Application of dynamic differential scanning calorimetry to study of phase transitions

A dynamic differential scanning calorimeter (dynamic DSC) works simultaneously as a differential scanning calorimeter and an AC calorimeter, using the same apparatus with AC temperature modulation superposed on a linear temperature ramp. From AC calorimetric (ACC) operation in which the part responding to AC temperature modulation is taken into account, we can determine the absolute value of heat capacity by making the best of the twin-type apparatus, although the temperature resolution is a little lower than in high precision AC calorimetry. Therefore, at a second order phase transition, ACC operation is useful for obtaining temperature dependence of linear heat capacity and furthermore, the frequency dispersion of the linear heat capacity is obtainable. On the other hand, at a first order phase transition, the situation is complicated. Nevertheless, latent heat at a first order phase transition can be estimated precisely from the both results of DSC operation and ACC operation. For further study, we have to pay attention to non-linear heat capacity. When a first order transition takes place over a certain temperature range, the analysis of the waveform of AC temperature observed at a steady temperature in this range gives evidence for a kinetic mechanism. The experimental test is performed for sodium nitrite, which undergoes both first and second order phase transitions.