Effect of glass transition: density and thermal conductivity measurements of B2O3

The role of B2O3 as a fluxing agent for developing fluoride-free fluxes has been accentuated in recent years. Therefore, knowledge about the thermophysical properties of the oxide is essential to find the optimal chemical composition of the mold fluxes. In the present study, the density and thermal conductivity of B2O3 were measured by means of the buoyancy method, the maximal bubble pressure (MBP) method and the hot-wire method in the temperature range of 295 – 1573 K. The results are discussed in the context of the chemical stability of the B2O3 as well as the effect of glass transition on the thermal conductivity. The density of the B2O3 decreases non-linearly with increasing temperature in the temperature range of 973 – 1573 K. The MBP method was successfully applied for the density measurements with a viscosity up to 91 Pa.s. The thermal conductivity of the B2O3 in the solid and molten states increases with increasing temperature. Based on the Kittel’s equation, the temperature dependence of the thermal conductivity through the glass transition temperature of B2O3 was discussed.