Dynamics of iron-bearing borosilicate melts: Effects of melt structure and composition on viscosity, electrical conductivity and kinetics of redox reactions

Abstract The dynamic properties of a series of iron-bearing sodium borosilicate melts have been investigated to determine how structure and composition control viscosity, electrical conductivity and the kinetics of iron redox reactions and, thus, atomic mobility as involved in these different processes. For this purpose, four compositions with 67 mol% SiO 2 and B 2 O 3 contents ranging from 0 to 22 mol% have been studied. In addition to viscosity and electrical conductivity, we have determined the kinetics of the iron redox reaction by isothermal iron K-edge XANES and Raman spectroscopy experiments performed as a function of time from 710 to 1570 K. Substitution of sodium for boron at constant SiO 2 content first causes transformation of BO 3 triangles into BO 4 tetrahedra until an excess of sodium induces instead melt depolymerization. These changes in the degree of polymerization and boron coordination lead to a maximum in oxygen diffusivity at around 18 mol% B 2 O 3 , and correlatively, to a viscosity minimum. Because this change of trigonal into tetrahedral boron requires charge compensation of B 3+ by cations such as Na + ions, the mobility of Na + decreases and reduces the rate of oxidation. In addition, the decreasing fraction of Na + ions and their change from a free to a charge compensating role explain the decreasing redox diffusivities and electrical conductivities of the samples.