Intermediate carbides

Crystal Structure and Lattice Parameters (nm) Orthorhombic, a = 0.283, b = 0.554, c = 1.1470 Cr3C2 is an intermediate carbide having carbon chains with C-C distance approximately 0.165 nm running through distorted metal lattice where the Cr atoms are at the corners of trigonal prisms and the carbon atoms in the center of the prisms.i li" ... 

Carbides. As might he expecled from its position in the periodic table, carbon forms binary compounds with the metals in which it exhibits a negative valence, and binary compounds with the non-metals in which it exhibits a positive valence. A convenient classification of the binary compounds of carbon is into ionic or salt-like carbides, intermediate carbides, interstitial carbides, and cuvalent binary carbon compounds. 

The fomation of carbon on iron and iron-copper catalysts by the reaction 2C0 = C02+C has been studied by several investigators (70-73). The most significant result of this work (in so far as the Fischer-Tropsch synthesis is concerned) is the fact that neither an iron-free nor a copper-free carbon deposit was obtained. The data show that cai-bon is deposited in the crystal lattice of the catalyst and the inability to obtain a copper-free carbon deposit from tests with an iron-copper catalyst shows that iron carbonyl formation will not explain the results. It is very probable that carbon is formed from carbon monoxide b3 way of iron carbide as an intermediate. Carbidic carbon diffuses rapidly throughout the crystal lattice and subsequently decomposes to yield elemental carbon, thus disrupting the lattice structure. 

Since 1958, seven representatives of the intermediate carbides (Spedding et al. 1958), ScijCig (Jedlicka et al. 1971), Y15C19 (Bauer and Nowotny 1971), ErijCig (Bauer 1974), YbijCig (Haschke and Eick 1970a, Bauer and Bienvenu 1980, Hajek et al. 

As described above, in the dicarbides and the sesquicarbides of the rare earth elements there are pairs of carbon atoms, while in the hypocarbides, either the cubic structure or the trigonal one, the carbon atoms are isolated and no longer appear as pairs. In the intermediate carbides, R15C19, the tetragonal structure is made up of distorted metal octahedra, carbon pairs and also single carbon atoms. 

While the dominant substrate for low pressure growth of diamond films is single crystal silicon, several different materials such as Ta, Mo, W, Cu, Au, Ni, SiC, Si02, and Si3N4 have been coated with polycrystalline diamond or diamond-like deposits. The nucleation rates and the adhesion of the generated films vary with the type of substrate material, which is related to the formation of intermediate carbide layers such as SiC or WC. 

Intermediate carbides (formed by transiton metals of Groups VII and VIII)... 

Coordination Number (CN)= 12 C = covalent carbide IM = intermediate carbide S = salt-like carbide... 

Chromium carbide, with a carbon/metal atomic-radii ratio of 0.61, is a borderline case and, strictly speaking, belongs to the intermediate class of carbides (reviewed in Ch, 2, Sec. 5.3). Yet, unlike other intermediate carbides, it meets the refractory criteria and is a material of major industrial importance. For these reasons, it is included in this book (see Ch. 7). 

The late-transition metals (Group VII and VIII) either do not form nitrides at all, such as the precious metals, or else form nitrides with intennediate (distorted) interstitial structures. These materials decompose readily and are not chemically stable. Examples are manganese, iron, cobalt, and nickel nitrides. In this respect, they are similar to the intermediate carbides (see Ch. 2, Sec. 6.3). 

The nucleation rate and the adhesion vary with the nature of the substrate and appear to be related to the ability of the substrate material to form an intermediate carbide. Surface treatments such as etching or mechanical working (scratching with a diamond powder or diamond polish) help promote adhesion. 

Substrate Materials. Cemented tungsten carbide (WQ is a major substrate cutting tool material. It has been successfully coated with CVD diamond with good adhesion especially with an intermediate carbide-former layer.l JI ... 

Of these producers, Atochem, Degussa, and Daicel are reported to be in the merchant acrolein business. Union Carbide suppHes only the acrolein derivative markets. Rhc ne-Poulenc also produces acrolein, primarily as a nonisolated intermediate to make methionine. A number of other small scale plants are located worldwide which also produce acrolein as an intermediate to make methionine. 

Calcium carbide has been used in steel production to lower sulfur emissions when coke with high sulfur content is used. The principal use of carbide remains hydrolysis for acetylene (C2H2) production. Acetylene is widely used as a welding gas, and is also a versatile intermediate for the synthesis of many organic chemicals. Approximately 450,000 t of acetylene were used aimuaHy in the early 1960s for the production of such chemicals as acrylonitrile, acrylates, chlorinated solvents, chloroprene, vinyl acetate, and vinyl chloride. Since then, petroleum-derived olefins have replaced acetylene in these uses. 

Most hafnium compounds have been of slight commercial interest aside from intermediates in the production of hafnium metal. However, hafnium oxide, hafnium carbide, and hafnium nitride are quite refractory and have received considerable study as the most refractory compounds of the Group 4 (IVB) elements. Physical properties of some of the hafnium compounds are shown in Table 4. 

P/M Tool Steels. In conventionally produced high alloy tool steels (slowly cooled cast ingots), carbide tends to segregate (48). Segregated clusters of carbide persist even after hot working, and cause undesirable effects on tool fabrication and tool performance. P/M tool steels, on the other hand, provide very fine and uniform carbides in the compact, the final bar stock, and the tools. Several tool steel suppHers consoHdate gas-atomized tool steel powder by HIP to intermediate shapes, which are then hot-worked to final mill shapes. Water-atomized tool steel powder is also available (see also T OOL materials). ... 

The most important process to produce 1-naphthalenol was developed by Union Carbide and subsequently sold to Rhc ne-Poulenc. It is the oxidation of tetralin, l,2,3,4-tetrahydronaphthalene/719-64-2] in the presence of a transition-metal catalyst, presumably to l-tetralol—1-tetralone by way of the 1-hydroperoxide, and dehydrogenation of the intermediate ie, l-tetralol to 1-tetralone and aromatization of 1-tetralone to 1-naphthalenol, using a noble-metal catalyst (58). 1-Naphthol production in the Western world is around 15 x 10 t/yr, with the United States as the largest producer (52). 

Cast-Cobalt Alloys. Cast-cobalt alloys were introduced about the same time as HSS for cutting tool appHcations. Popularly known as StelHte tools, these materials are Co-rich Cr—W—C cast alloys having properties and appHcations in the intermediate range between HSS and cemented carbides. 

The reaction of metals with gas mixtures such as CO/CO2 and SO2/O2 can lead to products in which the reaction of the oxygen potential in the gas mixture to form tire metal oxides is accompanied by the formation of carbon solutions or carbides in tire hrst case, and sulphide or sulphates in the second mixture. Since the most importairt aspects of this subject relate to tire performairce of materials in high temperature service, tire reactions are refeiTed to as hot corrosion reactions. These reactions frequendy result in the formation of a liquid as an intermediate phase, but are included here because dre solid products are usually rate-determining in dre coiTosion reactions. 

At the end of the 1970s considerable interest developed in what became known as linear low density polyethylenes (LLDPE) which are intermediate in properties and structure to the high pressure and low pressure materials. While strictly speaking these are copolymers it is most convenient to consider them alongside the homopolymers. The LLDPE materials were rapidly accepted by industry particularly in the manufacture of film. The very low density polyethylenes (VLDPE) introduced by Union Carbide in 1985 were closely related. 

The first commercial polymer Table 21.3, II) was offered in 1965 by Union Carbide as Bakelite Polysulfone, now renamed Udel. In 1967 Minnesota Mining and Manufacturing introduced Astrel 360 Table 21.3, V), which they referred to as a polyarylsulfone. In 1972 ICI brought a third material onto the market which they called a polyethersulphone (III) and which they then marketed as Victrex. They also introduced a material intermediate between III and V known as Polyethersulphone 720P (IV) but which has now been withdrawn. In the late 1970s Union Carbide introduced Radel (VI), which has a higher level of toughness. Around 1986 Union Carbide sold their interest in polysulphones to Amoco. In addition the Astrel materials were produced by Carborundum under licence from ICI. 

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