Abstract
The reduction of boron trichloride to boron by hydrogen in a radio-frequency plasma has been investigated. Published work on chemical synthesis and processing in radio-frequency plasmas has been reviewed and conventional methods of boron preparation from boron halides discussed. A radio-frequency plasma reactor and ancillary apparatus were developed which could be operated continuously on a BCl[3] feed at power levels up to 30 kW. Boron formed in the plasma reactor was found to be similar to other amorphous borons. It was brown in colour, particle size, surface area (12.4 m2/g), and was non-pyrophoric. Purity was approximately 99% with traces of oxygen and nitrogen probably arising from content with air. X-ray and electron diffraction photographs of the boron revealed that the particles were microcrystalline and similar to beta-rhombohedral boron. Individual particles were random in shape and consisted mainly of platelets between 200A and 7500A in diameter. The presence of regular dodecagonal platelets was also observed. These had a unit cell lattice constant of 30A, and are believed to be a further modification of known boron structures. The growth time for a 7000A platelet was calculated to be of the order of 2 msec, which is comparable to the residence times of reactants in the plasma reactor. The ultraviolet emission spectra of various plasmas were examined and an excitation temperature of 20000°K was calculated for a 10 kW argon plasma. Species detected in the emission spectra of Ar/BCl[3]/H[2] plasmas included B, BCl, BCl[2], Cl[2], Cl[2+], B[+], Ar[+], H and BH. The variation of BCl[3] conversion with reactant concentration, residence time and reactant feed configuration was investigated. The reaction was approximately first order with respect to BCl[3] and very low order with respect to hydrogen suggesting that the latter acts merely as a scavenger for chlorine produced by plasma-induced dissociation of BCl[3]. These and other data were discussed in terms of three different types of mechanism. The results could be interpreted partly in terms of a theory in which thermo dynamic equilibrium of all species at a certain temperature was approached in the tail of the plasma, and partly in terms of a mechanism involving gaseous ions.