Carbides covalent

When carbon forms compounds with other atoms having rather high electronegativity (Si, B, etc.), the bonds are considered to be covalent. The compounds formed, especially SiC, have the characteristics of being hard, unreactive refractory materials. Silicon carbide has a structure similar to diamond, and it is widely used as an abrasive material. It is prepared by the reaction of Si02 with carbon  

The difference in electronegativity between the two elements of the covalent carbides is small. The carbon atom is only slightly smaller than the other atom. The bonding is essentially covalent.1 1 Only two covalent carbides, silicon carbide and boron carbide, fully meet the refractory criteria. Other carbides such as beryllium carbide, Be2C, are only partially covalent and, while they have a high melting point, are generally not chemically stable and are not considered here. 

---

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 ... 

Carbon is the only Group 14/IV element that forms both monatomic and polyatomic anions. There are three classes of carbides saline carbides (saltlike carbides), covalent carbides, and interstitial carbides. The heavier elements in Group 14/IV form polyatomic anions, such as Si44 and Sn52, in which the atoms form a tetrahedron and trigonal bipyramid, respectively. 

The covalent carbides include silicon carbide, SiC, which is sold as carborundum ... 

Carbon forms ionic carbides with the metals of Groups 1 and 2, covalent carbides with nonmetals, and interstitial carbides with d-block metals. Silicon compounds are more reactive than carbon compounds. They can act as Lewis acids. 

The two covalent carbides have low density, low atomic weight, and useful semiconductor properties. They are extremely hard and strong materials which exhibit typical ceramic characteristics. 

Silicon carbide (SiC) is a major industrial material with a considerable number of applications. CVD plays a significant role in its development and production, SiC is a covalent carbide with two phases a and [3. The phase of major interest here is pSiC, which has a cubic zinc blend structure. It is the one reported here. 

The three covalent nitrides have the following common features and are in many ways similar to the covalent carbides reviewed in Ch. 9 ... 

There are three classes of carbides saline carbides (or saltlike carbides), covalent carbides, and interstitial carbides. 

In carbides, carbon is bound to elements with lower or similar EN-values. We distinguish three types of carbides. The salt-like carbides with elements from groups 1, 2 and 3 are decomposed by water A14C3 +12 H20 — 4 Al(OH)3 + 3 CH4. In addition, there are the covalent carbides like SiC and B4C and a intermediate group with most transition metals. In the intermediate group C atoms are located in the octahedral cavities of metal close packings. The melting points vary from 3000 to some extreme values of about 4800 °C and their hardness lies between 7 and 10 on the Mohs scale. Furthermore, the... 

As with the hydrides (Chap. 2), the carbides are divided into three classes—the covalent, the saltlike, and the metallic (or interstitial). The volatile covalent carbides (for example, CC14, (CN)2, CH4, and CS2) are discussed elsewhere of the nonvolatile covalent carbides, silicon carbide (carborundum, SiC), is by far the most important. Although there are three known crystal forms of this compound, we may, for simplification, imagine it as a diamond structure in which every alternate carbon atom is replaced by a silicon atom. Thus it is not surprising that this compound is almost as hard and chemically inert as is diamond itself. 

Carbides can be covalent or metal-like, the most important of the covalent carbides being SiC which like carbon crystallizes in both hexagonal and cubic structures. However, contrary to carbon, the basal planes in the hexagonal structure and the (111) faces of the cubic variant are linked by chemical bonds, so the corresponding surface energies should be about 103 rather than 102 mJ/m2. Estimated values for the surface energy of both faces are close to 1500 mJ/m2 (Appendix F). 

Covalent Carbides. Although other carbides (e.g., Be2C) are at least partially covalent, the two elements that approach carbon closely in size and electro-... 

Covalent carbides are formed by metals with 1 values similar to that of C, such as Si and B. 

Silicon carbide, SiC carborundum), an extremely hard covalent carbide and an excellent abrasive, is produced by the reaction of silicon dioxide with carbon at 2000°C ... 

Refractory metal carbides (TaC, WC, M02C) and some covalent carbides (SiC, B4C) have a positive effect on diamond nucleation, while effects of ionic carbides (AI4C3, liquid salts, etc.) are still less known.l ... 

Covalent carbides, which have giant-molecular structures, as in silicon carbide (SiC) and boron carbide (B4C3). These are hard high-melting solids. Other covalent compounds of carbon (CO2, CS2, CH4, etc.) have covalent molecules. 

Covalent carbides are formed by boron and silicon. Silicon carbide (SiC), known as Carborundum , is used as an abrasive and in cutting tools. Almost as hard as diamond. Sic has a diamondlike structure with alternating Si and C atoms. 

Figure 9. Different grain size effects in the hardness of ceramics with highly ionic (AI2O3) and highly covalent (carbides) characters of bonding.

---