Chemical transition metal carbides/nitrides

Transition metal carbide, nitride and carbonitride layers of the IVB group made by CVD (chemical vapor deposition) and PVD (physical vapor deposition) are produced in order to increase the service life of hardmetal tools [18,21,108] for cutting and milling operations. About 90% of cutting tools are coated. 

A new method of interpreting Auger electron spectroscopy (AES) sputter profiles of transition metal carbides and nitrides is proposed. It is shown that the chemical information hidden in the shape of the peaks, and usually neglected in depth profiles, can be successfully extracted by factor analysis (FA). The various carbide and nitride phases of model samples were separated by application of FA to the spectra recorded during AES depth profiles. The different chemical states of carbon, nitrogen and metal were clearly identified. 

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. 

Thermally, elastically, chemically and electrically they share many of the advantageous attributes of their respective stoichiometric binary transition metal carbides or nitrides they are electrically and thermally conductive, chemically stable. Mechanically they cannot be more different, however they are most readily machinable (Fig. 1.4b) and relatively soft. It is the ability of the basal planes to readily delaminate from each other, instead of fracturing, that renders them unique and why they have been labeled nanolaminates (Fig. 1.4a). 

Oyama, S. T., Crystal Structure and Chemical Reactivity of Transition Metal Carbides and Nitrides, J. Solid State Chem., 96 442-445 (1992)... 

Rapid development of nonempirical quantum-chemical methods (both in the band and cluster approaches) made it possible to perform detailed investigations of both the electronic structure of the vacancies and the effects of nonstoichiometry on the crystal energy spectra and properties. This chapter reviews the relevant results obtained for transition metal carbides and nitrides. 

In this chapter we shall discuss the results of the theoretical modelling of electronic spectra, chemical bonding, charge distributions and properties of transition metal carbides and nitrides, doped by s and p impurities, as well as carbide- and nitride-based multicomponent solid solutions -carbonitrides, oxycarbides, etc. 

This book presents a systematic description of the electronic and physicochemical properties of transition-metal carbides and nitrides. These materials possess remarkable physical and chemical properties, including extremely high hardness and strength, and high melting points, metallic conductivity and superconductivity. As a result, they have been extensively studied by scientists, and their properties widely exploited by engineers. 

This is the first book devoted to the theoretical modelling of refractory carbides and nitrides and alloys based on them. It makes use of computational methods to calculate their spectroscopic, electric, magnetic, superconducting, thermodynamical and mechanical properties. Calculated results on the electronic band structure of ideal binary transition-metal carbides and nitrides are presented, and the influences of crystal lattice defects, vacancies and impurities are studied in detail. Data available on chemical bonding and the properties of multi-component carbide- and nitride-based alloys, as well as their surface electronic structure, are described, and compared with those of bulk crystals. 

Many refractory transition-metal carbides and nitrides can be described as a close-packed metal lattice (f.c.c. of B1 (cF8) NaCl type or h.c.p.), with the carbon or nitrogen atoms in the center of the octahedral interstices. A large fraction of these octahedral sites (up to 50<7o in the cases of TiCo j and TiNoj) are unoccupied and may be considered as structural vacancies. Chemical bonding and transport properties depend strongly on the structural-vacancy content. 

Catalysis Control of chemical reactions Transition metal carbides and nitrides... 

The transition metal monocarbides and mononitrides typically exhibit wide homogeneity regions, caused by the formation of vacancies, mainly on the nonmetal sites and, to a much lesser extent, also on the metal sites. Because the vacancies have a strong influence on the physical and chemical properties of the substances, a large number of theoretical and experimental investigations have been devoted to the study of the electronic structure of vacancy-containing transition metal carbides and nitrides. 

K Schwarz. Band structure and chemical bonding in transition metal carbides and nitrides. CRC Crit Rev Solid State Mater Sci 13 211, 1987. 

W Lengauer, H Wiesenberger, M Joguet, D Rafaja, P Ettmayer. Chemical diffusion in transition metal-carbon and transition metal-nitrogen systems. In ST Oyama, ed. The Chemistry of Transition Metal Carbides and Nitrides. London Blackie Academic Professional, 1996, p 91. 

Reactions of boron ttihalides that are of commercial importance are those of BCl, and to a lesser extent BBr, with gases in chemical vapor deposition (CVD). CVD of boron by reduction, of boron nitride using NH, and of boron carbide using CH on transition metals and alloys are all technically important processes (34—38). The CVD process is normally supported by heating or by plasma formed by an arc or discharge (39,40). 

INTERSTITIAL. < 11 Descripiive of a nnnstoichiuiiiclric compound ol a metal and a nometal whose structure conforms to. 1 simple chemical formula, bill exists over a limited range of chemical composition. Imersiinal compounds are represented by borides, nitrides, and carbides ol the transition metals. 21 Descriptive n an atom of an impurity lhai causes a del cel... 

Such a dependence is frequently observed experimentally in the oxida-tion of transition metals forming volatile oxides, and also of their alloys, carbides, nitrides, borides, silicides and other chemical compounds.350 365 An example taken from the work of E.A. Gulbransen and K.F. Andrew389 is presented in Fig. 5.22. 

The crystal structures adopted by the binary carbides and nitrides are similar to those found in noble metals. The resemblance is not coincidental, and has been explained using Engel-Brewer valence bond theory [5]. Briefly, the main group elements C and N increase the metal s effective s-p electron count, so that structures and chemical properties of the early transition metals resemble those of the Group 8 metals. This idea was first introduced by Levy and Boudart [6] who noted that tungsten carbide had platinum-like properties. 

Most borides are chemically inert in bulk form, which has led to industrial applications as engineering materials, principally at high temperature. The transition metal borides display a considerable resistance to oxidation in air. A few examples of applications are given here. Titanium and zirconium diborides, alone or in admixture with chromium diboride, can endure temperatures of 1500 to 1700 K without extensive attack. In this case, a surface layer of the parent oxides is formed at a relatively low temperature, which prevents further oxidation up to temperatures where the volatility of boron oxide becomes appreciable. In other cases the oxidation is retarded by the formation of some other type of protective layer, for instance, a chromium borate. This behavior is favorable and in contrast to that of the refractory carbides and nitrides, which form gaseous products (carbon oxides and nitrogen) in air at high temperatures. Boron carbide is less resistant to oxidation than the metallic borides. 

The carbides of the early transition metals exhibit chemical and catalytic properties that in many aspects are very similar to those of expensive noble metals [1], Typically, early transition metals are very reactive elements that bond adsorbates too strongly to be useful as catalysts. These systems are not stable under a reactive chemical environment and exhibit a tendency to form compounds (oxides, nitrides, sulfides, carbides, phosphides). The inclusion of C into the lattice of an early transition metal produces a substantial gain in stability [2]. Furthermore, in a metal carbide, the carbon atoms moderate the chemical reactivity through ensemble and ligand effects [1-3]. On one hand, the presence of the carbon atoms usually limits the number of metal atoms that can be exposed in a surface of a metal carbide (ensemble effect). On the other hand, the formation of metal-carbon bonds modifies the electronic properties of the metal (decrease in its density of states near the Fermi level metal—>carbon charge transfer) [1-3], making it less chemically active... 

TABLE 5.2 Physical and Chemical Properties of Common Transition Metal Nitrides and Carbides... 

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