Thermodynamic optimization of the Na2O-B2O3 pseudo-binary system

Abstract

The Na 2 O-B 2 O 3 system is thermodynamically optimized by means of the CALPHAD method. A two-sublattice ionic solution model, (Na +1 ) P (O −2 ,BO 3 −3 ,B 4 O 7 −2 ,B 3 O 4.5 ) Q , has been used to describe the liquid phase. All the solid phases were treated as stoichiometric compounds. A set of thermodynamic parameters, which can reproduce most experimental data of both phase diagram and thermodynamic properties, was obtained. Comparisons between the calculated results and experimental data are presented.

Borate melt structure: Temperature‐dependent B–O bond lengths and coordination numbers from high‐energy X‐ray diffraction

“…The mass losses and compositional changes were kept to a minimum by limiting both the measurement times and the maximum temperatures reached. Although the same procedures were applied to the nominally 20 mol% Na 2 O melts of this study, larger mass losses were incurred owing partly to the higher congruent melting point of 1087 K for Na 2 B 8 O 13 compared to 1016 K for Na 2 B 4 O 7 . For the slowly cooled melt, Δ m = 0.7 mg loss from a total starting sample mass of m s = 30.0 mg was observed, whereas for the rapidly cooled melt, Δ m = 1.4 mg and m s = 30.4 mg.…”

“…B–O coordination numbers, n BO , in glassy, supercooled liquid and stable liquid low‐sodium borates. The glass‐transition region and melting point are indicated for 13 mol% Na 2 O, as interpolated from existing data . X‐ray diffraction bond length‐based determinations of this study: filled diamonds.…”

“…B–O coordination numbers, n BO , in glassy, supercooled liquid and stable liquid sodium borates, close to the diborate composition, x = 33.3, as indicated. T g and T melt are taken from the literature . X‐ray diffraction bond length‐based determinations of this study: filled diamonds (error bars encompass the full thermal expansion correction in this case).…”

“…Samples of nominally 50 and 70 mol% Na 2 O were also synthesized, but consistent diffraction measurements were not obtained. In the case of molten NaBO 2 , this was due to the high melting point (1241 K) which exacerbated soda volatilization and compositional drift, as well as the apparently low surface tension which made stable levitation challenging. The lack of glass formation at the metaborate composition furthermore limits the degree of supercooling and thereby the accessible T range.…”

“…The lack of glass formation at the metaborate composition furthermore limits the degree of supercooling and thereby the accessible T range. At 70 mol% Na 2 O, the melting point is low (844 K) and levitation was stable; however, both carbonate retention within the melts and apparently high hygroscopicity proved problematic.…”