Group VI carbides

Table 6.3 (after Ref 1) Thermodynamic Values of Group VI Carbides. 

Specific Heat. The specific heat (C of the Group VI carbides increases essentially linearly with increasing temperature. Figure 6.1 shows this relationship for WC.I I... 

Thermal Expansion. Like the carbides of Groups IV and V, the Group VI carbides have a relatively low thermal expansion which varies with temperature as shown in Fig. 6.2.Pi For discussion see Ch. 4, Sec. 2.4. 

Figure 6.2 Linear thermal conductivities of Group VI carbides as a function oftenqjeiatuie.t I...

The mechanical properties of Group VI carbides are summarized in Table 6.5. The values are average values reported in the recent litera-turel lPlI W7] (see Sec. 4.1 of Ch. 4). 

The Group VI carbides are chemically stable and have a chemical resistance similar to that of the Group IV carbides. 1 1... 

Ethylene oxide (qv) was once produced by the chlorohydrin process, but this process was slowly abandoned starting in 1937 when Union Carbide Corp. developed and commercialized the silver-catalyzed air oxidation of ethylene process patented in 1931 (67). Union Carbide Corp. is stiU. the world s largest ethylene oxide producer, but most other manufacturers Hcense either the Shell or Scientific Design process. Shell has the dominant patent position in ethylene oxide catalysts, which is the result of the development of highly effective methods of silver deposition on alumina (29), and the discovery of the importance of estabUshing precise parts per million levels of the higher alkaU metal elements on the catalyst surface (68). The most recent patents describe the addition of trace amounts of rhenium and various Group (VI) elements (69). 

Carbides oxidize readily although less rapidly than the nitrides but more so than the borides. Oxidation becomes more rapid going from the Group IV carbides (TiC, ZrC, HfC) to those of Group VI (Cr3C7, MoC, WC). In some cases, a protective film of the metal oxide is formed. Such is the case with SiC, as reviewed in Sec. 5.7 below. 

Inorganic non-oxide materials, such as III-V and II-VI group semiconductors, carbides, nitrides, borides, phosphides and silicides, are traditionally prepared by solid state reactions or gas-phase reaction at high temperatures. Some non-oxides have been prepared via liquid-phase precipitation or pyrolysis of organometallic precursors. However, amorphous phases are sometimes formed by these methods. Post-treatment at a high temperature is needed for crystallization. The products obtained by these processes are commonly beyond the manometer scale. Exploration of low temperature technique for preparing non-oxide nanomaterials with controlled shapes and sizes is very important in materials science. 

Another type of LTS catalyst includes nitrides, carbides, and borides of Group VI and Group VIII metals, with molybdenum carbide apparently being the most active of this group. Those are highly experimental catalysts and are not in production. 

Table 2 also shows that the group V transition metal carbides, NbCx and TaC, are less active than the carbides of the group VI metals. At 1223 K, both NbCx and TaCx deactivated rapidly, even at elevated pressure, while catalyst stability and high reactant conversions were achieved at 1373 K with NbCx, where autothermal gas-phase reactions are likely to play a significant role in the reaction and carbon deposition was observed, as is demonstrated by the high H2/CO ratio. The reason for the low activity of these materials is the relative ease of their conversion back to the oxide, i.e. the rate of carbidation over these materials is slower than the rate of oxidation. 

Fig. 6. Carbon K emission bands (peaks normalized) for group VI and higher transition metal carbides compared to the C K band from graphite. The target potential was 4000 V, the beam current was 1.4 mA, and the deviation was 1%.

Results formed the base for the following electrochemical studies of electroreduction processes of Group VI metals. " These studies developed theoretical bases and principles to control electrochemical processes of metals and their compoxmds (carbides, borides, sili-cides) deposition from ionic melts. ... 

As shown in the above table, the Group IV carbides (and Groups V and VI carbides as well) are good electrical conductors and have an electrical resistivity only slightly higher than that of the parent metals, reflecting the metallic character of these compounds. The nitrides and especially the borides have even lower resistivity. The large spread in the reported values may be attributed to differences in composition and the presence of defects and impurities. 

Hardness. Table 5.6 shows that carbides are die hardest, followed by the borides and the nitrides. The Group V carbides have higher hardness than those of Group VI but are not quite as hard as those of Group IV (see Ch. 4, Sec. 4.4 and Ch. 6, Sec. 4.0). This reflects the intermediate strength of M-C bonds found in these carbides. 

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