Synthesis of boron carbide and boron in an RF plasma.

The formation of boron carbide from BCl[3]-CH[4] and BCl[3]-H[2]-CH[4]-Ar mixtures in an RF plasma was studied and compared with a previous study of the formation of boron from BCl[3]-H[2]-Ar mixtures. The yields of boron carbide were determined under a variety of conditions and the product examined for composition and crystal structure. The products were finely divided powders. X-ray diffraction analysis of boron carbide showed that up to 14.6 wt. % carbon, B[13]C[2] was formed Between 14.6 wt. % and 21.7 wt. % carbon the additional carbon was incorporated into the lattice by random substitution for boron atoms in the B[12] icosahedra. Further carbon appeared as free graphite. The yields of boron and boron carbide were compared with thermodynamic calculations. It appears that a solid boron phase starts to settle out as the temperature in the plasma fail-flame falls to 2300 - 2500 K and continues to settle out until the temperature has fallen to approximately 1900 K. At this temperature the equilibrium conversion of boron trichloride to boron or boron carbide is a maximum. At low reactant flow rates, the experimentally determined amount of reaction was in close agreement with maximum equilibrium conversion. At high flow rates, the amount of reaction decreased. Below 1900 K the solid phase becomes unstable with respect to the gaseous reactants, but the reverse reactions did not appear to take place. Spectroscopic and thermodynamic studies indicated the most important radical was BCl. The radicals BH, B and BCl[2] were also detected. Dichloroborane, BHCl[2], was also formed as a reaction product of mixtures containing methane. The formation of boron appears to be initiated by the condensation of small boron nuclei followed by chemical vapour deposition by reaction of BCl with the nuclei. The role of hydrogen is purely as a scavenger for chlorine. In the formation of boron carbide, there appear to be two competing reactions, (a) BCl[3] + nCH4 → B[4]C (n-1/4) C (b) CH[4] → C and the driving force is again the formation of HCl. The formation of boron carbide is initiated by the formation of a carbon-rich nucleus which then undergoes chemical vapour deposition reaction with BCl molecules. The final composition of the product is determined by the composition and flow rate of the reactants