Compositional dependence of thermophysical properties in binary alkaline earth borate melts: Insights from structure in short-range and intermediate-range order

In this work, the thermal conductivity of alkaline earth borate melts was measured using hot-wire method from 1323 to 1623 K, and the thermal diffusivity was extrapolated from the thermal conductivity and heat capacity. The compositional dependence of thermophysical properties was interpreted according to the structure in short-range and intermediate-range order. Based on the Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and ¹¹B magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectra, modifier cation with higher field strength prefers the formation of non-bridging oxygens (NBOs) for the charge compensation at high BO_1.5/MO ratios. A lower amount of covalent bond and greater isolation of large borate groups lead to a lower thermal conductivity in calcium borate melt compared with strontium and barium borates. But the large size of Ba²⁺ encounters difficulty in fitting around B^[4]-O-B^[4] linkages inside the overcrowded large borate groups when BO_1.5/BaO = 2.5, promoting the formation of NBOs on the edge of borate groups for the charge compensation of modifier cations and leading to the decline in the thermal conductivity. Thermal conductivity plays a major role in regulating the thermal diffusivity at a given temperature since the compositional dependence of volumetric heat capacity is relatively weak compared with that of thermal conductivity.