Metal carbides diffusion

Similar methods are applied by Kuczynski and Landauer (K7) to the diffusion of carbon in metallic carbides, in which a metal-carbon... 

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). 

The present study made use of diffusion couples where metals were reacted with carbon or nitrogen with the purpose of determining both phase equilibria and nonmetal diffusivities for Group 4 and 5 transition metal carbide and nitride systems. 

Review of diffusion and vaporization of Group 4 and 5 transition metal carbides... 

Group 5 transition metal-carbon systems. The diffusion bands of the phases of the transition metal carbides are generally rather narrow so that an unambiguous proof of their presence or absence is difficult to obtain. Therefore wedge-type or thin plane-sheet samples were... 

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 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... 

Table 3 Diffusivity data for nonmetal diffusivity of transition metal carbides, for compounds with a = 0 no significant concentration dependency is present...

Figure 8 Diffusivity of carbon and metal atoms in some transition metal carbides. (Ref. 38. Reproduced by permission of Springer Verlag)...

Transition metal carbides can be used as diffusion barriers like transition metal nitrides in multilayer metallization schemes for integrated circuits. Layers on the order of lOOnm are applied and are produced by sputtering methods. The high chemical stability of these transition metal carbides, especially those of group 4, are exploited to prevent interaction of metal or component layers such as silicon, aluminum, and silicides upon thermal load in production processes. This load would cause electrical or even structural deterioration of the multilayer packages. 

The mixed carbides are prepared either by arc melting the elements or carbides or by heating the mixed, compressed powders for a time sufficient to insure a uniform product. Since metal atom diffusion is slow, the homogenization T must be high if excessive times are to be avoided. The conditions depend on the system and whether a binder is used, which accelerates the reaction. 

The formation of carbon to carbon bonds in the lattice of the catalyst must occur to obtain elemental carbon from metal carbide. If the diffusion of carbidic carbon to nuclei where such carbon to carbon bonding has started is retarded by the penetration of the lattice by atomic hydrogen, the rate of elemental carbon formation would decrease with increasing partial pressure of hydrogen. This explanation, based on rate of diffusion of carbidic carbon in the lattice, may serve also to account for the effect of alkali and other promoters or impurities. Thus the presence of alkali tends to preserve a structure similar to that of the spinels, and it is possible that carbidic carbon diffuses more readily through such a lattice than through that obtained when little or no alkali is present. 

The breakdown of chromized coatings during oxidation has been attributed in part to the phenomenon of secondary diffusion, which decreases the chromium concentration at the metal surface. The mechanism of oxidation is therefore complex [167], although probably similar to that of iron chromium alloys, a subject that has been reviewed by Wood [168], among others. Protection is therefore dependent on the formation of a coherent barrier scale or, in the case of carbon steels, a carbide diffusion barrier. 

In transition metal carbides, nitrides, and carbonitrides, nitrogen diffusion occurs via a vacancy mechanism in the nonmetal sublattice. Because the vacancy concentration... 

Further concentration-independent diffusion coefficients were measured for vanadium nitrides [46], hafnium nitrides [88], and chromium carbides, 5TiNi c [89] and 5ZrNi c [90]. Although the metal dilfusivity in transition metal nitrides has not yet been investigated, the activation energy of that process is much higher than for nitrogen diffusion and can be estimated to be of the order of about 8 eV. Tables 3a and 3b summarize some nonmetal diffusivity data of transition metal carbides and nitrides. 

Table 3. (b) Concentration independent non-metal diffusivities in transition metal carbides and nitrides. ... 

In transition metal carbides, carbon diffuses faster than the metal atoms. Why ... 

Because in the metal oxygen diffuses less readily than hydrogen, this reaction normally takes place only near the surface. During steel production, it can lead to carbon depletion at the surface, requiring a thermal treatment to restore a uniform carbide concentration. From a corrosion point of view the reaction (9.14) is usually not critical. 

A method which combines laser vaporization of metal targets with controlled condensation in a diffusion cloud chamber is used to synthesize nanoscale metal oxide and metal carbide particles (10-20 nm). The silica nanoparticles aggregate into a novel web-like microstructure. These aggregates are very porous and have a very large surface area (460 m /g). Bright blue photoluminescence from the nanoparticle silica has been observed upon irradiation with UV light. The photoluminescence is... 

Because very little is known about diffusion in diboride, this discussion is restricted to carbides and nitrides. The mechanism of diffusion in transition metal carbides and nitrides differs for metal and nonmetal sublattices. The nonmetal diffusion proceeds via a vacancy mechanism in the nonmetal sublattice because the vacancy concentration is a function of composition within the homogeneity range of fee phases (9,10). 

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