Study of the Reaction of B, B2O3, and B4C with Na2CO3 in a Thermogravimetric Apparatus and of the Products by Transpiration Thermogravimetry, Thermal Ionization Mass Spectrometry, and Knudsen Effusion Mass Spectrometry

Abstract

A thermogravimetric (TG) study of the reaction of sodium carbonate with elemental boron, B2O3, and B4C was conducted under flowing commercial argon. The reaction products (when initial B-to-Na atomic fraction ratio = 1) were further subjected to transpiration thermogravimetric (TTG) measurements. Some residues were subjected to Knudsen effusion mass spectrometry (KEMS) and thermal ionization mass spectrometry (TIMS) as well. The present study is a sequel to two experimental observations, related to (1) a steep fall in vacuum in TIMS as the filaments containing the mixture of Na2CO3 and B (or H3BO3 or B4C) were being heated to measure the boron isotope ratios and (2) the detection of Na2BO2+ as the only boron-containing positive ion in TIMS, at variance with the KEMS results on NaBO2 (s). KEMS showed NaBO2+ as the principal ion, originating from the major species NaBO2(g), and Na2BO2+ as a much less intense ion, originating from the minor species (NaBO2)2 (g). The thermograms recorded over (B + Na2CO3), (B2O3 + Na2CO3), and (B4C + Na2CO3) mixtures consistently revealed onset of chemical decomposition of Na2CO3 at temperatures much lower than the temperatures at which pure Na2CO3 would undergo thermal decomposition. The results of the TTG measurements were in accord with those reported for NaBO2, thereby confirming its formation and also indicating as a corollary that the oxidation of B and B4C (which are known to be kinetically hindered) proceeded through completion with Na2CO3 quickly reacting with the oxidation product. The mass spectrometric examination of the residues reaffirmed the predominance of NaBO2+ in KEMS and Na2BO2+ in TIMS, leading to the inference that, even in TIMS, it is only the monomeric species NaBO2(g) that yields Na+ as well as Na2BO2+ but through two processes that occur on the filament surface: NaBO2 → Na+ + (BO2)− and NaBO2 + Na+ → Na2BO2+.