Borides properties

Petit C and Pileni M P 1997 Nanosized oobalt boride partioles oontrol of the size and properties J. Magn. Magn. Mater. 166 82... 

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

Borides are inert toward nonoxidizing acids however, a few, such as Be2B and MgB2, react with aqueous acids to form boron hydrides. Most borides dissolve in oxidizing acids such as nitric or hot sulfuric acid and they ate also readily attacked by hot alkaline salt melts or fused alkaU peroxides, forming the mote stable borates. In dry air, where a protective oxide film can be preserved, borides ate relatively resistant to oxidation. For example, the borides of vanadium, niobium, tantalum, molybdenum, and tungsten do not oxidize appreciably in air up to temperatures of 1000—1200°C. Zirconium and titanium borides ate fairly resistant up to 1400°C. Engineering and other properties of refractory metal borides have been summarized (1). 

Table 1 fists many metal borides and their observed melting points. Most metals form mote than one boride phase and borides often form a continuous series of solid solutions with one another at elevated temperatures thus close composition control is necessary to achieve particular properties. The relatively small size of boron atoms facilitates diffusion. 

Uses. In spite of unique properties, there are few commercial appUcations for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses ia the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particularly useful as cathodes ia electronic devices because of their high thermal emissivities, low work functions, and resistance to poisoning. 

Table 3 summarizes the properties of the so-called nonmetallic hard materials, including diamond and the diamondlike carbides B C, SiC, and Be2C. Also iacluded ia this category are comadum, AI2O2, cubic boroa nitride, BN, aluminum nitride, AIN, siUcon nitride, Si N, and siUcon boride, SiB (12). 

Attempts to classify carbides according to structure or bond type meet the same difficulties as were encountered with hydrides (p. 64) and borides (p. 145) and for the same reasons. The general trends in properties of the three groups of compounds are, however, broadly similar, being most polar (ionic) for the electropositive metals, most covalent (molecular) for the electronegative non-metals and somewhat complex (interstitial) for the elements in the centre of the d block. There are also several elements with poorly characterized, unstable, or non-existent carbides, namely the later transition elements (Groups 11 and 12), the platinum metals, and the post transition-metal elements in Group 13. 

In general, many metals and alloys (e.g. of Al, Ta and Mo) can be deposited on metallic and some non-metallic substrates. M may also be a metal compound having special useful properties (e.g. borides, nitrides, oxides, silicides and carbides), or even a non-metal such as Si (as in Ihrigising ). 

The refractory-metal borides have a structure which is dominated by the boron configuration. This clearly favors the metallic properties, such as high electrical and thermal conductivities and high hardness. Chemical stability, which is related to the electronic... 

The borides listed above can all be produced by CVD. With a few exceptions, they have found only limited industrial applications so far, in spite of their excellent properties of hardness, erosion resistance, and high-temperature stability. 

The CVD coating materials for wear and corrosion resistance consist mostly of carbides and nitrides and, to a lesser degree, borides. Table 17.1 compares the relative properties of these materials. 

Selected Chemical Properties of Transition-Metal Borides... 

The crystal structures of the borides of the rare earth metals (M g) are describedand phase equilibria in ternary and higher order systems containing rare earths and B, including information on structures, magnetic and electrical properties as well as low-T phase equilibria, are available. Phase equilibria and crystal structure in binary and ternary systems containing an actinide metal and B are... 

Because they exhibit interplay of magnetic and superconducting properties, the formation and crystal chemistry of MRgMy4B4 compounds have been examined. Ternary rare-earth and actinide (Th, U, Pu)-transition metal borides of the approxi-... 

Rare-earth (and actinide)-B-carbon compounds resemble metal borides in B-rich carboborides, whereas the physical and structural properties of C-rich borocarbides tend to a more earbide-like behavior (which will not be covered in this context). 

Most of the known borides are compounds of the rare-earth metals. In these metals magnetic criteria are used to decide how many electrons from each rare-earth atom contribute to the bonding (usually three), and this metallic valence is also reflected in the value of the metallic radius, r, (metallic radii for 12 coordination). Similar behavior appears in the borides of the rare-earth metals and r, becomes a useful indicator for the properties and the relative stabilities of these compounds (Fig. 1). The use of r, as a correlation parameter in discussing the higher borides of other metals is consistent with the observed distribution of these compounds among the five structural types pointed out above the borides of the actinides metals, U, Pu and Am lead to complications that require special comment. 

Because of its stability and unusual properties, the crystal growth of TiB2 is the most studied process of boride preparation by CVD. It is interesting from both a scientific and an engineering viewpoint, and because of the availability, low cost and... 

This soft, silver white metal reacts with air and water. The oxide is applied in optical glasses with high refractive indices (special lenses for powerful cameras and telescopes). Used for special effects in optoelectronics and electronics. Lanthanum exhibits catalytic properties. It is a component of flint and battery electrodes. Lanthanum boride (LaB6) is the superior electron-emitter for electron microscopes. Lanthanum is the first of the series of 14 lanthanides, also called the "rare-earth" metals, whose inner N shells are filled with electrons. They do not belong on the "red list" of endangered species they are neither rare nor threatened with depletion. China is particularly rich in lanthanide ores. 

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... 

Considering other families of similar compounds, the contributions given by Guillermet and Frisk (1992), Guillermet and Grimvall (1991) (cohesive and thermodynamic properties, atomic average volumes, etc. of nitrides, borides, etc. of transition metals) are other examples of systematic descriptions of selected groups of phases and of the use of special interpolation and extrapolation procedures to predict specific properties. 

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