Borides

Borides. The preparation of ZrB12 has been reviewed67,68 and an electrochemical method for its preparation has been patented.69 

Borides. An X-ray examination of the tetragonal berthollide phases(Bi2)4B2Vi 5 i 9 has shown that the cell constants do not depend significantly on the vanadium content. 

spectra of unstable ion centres produced in polycrystalline MgO and CaO have been studied. A theoretical model has been described which provides a satisfactory basis for the interpretation of the electronic spectrum of the [V(H20) ] ion.  

Heating a benzene solution of VL2 (L = mesitylene) and furanecarboxylic acid (HL ) gave V3Lg,3HL, 0.5L. With pyridine as solvent, the product was py3V3L.  

Sakakai, H. Kikkawa, H. Kato, and S. Yoshida, Bull Chem. Soc. Japan, 1976,49, 76. 

LeFlem, and M. Pouchard, Compt. rend., 1975, 280, C, 1093. 

Metal Borides.—Chemical and ESCA studies on hydrogen adducts of cobalt and nickel borides are consistent with the formulations (Co2B)5H3 and (Ni2B)2H3.328 

The structures of CeCo4B, Ce3ConB4, and Ce2Co7B3 are built up from layers of composition CeCo5 alternating with 1, 2, or 3 layers of composition CeCo3B2.329 

The new k-borides M9Re4B (M = Zr or Hf) and Hf9Os4B have been prepared and identified.330 

35 has a crystal structure that may be described in terms of puckered layers of B and puckered double layers of Ir which also contain B atoms in trigonal-prismatic holes. There is no three-dimensional B network.331 

Biborides of holmium and thulium have been prepared from the elements.332 They belong to the same class of structures as A1B2. The structures of HoCo2B2, PrCo2B2, and Y bCo3B2 have also been determined.333 

In metal borides the metal atoms are not stacked as in the corresponding metal lattices (hcc, fee, or bcc) but form stacks to accommodate the boron nets or chains. In MB2 the metal atoms form face-linked trigonal prisms. The perovskite structure is rare in borides but it does occur in some ternary subborides, e.g., in LnRh3Bi in which Ln is a lanthanide. 

Isolated atoms Isolated atoms Isolated atoms Isolated atoms Isolated atoms Pairs 

Isolated and chains Branched chains Single chains Double chains Hexagonal plane Pluckered plane Octahedra and atoms Linked octahedra Cuboctahedra 

Monoborides of transition metals contain single boron chains in the matrix of metal atoms. The monoborides of Zr, Hf, and Ta are superconductors that have a critical temperature of 3-4°C. LuRh4B4 is a superconductor having a transition temperature of ll.T C and the superconducting transition temperature of YPdjBjC (0.3 X 0.4) is 23 K. NiB is used as a catalyst in fuel cells to lower the overvoltage. 

The metal diborides, MB2, have two-dimensional networks of boron atoms in a hexagonal crystal symmetry. They are very hard, brittle, metallic conductors, and are chemically inert (especially TiB2 and ZrB2). Transition metal diborides are resistant against carbon at lOOO C and are therefore used to passivate steel in coal liquefaction equipment. The diboride alloys Zro.13Moo.87B2 (Tc=10K) and SC0.1Nb1.9B2 (7 = 6.6K) are superconductors. 

The B-B bond lengths in borides is close to that in pure boron crystals, and the latter are quite hard (=3000kg/mm2). Furthermore, the relative bond lengths in the borides are different from the carbides. For example, in TiB2 the 

The CVD of Ceramic Materials Borides, SUicides, III—V Compounds and II—VI Compounds (Chalcogenides) 

The non-oxide ceramics comprise essentially borides, carbides, nitrides, and silicides. Like oxide ceramics they have two kinds of uses, which frequently overlap application of their physical properties and of their refractory high-temperature properties. Extensive accounts can be found in [2.1-3,8,10], 

b Temperature dependence of (a) the hardness and (b) the thermal condnctivity of ceramic materials in comparison with diamond and cubic boron nitride (CBN) [2.3] 

Young s modulus ( ,GPa) Flexural strength (x,MPa) Compressive strength (a, MPa) Vickers hardness HV (Mohs hardness HM) Other physicochemical properties, corrosion resistance, and uses lUPAC name 

320 2055 (11) Brown or dark powder, unreactive to oxygen, water, acids and alkalis. AHy i = 480 Id mol . Boron 

The two dominating design variables to be considered Borides and boride-based high-temperature refractories 

Beryllium hemiboride Be2B [12536-51-5] Cubic a = 467.00 pm Cl, cF12, FmZm, Cap2 type (Z = 4) 1890 1000 1520  

Boron is rather unreactive, but under certain conditions it forms one or more borides with most metals. For example, the reaction between magnesium and boron produces magnesium boride, Mg3B2  

This product is hydrolyzed by acids to produce diborane, B2H6  

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. 

It is thus possible to distinguish five types of boron compound, each having its own chemical systematics which can be rationalized in terms of the type of bonding involved, and each resulting in highly individualistic structures and chemical reactions  

The chemical reactivity of boron itself obviously depends markedly on the purity, crystallinity, state of subdivision and temperature. Boron reacts with F2 at room temperature and is superficially attacked by O2 but is otherwise inert. At higher temperatures boron reacts directly with all the non-metals except H, Ge, Te and the noble gases. It also reacts readily and directly with almost all metals at elevated temperatures, the few exceptions being the heavier members of groups 11-15 (Ag, Au Cd, Hg Ga, In, Tl Sn, Pb Sb, Bi). 

Fibre curling can be eliminated by heat treatment under tension near the mp, and the resulting fibres have a tensile strength of 3.S X 10 psi (I psi = 689S N m ) and an elastic modulus of SO x l( psi at a density of 2.35 gcm - the form was 

720 filament yam with a filament diameter of 11 -12/tm. The fibres are inert to hot acid and alkali, resistant to 

Boron itself has been used for over two decades in filament form in various composites BO3/H2 is reacted at 1300° on the surface of a continuously moving tungsten fibre 12/tm in diameter. US production capacity is about 20 tonnes pa and the price in about 80(. The primary use so far has been in military aircraft and space shuttles, but boron fibre composites are also being studied as reinforcement materials for commercial aircraft. At the domestic level they are finding increasing application in golf shafts, tennis rackets and bicycle frames. 

Rouxel, A. Le Blanc, and A. Royer, Bull. Soc. chim. France. 1971, 2019. 

Deyris, J. Roy-Montreuil, A. Rouault, R. Fruchart, and A. Michel, Compt. rend., 1971, 273, 

Ta3Re3B4 and TaReB3 have been identified in the Ta-Re-B system at 1400°C and characterized by X-ray studies.  

A number of stoichiometries obtain, such as LnB2, LnB4, LnBe, LnBia, and LnBee. 

The monophosphide is made by heating boron with red phosphorus at about 900°C in sealed tubes. It is cubic, isostructural with AlP (Table 8.10) and stable at ordinary temperatures and up to 2500°C under pressure. Heating in vacuo above 1100°C induces decomposition to the icosahedral boride (4.333). 

Thermal decomposition of some boron trihalide addition compounds will yield the monophosphide (4.334) and displacement from another metal phosphide may also be used (4.335,4.336). The compound can also be obtained directly from white P by the reaction (4.337) or by co-reduction of P halides [4]. 

The crystalline monophosphide is inert and has a reported melting point of 3000°C. It is harder than most metal borides and is as hard as silicon carbide and nearly as hard as boron nitride. A potential use is as a refractory semiconductor. 

Crystalline BP resists oxidation up to 800°C and is not dissolved by boiling mineral acids or cold concentrated alkali. Boiling with the latter produces phosphine and with steam above 400°C, some phosphine and boric acid are formed. Boron phosphide reacts on heating with halogens to form addition compounds. When heated to high temperatures in an atmosphere of ammonia, cubic boron nitride and phosphine are formed. 

FIGURE 4.22 Structures of boron phosphides (a) BP and (b) BijPj. Open circles = P, filled circles = B. In (b) the open circles represent end view of P-B-P ehains, or P-P units if the formula is B 2P2- 

The Bi2 and 654 units of boron. (0) The icosahedron unit common in elemental boron (the icosahedron has 12 vertices and 20 triangular feces), (b) The Bg4 unit found in some forms of boron is composed of a central B12 unit with an outwardly directed pentagonal pyramid associated with each of the 12 atoms of the central unit. 

A schematic representation of the variety of structures found in metal borides, (a) 

The structure of metal diborides, MBa. It consists of alternating parallel hexagonal layers of metal atoms and boron atoms. (From A. G. Sharpe, Inorganic Chemistry, 2nd edition. Copyright 1981, 1986. Reprinted by permission of Pearson Education Limited.) 

Besides these physical and chemical properties, boron has nuclear properties that add to its usefulness. Boron-10 absorbs neutrons efficiently. Note that the products of this absorption, shown in Equation (14.12), are nonradioactive isotopes of hehum and Hthium  

This abihty to absorb neutrons, together with their inertness and structural stability, makes the borides suited to a large number of nuclear applications, such as control rods and neutron shields. Equation (14.12) is also the basis of a neutron counter that either has boron on its inner walls or is filled with a gas such as boron trifluoride. 

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

Metalation Metalations Metalaxyl [137414-52-9] Metal borides Metal carbonyls... 

Adiponitrile undergoes the typical nitrile reactions, eg, hydrolysis to adipamide and adipic acid and alcoholysis to substituted amides and esters. The most important industrial reaction is the catalytic hydrogenation to hexamethylenediarnine. A variety of catalysts are used for this reduction including cobalt—nickel (46), cobalt manganese (47), cobalt boride (48), copper cobalt (49), and iron oxide (50), and Raney nickel (51). An extensive review on the hydrogenation of nitriles has been recendy pubUshed (10). 

The electron sources used in most sems are thermionic sources in which electrons are emitted from very hot filaments made of either tungsten (W) or lanthanum boride (LaB ). W sources are typically heated to ca 2500—3000 K in order to achieve an adequate electron brightness. LaB sources require lower temperatures to achieve the same brightness, although they need a better vacuum than W sources. Once created, these primary electrons are accelerated to some desired energy with an energy spread (which ultimately determines lateral resolution) on the order of ca 1.5 eV. 

Boron trifluoride has been used in mixtures to prepare boride surfaces on steel (qv) and other metals, and as a lubricant for casting steel (see... 

Contrary to previous indications apparendy gallium boride does not exist. 

Hafnium Boride. Hafnium diboride [12007-23-7] HfB2, is a gray crystalline soHd. It is usually prepared by the reaction of hafnium oxide with carbon and either boron oxide or boron carbide, but it can also be prepared from mixtures of hafnium tetrachloride, boron trichloride, and hydrogen above 2000°C, or by direct synthesis from the elements. Hafnium diboride is attacked by hydrofluoric acid but is resistant to nearly all other reagents at room temperature. Hafnium dodecaboride [32342-52-2] has been prepared by direct synthesis from the elements (56). 

Hafnium dioxide is formed by ignition of hafnium metal, carbide, tetrachloride, sulfide, boride, nitride, or hydrous oxide. Commercial hafnium oxide, the product of the separation process for zirconium and hafnium, contains 97—99% hafnium oxide. Purer forms, up to 99.99%, are available. 

AH of the alloys Hsted in Tables 4 and 5 are austenitic, ie, fee. Apart from and soHd-solution strengthening, many alloys benefit from the presence of carbides, carbonitrides, and borides. Generally the cubic MC-type monocarbides, which tend to form in the melt, are large and widely spaced, and do not contribute to strengthening. However, the formation, distribution, and soHd-state reactions of carbides are very important because of their role... 

Reactions of HCl and nitrides, borides, silicides, germanides, carbides, and sulfides take place at significant rates only at elevated (>650° C) temperatures. The products are the metal chlorides and the corresponding hydrides. The reactions most studied are those involving nitrides of aluminum, magnesium, calcium, and titanium, where ammonia (qv) is formed along with the corresponding metal chloride. 

Metal-Matrix Composites. A metal-matrix composite (MMC) is comprised of a metal ahoy, less than 50% by volume that is reinforced by one or more constituents with a significantly higher elastic modulus. Reinforcement materials include carbides, oxides, graphite, borides, intermetahics or even polymeric products. These materials can be used in the form of whiskers, continuous or discontinuous fibers, or particles. Matrices can be made from metal ahoys of Mg, Al, Ti, Cu, Ni or Fe. In addition, intermetahic compounds such as titanium and nickel aluminides, Ti Al and Ni Al, respectively, are also used as a matrix material (58,59). P/M MMC can be formed by a variety of full-density hot consolidation processes, including hot pressing, hot isostatic pressing, extmsion, or forging. 

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