Borides


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Borides

 

compounds of boron with metals. Borides have physical properties characteristic of metallic substances (increase of the coefficient of electrical resistance with increase in temperature, high values for electrical and heat conductivity, and a metallic luster), as well as those of nonmetallic substances with semiconductor properties. Borides of the transitional metals are an intermediate class between inter-metallic compounds (of the beryllide type) and the so-called embedded phases. A characteristic crystallochemical trait of borides is the presence of isolated configurations of boron atoms in their structures. The chemical stability of borides is determined largely by the strength of the boron-to-boron bonds in the boride lattices; it increases with increased boron content in them. The greatest chemical stability (according to the speed of hydrolytic decomposition) is observed in hexaborides and dodecaborides. Most borides are resistant to acids—for example, even boiling aqua regia has no effect on TaB2.

Figure 1. Dependence of heat capacity of diborides on temperature

The diborides (MeB2) are used most widely in engineering. The most important indicator of these materials is the change in their basic properties with temperature (see Figures 1, 2, and 3). The most important physical properties of some borides of refractory metals are shown in Table 1. A large group is formed by borides of rare-earth metals-the lantha-nides and scandium and yttrium, whose properties are similar to those of the lanthanides. The most interesting in this group are the hexaborides (MeB6). (See Table 2.) The structure of hexaborides is double in character. The crystal lattice of hexaborides may be regarded as a simple cubic lattice of metal atoms centered by an octahedron of boron atoms or as a cubic lattice of boron atom complexes with the metal atoms located freely in the center.

Borides have negligible plasticity and very great hardness

Figure 2. Dependence of coefficient of linear expansion of diborides on temperature

(microhardness 20–30 giganewtons per sq m [GN/m2]). The tensile strength of TiB2 at a porosity of 2–3 percent is 380 meganewtons per sq m (MN/m2), and at a porosity of 7–9 percent, 140 MN/m2 (1 GN/m2 = 100 kilograms-force per sq mm [kgf/mm2]; 1 MN/m2 = 0.1 kgf/mm2). The high heat resistence of this diboride is characterized by a relatively slow creep rate (at a tension of 90 MN/m2 the creep rate at temperatures of 1920°, 2080°, and 2270° C is 1.5, 9.2, and 57 microns/min respectively). The modulus of elasticity obtained in nonporous specimens by measuring the velocity of longitudinal ultrasonic oscillations is 650 GN/m2 for NbB2, 700 for TaB2, and 685 for Mo2B5; and 790 GN/m2 for W2B5.

Borides are obtained by several methods, the most important of which include the reduction of metal oxides with a mixture of boron carbide and carbon black according to the reaction MeO + B4C + C → MeB + CO; the reduction of mixtures of metal oxides with boric anhydride by carbon black according to the reaction MeO + B2O3 + C → MeB + CO; and reduction by the magnesium-thermal method according to the reaction MeOx + nBO1.5 + (1.5n + x)Mg → MeBn + (1.5n + x) · MgO.

Solid products are obtained from boride powder by pressing with subsequent sintering or by hot forming. Borides are

Figure 3. Dependence of heat conductivity of diborides on temperature

widely used in technology. Because of their emission properties, they are used in radio electronics—for example, lanthanum hexaboride is used to make cathodes of high-powered generator apparatus and instruments. Because of their high neutron capture cross section, borides are used in nuclear engineering as regulating materials and for protection against nuclear radiation. Their great hardness, wear resistance, and grinding capability make them useful in machine and instrument building. The ability of some borides to maintain their properties in a medium of molten metals has made it possible to use zirconium boride—for example, in metallurgy for making thermocouple tips, which provided the possibility of automatic control of the temperature of steel in open-hearth furnaces. Future uses of borides include in high-strength and high-modulus continuous fibers and threadlike crystals for reinforcing composition materials.

REFERENCES

Tugoplavkie materialy v mashinostroenii: Spravochnik. Edited by A. T. Tumanov and K. I. Portnoi. Moscow, 1967.
Samsonov, G. V. Tugoplavkie soedineniia: Spravochnik po svoi-stvan i primeneniiu. Moscow, 1963.
K. I. PORTNOI
Table 1. Physical properties of borides of refractory metals
 Density (g/cm3)Melting point (°C)Molar heat capacity at 20 °C (kJIkmol·°K) [cal/(mol·°C)]Heat conductivity 20 °C (W/m·°K) [cal/(cm·sec·°C)]Specific electrical resistance at 20°C (microhm·m)Temperature coefficient of linear expansion (106α·°C–1)
TiB ................ 24.52298054.5 [13.02]24.3 [0.058]0.209.5 (20°–2000°C)
ZrB ................ 26.09304050.2 [12.0]24.3 [0.058]0.3885.0 (20°–2000°C)
HfB ................ 211.232500.33 [0.08]0.125.1 (20°–1000°C)
VB ................ 25.1024000.197.5 (20°C–10000°C)
NbB ................ 27.0300016.7 [0.040]0.327.9–8.3 (20°–1100°C)
TaB ................ 212.62310030.4 [7.25]106 [0.254]0.375.6 (20°–1000°C)
CrB ................ 25.6220051.2 [12.24]22.2 [0.053]0.5711.1 (20°–1100°C)
Mo ................ 2B37.482200128.7 [30.75]26.8 [0.064]0.18
W ................ 2B513.10237031.8 [0.076]0.43
Table 2. Physical properties of hexaborides of rare-earth metals
 Density (g/cm*)Melting point (°C)Temperature coefficient of linear expansion (106α·°C-1)Specific electrical resistance at 20°C (microhm · m)Temperature coefficient of electrical resistance (αp·103°C-1)Hall’s coefficient (R × 104 cm3/coulomb)Thermal electro motive force μV·°C-1Work function (eV)
LaB ................ 64.7322006.40.1742.68—5.04.62.68
CeB ................ 64.8121907.30.6051.0—4.21.12.93
NdB ................ 64.9425407.30.281.93—4.48.73.97
SmB ................ 65.0825806.83.884.21.543.44.4
EuB ................ 64.9526006.90.85—0.90—50.2—17.74.9
GdB ................ 65.2725108.70.5151.40—4.390.12.05
YbB ................ 65.5723705.80.3652.34—83.6—25.53.13
YB ................ 63.7623006.20.4041.24—4.64.62.22
References in periodicals archive ?
Zirconium boride (Zr[B.sub.2]) is a new type of ceramic material, whose lamellar crystal structure is similar to graphite, exhibiting high performance including high melting point (>3,000[degrees]C), high hardness (36.0 GPa), high elastic modulus (around 500 GPa) and good friction performance [12,13].
Researchers demonstrated that the addition of hard boride particles to an aluminum matrix can enhance the composite strength.
This method is usually applied for producing the nanocomposite coatings with Niken matrix and nanofillers are carbide, nitrite, boride, or PTFE [49, 81, 82, 111-117].
Such new types of composites as functional gradient materials (FGM) are alloys consisting of hard grains of carbides, nitrides and borides of transition metals (for example, tungsten carbide, titanium carbide, titanium carbonitride, titanium diboride, etc.) forming a strong continuous framework matrix), and a metal bond (from cobalt, nickel, titanium, aluminum, etc.), the content of which varies continuously in the volume of such CM [12].
In all considered titanium borides, the shortest bond lengths (-1.8 [Angstrom]) are found between B atoms.
When boriding duration increases from 30 to 120 minutes, boride and diffusion zones are increased due to greater penetration of atomic boron deep into the alloy, as well as to the formation and growth of increasingly more amount of borides and increase of the diffusion zone (Fig.
Borides can exist as a wide range of compositions and display structural features, which depends strongly on the metal and boron ratio.
More than hundreds of kinds of materials, including borides, carbides, and silicides, are reported to be synthesized [1-5] by applying this process, while some of them are quite difficult to synthesize in conventional ways.
Even fluoropolymers are no threat to these barrels, which have a protective layer of nickel-rich boron alloy containing molybdenum and a matrix of borides and carbides.
The overlay welded layers wear less when the structure was formed of martensite or layers consisting of carbides, borides and others.
(15.) Nag S, Samuel S et al., Characterization of novel borides in Ti-Nb-Zr-Ta+ 2B metal-matrix composites.