Borides refractory materials

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 ... 

Hydrides of the types AnHi (An = Th, Np, Pu, Am, Cm) and AnHs (Pa —> Am), as well as ThaHis (i.e. ThHs.yj) have been so obtained but are not very stable thermally and are decidedly unstable with respect to air and moisture. Borides, carbides, silicides and nitrides (q.v.) are mostly less sensitive chemically and, being refractory materials, those of Th, U and Pu in particular have been studied extensively as possible nuclear fuels.Their stoichiometries are very varied but the more important ones are the semi-metallic monocarbides, AnC, and mononitrides, AnN, all of which have the rock-salt structure they are predominantly ionic... 

It is impossible to find inert containers owing to both the high mp of borides (> 2000°C) and their reactivity with refractory materials. The problem is solved by melting borides either in water-cooled boats or pedestals. Water-cooled, metal... 

Solid-state metathesis reactions. For a number of compounds, solid-state metathesis (exchange) reactions have the advantages of a rapid high-yield method that starts from room-temperature solids and needs little equipment. The principle behind these reactions is to use the exothermicity of formation of a salt to rapidly produce a compound. We may say that for instance a metal halide is combined with an alkali (or alkaline earth) compound of a /7-block element to produce the wanted product together with a salt which is then washed away with water or alcohol. Metathesis reactions have been used successfully in the preparation of several crystalline refractory materials such as borides, chalcogenides, nitrides. 

The problems associated with direct reaction calorimetry are mainly associated with (1) the temperature at which reaction can occur (2) reaction of the sample with its surroundings and (3) the rate of reaction which usually takes place in an uncontrolled matmer. For low melting elements such as Zn, Pb, etc., reaction may take place quite readily below S00°C. Therefore, the materials used to construct the calorimeter are not subjected to particularly high temperatures and it is easy to select a suitably non-reactive metal to encase the sample. However, for materials such as carbides, borides and many intermetallic compounds these temperatures are insufficient to instigate reaction between the components of the compound and the materials of construction must be able to withstand high temperatures. It seems simple to construct the calorimeter from some refractory material. However, problems may arise if its thermal conductivity is very low. It is then difficult to control the heat flow within the calorimeter if some form of adiabatic or isothermal condition needs to be maintained, which is further exacerbated if the reaction rates are fast. 

Solid borides have high melting points, exceptionally high hardness, excellent wear resistance, and good immunity to chemical attack, which make them industrially important with uses as refractory materials and in rocket cones and turbine blades. Some metal borides have been found to exhibit superconductivity. 

The fact that the expected product BH3 is not obtained will be discussed in a later section. Some metals form borides containing the hexaboride group, B62. An example of this type of compound is calcium hexaboride, CaB6. In general, the structures of compounds of this type contain octahedral B62 ions in a cubic lattice with metal ions. Most hexaborides are refractory materials having melting points over 2000 °C. 

Elemental boron is a refractory material that is usually isolated either as a shiny black crystalline solid or a softer, browner, more impure amorphous solid. Reduction of readily available boron compounds containing boron oxygen bonds to elemental boron is energy intensive and costly. This has limited the extent of conunercial use of this material. Many related refractory boron compounds have been prepared and characterized including metal borides, boron carbides, boron nitrides, and various boron metal alloys. These refractory materials and elemental boron are also discussed in some detail in the article Borides Solid-state Chemistry. Other general references are available on elemental boron and other refractory boron compounds. " ... 

In 1958, Hall [141] discussed the desirability of preparing a cemented diamond composition analogous to WC and hinted that experiments to produce polycrystalline diamonds were underway. But it was not until 1970, when he reported details of his procedures [142], that he established experimentally practical pressure and temperature fields where pure diamond powder can be sintered within times ranging from several days down to about one second. He mentions hard refractory materials like borides, carbides, nitrides and oxides as suitable binders. 

Deposition from the gas phase can be performed by a wide variety of physical (PVD) and chemical (CVD) processes, with or without assistance of a plasma. These processes are particularly well suited for the fabrication of thin coatings of refractory materials such as carbides, nitrides, borides or diamond like carbon (DEC). The resulting coatings are useful for tribological and other functional applications, but they generally offer only a limited corrosion resistance. 

Non-metallic Inclusion. A non-metallic particle that has become embedded in steel during its processing. Tests, particularly those employing radioactive tracers, have shown that (contrary to earlier belief) these inclusions rarely originate from the refractory materials of the furnace, ladle or casting-pit. Non-Oxide Ceramics. A useful general term for ceramies (sueh as borides, carbides, nitrides) in whose structure oxygen plays no role. 

J.J. Melendez-Martfnez, A. Dominguez-Rodn guez, F. Monteverde, C. Melandri and G. De Portu Characterisation and high temperature mechanical properties of zirconium boride-based materials. Journal of the European Ceramic Society 22, 2543-2549 (2002). W.G. Fahrenholtz, G.E. Hilmas, I.G. Talmy and J.A. Zaykoski Refractory diborides of zirconium and hafnium. Journal of the American Ceramic Society 90, 1347-1364 (2007). 

The materials deposited by PVD techniques include metals, semiconductors (qv), alloys, intermetaUic compounds, refractory compounds, ie, oxides, carbides, nitrides, borides, etc, and mixtures thereof. The source material must be pure and free of gases and inclusions, otherwise spitting may occur. 

Even though TiC is much harder than WC at room temperature (3200 kg/mm for TiC, vs 1800 kg/mm for WC), at higher temperatures, TiC oxidi2es and loses its hardness rapidly. Figure 17 is a plot of the variation of hardness of single crystals of various monocarbides with temperature (44). No similar data is available for multicarbides or other refractory hard materials, such as nitrides, borides, oxides, or any combination of them. 

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