Borides thermal stability

The thermal stability of alkali-metal borides is relatively low, which is expected from the high vapor pressures of the corresponding metals at high T. Consequently, the alkali-metal vapor pressure is an important parameter, and synthesis of alkali-metal boride is carried out in isothermal reactors that permit maximum alkali-metal pressure and hence optimum preparation conditions. 

The B-Na system includes two phases with different thermal stabilities. Either of these two borides can be obtained by direct synthesis on adjusting the alkali-metal pressure in the vapor phase. Thus, the preparation of NaB can be carried out in isothermal reactors at < 1100°C (p a = 45 X 10 N m" ) where the equilibrium... 

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

Borides have been prepared as single crystals by making use of gas-phase, liquid-phase and flux methods, depending on the thermal stability of the boride and on the required size and perfection of the single crystals. 

The metal-rich transition metal phosphides (<60% P) are dark coloured and insoluble in water, they have high chemical and thermal stability, they are dense, hard and brittle and have high thermal and electrical conductivities. These properties they have in common with the transition metal borides and silicides (and in some cases carbides and nitrides) to which they are often structurally related. With few exceptions, the transition metal phosphides, borides and silicides are not attacked by dilute acids and bases and may remain unaffected by hot concentrated mineral acids. 

Under vacuum conditions the thermal stability of boride layers is very high exceeding temperatures of 2500 K, and remaining extremely hard [6]. Oxydative environments reduce this stability dramatically to temperature limits of 900K where the boride dissolves. A positive aspect of this enhanced reactivity in air is the possibility of recycling substrate material by oxy dative tempering at 1000 K. 

Loboda, P. I., Bogomol, 1.1. (2002). The thermal stability of the directionally reinforced boride ceramics microstructure. Ceramics, 69,117-124. 

Kie] Kiessling, R., Liu, Y.H., Thermal Stability of the Chromium, Iron, and Tungsten Borides in Streaming Ammonia and the Existenee of a New Tungsten Nitride , Trans. Met. Soc. AIMS, 191(8), 639-642 (1951) (Experimental, Phase Relations, 11)... 

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... 

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 silicon, have high melting points, great hardness, low coefficients of thermal expansion, and good chemical stability. 

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