High Temperature Calorimetric Study of Mixing, Phase Separation, and Crystallization in Silicate Glasses
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Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society
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Temperature (K) Fig. 1 Heat capacity of diopside, CaMgSi206, obtained by step scanning calorimetry on heating (10 K steps). [10]
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Fig. 2 Change in calorimetric signal during continuous cooling of diopside melt. Exotherms show crystallization [ 10].
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equilibrate. It is not suitable for in situ studies of most silicate melts. We are finishing construction of an ultrasensitive solution calorimeter for samples in the 1-2 mg range. Controlled atmosphere can be passed over the samples in solution calorimeters. A commercial Setaram DSC 111 calorimeter, with miniaturized thermopile detectors, measures heat capacities and enthalpies of transition (including glass transitions), [6] at 300-1073 K. Though somewhat delicate, it is sensitive, accurate, and precise (+1% in Cp). Calorimetric samples can be sealed in capsules and/or run in controlled atmosphere. A thermogravimetric apparatus (TGA), originally part of this instrument, now is run separately with a furnace capable of 1373 K. We found that running the calorimeter in a vertical position, necessary for TGA, compromised calorimetric accuracy severely. A commercial Setaram HT 1500 calorimeter, whose temperature can be varied from day to day in the 900-1773 K range, can be used for transposed temperature drop calorimetry, in which a sample is dropped from room temperature to high temperature in the calorimeter. A home-built "hybrid calorimeter" combines features of the Setaram and Calvet designs [7]. These instruments have been the workhorses for determining heats of fusion of silicates and heats of mixing in situ in silicate melts [7-9]. A recent development has been to use these calorimeters in scanning and step-scanning mode for measurement of enthalpies of fusion [10], of heat capacities of silicate melts [6, 11], and kinetic studies of melting and crystallization [12]. The accuracy of Cp measurements appears to be 3 % at 900-1400 K, falling off to 5-8 % at 1500-1773 K. The experiments are time-consuming, and 1-3 g samples are required. The heat capacity of diopside, obtained by heating in step scanning mode [10] is shown in Fig. 1. The increase in C at 1600-1680 K suggests incongruent melting and/or "premelting" phenomena in the solid. 'he major melting event at 1691 K is followed by a region of constant heat capacity in the liquid. Cooling results in a different sequence of events (see Fig. 2). At a continuous scan rate, diopside does not crystallize at its equilibrium melting point; rather, the liquid supercools, and crystallization to a complicated phase assemblage is seen [10]. This happens at variable temperatures in nominally identical experiments, suggesting that nucleation may be somewhat poorly controlled and presumably heterogeneous. A recent advance is the availability of a more conventional differential scanning c
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