Synthesis boron carbides

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). 

For the first time we have discovered transparent (painted in various colours) thread-like crystals of carbon among the products of hydrocarbon pyrolysis and during synthesis of silicon and boron carbides (Fig. 3.6) [12]. The X-ray spectral analysis has shown that the transparent threads consist of carbon (Fig. 3.7). 

Starting from fine boron and carbon powders, the direct synthesis of stoichiometric boron carbide is possible -, either under vacuum at 2073K in an electric furnace, or at 2273 K by hot pressing under Ar. This method is inefficient economically and finds no practical application. The compositions in the phase homogeneity range B10.4C-B4C can be obtained by hot pressing mixtures of B C with boron . The diffusion diagram between B and C is established. ... 

Carbides of metals and other elements have been produced by this approach, including carbides of boron, silicon, titanium, zirconium, hafnium, vanadium, niobium, molybdenum, tungsten, tantalnm, and thorium (Funke, Klementiev, Kosukhin, 1969 Sheppard Wilson, 1972 MacKinnon Wickens, 1973 Chase, 1974 Steiger Wilson, 1974 Swaney, 1974 MacKinnon Renben, 1975). The produced caibide particles are very small their diameter is usually about 20-200 mu. Halides are mixed with hydrocarbons, usually in a ratio H2 Me = 2-30. The gas mixture is heated up in plasma to temperatures of 1300-4000 K time of synthesis exceeds 50 ms. As an example, production of submicron boron carbide powder from gaseous boron trichloride and methane occurs in the strongly endothermic process ... 

An interesting n thod similar to Matsumoto s has been proposed by Wickens for boron carbide synthesis. The experimental arrangement is shown Fig. 81 a hi intensity arc is struck between a graphite cathode and a composite consumable anode made of boron oxide or sodium borate which are more readily available and cheaper than boron trichloride. B4C was obtained by this method, but the author indicates that the power comsumption (kWh/kg B4C) was high owing to insufficient erosion of the consumable anode. 

This material usually contains only small amounts of residual carbon or boron carbide but no metals, and is thus the favored process for the technical synthesis of less contaminated borides. The process is carried out in tunnel furnaces under hydrogen or in a vacuum at 1600-2000°C, i.e., below the melting point of the boride. It is thus a reaction sintering procedure yielding a high-porosity product which can easily be crushed and milled. Additional refinement is obtained by multiple vacuum treatments with metallic or B4C additives to compensate nonstoichiometries. The final product is then called vacuum quality . 

The first examples of catalytic hydroborations were reported in the 1980s. Sneddon published a series of papers on the additions of the B-H bonds in boron clusters to alk5mes catalyzed by transition metal complexes. " An example of these processes is shown in Equation 16.42. These reactions provided precursors to new boron cages and to boron-carbide materials. In 1985, Noth published the hydroboration of olefins with catecholborane catalyzed by Wilkinson s catalyst. One example of this process (Equation 16.43) shows the difference in chemoselectivity between the catalyzed and uncatalyzed processes. This report by Noth led to the development of catalytic hydroboration as a method for organic synthesis. Studies on both early and late transition metal catalysts have been conducted, and these studies included experiments to probe for differences in selectivities between catalyzed and uncatalyzed processes. 

Wang, H.L. and Gao, Q.M. Synthesis, characterization and energy-related applications of carbide-derived carbons obtained by the chlorination of boron carbide. Carbon Al, 820-828, 2009. 

A template-assisted synthesis of boron carbide nanowires was published by Pender and Sneddon. In this method, a linked cage 6,6 -(CH2)6(BioHi3)2 was selectively chosen as the single-source molecular precursor due to the fact that the ratio of boron to carbon in it closely matches that of boron carbide (B4C). This precursor compound is air stable with a melting point of 96°C, ° which helped... 

Pender, M. J. and Sneddon, L. G. An efficient template synthesis of aligned boron carbide nanofibers using a single-source molecular precursor. Chem. Mater, 2000,12, 280-283. 

Jazirehpour, M. and Alizadeh, A. Synthesis of boron carbide core-shell nanorods and a qualitative model to explain formation of rough shell nanorods. J. Phys. Chem. C, 2009,113,1657-1661. 

Li-Hong, B. Chen, L. Yuan, T. Ji-Fa, T. Chao, H. Xing-Jun,W. Cheng-Min, S. and Hong-Jun, G. Synthesis and photoluminescence property of boron carbide nanowires. Chin. Phys. B, 2008,17,4585-4591. 

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