[S-MP47_1PM2] Physics and Chemistry of Minerals

[SMP47-22] Temperature-dependent thermal expansivities of aluminum-free silicate melts and borosilicate melts

*Toru SUGAWARA¹, Junki KATSUKI², Satoshi YOSHIDA², Jun MATSUOKA², Kazuhiro MINAMI³, Eiji OCHI³ (1.Akita University, 2.The University of Shiga Prefecture, 3.Japan Nuclear Fuel Limited)

Keywords:silicate melt, thermal expansivity, densitometry

Thermal expansivities (dV/dT) of silicate melts are essential in a thermodynamic calculation of phase equilibira in magmatic system as a function of pressure and temperature and in a numerical simulation of flow and thermal structures in glass melting furnace. Previous studies have been suggested that the dV/dT of alumino-silicate melts (Lange, 1996; Potuzak et al., 2006) and magmatic silicate melts (Lange, 1997; Ghiorso and Kress, 2004) is a function of composition, but independent of temperature. On the other hand, it has been reported that the dV/dT of SiO₂-TiO₂-Na₂O melt (Liu and Lange, 2001) and 50SiO₂-25CaO-25MgO melt (Gottsmann and Dingwell, 2000) decrease with increasing temperature. Recently, we found that simulated-radioactive waste glass melt which has sodium-borosilicate composition also shows negative temperature-dependent dV/dT (Sugawara et al., 2013). We carried out density measurements for sodium-silicate melts ((100-x)SiO2-xNa2O, x=23 or 32.3 mol%), commercial soda-lime silicate melt (71SiO₂-6MgO-9CaO-14Na₂O, mol%) and borosilicate melts (66.6SiO₂-yB₂O₃-(33.33-y)Na₂O where y=8.3, 16.6, 25; 66.6SiO₂-(12.5+z)B₂O₃-(4.2-z)Al₂O₃-zCaO-(16.7-z)Na₂O where z=0 or 4.2 mol%). The temperature and compositional dependences of the dV/dT are discussed based on the new density data and the literature data. The high-temperature density (dHT) measurement has been made by double-bob Archimedean method between 1173K and 1665K. The glass samples were annealed around glass transition temperature (Tg) for 6-396 hours and quenched. Then the density of annealed glasses at 298K (d298) and linear thermal expansivity (dL/L) were determined by Archimedean method and TMA. respectively. The densities of supercooled melt around Tg (dTg) were calculated from the d298 and the dL/L of glasses. Then, molar volume as a function of temperature and the dV/dT of melts were obtained by combining the dTg and the dHT. The dV/dT values of all samples examined in this study show negative temperature dependence. In the sodium silicate melts, the temperature dependence of the dV/dT is remarkable when the SiO2 content increases from 50 to 67 mol%, while the dV/dT becomes close to zero as further increase in the SiO2 content. The negative temperature-dependent dV/dT observed in the 71SiO₂-6MgO-9CaO-14Na₂O melt can be reproduced by an additive sum of the dV/dT of 67.8SiO₂-32.2Na₂O, diopside (Gottsmann and Dingwell, 2000) and wollastonite (Potuzak et al., 2006) melts. High-temperature Raman spectroscopy for the SiO₂-Na₂O and SiO₂-Na₂O-MgO melts has been indicated that amount of Q4 species increases with increasing temperature and SiO₂ and MgO contents (Maehara et al., 2004, 2005). Therefore, the temperature dependent dV/dT for the sodium-silicate, commercial soda-lime silicate and diopside melts can be rationalized by an increase of rigid Q4 species at high temperature. The temperature dependence of the dV/dT is most remarkable in the 66.6SiO₂-8.3B₂O₃-25Na₂O melt among the borosilicate melts. The dV/dT decreases with replacement of Na₂O by B₂O₃ or CaO and of B₂O₃ by Al₂O₃, suggesting that partial molar dV/dT of B₂O₃ depends on temperature-induced coordination change of boron and their composition dependence (Wu and Stebbins, 2010). Acknowledgements: This work was a part of the research supported by Japan Nuclear Fuel Limited with Grant-in-Aid by the Ministry of Economy, Trade and Industry.