Boride melting points

Refractory Compounds. Refractory compounds resemble oxides, carbides, nitrides, borides, and sulfides in that they have a very high melting point. In some cases, they form extensive defect stmctures, ie, they exist over a wide stoichiometric range. For example, in TiC, the C Ti ratio can vary from 0.5 to I.O, which demonstrates a wide range of vacant carbon lattice sites. 

Borides have metallic characteristics such as high electrical conductivity and positive coefficients of electrical resistivity. Many of them, particularly the borides of metals of Groups 4 (IVB), 5 (VB), and 6 (VIB), the MB compounds of Groups 2(11) and 13(111), and the borides of aluminum and siUcon, have high melting points, great hardness, low coefficients of thermal expansion, and good chemical stabiUty. 

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. 

Boride Density g/cm Melting Point Point°C Hardness Kg/mm (VHN50) Electrical Resistivity pohm-cm Thermal Conduc. w/cm °C Thermal Expans. 10-6/°C (300- 1000°C)... 

Crystal growth is relatively difficult for borides because they have high melting points and sometimes low thermal stability, as indicated in 6.7.2. 

Solid borides have high melting points, exceptionally high hardness, excellent wear resistance, and good immunity to chemical attack, which make them industrially important with uses as refractory materials and in rocket cones and turbine blades. Some metal borides have been found to exhibit superconductivity. 

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. 

Nitrides, borides and silicides. Nitrides have the highest melting points next to carbides. They are brittle, hard, and oxidize above 1000 °C. The nitrides mentioned below are efficient electrical insulators even at high temperatures, whereas others (e.g. TiN) are good conductors. 

The highest melting points of all the known compounds are exhibited by carbides, nitrides, borides and oxides. Since the first three groups of substances are unstable in an oxidizing atmosphere at high temperatures, most industrial refractories are based on oxides. 

Refractory borides with melting points of 1900—3000 °C are promising materials for high-temperature application. They are very stable, only slightly volatile up to about 2500 so that next to carbides they are the only suitable materials for application in vacuum at these temperatures. Their resistance to oxidation is somew hat superior to that of carbides (up to 1300—1500 °C). They also exhibit hardness and electrical conductivity which makes them suitable materials for electrical contacts and electrodes. 

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 ... 

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. 

It is exceedingly difficult to prepare elemental boron in a high state of purity because of its high melting point and the corrosive nature of the liquid. It can be prepared in quantity but low purity (95-98%) in an amorphous form by reduction of B203 with Mg, followed by vigorous washing of the material so obtained with alkali, hydrochloric acid, and hydrofluoric acid. This amorphous boron is a dark powder that may contain some microcrystalline boron but also contains oxides and borides. 

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