Webb, Sharon L. and Knoche, Ruth and Dingwell, Donald B.
Determination of silicate liquid thermal expansivity using dilatometry and calorimetry.
In: European Journal of Mineralogy, Vol. 4, No. 1: pp. 95-104
A method for the determination of relaxed silicate liquid molar volume and expansivity at temperatures
just above the glass transition is discussed. The method involves the comparison of heat capacity and molar
expansivity in the glass transition region. Glassy and liquid heat-capacity data are obtained using differential
scanning calorimetry, and glassy thermal expansion data are obtained using scanning dilatometry. The molar
expansivity of the liquid is calculated by a fictive temperature normalization of the relaxation behavior of both
the heat capacity and the molar expansivity in the glass transition region, with the normalized heat capacity curve
being used to extend the dilatometric data into the liquid temperature range. This comparison is based upon the
assumed equivalence of the parameters describing the relaxation of volume and enthalpy.
The molar expansivity of relaxed sodium trisilicate (Na2Si3O7) has been determined in this manner at temperatures
above the glass transition temperature. This low-temperature determination of liquid molar expansivity has
been tested against high-temperature liquid expansivity data obtained from high temperature Pt double bob Archimedean
buoyancy measurements. The low-temperature molar expansivity (26.43±0.83xl0~4 cm3 mole"lβC_1
at 540°C) determined in this manner agrees within error with the high-temperature molar expansivity
(23.29±1.39xl0~4 cm3 mole^ºC1 at 1400°C). This dilatometric/calorimetric method of liquid molar expansivity
determination greatly increases the temperature range accessible for thermal expansion measurements. A
weighted linear fit to the combined low and high temperature volume data gives a molar expansivity of
23.0010.25x10^ cm3 mole^ºC"1. The volume-temperature relationship thus derived reproduces the measured
volumes from both dilatometry and densitometry with a RMSD value of 0.033 cm3 mole"1 or 0.14%. This
represents a substantial increase in precision, which is especially important for liquids whose high liquidus
temperatures restrict the temperature range accessible to liquid volume determinations.