Physical properties borides

Table 3. Physical Properties of Titanium Borides, Carbides, and Nitrides ...

Ditrimethylolpropane, 2 47 physical properties of, 2 48t Ditungsten boride, 25 386 Ditungsten trisilicide, 25 386 Diucardin, molecular formula and structure, 5 162t Diuloses, 4 711 Diuretics, 22 867 Diuril, 5 168... 

Titanium anodes, 15 591 Titanium aryloxides, 25 78 Titanium beach-sand mining, 24 847 Titanium borides, 25 5-6 physical properties of, 25 7 Titanium bromide... 

Incidentally, the B6 octahedron (Longuet and Roberts, 1955) and B12 cuboocta-hedron (Lipscomb and Britton, 1960) are also found to be two-electron deficient. This electron deficiency of the clusters leads to dramatically different physical properties for trivalent and divalent metal borides as illustrated for example in the next section on the dodecaborides. More on the bonding requirements in the higher borides will be discussed in later sections. 

An attractive method to produce single crystals (in the dimension range from pm to several nun) is the high-temperature solution (flux) method, becanse of its simphcity and the low temperature required. The elements are dissolved in the solvent metal (often Al) and subseqnently the solntion is slowly cooled to room temperature. A prerequisite is, of course, that the solubility of the used solvent in the desired boride is insignificant. The solubility of Al in most boron-rich binary borides has been found to be extremely small. Crystals prepared in this manner are suitable for measurement of physical properties, for instance, microhardness, electrical resistivity, and so on. 

A large amount of systematic development work on new high-performance mixed carbide, nitride, boride and oxide coating materials produced by physical vapour deposition (PVD) techniques is being carried out, because the mixed coatings often display significantly improved properties (e.g. wear-, corrosion-, oxidation-resistance or certain physical property behaviour) compared to the properties of the individual constituents. 

In the so-called interstitial nitrides the metal atoms are approximately, or in some cases exactly, close-packed (as in ScN, YN, TiN, ZrN, VN, and the rare-earth nitrides with the NaCl structure), but the arrangement of metal atoms in these compounds is generally nor the same as in the pure metal (see Table 29.13, p. 1054), Since these interstitial nitrides have much in common with carbides, and to a smaller extent with borides, both as regards physical properties and structure, it is convenient to deal with all these compounds in Chapter 29. 

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. 

The interstitial structures comprise the compounds of certain metallic elements, notably the transition metals and those of the lanthanide and actinide series, with the four non-metallic elements hydrogen, boron, carbon and nitrogen. In chapter 8 we discussed the structures of a number of hydrides, borides, carbides and nitrides of the most electropositive metals, and these we found to be typical salt-like compounds with a definite composition and with physical properties entirely different from those of the constituent elements they are generally transparent to light and poor conductors of electricity. The systems now to be considered are strikingly different. They resemble... 

Table 7-2. Physical properties of important transition metal borides [lOj.

Guldamashvili, A., G. Bokuchava, R. Kutelia, G. Darsavelidze, O. Tsagareishvih, and M. Antadze. 2008. Electro-physical properties of ion-implanted heta-rhomhohedral horon. In Abstracts of the 17th International Symposium on Boron, Borides and Related Materials, Matsue, Japan, p. 136. 

Densities above 95% have been achieved by axial hot-pressing at pressures exceeding 20 MPa and temperatures above 1800°C. The microstructures consist typically of particles of > 20 qm in size. Another problem is related to the hexagonal layered structure of the AlB2-type borides. Because of the strong anisotropic behavior of the physical properties, especially of the coefficients of thermal expansion, the... 

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