# Thermal Characterization of Phase Change Materials by Differential Scanning Calorimetry: A Review
## 3 Calorimetric Characterization
### DSC Tests for PCMs  
There are several considerations during a DSC test that should be applied to any materials, such as sample size, calibration, and letting the sample be in a thermal equilibrium state by customizing the temperature program and sample mass. However, one faces additional challenges in calorimetry of PCMs, such as subcooling, high enthalpy and low thermal conductivity. PCMs usually show a phase change temperature range instead of a phase change temperature point [46]. Therefore, it is more challenging to describe their state and related thermal properties. It is also difficult to achieve a thermodynamic equilibrium in the sample due to subcooling during solidification. Furthermore, the internal thermal gradient in PCMs is larger than for any other materials due to the high enthalpy during the phase change and low thermal conductivity. This thermal gradient reaches its maximum near the phase change temperature, where a high precision is needed [22]. Thus, it is time-consuming to get the sample under isothermal conditions. Heat enters the sample from outside of the pan and the sample temperature is measured at the surface of the pan, so the real sample temperature, especially during the phase transition, is always overestimated in heating and underestimated in cooling (Figure 5a,b). Therefore, different results during the heating and cooling processes could be obtained, giving rise to a so-called hysteresis loop. [PERSON] and [PERSON] [18] distinguished two definitions of hysteresis: the hysteresis caused by a material property such as subcooling (supercooling or undercooling), called real hysteresis, and the hysteresis caused by measurement conditions, referred to as apparent hysteresis. One can see the subcooling hysteresis of adipic acid (equal to 5 \({}^{\circ}\)C) in Figure 6a. [PERSON] et al. [48] observed the same degree of subcooling for adipic acid of 99% purity. Generally, the real hysteresis cannot be avoided while the apparent hysteresis can be minimized by considering small size samples, low heating and cooling rates, and using instruments with higher reliability. However, it should be mentioned that small size samples might not be good representatives of PCMs and real hysteresis caused by subcooling is larger in small volumes [49]. Thus, DSC might not be a suitable method to find the actual subcooling of a PCM. For example, the subcooling of erythroid (sugar alcohols family) can increase from 14 \({}^{\circ}\)C for a sample mass of 200 g to 54-82 \({}^{\circ}\)C for few milligrams [26]. In a larger volume, there is more chance for nucleation which leads to a lower subcooling. In addition, low heating rates lead to low signal to noise ratios. Thus, there is always a balance when choosing between the suitable mass of PCM and the heating rate. Subcooling causes problems in recovering stored heat in the next thermal cycles. It  
Figure 4: Modes of heating and the corresponding DSC signals: (**a**) dynamic mode and (**b**) step mode after [1]. can be controlled by adding nucleating agent into the PCM. Detailed information about nucleation and subcooling can be found in the review article published by [PERSON] et al. [50]. Recently, methods like the one developed by IEA provide guidance to select a suitable heating rate. Sometimes, PCMs are not pure and they are mixtures of several constituents. Each constituent might melt/solidify at different temperature ranges that lead to the formation of multiple transition peaks, especially during solidification. For example, commercial PCMs, such as paraffin waves, are made of various kinds of alkanes, and paraffins mostly show congruent transition process because the molecular interactions between constituents overcome the molecular weight. However, if the molecular weights of various constituents are significantly different, peak separation will appear [46]. Scanning these kinds of PCMs with large heating rates results in disappearing peaks or the merging of small peaks into a greater one. As stated by [PERSON] et al.