Thermal conductivity and structure of alkali silicate melts containing fluorides

Abstract

The thermal conductivities of alkali silicate melts containing fluorides have been determined using the non-stationary hot wire method. Measurements were carried out over the temperature range 1050–1550 K on the following six samples: Li₂O–2SiO₂, Na₂O–2SiO₂, 0·7Li₂O–0·6LiF–2SiO₂, 0·9Na₂O–0·2NaF–2SiO₂, 0·7Na₂O–0·6NaF–2SiO₂, and 0·5Na₂O–NaF–2SiO₂. In the temperature range investigated, the thermal conductivities of these silicates decreased with increasing temperature. Additions of fluorides were found to cause decreases in thermal conductivity. This supports the view that fluorides depolymerise the network structure of silicates to form bonds terminating in non-bridging oxygen and fluorine ions. It was concluded on the basis of this structural model that, from the viewpoint of the thermal resistance, the alkali fluorosilicate melts contain three types of chemical bonds: (i) primary covalent bonds (denoted by Si–O) between silicon and oxygen in the network structure, including Si–O⁻ and Si–F bonds because both are covalent, (ii) ionic bonds (denoted by O–O) between non-bridging oxygens via sodium or lithium ions, and (iii) F–F bonds which are actually dissociated by the formation of Si–F bonds. The magnitudes of the contributions to the thermal resistivities of the silicates r from these bonds were in the hierarchy: ρ _F–F > ρ _O–O in Na₂O–SiO₂ > ρ _O–O in Li₂O–SiO₂ > ρ _Si–O. The contribution from the F–F bonds was the greatest.