Boron Nitride and Metal Borides

Boron Nitride and Metal Borides.—A new high-precision volumetric adsorption apparatus has been used to determine the energies of interaction of Ne, Ar, Kr, and Xe with hexagonal BN at 273 K. 

New mixed borides of Hf and Mo (HfgMo B) and Zr and Mo (Zr9Mo4B) have been prepared and characterized as fc-borides by 2f-ray methods. The hafnium species will dissolve up to 14 atom% A1 at 1400 °C. 

A new monoboride phase (CrB) has been detected in alloys prepared at relatively low temperatures together with a chromium-rich phase which has not yet been characterized. Investigations in the Cr-B-C system show that mutual solubility of the Cr carbides and borides is insignificant up to 1000 °C. Carbon/boron substitution in Mn23Cg extends almost to MnaaCgBg. 

The complex borides Vg gRug 5B and Nig gRug 5B have been obtained for the first time—they crystallize with the FeB-type structure, by contrast with Fca.aRho.sB i d Fea.alrg.sB, which have the FegB structure. The stabilizing influence of a number of metals on these was investigated.  

Amorphous, non-stoicheiometric NigB is produced by the addition of NaBH4 to Ni(OAc)2 solution. The product contains excess Ni, which reacts on heating to produce crystalline NigB.  

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Schwetz, K. A., andLipp, A., Boron Carbide, Boron Nitride, and Metal Borides, in Ullmann s Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A4, VCH (1985)... 

Greim, J. and Schwete, K.A. (2009) Boron carbide, boron nitride, and metal borides, in Ullmann s Encyclopedia of Industrial Chemistry, (7th edn on CD ROM), Wdey-VCH, Weinheim, Germany. 

Boric oxide is used as a catalyst ia many organic reactions. It also serves as an iatermediate ia the production of boron haUdes, esters, carbide, nitride, and metallic borides. 

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. " ... 

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. 

Next to Cr C2, TiC is the principal component for heat and oxidation-resistant cemented carbides. TiC-based boats, containing aluminum nitride, AIN, boron nitride, BN, and titanium boride, TiB2, have been found satisfactory for the evaporation of metals (see Boron compounds, refractory boron compounds Nitrides). 

Structures of the lanthanide nitridoborates appear as layered structures with approximate hexagonal arrangements of metal atoms, and typical coordination preferences of anions. As in many metal nitrides, the nitride ion prefers an octahedral environment such as in lanthanum nitride (LaN). As a terminal constituent of a BNx anion, the nitrogen atom prefers a six-fold environment, such as B-N Lns, where Ln atoms form a square pyramid around N. Boron is typically surrounded by a trigonal prismatic arrangement of lanthanide atoms, as in many metal borides (Fig. 8.10). All known structures of lanthanide nitridoborates compromise these coordination patterns. 

In finely divided form, hafnium is pyrophoric, igniting in air spontaneously. However, bulk metal reacts slowly in oxygen or air above 400°C. The rate of oxidation increases with temperature. The product is hafnium dioxide, Hf02. It combines with nitrogen, carbon, boron, sulfur and silicon at very high temperatures to form hafnium nitride HfN, hafnium boride HfB, hafnium sulfide HfSi2, respectively. Nitride formation occurs at 900°C. 

Thermal Evaporation The easiest way of evaporating metal is by means of resistance evaporators known commonly as boats . Boats, made of sintered ceramics, are positioned side by side at a distance of approximately 10 cm across the web width (Fig. 8.1). Titanium boride TiB2 is used as an electrically conductive material with boron nitride BN (two-component evaporator) or BN and aluminum nitride AIN (three-component evaporator) as an insulating material [2]. By combination of conductive and insulating materials, the electrical properties of evaporators are adjusted. 

As for hydrides, borides, and carbides, different types of nitrides are possible depending on the type of metallic element. The classifications of nitrides are similarly referred to as ionic (salt-like), covalent, and interstitial. However, it should be noted that there is a transition of bond types. Within the covalent classification, nitrides are known that have a diamond or graphite structure. Principally, these are the boron nitrides that were discussed in Chapter 8. 

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. 

More than one boride phase can be formed with most metals, and in many cases a continuous series of solid solutions may be formed. Several methods have been used for the relatively large-scale preparation of metal borides. One that is commonly used is carbon reduction of boric oxide and the appropriate metal oxide at temperatures up to 2000 °C. Fused salt electrolysis of borax or boric oxide and a metal oxide at 700 1000 °C have also been used. Small-scale methods available include direct reaction of the elements at temperatures above 1000 °C and the reaction of elemental boron with metal oxides at temperatures approaching 2000 °C. One commercial use of borides is in titanium boride-aluminum nitride crucibles or boats for evaporation of aluminum by resistance heating in the aluminizing process, and for rare earth hexaborides as electronic cathodes. Borides have also been used in sliding electrical contacts and as cathodes in HaU cells for aluminum processing. 

Numerous ceramics are deposited via chemical vapor deposition. Oxide, carbide, nitride, and boride films can all be produced from gas phase precursors. This section gives details on the production-scale reactions for materials that are widely produced. In addition, a survey of the latest research including novel precursors and chemical reactions is provided. The discussion begins with the mature technologies of silicon dioxide, aluminum oxide, and silicon nitride CVD. Then the focus turns to the deposition of thin films having characteristics that are attractive for future applications in microelectronics, micromachinery, and hard coatings for tools and parts. These materials include aluminum nitride, boron nitride, titanium nitride, titanium dioxide, silicon carbide, and mixed-metal oxides such as those of the perovskite structure and those used as high To superconductors. 

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... 

Boron Nitride, Metal Borides, and Related Spedes.— Alo.o6BeB3 05, Le. BeB3 belongs to the space group P6/mmm, and contains B12 icosahedra and other polyhedral units of Be and B atoms. The linkages between the polyhedra resemble those in /3-rhombohedral boron. Aluminium atoms occupy interstitial... 

Boron is inert under normal conditions except for attack by F2. At high temperatures, it reacts with most non-metals (exceptions include H2), most metals and with NH3 the formations of metal borides (see Section 12.10) and boron nitride (see Section 12.8) are of particular importance. 

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