# Thermal Characterization of Phase Change Materials by Differential Scanning Calorimetry: A Review
## 4 PCM Properties Achieved by DSC
### Transition Temperatures  
The most important property of a PCM for a thermal storage application is the phase change temperature [22; 54]. Transition temperatures of PCMs are mostly determined by DSC. When a PCM goes through a phase transition from solid to liquid or vice versa, one can observe an endothermic or exothermic peak for melting and solidification, respectively. This peak (heat flow graph) provides us with important information regarding the PCM's thermal properties (before, during and after the transition), such as transition temperatures, enthalpy and heat capacities (mostly outside the transition range). In this section, we specifically look at the transition temperatures. As shown in Figure 8a for adipic acid, the transition peak (melting or solidification) can provide the transition temperatures, namely, transition onset (beginning of the peak), peak temperature (maximum of the peak) and endset temperature (end of the transition). [PERSON] et al. [48] report the transition enthalpy of adipic acid 254 J/g, which agrees with the values obtained for a heating rate over 1 \({}^{\circ}\)C/min in Figure 8b. Among these temperatures, melting onset is the most important one. In the literature, wide temperature ranges are reported on PCM's melting point [54] and sometimes the procedure behind the measurement is not mentioned. Thus,  
Figure 7: PCMs representing stored heat as a function of temperature during the melting phase and solidification with subcooling (personal data for adipic acid). one does not know the reason behind this wide temperature range. For example, there is about 7 \({}^{\circ}\)C range in the reported melting point of myristic acid from the family of fatty acid [55]. As shown in Figure 8b, among the transition temperatures, transition onset is less affected by mass and heating rate, so it can be considered as a melting point of the material. However, the peak and endset temperatures (transition range) shift to higher temperatures by increasing heating rate and sample mass [22; 56; 57]. The onset temperature is the intersection of a flat baseline and tangent of the melting peak in the heat flow curve (extrapolation of onset temperature) [54; 58]. The shift to higher temperatures is due to the thermal resistance of the sample that causes nonuniform temperatures. This means that the inner temperature of the sample is lower than the surface temperature where the DSC temperature sensor is placed. By increasing the heating rate, this temperature difference increases and the real temperature of the sample is always lagging behind the surface temperature. Therefore, the sample is still in the transition state and adsorbs heat while the surface temperature shows the end of transition. The measurement parameters such as sample mass, thermal contact between the sample and the crucible, thermal contact between crucible and sensor, sample thermal conductivity and the heating rate affect the difference between the measured and the real temperature of the sample. For PCMs, this temperature difference is higher compared to other materials due to their very low thermal conductivity and large heat capacity, especially during the phase change. Thus, the large amount of heat in narrow phase change temperature and low thermal conductivity leads to a thermal gradient inside the sample. As the heating rate and sample mass increases, this gradient also increases [22]. Therefore, the peak and endset temperatures shift to higher values by increasing the heating rate, and this leads to an increase of the temperature range for the transition, as already observed by [22; 42]. In order to measure the transition temperature accurately, scanning the sample with low heating/cooling rate is recommended [8; 54; 59]. However, selecting a heating/cooling rate is always a compromise between accuracy in terms of enthalpy and temperature and representativity.