Metal carbides typical examples

In some metal components it is possible to form oxides and carbides, and in others, especially those with a relatively wide solid solubility range, to partition the impurity between the solid and the liquid metal to provide an equilibrium distribution of impurities around the circuit. Typical examples of how thermodynamic affinities affect corrosion processes are seen in the way oxygen affects the corrosion behaviour of stainless steels in sodium and lithium environments. In sodium systems oxygen has a pronounced effect on corrosion behaviour whereas in liquid lithium it appears to have less of an effect compared with other impurities such as C and Nj. According to Casteels Li can also penetrate the surface of steels, react with interstitials to form low density compounds which then deform the surface by bulging. For further details see non-metal transfer. 

The various methods of preparation employed to prepare nanoscale clusters include evaporation in inert-gas atmosphere, laser pyrolysis, sputtering techniques, mechanical grinding, plasma techniques and chemical methods (Hadjipanyas Siegel, 1994). In Table 3.5, we list typical materials prepared by inert-gas evaporation, sputtering and chemical methods. Nanoparticles of oxide materials can be prepared by the oxidation of fine metal particles, by spray techniques, by precipitation methods (involving the adjustment of reaction conditions, pH etc) or by the sol-gel method. Nanomaterials based on carbon nanotubes (see Chapter 1) have been prepared. For example, nanorods of metal carbides can be made by the reaction of volatile oxides or halides with the nanotubes (Dai et al., 1995). 

The types of molecules considered in this work are those that have structural or chemical features that are manifestly different than are those of their more common oxidation state counterparts. Because of the breadth of this subject, selected examples are presented to illustrate typical behavior. The properties of the types of compounds containing the elements in more typical oxidation states may be found in the Inorganic and Organometallic sections describing each element or gronp and will not be discussed in this article. Similarly, minerals, metal phosphides, metal carbides, and compounds where the oxidation state of the element is low based on formal electron counting techniques (as in some catenated Catenation group 14 compounds), but that do not result in unusual chemistry, are not included. 

Metal carbides with the NaCl structure have been paid much attention to its remarkable physical properties, such as extreme hardness, very high melting points, and metallic-like electric conductivity [1,2]. Being significantly less reactive than most metals, the compounds have been used as cold cathode emitters. It is essential to understand the bond nature of these compounds for practical use of their physical properties. However, few attempts to study the chemical bonding of these compounds have been made until now. The aim of the present work is to examine TiC and UC as typical examples of transition and heavy metal carbides and to compare their bond natures in connection with their physical properties. 

Typical examples, which will be discussed in more detail below, are the zinc process, or the Coldstream process for reclamation of cemented carbide scrap, and the oxidation and subsequent reduction of tungsten heavy metal turnings or thoriated tungsten (see Table 11.3). 

Stable compound formation will always cause a depressive effect. Typical examples are the lowering of alkaline earth metal absorbances in the presence of phosphate, aluminate, silicate and some other oxo anions, the low sensitivity of metals which form thermally stable oxides (refractory oxide elements), and the depression of the calcium signal in the presence of proteins. In addition, some refractory oxide elements may also form stable carbides, especially in rich hydrocarbon flames. 

Unpredictable interactions can result between wear and corrosion when the surfaces in contact have complex, multiple-phase microstructures that can lead to microgalvanic activity and selective phase corrosion (a localized attack), as well as three-body wear modes. Examples of such surfaces include composites or surfaces that undergo compositional changes induced by tribological interactions. For instance, the presence of carbides in a metallic surface, typically formed for improved wear resistance, establishes a microgalvanic corrosion cell as the carbide is likely to be cathodic with respect to the surrounding metallic matrix [4]. This can result in a preferential anodic dissolution of the metallic matrix close to or at the matrix/carbide interface, and thereby accelerate carbide removal from surfaces and reduce the antiwear properties of the surface. 

Another solid electrolyte that has recently found fairly wide application in thermochemical e.m.f. work is calcium fluoride which is reversible to fluoride ions. Galvanic cells employing this electrolyte have been used to obtain thermochemical data for the Gibbs energy of formation of metal fluorides, carbides, borides, and phosphides at temperatures of 875 to 1120 K. Typical examples are the cells ... 

In principle all materials available in a sinterable powder can be mixed with an appropriate binder and processed on injection moulding machines. Therefore in addition to the traditional oxide ceramics it is also possible for example to use metals, carbides and nitrides. Some typical materials are shown in Table 7.11. 

Sihcon carbide fibers exhibit high temperature stabiUty and, therefore, find use as reinforcements in certain metal matrix composites (24). SiUcon fibers have also been considered for use with high temperature polymeric matrices, such as phenoHc resins, capable of operating at temperatures up to 300°C. Sihcon carbide fibers can be made in a number of ways, for example, by vapor deposition on carbon fibers. The fibers manufactured in this way have large diameters (up to 150 P-m), and relatively high Young s modulus and tensile strength, typically as much as 430 GPa (6.2 x 10 psi) and 3.5 GPa (507,500 psi), respectively (24,34) (see Refractory fibers). 

The most extensive group of nitrides are the metallic nitrides of general formulae MN, M2N, and M4N in which N atoms occupy some or all of the interstices in cubic or hep metal lattices (examples are in Table 11.1, p. 413). These compounds are usually opaque, very hard, chemically inert, refractory materials with metallic lustre and conductivity and sometimes having variable composition. Similarities with borides (p. 145) and carbides (p. 297) are notable. Typical mps (°C) are ... 

Mechanochemical processing has been used to manufacture nanocrystalline powders of nitride and carbide ceramics. The majority of systems involve milling of the metal precursor with a source of carbon or nitrogen. The source of carbon or nitrogen has typically taken the form of the element itself. However, a variety of other reagents have also been used. For example, Zhang et al. reported the synthesis of titanium nitride by milling titanium metal with pyrazine in a benzene solution. 

Sintering involving a chemical reaction is typical for the manufacture of some carbides and nitrides. For example, dense SiC is obtained by firing carbon compacts in silicon vapours. A similar example is provided by the manufacture of densified Si3N4 w here the nitride is formed in the powdered metal compact by reaction with gaseous nitrogen the product fills up the pores and thus gives low porosity. 

Although the silicon atom has the same outer electronic structure as carbon its chemistry shows very little resemblance to that of carbon. It is true that elementary silicon has the same crystal structure as one of the forms of carbon (diamond) and that some of its simpler compounds have formulae like those of carbon compounds, but there is seldom much similarity in chemical or physical properties. Since it is more electro-positive than carbon it forms compounds with many metals which have typical alloy structures (see the silicides, p. 789) and some of these have the same structures as the corresponding borides. In fact, silicon in many ways resembles boron more closely than carbon, though the formulae of the compounds are usually quite different. Some of these resemblances are mentioned at the beginning of the next chapter. Silicides have few properties in common with carbides but many with borides, for example, the formation of extended networks of linked Si (B) atoms, though on the other hand few silicides are actually isostructural with borides because Si is appreciably larger than B and does not form some of the polyhedral complexes which are peculiar to boron and are one of the least understood features of boron chemistry. 

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