Carbides, point defects

The treatment assumes that the point defects do not interact with each other. This is not a very good assumption because point defect interactions are important, and it is possible to take such interactions into account in more general formulas. For example, high-purity silicon carbide, SiC, appears to have important populations of carbon and silicon vacancies, and Vsj, which are equivalent to Schottky defects, together with a large population of divacancy pairs. 

There is increasing experimental evidence for the superlattice ordering of vacant sites or interstitial atoms as a result of interactions between them. Superlattice ordering of point defects has been found in metal halides, oxides, sulphides, carbides and other systems, and the relation between such ordering and nonstoichiometry has been reviewed extensively (Anderson, 1974, 1984 Anderson Tilley, 1974). Superlattice ordering of point defects is also found in alloys and in some intermetallic compounds (Gleiter, 1983). We shall examine the features of some typical systems to illustrate this phenomenon, which has minimized the relevance of isolated point defects in many of the chemically interesting solids. 

Point defects can drastically lower the thermal conductivity of the important carbide and nitride high thermal conductivity ceramics. In this respect, oxygen, which is a common impurity, has been found to be very important. For example, silica (Si02), an impurity in silicon nitride (Si3N4), formed by oxidation at high temperatures in air, can react to produce substitutional defects and vacancies in the following way ... 

Because boron carbide can be used as the control rod material in a nuclear reactor, in order to interpret its performance it is necessary to establish nature of grown-in and neutron-radiation-induced lattice defects in boron carbide. It was found that the dose received by the irradiated specimen corresponds to transmutation of about eight B nuclei per unit cell in equal number of both Li and " He nuclei (Ashbee 1971). It is believed that the formation of the partial dislocation loops resulting from the agglomeration of point defects are introduced during neutron irradiation. 

Table 4.7 Boron carbide Comparison between theoretical electronic properties, experimental characterization and intrinsic point defects determined experimentally. 

In order to elucidate whether such a precipitate can trap positrons, the positron affinities A+ for the host material and the precipitate were calculated [154], The A+ values were found to be relatively high and the positron lifetimes very short for perfect MC carbides. This fact confirms that perfect MC (M s Cr, V, Ti, Mn, Fe, Zr, Nb) carbides are very dense materials that cannot trap positrons when embedded in the Fe matrix. In general, from a PAS point of view, radiation damage can be interpreted as a combination of radiation-induced point defects, dislocations and small vacancy clusters [129,130] that occur mainly in the region of the precipitate-matrix interface. 

Other phases which are found to precipitate under neutron irradiation in FM steels are described in references [48,75,83] (1) Diamond cubic r] (MsC) carbide, was frequently observed in irradiated FM steels containing more than about 0.3% Ni [47,75,83] (2) bcc x intermetallic phase [47,77,78,84] (see Fig. 9.4(b)) (3) fee G (MyNiigSi , where M = Mn, Cr, or Nb) silicide phase [6,83] (4) a phase and phosphides of M3P and MP types were infrequently reported [82]. The formation of these phases, enriched in minor solutes such as Ni, Si, and P, is thought to be irradiation-induced, i.e., due to RIS of these elements which are known to segregate to point defect sinks (see above). 

Up to now, we have discussed mostly model intermetallic compounds with simple crystal structures (generally cubic LI2, B2,. . . ) and containing two metal species. We shall now present briefly some properties of point defects in more exotic systems, of considerable interest the A15 superconductors, transition-metal carbides and nitrides, and III-V semiconductors (e.g. GaAs). 

Transport properties of these compounds are reviewed. The resistivity of MC, single crystals except for VQ at room temperature increases with decreasing x but begins to saturate (1). The contribution of phonon scattering for TiC was found to be 30,20, and 10% at a composition (C/Ti) of 0.95. 0.9, and <0.8, respectively (16). Also the inverse mobility for nonstoichiometfic carbides is proportional to the vacancy concentration, showing evidence that carbon vacancies act as scattering centers for electrons (17). This relationship can be revised to obtain the point-defect concentration and the chemical composition of a carbide from its resistivity (18). 

J Schneider, K Maier. Point defects in silicon carbide. Physica B185 199, 1993. 

Refractory Compounds. Refractory compounds resemble oxides, carbides, nitrides, borides, and sulfides in that they have a very high melting point. In some cases, they form extensive defect stmctures, ie, they exist over a wide stoichiometric range. For example, in TiC, the C Ti ratio can vary from 0.5 to I.O, which demonstrates a wide range of vacant carbon lattice sites. 

Localized corrosion can occur in the otherwise passive region of potentials. In the case of localized corrosion, the passive film is locally breached, due to mechanical or chemical compromise of the oxide film or structural defects, such as grain boundaries, triple points, surfacing dislocations, or intermetallic phases such as carbide and/or sulfides. Localized corrosion manifests itself in the plot of corrosion current versus electrode potential (the polarization curve) as noisy peaks in the passivation region, due to the stochastic nature of this mode of corrosion. 

An example of this behavior occurs during the reaction with H2. All of the carbon-saturated carbides will lose carbon as hydrocarbons at high temperatures. This lowers the carbon concentration, hence its activity, and raises the hydrocarbon concentration in the gas. At some point, depending on the carbide system and the temperature, these processes will come to equilibrium, but at a reduced carbide stoichiometry. If, on the other hand, the H2 is made to flow and the hydrocarbons are swept away, the carbide will continue to lose carbon at a rate which will depend on the diffusion rate of carbon through the carbide. Fortunately for many applications, this rate is small. At low temperatures, H2 can dissolve in the defect lattice forming a carbohydride. 

---