Monocarbide phases

During vaporization of non-stoichiometric refractory carbides each element vaporizes at a different rate which is dependent on surface composition or relevant activities at the surface. When the initial bulk composition is near C/M = 1, the vaporization of C is much greater than that of M. As a result, the surface C content decreases and eventually approaches a constant value, which we will call the steady-state CVC (ssCVC). At the ssCVC, the vapor composition is nearly equal to the initial bulk composition. As C diffusion to the vaporizing surface reduces the C content of the bulk material, the surface composition asymptotically approaches the equilibrium CVC (eCVC). The rate at which eCVC is approached depends on the relative magnitudes of C vaporization and diffusion. When the eCVC has been reached, the surface and bulk C/M ratios are equal to the vapor composition. The intersection of the solid eCVC map with the solidus boundary of the monocarbide phase determines where melting occurs under equilibrium conditions for a particular atmosphere. 

These carbides also have similar properties and characteristics. Only the monocarbide phases are of industrial importance. TaC is produced on a relatively large scale while the importance of VC and NbC is still limited. Their fabrication processes and applications are summarized in Sections 6, 7, and 8 and reviewed in more detail in Chs. 14, 15, and 16. 

Samples were tested on in a melt of salts (75% Na SO, 25% NaCl) at 950°C in an air atmosphere for 24 hours. Micro X-rays spectrum by the analysis found that the chemical composition of carbides of an alloy of the ZMI-3C and test alloys differs noticeably. In the monocarbide of phase composition of an alloy of the ZMI-3C there increased concentration of titanium and tungsten is observed in comparison with test alloys containing chemical composition tantalum. The concentration of more than 2% of tantalum in test alloys has allowed mostly to deduce tungsten from a mono carbide phase (MC) into solid solution. Thus resistance of test alloys LCD has been increased essentially, as carbide phase is mostly sensitive aggressive environments influence. The critical value of total molybdenum and tungsten concentration in MC should not exceed 15%. 

Structure. The structure of the refractory-metal carbides increases in complexity with increasing group number. Thus the carbides of Group IVa are characterized by a single cubic monocarbide. In those of Group Va, a M2C phase exists as well as the monocarbide. The carbides of Group Via are far more complex and have several compositions. 

Niobium carbide, also known as columbium carbide, is a important refractory material with a high melting point. It is used as a CVD coating mostly on an experimental basis. Niobium carbide has two phases Nb2C and the monocarbide NbC. The latter is the only phase of industrial importance and the only one reviewed here. Its characteristics and properties are summarized in Table 9.5. 

Tungsten carbide has a complex crystal structure with three phases Wq (subcarbide), the monocarbide WC (also called a-WC), and P-WCj.x, which is unstable and forms only above 1530°C. The monocarbide WC is the most important phase and the one reported here. Its characteristics and properties are summarized in Table 9.9. 

A system particularly studied from a thermodynamic point of view is the monocarbide (AnCi x) system. A range of stability for carbon deficient compositions in the monocarbides is present in many metal-carbon phase diagrams. Table 6 shows the composition range at room temperature for actinide monocarbides. The non-stoichiometry range is very limited for uranium monocarbide for neptunium and plutonium monocarbides, the stoichiometric AmCj oo composition is not stable. 

In practically all monocarbide and mononitride phases a non-linear diffusion profile was found. These profiles will be the object of an... 

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. 

Tungsten carbide occurs in two fundamental compositions, WC and W2C. Less common is the phase of the stoichiometry W3C. The monocarbide has a melting point of... 

The chenucal potential of thorium relative to Th(cr) were calculated from the emf measurements in the single phase monocarbide region between ThCo ee and ThCo ge- The cells were of the type Th, ThF4 CaF2 ThF4 ThCx,. Measurements were made at 1073 to 1223 K. The chenucal potential at 1173 K decreases from -1.7 kJ-mol at ThCo ee to - 83.3 kJ mol at ThCoee The thermodynanuc data are discnssed in coimection with theories of chemical bonding. 

Monocarbides. Brewer and Krikorian (1956) and Dancy et al. (1962) had reported the monocarbide of cerium, but its existence has been discredited. Spedding et al. (1958) found that the lattice parameter reported by Brewer and Krikorian (1956) for CeC was identical with that of Ce metal saturated with C, and the reported CeC was most likely a solid solution of C in Ce. Anderson et al. (1969) re-examined the binary cerium-carbon system, and gave no evidence for the existence of any phase structurally based on monoatomic (methanide) anions. 

The formation of the rare-earth-nitrogen-carbon compounds has been investigated only for the light lanthanide systems with the emphasis on the existence of the monocarbide of La, Ce, Pr and Nd, in the presence of nitrogen. It is well known that no evidence for the existence of a face-centered cubic phase, assumed to be the monocarbide, was found in the detailed work on the light-lanthanide-carbon systems (Spedding et al. 1958, Dancy et al. 1962, Anderson et al. 1969). 

The crystal structures of the phases in these systems are characterized by destabilization of the octahedral structure elements and the appearance of trigonal prismatic structural elements. F.c.c. monocarbides MeCj v in the Mo-C and W-C system exist only at high temperatures and with substoichiometric compositions. The stoichiometric monocarbides WC and MoC are hexagonal. 

Microstructures of glassy and metastable crystalline phases formed from Fe69Vi2Ci9 alloy were studied by [1982Hor]. It was shown that the non-stoichiometric monocarbide VCi t could be modified by splat cooling into a mixed carbide if V was partially substituted by Fe. 

Cuboid and cycloid niobium monocarbide (NbC) whiskers, 0.1-2.0 pm in diameter and 5-100 pm in length, and having a square-shaped tip, were recently synthesized by heating mixtures of niobium oxide (Nb203) and carbon black at temperatures over 1100°C [38]. Silicon carbide nano-whiskers, 20-50 nm in diameter and 2-5 pm in length, were carbothermally synthesized by reducing ultrafine precipitated silica powders with ultrafine carbon black by microwave heating [29]. These processes proceed without addition of metal particle catalysts, and therefore by a VS phase transformation [14] [29] [38]. 

The monocarbide VC is the only phase produced industrially but its use is limited. The following is a summary of its applications in production or development (see Ch. 16). 

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