Composites transition metal carbides

This volume, which is unique in its coverage, provides a general introduction to the properties and nature of transition metal carbides and nitrides, and covers their latest applications in a wide variety of fields. It is directed at both experts and nonexperts in the fields of materials science, solid-state chemistry, physics, ceramics engineering and catalysis. The first chapter provides an overview, with other chapters covering theory of bonding, structure and composition, catalytic properties, physical properties, new methods of preparation, and spectroscopy and microscopy. 

The purpose of this paper is to (1) document published studies of vaporization in vacuum of the Group 4 and 5 transition metal carbides and uranium carbide, and determine the temperature dependence of their equilibrium CVCs (C/MeCVC) and vaporization rates (Vecvc, g/cmzs) (2) document published studies on chemical diffusion of these carbides, and develop and compare data to a model describing the concentration dependence of the chemical diffusion coefficients and (3) develop diffusion-coupled vaporization equations which predict changes of surface composition (Cs, units of C/M and g/cm3 of C), average composition (Cavg units of C/M and g/cm3 of C), and C mass loss (M, units of g/cm2 of C). 

In earlier work, it was found for borides, silicides and nitrides that specific activity, expressed as total rate of methane consumption per unit surface area, plummeted with increasing surface area of the catalyst samples.1718 The same relationship was also found for transition metals carbides (Figure 16.4). It should be noted the dependence of specific activity on surface area rather than catalyst composition is unusual for heterogeneous catalytic reactions. In addition, it can be found that the reaction order in the oxidant is perceptibly in excess of 1 (Tables 16.8 and 16.9). Such an order is hard to explain in terms of common mechanism schemes for heterogeneous catalytic oxidative reactions. 

Many binary transition metal carbides, especially the 5-phases of group 4 elements, exist over a broad range of composition with an upper limit of the carbon to metal ratio near 1. Practically all solid-state properties show a gradual change with the [C]/[T] ratio (T = transition metal). Only some carbides such as tungsten monocarbide, WC, and the chromium carbides have a very narrow homogeneity region. 

In the preparation of standards, diffusional techniqnes are of special importance. Using snch techniqnes diffusion layers are formed. With a special modification by use of wedge-type samples, these layers are broadened and are thus better accessible for the microprobe.The samples consist of several diffusion bands because of the various phases in the respective carbide system. Usually, however, the samples should be single phase and homogeneous in order to make chemical gross compositional analysis applicable. Only with a comparison of different techniques a real standardization is possible. Such transition metal carbide standards have been made from the phases VC, NbC, TaC and Cr3C2. ... 

The thermal conductivity of is much higher than that of the fee transition metal carbides. The lower thermal conductivity of fee carbides has been attributed to their generally nonstoichiometric composition and hence the higher concentration of phonon scattering defects in the lattices, but can probably be explained by the stronger covalent bonding in WC and hence the higher efficiency of the heat transport by phonons. 

Thermal expansion is an important property of transition metal carbides. " They are practically never used in pure form but mostly in composite materials with matrices of other materials (metals). Upon thermal load, the difference in the thermal expansion coefficients of the carbide phase and the matrix may cause degradation of the composite. Generally, the thermal expansion of transition metal carbides is higher than that of the pure metal component. Table 1 gives average thermal expansion coefficients of various carbides. For WC, the thermal expansion has even been measured at various pressures. ... 

The diffusion of carbon in transition metal carbides proceeds via a vacancy mechanism and thus the diffiisivity increases with increasing vacancy concentration in the nonmetal sublattice. However, carbon diffusivities versus composition were measured only for very few types of... 

The Young s moduli E of group 4 transition metal carbides are on the order of about several hundred GPa (Table 1) and appear to depend on the carbon/metal ratio. This is at least trae for 5-TiCi x, where E increases with increasing [C]/[Ti] approximately linearly from 200 GPa at TiCo.50 to 460 GPa at TiCo.98 (Figure 9). Values for 5-ZrCi x and 5-HfCi x are 370-480 and 420-460 GPa, respectively, for unspecified but probably near-stoichiometric compositions. The highest Young s modulus is that of WC (Table 1) and has been measured for temperatures of up to 2200 It drops almost linearly from about 710 GPa at room temperature to 557 GPa at 2200 K. 

Many of the transition metal carbides such as TiC are very hard compounds. In 5-TiCi x the microhardness increases with increasing carbon content, a phenomenon that is probably closely related to the VEC, with the maximum stability at VEC = 8 at the composition TiC. Figure 11 shows this behavior for S-TiCi-, other data are contained in Table 1. Both microhardness and nanohardness were recently measured for Ti, Zr, and Hf carbonitrides as a function of the C/N ratio. The data are... 

Transition metal carbides (mainly of W and Mo) have been shown to be effective catalysts in some chemical reactions that are usually catalyzed by noble metals such as Pt and Pd (ref.1). Their remarkable physical properties added to lower cost and better availability could make them good candidates for substitute materials to noble metals in automobile exhaust catalysis. Hence, for this purpose, we have prepared several catalysts of tungsten carbide and W,Mo mixed carbides supported on y alumina with different Mo/W atom ratios. The surface composition has been studied by XPS while the quantitative determination of catalytic sites has been obtained by selective chemisorption of hydrogen and of carbon monoxide. The catalytic performances of these catalysts have been evaluated in the simultaneous conversion of carbon monoxide, nitric oxide and propane from a synthetic exhaust gas. 

In the absence of experimental thermochemical evidence about the strength of the metal-carbon bonds in metal carbonyl carbide systems, we can turn to the binary compounds formed between transition metals and carbon for information about the last point, the strength of metal-carbon bonds to core carbon atoms. Transition metal carbides are important. They include, in substances such as tungsten carbide, WC, some of the hardest substances known, and the capacity of added carbon to toughen metals has been known since the earliest days of steel-making. Information about them is, however, patchy. They are difficult to prepare in stoichiometric compositions of established structure and thermochemistry the metals we are most interested in here (osmium, rhenium, and rhodium) are not known to form thermodynamically stable binary phases MC and the carbides of some other metals adopt very complicated structures. Enough is, however, known about the simple structures of the carbides of the early transition metals to provide some useful pointers. 

Interest in new compositions and new synthetic routes in the context of catalysis is growing, and recent examples of the synthesis and use of non-oxidic, ceramic compositions in catalysis include SiC as a support for Ni or Pt in CO hydrogenation (2), SiC as a support for Co and Mo for thiophene hydrodesulphurisation (3), transition metal (Ti, Ta, Mo or W) carbides for methanol decomposition (4), early transition metal carbides, nitrides or borides for hydrodenitrogenation of quinoline (5), and the synthesis of high surface area molybdenum carbide (6). 

Table 1. Bulk properties at room temperature of some important transition metal carbides (disordered state). For f.c.c. carbides (Pearson symbol cF8) the composition is near 50at-% C, except for VCo.ijs- The melting points do not apply to this composition. 

A mixture of metallic, covalent and ionic components prevails in the bonding of transition metal carbides, nitrides, and carbonitrides. The metallic character is shown by the high electrical conductivities of these compounds. The bonding mechanism has been described extensively by a variety of approaches for calculating the density of states (DOS) and hence the electron density in f.c.c. transition metal carbides, nitrides, and oxides [11]. In the DOS of these compounds there is a minimum at a valence electron concentration (VEC) of 8, which corresponds to the stoichiometric composition of the group ivb carbides TiC, ZrC, and HfC. Transition metal carbides have a lower DOS at the Fermi level than the corresponding transition metal nitrides, hence the electrical properties such as electrical and thermal conductivity and the superconducting transition temperature, T, are lower than those of the nitrides. 

Hardness data for transition metal carbides near the composition indicated by the formula are contained in Table 1. 

The most widely used transition metal carbide is tungsten carbide, hexagonal WC, which is employed as the hard constituent in WC-Co hardmetals. Such hardmetals are sintered composite materials with 80-90% of hard particles such as WC embedded in a ductile binder phase such as Co. For these apphcations WC combines a number of... 

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