Heat flux DSCs glass transition

Figure 3.27 Calculation of heat capacity of an unknown using a Netzsch DSC200 heat-flux DSC [7]. The distinct shift in heat capacity at 690°C corresponds to the glass transition temperature (see section 7.6). A 191 mg sapphire standard was used as calibrant for a 130 mg (laser special) glass sample. All heating ramps were at 20°C/min (faster heating rates permit greater temperature lags). The right hand scale, in the original units of the differential thermocouple, is inverted in exothermic and endothermic directions as compared to the usual convention in this book.

Figure 7.11 Glass transition of B2O3 glass as determined by heat-flux DSC. Silicate glasses, because of their three dimensional network tend to have smaller volume changes at Tg and hence DTA/DSC traces of this transformation in those glasses are less distinct (13].

The principle of heat-flux calorimetry is illustrated with a schematic of a classical DTA in Appendix 9. Accuracies of heat measurements by DSC range from 10% to 0.1%. Temperature can be measured to 0.1 K. Typical heating rates vary between 0.1 and 200 K min. Sample masses can be between 0.05 and 100 mg. The smaller masses are suitable for large heat effects, such as chemical reacdons (explosions), phase transidons, or when fast kinetics is studied. The larger masses are necessary for assessment of smaller heat effects as in studies of heat capacity or glass transitions. Sensitivities are hard to estimate, but effects as small as 1.0 pi s are observable. 

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