The formation of boron, silicon and calcium containing molecular species in a graphite furnace in Ar/O2 mixtures

Abstract

The composition of the Ar/O2/C gas system in the presence of traces of either B, Si or Ca was calculated under the assumption of thermal equilibrium in the temperature range 500-5500 K. The mole concentration of oxygen was taken to be 1-4 %. Two sets of calculations were carried out. In the first one the presence of solid phase (graphite) was ignored and the calculations were performed for a single-phase (gas) system, at variable ratios C/O (0.5, 0.96, 1 and 2). In the second set of calculations the presence of solid carbon (graphite) was taken into account and the composition of the gas system in equilibrium with solid carbon, at p = 1 atm, was determined. The results presented show that the equilibrium composition, particularly the concentration of different compounds involving the trace elements, is very sensitive to the amounts, and the ratio of the amounts of oxygen and carbon. Increasing the O/C ratio results in increasing partial pressures of molecular and atomic oxygen, which favours the formation of oxides of the trace elements and moves their atomization temperatures to higher values. On the other hand, increasing the C/O ratio (C/O >1) favours atomization, but also carbide formation in the lower-temperature region. It was found that, over a relatively wide temperature interval (1000 < T < 3500 K), the composition of the Ar/O2/C/X (X = B, Si, Ca) system, with comparable amounts of oxygen and carbon (C/O = 1), does not significantly depend on the presence of the solid phase. The results of calculations enable a reasonable interpretation of numerous experiments carried out on similar systems.