Thermal Conductivity of Molten B ₂ O ₃ , B ₂ O ₃ –SiO ₂ , Na ₂ O–B ₂ O ₃ , and Na ₂ O–SiO ₂ Systems

Abstract

With the aid of the transient hot‐wire method, the thermal conductivity of molten B 2 O 3 , B 2 O 3 –SiO 2 , Na 2 O–SiO 2 , and Na 2 O–B 2 O 3 systems was measured along with their temperature and composition. It was observed that the thermal conductivity of pure B 2 O 3 increased with temperature, until about 1400 K, and then decreased subsequently. Using the MAS ‐ NMR , 3Q‐ MAS , and Raman spectroscopy, the structure of B 2 O 3 and SiO 2 in the B 2 O 3 –SiO 2 system was confirmed. Findings show that an addition of B 2 O 3 into the pure SiO 2 system causes a significant decrease in thermal conductivity, due to the formation of boroxol rings. The thermal conductivity of the Na 2 O–SiO 2 system was measured and its phonon mean free path was calculated. In addition, a positive linear relation between viscosity and thermal conductivity was observed. In the Na 2 O–B 2 O 3 system, it was found that a change in the relative fraction of 4‐coordinated boron has an influence on the thermal conductivity when the concentration of Na 2 O is between 10 and 30 mol%, in which case the tetraborate unit is dominant.