Melting boron compounds

The melting and boiling points of the aluminium halides, in contrast to the boron compounds, are irregular. It might reasonably be expected that aluminium, being a more metallic element than boron, would form an ionic fluoride and indeed the fact that it remains solid until 1564 K. when it sublimes, would tend to confirm this, although it should not be concluded that the fluoride is, therefore, wholly ionic. The crystal structure is such that each aluminium has a coordination number of six, being surrounded by six fluoride ions. 

Heating triacetylboron at temperatures above its melting poiat, 123°C, causes a rearrangement to B20(0CCH2)4 (15). An explosive hazard is also generated by dissolving BF ia anhydride (see Boron compounds). 

A product of higher purity is obtained by reducing a volatile boron compound, such as BCli or BBr3, with hydrogen on a heated filament (tantalum is used for the filament because it has a very high melting point) ... 

B4C boron carbide has a melting point of 2450 °C and a hardness somewhere between those of SiC and diamond. This makes the material a suitable abrasive. It is used in heads of sand blasting equipment, in mortars and in armour plating. For the latter application a B4C plate is provided on both sides with a plastic which has been reinforced with glass fibre. This is done to reduce the risk of splintering. Boron carbide is also used as the raw material for many other boron compounds ... 

Boron. Boron compounds have been used to treat wood for fire retardancy. Borax and boric acid, the primary fire-retardant compounds, have low melting points and form glassy films on exposure to high temperature. Borax, also known as sodium tetraborate deca-hydrate, is available in other hydrated states. Sodium tetraborate pentahydrate can be used in place of the decahydrate at a weight ratio of 74 (pentahydrate) to 100 (decahydrate) (81). 

BjHg, a colourless, volatile compound that is soluble in ether boiling point —92.5 C, melting point—165.5"C can be used to produce pentaboranc and decaboranc, proposed for use as rocket fuels also used to syn e-size organic boron compounds. Also known as boroethane diboron hexahydride. 

The element boron has an atomic number of 5, a molecular weight of 10.811, an oxidation state of 3 for simple compounds (but other oxidation states for carboranes and other polyhedral cage boron compounds), a specific gravity of 2.34, a melting point of 2300°C, sublimation at 2550°C, and is almost insoluble in water. Boron exists as B-10 (19.78%) and B-11 (80.22%) isotopes, and it contributes about 0.001% to the earth s crust, although it does not occur free in nature. The chemistry of boron is exceedingly con tlex and rivals that of carbon in diversity. 

The monophosphides MP, where M = B, Al, Ga or In, form an important group of phosphides in which each attmi is tetrahedrally coordinated by atons of the opposite kind in a cubic zinc blend-type similar to those of diamond, silicon and boron nitride (Figure 8.11). These monophosphides are hard high melting point compounds which have important semiconductor properties, and the system. GaP-... 

Table 4.1-29 Melting point Tm or decomposition temperature of boron compounds...

Chemical Minerals and Fertilizers. A number of nonmetallic minerals such as halite, baking soda, and sylvite are used in food. Halite and sylvite are salts used for flavoring. Halite is also used to soften water and to melt ice on roads. Borox is a boron compound that is used in some detergents and cosmetics. 

Because the solubility coefficients of carbon in the solid and the liquid phase are almost the same, zone melting, which is used to prepare high-purity crystals of many other elements, is not suitable in the case of boron. Technical boron, which is often taken as the ingredient for the preparation of boron compounds, contains up to about 0.5% carbon. However, in several preparative methods for boron compounds the carbon content may be reduced by secondary chemical or physical reactions. The purest P-rhombohedral boron crystals that have become available up to now were produced by Wacker-Chemie, Munich, FRG. Despite the claimed purity of 99.9999% with respect to other elements, even this high-purity boron contains carbon in concentrations of typically 30 to 80 ppm. Therefore, apart from boron carbide containing carbon as a determining bonding partner, in the assessment of the properties of boron and boron compounds attention must be paid to the fact that a certain, usually unknown carbon content could have influenced the properties determined. 

Boron is a highly significant element for nuclear engineering due to its capacity of neutron absorption. Therefore, boron compounds are crucial components in control rods, shielding and waste disposal or transport, but they can be also used in core catcher construction. So far, B4C, due to its highest boron content, is widely used in order to control fusion in nuclear reactors, typically as a composite in a steel matrix, or inserted into steel pipes. However, comparing to other borides, B4C is characterized by its poor chemical stability, and because of that, sometimes it is used as a substrate for other reactions with metals. It is also characterized by very poor corrosion resistance, since no protective oxides scale is formed. For instance, carbon is oxidized to COx(gas), while the only solid product of oxidation B2O3 melts at low temperature of 510°C. 

Boron nitride is chemically unreactive, and can be melted at 3000 K by heating under pressure. It is a covalent compound, but the lack of volatility is due to the formation of giant molecules as in graphite or diamond (p. 163). The bond B—N is isoelectronic with C—C. 

Lithium Nitride. Lithium nitride [26134-62-3], Li N, is prepared from the strongly exothermic direct reaction of lithium and nitrogen. The reaction proceeds to completion even when the temperature is kept below the melting point of lithium metal. The lithium ion is extremely mobile in the hexagonal lattice resulting in one of the highest known soHd ionic conductivities. Lithium nitride in combination with other compounds is used as a catalyst for the conversion of hexagonal boron nitride to the cubic form. The properties of lithium nitride have been extensively reviewed (66). 

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