Production of B Atoms and BH Radicals from B₂H₆/He/H₂ Mixtures Activated on Heated W Wires

Abstract

B atoms and BH radicals could be identified by laser-induced fluorescence when B2H6/He/H2 mixtures were activated on heated tungsten wires. The densities of these radical species increased not only with the wire temperature but also with the partial pressure of H2. The densities in the presence of 0.026 Pa of B2H6 and 2.6 Pa of H2 were on the order of 10(11) cm(-3) both for B and BH when the wire temperature was 2000 K. Densities in the absence of a H2 flow were much smaller, suggesting that the direct production of these species on wire surfaces is minor. B and BH must be produced in the H atom shifting reactions, BH(x) + H → BH(x-1) + H2 (x = 1-3), in the gas phase, while H atoms are produced from H2 on wire surfaces. The B atom density increased monotonously with the H atom density, while the BH density showed saturation. These tendencies could be reproduced by simple modeling based on ab initio potential energy calculations and the transition-state theoretical calculations of the rate constants. The absolute densities could also be reproduced within a factor of 2.5.

Decomposition processes of H3NBH3 (borazane), (BH)3(NH)3 (borazine), and B(CH3)3 (trimethylboron) on heated W wire surfaces

“…Mass spectrometric measurements showed that its decomposition efficiency is 86% when the wire temperature is 2.23 × 10 3 K and the decomposition efficiency increases even more with the addition of H 2 . These efficiencies are higher than those for B 2 H 6 , which has an efficiency of around 70% at 2.0 × 10 3 K and depends little on the wire temperature over this temperature [3,4]. However, since the vapor pressure of H 3 NBH 3 , ∼0.01 Pa at room temperature, is much lower than that of B 2 H 6 , it is necessary to ascertain that a high enough amount of active species, such as B atoms, is produced from H 3 NBH 3 .…”

“…The experimental procedure and apparatus for this study were similar to those described elsewhere [3,4]. H 3 NBH 3 was effused into a cylindrical chamber from its reservoir, since its vapor pressure is low, on the order of 0.01 Pa at room temperature.…”

“…The reservoir temperature was measured with a chromelalumel thermocouple. (BH) 3 (NH) 3 diluted with He to 2.0%. The vacuum chamber was evacuated with a turbomolecular pump (Osaka Vacuum, TG350FCWB) backed with a dry scroll pump (Ulvac, DIS-251).…”

“…The decomposition mechanism of B 2 H 6 activated on heated W wire surfaces has recently been examined by one of the present authors and it has been shown that B and BH are produced efficiently in the presence of an excess amount of H 2 [3]. B 2 H 6 must be decomposed to BH 3 on wire surfaces, while B and BH are produced from BH 3 through H-atom shifting reactions, BH x + H → BH x−1 + H 2 , in the gas phase [3,4]. Although B 2 H 6 can be a useful dopant material, it is not only toxic, but also explosive.…”

“…However, since the vapor pressure of H 3 NBH 3 , ∼0.01 Pa at room temperature, is much lower than that of B 2 H 6 , it is necessary to ascertain that a high enough amount of active species, such as B atoms, is produced from H 3 NBH 3 . (BH) 3 (NH) 3 , which has a benzene-like ring structure, and B(CH 3 ) 3 are other safe candidate sources of B-atoms. The vapor pressures of these materials are much higher than that of H 3 NBH 3 , but it is uncertain if the B N and B C bonds can be broken on hot wire surfaces.…”