Mass Spectrometric Study of Gaseous Species in the B–B2O3 System

Abstract

A combination of Knudsen effusion and mass spectrometric techniques have been employed in studying the species existing in the vapor in thermodynamic equilibrium with a condensed mixture of boron and boric oxide in the temperature range 1300°K to 1500°K. Positive ions were produced by electron bombardment of vapor effusing from the Knudsen cell and were analyzed mass spectrometrically. Ion currents of B⁺, BO⁺, B₂O₂⁺, and B₂O₃⁺ were observed. The mixture, B+B₂O₃, was shown to vaporize mainly as the gaseous molecule, B₂O₂. The heat of vaporization of B₂O₂(g) from the condensed phase is ΔH₁₄₀₀⁰=94±8 kcal/mole. The ΔH₀⁰ for the reaction ⅔B(s)+⅔B₂O₃(g)→B₂O₂(g) is found to be 35.7±3.5 kcal. The BO⁺ and B⁺ ions observed were found to be formed mainly in processes of dissociative ionization. The concentration of BO(g) in the vapor is at least an order of magnitude lower than that of B₂O₂(g) showing the dimerization energy is greater than 99.6 kcal. Results obtained with pure B₂O₃ liquid in the Knudsen cell agree with earlier published work.