Configurational heat capacity of Na 2O–CaO–Al 2O 3–SiO 2 melts

Abstract

The determination of the glassy and liquid heat capacity of a series of Na 2O–Al 2O 3–SiO 2 and CaO–Al 2O 3–SiO 2 melts using differential scanning calorimetry shows a scattered trend to higher C p values with increasing amounts of Al 2O 3 for each series of melts. The determination of C p conf as a function of composition, however, reveals a minimum value as a function of composition when Na 2O = Al 2O 3, and a maximum value when CaO = Al 2O 3. This is the condition at which the melt structure must change as, with increasing amounts of Al 2O 3, there are not enough cations to charge balance the Al 3+ in tetrahedral co-ordination. Furthermore, there is a minimum in C p conf for the CaO–Al 2O 3–SiO 2 melts at the condition Ca atoms = Al atoms. This is the composition at which the Al 3+ in tetrahedral co-ordination must begin to share their charge-balancers as the number of Al atoms is increased. As C p conf is a measure of the energy required for the melt structure to rearrange to be in equilibrium with temperature, this data shows that less energy is required as the number of non-bridging oxygens in peralkaline Na 2O–Al 2O 3–SiO 2 melts decreases. There is then a minimum in C p conf and the value begins to increase as the number of non-charge-balanced Al 3+ in tetrahedral co-ordination increases in the peraluminous composition melts upon the further reduction of the amount of Na + in the melt. Similar behaviour occurs in the metaluminous CaO–Al 2O 3–SiO 2 melts. The C p conf decreases as the number of non-bridging oxygens is reduced by the removal of CaO. However, when Ca atoms < Al atoms (i.e. when there is no longer enough Ca 2+ available that each Al 3+ can have its own charge charge-balancer, and they must begin sharing Ca 2+) the C p conf begins to increase. There is another change in the C p conf trend as the melts become peraluminous and the Al 3+ no longer have enough Ca 2+ to be charge balanced; and C p conf begins to decrease. As the two melt series approach the end-member 33Al 2O 3·67SiO 2 composition, the C p conf values converge. The measurement of C p conf values for these melts together with viscosity data allows the calculation of the S conf( T g) term in the Adam–Gibbs equation as a function of composition. The configurational entropy at T g increases as the composition changes from the end-member peralkaline/metaluminous to the peraluminous compositions; indicating that the range of structures occurring in the melts increases as the (Na 2O + CaO)/Al 2O 3 ratio decreases.