Va-Metal Carbides

So far, we have been considering the electronic structure of binary rva, va-metal carbides which possess the simplest cubic structure, which include a large number of individual carbides with very different and rather complicated crystal lattices and which exhibit interesting mechanical, electric and catalytic properties. As is well known, the C content in d-metal-containing phases drops sharply as the valence of the transition... 

Carbides produced by CVD include the refractory-metal carbides and two important non-metallic carbides boron carbide and silicon carbide. The refractory-metal carbides consist of those of the nine transition elements of Groups IVa, Va, and Via and the 4th, 5th, and 6th Periods as shown below in Table 9.1. 

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. 

Metal carbides are generally prepared by the direct reaction of the elements at high temperatures (-2470 K). Reaction of metal oxides with carbon is another important route. Reaction of metal vapour with hydrocarbons also yields metal carbides. Phase relations in carbides of IVA, VA and VIA group elements as well as actinides have been reviewed by Storms [5], SiC has been prepared by the reaction of SiCl and CCl with Na, a similar reaction of CCl and BClj with Na gives B C [2]. SiC is formed by the decomposition of CH3S1H3 or (CHj)jSiClj. Pyrolysis of organosilicon polymer precursors has been employed to prepare SiC [6]. Some of the precursor reactions are discussed in Chapter 4 of this book. Various metal carbides have also been synthesized by sol-gel chemistry [7]. 

Stoichiometric cubic (NaCl-type) carbides of iva and va transition metals (referred to as d-metal carbides) have received most attention in the literature. Due to the simplicity of their crystal structure, these phases (especially TiC and VC) have been studied by a large number of quantum-chemical methods, as well as by X-ray emission, electron. 

Calculated and experimental values of some thermomechanical parameters for refractory carbides are compared in Table 2.1. Best agreement has been obtained for the lattice constants (the difference from the experimental values being not more than 0.05 au). Other parameters derived from the calculations are much less accurate, but in most cases they reproduce well the trends observed in experimental data. The most important of these is the increase in chemical bonding strength, when going from iva subgroup metal carbides to the carbides of va and via metals. This follows from the increase of the elasticity modulus and the hydrostatic breakdown tension values in this direction. 

The characteristic feature of the bands of cubic nitrides, which is different from those of the isostructural carbides (see Chapter 2), is the position of the N2s-bands, which, due to the lower energy of the 2s states of free N atoms is situated at a lower energy. Hybridised Mnd-bands have almost the same energy as their counterparts in the carbides. But their width in nitrides is essentially less than in the carbides, and this results in the large interval between the edges of the hybridised and metallic state bands. Moreover, in contrast to carbides, the Fermi level in iva and va metal nitrides is always located inside the Mnd-band. The lower covalency... 

The interstitial or refractory carbides are the most important of the three classes these are formed by the transition metals, the most stable being the carbides of the metals in Groups IVa, Va, and Via. Such carbides may be made from the elements at high temperatures under pressure and resemble the metals themselves. They tend, however, to be much harder and higher melting than the parent metals. The melting points, (admittedly approximate) of tantalum carbide, TaC, (4200° C) and zirconium carbide, ZrC, (3800° C) may be compared with those of tungsten... 

Metallic nitrides, sometimes termed interstitial compounds, are formed from combinations of N with transition metals of groups IVA, VA, and VIA. As the name implies, they exhibit electrical conductivity and most of the general characteristics associated with standard metals. They are also refractory and hard, and usually depart from the ideal stoichiometry ratios displayed above (see 17.3.9, Table 1). They readily form solid solutions with carbides and oxides, which gives rise to problems when it is necessary to obtain nitrides in pure form. Included in this category are numerous ternary nitrides of a transition metal with a group B metal. 

In February, 1980, Sumitomo from Japan filed the patent Sintered compact for a machining tool and a method of producing the compact [161]. This patent basically covers any compact with 10-80 vol% cBN and a balance of binder material that can comprise any carbides, nitrides, borides, or silicides of metals of groups IVa, Va, or Via. Specifically mentioned are titanium, zirconium, hafnium, vanadium, niobium. 

The refinements suggested by Brooks et al are as follows. The majority of refractory carbides and nitrides of iiia-va subgroup transition metals have no localized magnetic moments, i.e., the structure of their one-electron states may be described using methods that disregard the spin polarisation of atoms in the crystal. If an analogous spin-restricted... 

The main trends of phase formation in transition metal-carbon or transition metal-nitrogen systems are well known (see Goldschmidt (1967)). For example, ina-va subgroup metals form stable carbides, which are characterised by good thermomechanical properties, but on going to... 

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