Borides, synthesis

Zenin, A. A., Merzhanov, A. G., and Nersisyan, G. A., Thermal wave structure in SHS processes (by the example of boride synthesis). Combust. Explos. Shock Waves, 17,63 (1981). 

A processable polymeric precursor for boride synthesis can be obtained by dispersing a metal oxide in a boron carbide polymeric precursor. On heating, the mixture results in either in situ generation of boron carbide and carbon, followed by reaction to produce tbe boride or direct reaction of the polymer with the metal oxide to give the final boride product. The key requirements of the polymer component are given below ... 

Su, K., Sneddon, L. G. (1991). Polymer-precursor routes to metal borides Synthesis of TiBj, andZrBj. Chemistry of Materials, 3, 10-12. doi 10.1021/cm00013a005. 

Hafnium Boride. Hafnium diboride [12007-23-7] HfB2, is a gray crystalline soHd. It is usually prepared by the reaction of hafnium oxide with carbon and either boron oxide or boron carbide, but it can also be prepared from mixtures of hafnium tetrachloride, boron trichloride, and hydrogen above 2000°C, or by direct synthesis from the elements. Hafnium diboride is attacked by hydrofluoric acid but is resistant to nearly all other reagents at room temperature. Hafnium dodecaboride [32342-52-2] has been prepared by direct synthesis from the elements (56). 

Such reactions are discussed at appropriate points throughout the book as each individual compound is being considered. A particularly important set of reactions in this category is the synthesis of element hydrides by hydrolysis of certain sulfides (to give H2S), nitrides (to give NH3), phosphides (PH3), carbides (C Hm), borides (B Hm), etc. Useful reviews are available on hydrometallurgy (the recovery of metals by use of aqueous solutions at relatively low temperatures), hydrothermal syntheses and the use of supercritical water as a reaction medium for chemistry. 

Finally, the high-pressure synthesis favors the formation of borides that are difficult to prepare under standard conditions. In this way SmBj with the AlB2-type structure and GdB,2 and ThB,2 with the UB,2-type structure are prepared. ... 

Since metals react with B only at > 700°C, and since alkali-metal borides are thermally unstable, the preparation of these borides by direct synthesis can be carried out at high T only with an excess of metal in sealed containers. 

The formation of alkali-metal borides by direct synthesis requires temperatures of 750-1200°C, depending on the variety of boron utilized and the specific metal. This leads to two important consequences ... 

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... 

A.W. Weimer, Carbide, Nitride and Boride Materials, Synthesis and Processing, 1st Ed. Chapman Hall, London, 1997. 

Low-temperature solvents are not readily available for many refractory compounds and semiconductors of interest. Molten salt electrolysis is utilized in many instances, as for the synthesis and deposition of elemental materials such as Al, Si, and also a wide variety of binary and ternary compounds such as borides, carbides, silicides, phosphides, arsenides, and sulfides, and the semiconductors SiC, GaAs, and GaP and InP [16], A few available reports regarding the metal chalcogenides examined in this chapter will be addressed in the respective sections. Let us note here that halide fluxes provide a good reaction medium for the crystal growth of refractory compounds. A wide spectrum of alkali and alkaline earth halides provides... 

A reagent of nickel boride/hydrazine hydrate reduces both aromatic and aliphatic nitro compounds. For example, it has been used for synthesis of 4-(benzyloxy)indole and -alky ltryp-... 

Borides The electrolysis (at 950-1100°C) of a mixture of a borate and a transition metal fluoride (or alkaline-earth fluoride with the transition metal oxide) resulted in the synthesis of several borides (Ca, Sr, Y, Th, Ti, V, etc.). 

Application of carbo-thermal reduction. This is a synthesis process for the preparation of powders of carbides, nitrides and borides. Carbon may be graphite, coke, pyrolysed organic polymers. A reference process may be the Acheson process for the production of SiC ... 

Figure 3.35 Classification of chemical vapor infiltration processes. From Carbide, Nitride, and Boride Materials Synthesis and Processing, A. W. Weimer, ed. p. 563. Copyright 1997 by Chapman Hall, London, UK, with kind permission of Kluwer Academic Publishers.

In general, carbides, nitrides, and borides are manufactured in the vapor phase in order to form high-purity powders. This procedure is fundamentally different than a strict CVD process, since in powder synthesis reactors, deposition on seed particles may be desirable, but deposition on the reactor walls represents a loss of product material. As we will see, in CVD, heterogeneous deposition on a surface will be sought. Aside from this issue of deposition, many of the thermodynamic and kinetic considerations regarding gas phase reactions are similar. 

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