C-SiC and semi-empirical energy levels of the excited donor states of Nc and EMD (after [170]) 

The measured Is (Ai) - ls(E) valley-orbit splitting of the Nc donor is 8.36meV so that the ls(E) level energy is 45.83meV, slightly less than the one-valley EM value, but such a situation is also encountered for the Sb and Bi ls(E) levels in silicon (Table 6.5). For EMD, E10 is close to that calculated in the EMA and no valley-orbit splitting is detected. 

For the 4/f-SiC polytype, a detailed study of the donor level classification and selection rules for an EM donor at the hexagonal (h) site has been given by Ivanov et al. [114]. It has been applied to the N donor, for which 10 electronic lines between 38 and 56 meV have been reported by different groups, with an ionization energy Eh(N) of 61.4meV ([114] and references therein). This value of Eh N) contrasts with the value obtained for the ionization energy Ec(N) at the cubic site, which rises to 125.5 meV ([113]. 

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The formation of silicon carbide, SiC (carborundum), is prevented by the addition of a little iron as much of the silicon is added to steel to increase its resistance to attack by acids, the presence of a trace of iron does not matter. (Addition of silicon to bronze is found to increase both the strength and the hardness of the bronze.) Silicon is also manufactured by the reaction between silicon tetrachloride and zinc at 1300 K and by the reduction of trichlorosilane with hydrogen. 

The covalent carbides These include boron carbide B4C and silicon carbide SiC the latter is made by heating a mixture of silica and coke in an electric furnace to about 2000 K ... 

Silicon is important to plant and animal life. Diatoms in both fresh and salt water extract Silica from the water to build their cell walls. Silica is present in the ashes of plants and in the human skeleton. Silicon is an important ingredient in steel silicon carbide is one of the most important abrasives and has been used in lasers to produce coherent light of 4560 A. 

Moissanite, see Silicon carbide Molybdenite, see Molybdenum disulfide Molybdite, see Molybdenum(VI) oxide Molysite, see Iron(III) chloride Montroydite, see Mercury(II) oxide Morenosite, see Nickel sulfate 7-water Mosaic gold, see Tin disulfide Muriatic acid, see Hydrogen chloride, aqueous solutions... 

NaOH or KOH 320-380 Au, Ag, Ni For silicates, silicon carbide, certain minerals... 

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