کد مقاله کد نشریه سال انتشار مقاله انگلیسی نسخه تمام متن
4700568 1637719 2008 10 صفحه PDF دانلود رایگان
عنوان انگلیسی مقاله ISI
Configurational heat capacity of Na2O–CaO–Al2O3–SiO2 melts
موضوعات مرتبط
مهندسی و علوم پایه علوم زمین و سیارات ژئوشیمی و پترولوژی
پیش نمایش صفحه اول مقاله
Configurational heat capacity of Na2O–CaO–Al2O3–SiO2 melts
چکیده انگلیسی

The determination of the glassy and liquid heat capacity of a series of Na2O–Al2O3–SiO2 and CaO–Al2O3–SiO2 melts using differential scanning calorimetry shows a scattered trend to higher Cp values with increasing amounts of Al2O3 for each series of melts. The determination of Cpconf as a function of composition, however, reveals a minimum value as a function of composition when Na2O = Al2O3, and a maximum value when CaO = Al2O3. This is the condition at which the melt structure must change as, with increasing amounts of Al2O3, there are not enough cations to charge balance the Al3+ in tetrahedral co-ordination. Furthermore, there is a minimum in Cpconf for the CaO–Al2O3–SiO2 melts at the condition Caatoms = Alatoms. This is the composition at which the Al3+ in tetrahedral co-ordination must begin to share their charge-balancers as the number of Alatoms is increased.As Cpconf 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 Na2O–Al2O3–SiO2 melts decreases. There is then a minimum in Cpconf and the value begins to increase as the number of non-charge-balanced Al3+ 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–Al2O3–SiO2 melts. The Cpconf decreases as the number of non-bridging oxygens is reduced by the removal of CaO. However, when Caatoms < Alatoms (i.e. when there is no longer enough Ca2+ available that each Al3+ can have its own charge charge-balancer, and they must begin sharing Ca2+) the Cpconf begins to increase. There is another change in the Cpconf trend as the melts become peraluminous and the Al3+ no longer have enough Ca2+ to be charge balanced; and Cpconf begins to decrease. As the two melt series approach the end-member 33Al2O3·67SiO2 composition, the Cpconf values converge.The measurement of Cpconf values for these melts together with viscosity data allows the calculation of the Sconf(Tg) term in the Adam–Gibbs equation as a function of composition. The configurational entropy at Tg 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 (Na2O + CaO)/Al2O3 ratio decreases.

ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Chemical Geology - Volume 256, Issues 3–4, 15 November 2008, Pages 92–101
نویسندگان
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