Characterization of surface ionic arrangements of borate melts induced by structural relaxation

This is the authors' abstract. We don't add key points for this paper.

Sodium borate is the main component of molten flux for copper alloy production. Both B 2 O 3 and Na 2 O are recognized as surface‐active species, but their roles in surface ionic coordination have not been clarified. Here, the surface ionic arrangements of sodium borate melts, where Na 2 O content is from 0 to 37.5 mol% and temperature is around 1273 K, are characterized by classical molecular dynamics simulation using a polarizable ion model (PIM‐MD), x‐ray absorption spectroscopy, and grazing‐incidence x‐ray scattering. The results are summarized below. For pure B 2 O 3 melt, the BO 3 triangles in the surface region preferentially form ring structures larger than 4.5 Å in diameter in the same plane by structural relaxation. For sodium borate melts, the BO 3 proportion among BO 3 and BO 4 polymorphs in the surface region is 40% higher than that in the bulk. The BO 3 and BO 4 polymorphs in the surface region form a large ring structure. These short‐ and medium‐range ionic rearrangements are found to be the main surface relaxation in borate melts. The estimated surface excess energy of pure B 2 O 3 melt is initially higher than 100 J m −2 , but it significantly decreases by the structural relaxation, resulting in less than 0.3 J m −2 in the equilibrium state.