IVa-Metal Carbides

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. 

Finally, numerous ternary metal carbides containing Ln metals, transition metals, and discrete Q units are also well established.35 In many such carbides the C—C distance are in the range 1.32 to 1.47 A and have been described as double bonds. Thus, they are formally derivatives of the ethyl-enic tetraanion Q", e.g., Ln3M(Q)2 (M = Fe, Co, Ni, Ru, Rh, Os, and Ir). The Q units can bind to the transition metal atoms in a 1,1 (7-IVa),... 

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. 

Silicon (3), which resembles metals in its chemical behavior, generally has a valence of +4. In a few compounds it exhibits a +2 valence, and in silicides it exists as a negative ion and largely violates the normal valency rules. Silicon, carbon, germanium, tin, and lead comprise the Group 14 (IVA) elements. Silicon and carbon form the carbide, SiC (see Carbides). Silicon and germanium are isomorphous and thus mutually soluble in all proportions. Neither tin nor lead reacts with silicon. Molten silicon is immiscible in both molten tin and molten lead. 

In addition to the types of compounds discussed so far, the group IVA elements also form several other interesting compounds. Silicon has enough nonmetallic character that it reacts with many metals to form binary silicides. Some of these compounds can be considered as alloys of silicon and the metal that result in formulas such as Mo3Si and TiSi2. The presence of Si22 ions is indicated by a Si-Si distance that is virtually identical to that found in the element, which has the diamond structure. Calcium carbide contains the C22-, so it is an acetylide that is analogous to the silicon compounds. 

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. 

Toward the end of 1995, we identified a new class of solids best described as machinable, thermodynamically stable polycrystalline nanolaminates (Fig. 1.4a). These solids are ternary layered hexagonal carbides and nitrides with the general formula, M + AX , where = 1 to 3, M is an early transition metal, A is an A-group element (mostly IlIA and IVA) and X is C and or... 

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. 

Metal borides are generally prepared by the direct reaction of the elanents at high temperatures or by the reduction of metal oxides or halides. Thus, reduction of mixtures of BjOj and metal oxides by carbothermic reaction yields metal borides. Reaction of metal oxides with boron or with a mixture of carbon and boron carbide is another route. Some metal borides are prepared by fused salt electrolysis (e.g. TaBj). Borides of IVA-VIIA elements as well as ternary borides have been reviewed by Nowomy [1], The method employed to prepare TiB starting with TiCl is interesting [2], TiCl and BCI3 react with sodium in a nonpolar solvent (e.g. heptane) to produce an amorphous precursor powder along with NaCl. NaCl is distilled off and the precursor crystallized at relatively low temperatures (-970 K). 

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... 

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