High-Purity Crystalline Boron

Abstract

Vapor phase reduction of boron trichloride with hydrogen on electrically heated filaments in flow systems was investigated as a potential method for the preparation of high-purity boron. Experiments were conducted on laboratory and bench-scale flow reactors. Conversion efficiency was a function of mole ratio of reactants, residence time, mass of deposit, and deposition temperature.Massive deposits of crystalline boron analyzing as high as 99.6% by direct chemical analyses were obtained. Purity of boron appeared to be a function of both filament diffusion and boron trichloride purity. Titanium filaments were found to be most satisfactory, since contamination due to titanium could be removed by chlorination at 300~ X-ray diffraction studies indicated that titanium probably existed in solid solution with boron rather than as a boride.

Preferred orientation of TiB2 plates prepared by CVD of the TiCl4 + B2H6 system

“…Excess free B is easily codeposited whatever the gas systems used, while the lattice parameters are independent of the free B content. The density ofTiB 2 film containing free B is explained well by a rule of mixtures for TiB 2 (4.50 x 103 kgm -3) and B (2.33 X 103 kgm -3 [19]) [7]. It is widely believed that TiB 2 is strictly stoichiometric, and excess B is present as a dispersed second phase in amorphous or crystalline states [7].…”

Methods of Preparation of Amorphous Boron

“…The electrolyte consists of KBF4 (20%), NaCl (40%) and KCl (40%). After acid treatment, washed and dryed, the product contained 99.8% boron (STERN and LYNDS, 1958;STERN, 1960a,b).…”

“…Comparative thermodynamic calculations of the reduction of boron halogenides (BF 3 , BC1 3 , BBr3) by hydrogen have shown (CUEILLERON, 1944;EFREMKIN, 1957;STERN and LYNDS, 1958) that boron fluoride may be reduced only at very high temperatures (2000 0 C), thus excluding the possibility of its practical application. Hydrogen reduction of boron chloride to elementary boron takes place at temperatures above 1000 o C, according to the following reaction:…”

“…An increase of ratio of BC13 to Hz in the reactice mixture and a decrease of pressure increases the yield of boron (CUEILLERON, 1944;STERN and LYNDS, 1958;NISEL'SON et al, 1965;KRENEV et al, 1972). At lower temperatures (700-900 0 C) interaction of boron chloride with hydrogen leads more readily to formation of dichlorborane (ZHIGACH and STASINEVICH, 1968) according to equation:…”

Boron and Boron Alloys

“…Samples of very pure boron, > 99 % B, can be obtained by reduction of boron halides with hydrogen, especially BBr 3 [27], [28] and BCl 3 [29][30][31][32]. This is also the method of choice for laboratory synthesis [27], [29].…”