Borides metal

Metal borides are notable for their high melting points, high hardness, good electrical conductivity and chemical resistance (see Table 5.6-4). They are manufactured by the following processes  

Metal borides possess excellent hard material properties, but are difficult to work 

Borides cannot be processed with bonding metals to hard metal alloys 

Titanium diboride and the chromium borides are virtually the only borides utilized industrially  

Despite their interesting properties wide application of borides is not possible, because they cannot be processed with bonding metals to hard metal alloys and the more expensive hot press process must be used. Furthermore, borides form low melting point eutectics with ferrous metals. 

Solid state metal borides are characteristically extremely hard, involatile, high melting and chemically inert materials which are industrially important with uses as refractory materials and in rocket cones and turbine blades, i.e. 

Metal borides may be boron- or metal-rich, and general families include MB3, MB4, MBg, MBjo, MB12, M2B5 and M3B4 (B-rich), and M3B, M4B, M5B, M3B2 and M7B3 (M-rich). The formulae bear no relation to those expected on the basis of the formal oxidation states of boron and metal. 

The structural diversity of these materials is so great as to preclude a full discussion here, but we can conveniently consider them in terms of the categories shown in Table 

Although this summary of metal borides is brief, it illustrates the complexity of structures frequently encountered in the chemistry of boron. Research interest in metal borides has been stimulated since 2001 by the discovery that MgB2 is a superconductor with a critical temperature, T, of 39 K. We explore this property further in Section 27.4. 

Examples of metal borides adopting each structure type 

Preparative routes to metal borides are varied, as are their structures. Some may be made by direct combination of the elements at high temperatures, and others from metal oxides (e.g. reactions 13.74 and 13.75). 

Description of the boron atom Pictorial representation of the boron organization association 

A large number of metal borides have been prepared and characterized. Several hundred binary metal borides M Bj, are known. With increasing boron content, the number of B-B bonds increases. In this manner, isolated B atoms, B-B pairs, fragments of boron chains, single chains, double chains, branched chains, and hexagonal networks are formed, as illustrated in Fig. 13.3.1. Table 13.3.1 summarizes the stoichiometric formulas and structures of metal borides. 

In boron-rich compounds, the structure can often be described in terms of a three-dimensional network of boron clusters (octahedra, icosahedra, and cubo-octahedra), which are linked to one another directly or via non-cluster atoms. 

Idealized patterns of boron catenation in metal-rich borides (a) isolated boron atoms, (b) B-B pairs, (c) single chain, (d) branched chain, (e) double chain, 

Formula Example Catenation of boron and figure showing structure 

Compound mp. [°C] Density [g/cm l Hardness [kg/mm ] Resistivity [poem] Thermal conductivity [W/cm °C] 

We noted in Section 13.6 that InCl is virtually insoluble in most organic solvents. In contrast, the triflate salt, InSOsCFs, dissolves in a range of solvents, making it a more convenient source of In(I). The salt can be stabilized as a crown ether complex (Fig. 13.28), the solid state structure of which reveals an In-O(triflate) distance (237 pm) that is shorter than the In-O(ether) distances (average 287 pm). 

Solid state metal borides are characteristically extremely hard, involatUe, high melting and chemically inert materials which are industrially important with uses as refractory materials and in rocket cones and turbine blades, i.e. components that must withstand extreme stress, shock and high temperatures. The borides LaBg and CeB are excellent thermionic electron emission sources, and single crystals are used as cathode materials in electron microscopes (see Box 13.8). 

The structural diversity of these materials is so great as to preclude a full discussion here, but we can conveniently consider them in terms of the categories shown in Table 13.3, which are identified in terms of the arrangement of the B atoms within a host metal lattice. The structure of the MBg borides (e.g. CaBg) is similar to a CsCl-type structure with Bg-units (Table 13.3) replacing 

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

S. J. Sindeband and P. Schwart2kopf, "The MetaUic Nature of Metal Borides," 97th Meeting of the Electrochemical Society Cleveland, Ohio, 1950 Powder Metall Bull 5/3, 42 (1950). 

Boron forms B—N compounds that are isoelectronic with graphite (see Boron compounds, refractoryboron compounds). The small size also has a significant role in the interstitial alloy-type metal borides boron forms. Boron forms borides with metals that are less electronegative than itself including titanium, zirconium, and hafnium. 

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. 

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. 

CH2CN)4Yb[( J.-H)2BH]2, and (CgH N)4Yb[( J.-H)2BH4]2 have been stmcturally characterized by x-ray crystallography and shown to contain ytterbium to boron hydride Yb—H—B linkages. Thermal decomposition of lanthanaboranes can be used to generate lanthanide metal borides. 

The structural complexity of borate minerals (p. 205) is surpassed only by that of silicate minerals (p. 347). Even more complex are the structures of the metal borides and the various allotropic modifications of boron itself. These factors, together with the unique structural and bonding problems of the boron hydrides, dictate that boron should be treated in a separate chapter. 

Other electropositive elements have been used (e.g. Li, Na, K, Be, Ca, Al, Fe), but the product is generally amorphous and contaminated with refractory impurities such as metal borides. Massive crystalline boron (96%) has been prepared by reacting BCI3 with zinc in a flow system at 900°C. 

Boron is unique among the elements in the structural complexity of its allotropic modifications this reflects the variety of ways in which boron seeks to solve the problem of having fewer electrons than atomic orbitals available for bonding. Elements in this situation usually adopt metallic bonding, but the small size and high ionization energies of B (p. 222) result in covalent rather than metallic bonding. The structural unit which dominates the various allotropes of B is the B 2 icosahedron (Fig. 6.1), and this also occurs in several metal boride structures and in certain boron hydride derivatives. Because of the fivefold rotation symmetry at the individual B atoms, the B)2 icosahedra pack rather inefficiently and there... 

Figure 6.1 The icosahedron and some of its symmetry elements, (a) An icosahedron has 12 vertices and 20 triangular faces defined by 30 edges, (b) The preferred pentagonal pyramidal coordination polyhedron for 6-coordinate boron in icosahedral structures as it is not possible to generate an infinite three-dimensional lattice on the basis of fivefold symmetry, various distortions, translations and voids occur in the actual crystal structures, (c) The distortion angle 0, which varies from 0° to 25°, for various boron atoms in crystalline boron and metal borides.

Phosphides resemble in many ways the metal borides (p. 145), earbides (p. 297), and nitrides (p. 417), and there are the same diffieulties in elassifieation and deseription of bonding. Perhaps the least-eontentious proeedure is to elassify aeeording to stoiehiometry, i.e. (a) metal-rieh phosphides (M/P >1), (b) monophosphides (M/P =1), and (e) phosphorus-rieh phosphides (M/P < 1) ... 

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)... 

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... 

Pierson, H., A Survey of the Chemical Vapor Deposition of Refractory Transition Metal Borides, in Chemical Vapor Deposited Coatings, pp. 27-45, Am. Ceram. Soc. (1981)... 

Selected Chemical Properties of Transition-Metal Borides... 

Ranking metal borides as refractory compounds results from the formation of covalent B — B bonds by the electron-deficient B atoms ". As a result the metal lattice may be changed drastically, even for low B contents. 

Figure 1 presents a scheme for the formation of B—B bonds in binary metal borides, i.e., the occurrence of one-, two- and three-dimensional B aggregates as a function of the periodic group of the metal constituent. 

A. Metal frame structures and metal borides with isolated B atoms... 

Metal borides with B pairs, B chain fragments, and chains ... 

Metal-boron ratio Metal borides and structure types Boron aggregation... 

Figure 1. Scheme of the formation of B — B bonds in binary metal borides B(0), borides with isolated B atoms 1b(2), borides with one-dimensional infinite B chains, each B atom having two nearest-B-neighbor atoms iB(3), borides with infinite two-dimensional B nets (three B neighbors for each B atom) formation of a three-dimensional B network (more than three nearest-neighbor B atoms for each B atom). 

Uncharacterized ternary and binary metal borides, lacking precise composition or crystal chemistry data (see Fig. 2, Table 2), are not covered in this context. Further-... 

Table 2. Structure Types, Boron Coordination and Representatives op Metal Borides with Isolated B Atoms (Filled Metal Host Lattice Compounds)...

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