# Thermal Characterization of Phase Change Materials by Differential Scanning Calorimetry: A Review
## 4 PCM Properties Achieved by DSC
### Transition Temperatures
After measuring the sample with a low heating rate, it is recommended to conduct an experiment with a higher heating rate (lower signal to noise ratio) to check the value of enthalpy. [PERSON] et al. [60] carried out a study on the different sources of uncertainty (PCM, air conditions and geometric parameters) in an air-PCM thermal energy storage. They concluded that improving the determination of melting temperature range from \(\pm\)1 \({}^{\circ}\)C to \(\pm\)0.25 \({}^{\circ}\)C enhances the determination of air-heat exchange rate by 25%. [PERSON] et al. [8] used a combination of phase diagram and DSC curves to study the phase transition and related enthalpy of a binary mixture of paraffin-based tetradecane and hexadecane for a cooling application. It was concluded that the phase transition temperature range is a  
Figure 8: PCM’s transition peak: (**a**) transition temperatures, (**b**) shifting the end of transition toward a higher temperature. (Personal data for adipic acid). function of mixture composition. The lack of phase equilibrium, including thermal and chemical equilibria, is the reason why DSC measurements with high scanning rates could not provide correct information. They strongly recommend conducting a phase equilibrium study alongside the DSC measurement, especially in the case of binary mixtures of PCMs, and using low scanning rates in DSC tests. They also concluded that thermal relaxation takes longer for melting than solidification, leading to a larger temperature range during melting. Furthermore, with a higher heating rate, the endset temperature shifts to a higher value and the temperature range increases. The reason for this is heat transfer during the phase transition. During melting, the process transition starts by conduction and continues by natural convection, but during the solidification, it is the contrary. Thus, because the thermal conductivity is higher in the solid phase than in the liquid phase, solidification is faster than melting and transition occurs for a narrower temperature range. Apart from all approximations that should be considered, one should also minimize all resistance effects to be as close as possible to the true heat flow rate by subtracting baseline and improving the calibration constant. The temperature gradient inside the sample has an influence on the peak shape. When a transition takes place in the sample, there is no longer a steady state, and the temperature difference between the sample and the reference is delayed with time. [PERSON] et al. [19] carried out a numerical simulation based on the finite element method and observed the dependence of the peak shape on the heating rate, heat of transition and thermal conductivity of the sample. They concluded that only the onset of peak is independent of the test parameters. In Figure 9, [PERSON] and [PERSON] [61] numerically simulated heat flow curve of pure indium versus temperature and observed widening of the melting peak by increasing the heating rate. This finding agrees with the experimental results shown in Figure 10a,b, which are discussed in the next paragraph. Figure 10: The effect of (**a**) heating rates and sample mass on the transition temperature; (**b**) increasing the melting range with increasing heating rates for adipic acid (personal data). Figure 9: The effect of heating rate on peak shape for indium, after [61]. [PERSON] et al. [57] studied the phase transition temperatures of paraffin and hydrated salt with two different masses and three different heating rates. They considered a constant temperature for melting onset and observed the change in peak and endset temperatures. For each PCM, both peak and endset temperatures shift to higher temperatures because of temperature gradient. This temperature gradient increases by increasing either the sample mass or the heating rate. For the hydrated salt, they observed a step-like behavior, especially at lower heating rates and smaller masses, caused by phase separation. The same behavior was observed also by [PERSON] et al.