Ti and V carbides

Interatomic bonding parameters in TiC, and VC, as obtained from semiempirical cluster models reveal that in both ideal and defect compounds the main role in chemical bonding belongs to the hybridisation of metallic 3d AOs with 2p AOs of nonmetal atoms. With an increase in the number of vacancies the occupation of single M-C bonds (M = Ti, V) increases, but the total number of electrons in M-C bonds (per metal atom) decreases (from 2.45e in TiC,.o to 1.61e in TiCo.s). 

The increase in the number of vacancies also leads to an increase in the electronic charge in the metal sphere. As a consequence, the positive effective metal charge decreases gradually, in agreement with the experimental data of Rumqvist (1971), Johansson (1976) and Kharlamov (1986), 

When C defects appear in the crystal lattice, the p-like states contribute 

Detailed investigation of the C vacancy electronic structure has been carried out by both band and cluster methods. Gubanov, Ivanovsky, Shveikin and Ellis (1984) used the Ti6C,2Tig cluster ( is the vacancy) including three first coordination spheres of the defect in order to model defect TiC. The calculations were carried out by the X DV method in the imbedded cluster approximation. In order to study the vacancy charge, the AO basis included effective vacancy Is and 2p functions. The calculations showed that the additional Is and 2p functions contribute little to the MOs of the valence band, and the effective charge of the vacancy in TiC is very low ( 0.24c). 

The shape of individual valence states around the vacancy can be seen from the contour maps of the relevant MOs (Fig. 4.3). The spatial deformation of the wave functions near the C vacancy results in an increase in the electron density between the metal atoms closest to the vacancy. This can be interpreted as the formation of additonal Ti-Ti bonds. These peculiarities which were found in cluster calculations have been reproduced in band APW calculations of ordered TiC by Redinger et al (1985). 

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Table 4.3 Theoretical and experimental values of the lattice constant a and the bulk modulus B for some Ti and V carbides and nitrides.

Fig. 4.29 Vacancy formation energies (a) ,f, (b) Ey, and (c) E as functions of the number of valence electrons n, for Ti and V carbides, nitrides and oxides as calculated by the LMTO method.

As the excess of NH4NO3 dissociates at 350 °C, NH4C1 sublimates and NaN03 decomposes below 400 °C, all NaCl is removed at a temperature below 400 °C. Without the addition of NH4NO3 this would only be possible at the volatilization temperature of NaCl (1400 °C), by when analyte losses would be inevitable. Furthermore, in the case of the graphite furnace elements such as Ti and V form thermally stable compounds such as carbides, which lead to negative errors, because in this way fractions of the analytes are bound and do not contribute to the AAS signal. 

The existence of ternary carbides and nitrides was discussed in Ch. 4, Sec. 5.0. As shown in Fig. 4.7 (Ch. 4), the carbides of Group VI, Cr3C2, M02C, and WC, with their hexagonal or orthoifaombic structure, cannot readily accommodate the cubic structure of the Groups IV and V carbides in solid solution. However the reverse is possible. For instance W-C and Mo-C are sduble in Ti-C and Ta-C although die solubility is limited as shown below. 

SHS is particularly suited to the synthesis of refractory ceramic powders and compacts such as carbides of Ti, Si, Cr, Ta, and B, borides of Ti, V and Cr, nitrides of B, Ti, Al, Si, sihcides of Mo, Ti and V, or even more complex compounds such as YBa2Cu307-,. The thermodynamic basis of the process, the individual types of SHS processing techniques, and the equipment and post-synthesis processing to obtain powder compacts have been reviewed by Yukhvid (1992). 

As follows from the preceding chapters, the electronic structure characteristics of the MX phases will change sharply in the following cases (1) deviation of the composition from stoichiometric (2) partial replacement of X and M atoms by other atoms (3) the presence of both vacancies and impurities. Ivanovsky, Gubanov and Bekshaev (1988) have carried out a quantum-chemical analysis of the influence of these factors on the interatomic interactions in H-containing refractory phases, taking B1 carbides and nitrides of Ti and V as examples. 

Quantitative calculations of the chemical bonding parameters and the electronic spectra of the valence band for Ti Vi- Cj, (y 1) have been performed by Ivanovsky, Gubanov, Zhukov and Shveikin (1980) and Cherkashenko et al (1984) using cluster models. The experimental XES for seven ternary alloy compositions (from TiC(, 92 to VCq.s ) have also been recorded. This study revealed an inversion of the energy position of the Ti and V peaks for intermediate compositions of the alloy (relative to the initial binary carbides). When going from TiC to VC, a monotonic shift of the CK line to the short-wave region also occurs (Fig. 7.6). 

Silicon carbide has very high thermal conductivity and can withstand thermal shock cycling without damage. It also is an electrical conductor and is used for electrical heating elements. Other carbides have relatively poor oxidation resistance. Under neutral or reducing conditions, several carbides have potential usehilness as technical ceramics in aerospace appHcation, eg, the carbides (qv) of B, Nb, Hf, Ta, Zr, Ti, V, Mo, and Cr. Ba, Be, Ca, and Sr carbides are hydrolyzed by water vapor. 

Menstruum process (fluxprocess) The menstruum process was described for the preparation of carbides which are distinguished by good wettability and a very low content of impurities (such as N, O). As a most suitable auxiliary metal bath for the reaction between transition metals and carbon, a Fe-Ni alloy was suggested. The amount of this alloy (70 Fe-30 Ni mass%) was about four times the volume of the transition metal (Ti, Zr, V). After the reaction the product was crushed and put in a warm HC1 solution to dissolve the menstruum alloy. 

Fig. 6.3 Calculated adsorption energies of carbon monoxide (a), atomic sulfur (b), sulfur dioxide (c) and thiophene (d) on the metal carbides (Ti carbides, V carbides and Mo carbides) as a function of the C/M ratio. The C/M values of 0.5, 1 and 1.5 correspond to bulk M C, bulk MC, and metcar M Cj, respectively. Reprinted with permission from [16]. Copyright 2004 by the American Institute of Physics...

In comparing the L m emission band from the metal to that from its carbide, it was found that Wj /2 for the V Ljjj band is 25% larger in vanadium compared to VC in contrast to only a 10% difference between Ti and TiC. These differences in Wj/2 could be related to the similar differences in the Wj/2 of the C K bands between TiC and VC. It will be noted that the Lni bands of the metals have asymmetrical peaks while their carbides all have symmetrical peaks. The occurrence of asymmetrical peaks with an emission edge from metals and symmetrical peaks and no emission edge from their carbides is also characteristic of the My bands (5p — 3d transition) for ZrC, NbC, and M02C [15]. Lukirski et al. [17] recently reported symmetrical peaks for the My emission band from NbC. 

Chemical Interference. Chemical interference may be caused by losses of analyte during ashing, especially in the case of volatile elements such as As, Sb, Bi, Cd. but also for elements forming volatile compounds (e.g, halides). In this respect, removal of NaCl, which is present in most biological samples, is critical. Further element losses may occur from the formation of thermally stable compounds such as carbides or oxides which cannot be dissociated and hamper atomization of the analyte element elements such as Ti or V are especially difficult to determine by furnace AAS. Some of these sources of systematic error can be reduced in furnace AAS by the use of thermochemical reactions. 

The stmctures of the so-called interstitial carbides (formed by heating C with fi -block metals having > 130 pm, e.g. Ti, Zr, V, Mo, W) may be described in terms of a close-packed metal lattice with C atoms occupying octahedral holes (see Fig. 6.5). In carbides of type M2C (e.g. V2C, Nb2C) the metal atoms are in an hep lattice and half of the octahedral sites are occupied. In the MC type (e.g. TiC and WC), the metal atoms adopt a cep stmcture and all the octahedral holes are occupied. These interstitial carbides are important refractory materials they are very hard and infusible, have melting points >2800 K and, in contrast to the acetylide... 

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