# Thermal Characterization of Phase Change Materials by Differential Scanning Calorimetry: A Review
## 4 PCM Properties Achieved by DSC
### Specific Heat Capacity
[43] calculated the heat capacity using Equation (18), where \(\phi_{m}\) is the material heat flow signal, \(\phi_{s}\) is the sapphire heat flow signal, \(m_{s}\) is the material mass and \(m_{st}\) is the sapphire mass:  
\[C_{p(s)}=C_{p(st)}\frac{D_{s}}{D_{st}}\frac{W_{st}}{W_{s}}+C_{p(pan)}\left( \frac{D_{s}}{D_{st}}\frac{\left(W_{st}-W_{pan}\right)}{W_{s}}-\frac{\left(W_{ s}-W_{pan}\right)}{W_{s}}\right) \tag{17}\]  
\[C_{pmn}=\frac{\varphi_{m}\cdot C_{p,sapphire}\cdot m_{st}}{m_{s}\cdot\varphi_{ st}} \tag{18}\]  
By using the TA Q200 DSC and applying ASTM method [44], [PERSON] et al. [54] achieved 1.7% and 2.5% deviations in heat capacity of indium and magnesium, respectively. However, in case of PCM (paraffin-based Parafol 18-97), the deviation increased to 11.5% for solid state and 7% for liquid state. The determination of the Cp value from the heat flow curve obtained by DSC measurements is only possible over the flat region of the curve. One has to consider a margin before and after the transition to avoid even very small changes in the peak slope. More care has to be taken for PCMs exhibiting multiple peaks due to the phase transformation [67]. [PERSON] et al. [17] experimentally studied the effects of heating rate and sample mass on thermal properties of two commercially available PCMs, namely, RT15 and RT22 HC by DSC. These PCMs can be utilized for low temperature applications such as air conditioning systems. Their study is focused on heat capacity as a primary step to calculate the latent heat rather than an absolute value. The heat capacity is represented as a function of temperature based on heat flow measurements, and the strong dependency of heat flow curve on heating rates is observed. This means that by increasing the heating rate, the phase transition takes place over a wider temperature range. Therefore, to calculate latent heat, the specific phase transition range is determined for each heating rate and sample mass. For this purpose, a baseline (where there is no change in the sample properties)  
Figure 15: Three steps in Cp measurements for Parafol 18–97, after [54]. is defined based on heat capacity, and the point showing more than 2% change in heat capacity was considered as the starting point of transition. In order to determine the latent heat related to each heating rate, the enthalpy is calculated in the previously defined range for each PCM in a separate 1K temperature range. It was also observed that samples with a lower mass were less influenced by heating rate. [PERSON] et al. [68] characterized octadecanoic acid among some organic PCMs in the temperature range of 70 \({}^{\circ}\)C to 90 \({}^{\circ}\)C as a promising candidate for nuclear safety applications. They emphasized the need for a modulated DSC for measuring the heat capacity in the overlap and phase transition regions. Generally, the heat capacity in solid and liquid states can be measured by the conventional heat-flux DSC following the ASTM three-step method (measuring baseline, calibration with sapphire and sample measurement). However, during the thermal events such as solid-liquid phase transition, it would be impossible to predict the correct baseline due to the large curvature, slope and offset of the baseline. In a conventional heat-flux DSC, the difference in terms of heat transferred between the furnace environment and the sample and between the furnace environment and the reference is measured over the thermal resistance (\(R\)) of conducting connections (heating disk) as a temperature difference. The model used in conventional DSC only considers the first term of Equation (19), which is an over-simplified model compared to the actual thermal events in the DSC. In order to evaluate the measurement based on the conventional model, the value of the thermal resistance should be determined by a calibration. Upon calibration, this model is acceptable to measure transition temperatures and the area under the curve.