Nitrides and Carbides

One interesting approach that has been suggested is to improve the corrosion resistance of carbon blacks by coating them with WC.i Depositing Pt onto WC-treated carbon has been shown to improve cycle degradation resistance to 1.8 V. 

Titanium carbide has also been widely studied. Vinod and Frost prepared relatively high surface area forms of TiC (25-125 m g i) and showed lower corrosion currents at 1.0 V in 100% H3PO4 at 200°C than graphitized XC72. ° The oxygen reduction specific activity of Pt/TiC was superior to that of Pt/C, although this may have been influenced by the larger Pt particles deposited onto the TiC. 

Pt on TiC and TiN has also been reported to show good cycle stability (to 1.2 V), especially when mixed with uncatalyzed carbon.i° Steady-state corrosion tests of Pt/TiC at 1.2 and 1.4 V showed gradual oxidation to Ti02, although this was not dependent on potential. TiC has also been used by 3M as a support for a series of non-PM catalysts and has showed very stable performance over 1,000 h at 0.6 V. However, the overall activity was four times lower than that of an equivalent catalyst on carbon. 

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Carbon, Carbides, and Nitrides. Carbon (graphite) is a good thermal and electrical conductor. It is not easily wetted by chemical action, which is an important consideration for corrosion resistance. As an important stmctural material at high temperature, pyrolytic graphite has shown a strength of 280 MPa (40,600 psi). It tends to oxidize at high temperatures, but can be used up to 2760°C for short periods in neutral or reducing conditions. The use of new composite materials made of carbon fibers is expected, especially in the field of aerospace stmcture. When heated under... 

Table 3. Physical Properties of Titanium Borides, Carbides, and Nitrides ...

Dry hydrogen chloride gas readily attacks solid beryllium above about 500° C with the formation of volatile beryllium chloride. Beryllium carbide and nitride are similarly attacked, but not beryllium oxide this behaviour is of use in one method for the determination of beryllium-oxide in metallic beryllium. 

Similar free-energy diagrams, which can be interpreted in exactly the same way, have been constructed for sulphides , carbides and nitrides (Figs. 7.56 to 7.58). 

There have been few satisfactory demonstrations that decompositions of hydrides, carbides and nitrides proceed by interface reactions, i.e. either nucleation and growth or contracting volume mechanisms. Kinetic studies have not usually been supplemented by microscopic observations and this approach is not easily applied to carbides, where the product is not volatile. The existence of a sigmoid a—time relation is not, by itself, a proof of the occurrence of a nucleation and growth process since an initial slow, or very slow, process may represent the generation of an active surface, e.g. poison removal, or the production of an equilibrium concentration of adsorbed intermediate. The reactions included below are, therefore, tentative classifications based on kinetic indications of interface-type processes, though in most instances this mechanistic interpretation would benefit from more direct experimental support. 

Wicks, C. E. and Block, F. E. "Thermodynamic Properties of 65 Elements - Their Oxides, Halides, Carbides, and Nitrides", Bureau of Mines, Bulletin 605, 1963. 

HANDBOOKOF REFRACTORY CARBIDES AND NITRIDES by Hugh O. Pierson... 

Many CVD reactions are being investigated for the deposition of carbides and nitrides, particularly for titanium nitride for semiconductor applications, such as diffusion barrier. The following is a summary of the metallo-organic precursors and deposition condition presently used in development or production of these materials. 

Pierson, H., Handbook of Refractory Carbides and Nitrides, Noyes Publications, Park Ridge, NJ (1996)... 

Futamoto, M., Yuito, I, andKawabe, U., Hafnium Carbide and Nitride Whisker Growth by Chemical Vapor Deposition, /. Cryst. Growth, 61(l) 69-74 (Jan./Feb. 1983)... 

The CVD coating materials for wear and corrosion resistance consist mostly of carbides and nitrides and, to a lesser degree, borides. Table 17.1 compares the relative properties of these materials. 

Coatings of refractory carbides and nitrides and more recently carbon and DLC are used extensively in decorative applications on jewelry, eyeglasses, and similar products in attractive colors such as gold (TiN) and metallic grey, or charcoal (DLC, CrN, and TaN). They provide a surface which is hard and wear resistant, sweat resistant and, in the case of gold, less costly. They are usually applied by cathode sputtering and less frequently by CVD. 

Thermal spray is a well-established, relatively low-cost, industrial processes which is used widely for the deposition of metals and compounds, including the refractory carbides and nitrides. Examples are coatings of tungsten carbide with a cobalt binder which are of major industrial importance. PI... 

Borides, in contrast to carbides and nitrides, are characterized by an unusual structural complexity for both metal-rich and B-rich compositions. This complexity has its origin in the tendency of B atoms to form one- two-, or three-dimensional covalent arrangements and to show uncommon coordination numbers because of their large size (rg = 0.88 10 pm) and their electronic structure (deficiency in valence electrons). The structures of the transition-element borides are well established " . 

In contrast to the carbides and nitrides, there are a large variety of formulas and structure types for borides (from M4B to MB 5). Although it is possible to establish a comparison between metal-rich borides and carbides, B-rich borides have no counterpart in the carbides. 

The carbides and nitrides of the elements Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th, and U are considered to be typical interstitial compounds. Their compositions frequently correspond to one of the approximate formulas M2X or MX. As a rule, they are nonstoichiometric compounds with compositions ranging within certain limits. This fact, the limitation to a... 

A thorough review of the intermetallic compounds of mechanical interest has been written by Westbrook (1993). The borides, carbides, and nitrides will be considered separately in Chapter 10. 

The small atoms at the center of the first row of the Periodic Table (B, C, N, O, and to a lesser extent Al, Si, and P) can fit into the interstices of aggregates of larger transition metal atoms to form boride, carbide, and nitride compounds. These compounds are both hard and moderately good electronic conductors. Therefore, they are commonly known as hard metals (Schwarzkopf and Kieffer, 1953). 

The structures of the prototype borides, carbides, and nitrides yield high values for the valence electron densities of these compounds. This accounts for their high elastic stiffnesses, and hardnesses. As a first approximation, they may be considered to be metals with extra valence electrons (from the metalloids) that increase their average valence electron densities. The evidence for this is that their bulk modili fall on the same correlation line (B versus VED) as the simple metals. This correlation line is given in Gilman (2003). 

Nitrides are closely related to carbides. Several of them have the same NaCl crystal structure, and similar lattice parameters. Also, the carbide and nitride of the same metal are mutually soluble. Their hardnesses are similar. 

Leclercq, L., Almazouari, A., Dufour, M., and Leclercq, G. 1996. Carbide-oxide interactions in bulk and supported tungsten carbide catalysts for alcohol synthesis. In Chemistry of transition metal carbides and nitrides, ed. S. T. Oyama, 345-61. Glasgow Blackie. 

Hagg, G. 1931. Regularity in crystal structure in hydrides, borides, carbides and nitrides of transition elements. Z. Physik. Chem. 12B 33-56. 

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