# Thermal Characterization of Phase Change Materials by Differential Scanning Calorimetry: A Review
## 3 Calorimetric Characterization
### Calorimetric Methods  
In order to study PCMs from a calorimetric point of view, there are several methods and types of equipment, as listed in Table 2. These methods are different in terms of measurement principle, sample size and calibration process, leading to different result accuracy levels. A key parameter remains the size of the tested sample, which has to be large enough to be representative of the PCM. Furthermore, kinetic properties such as crystal growth, phase separation and subcooling are size dependent properties. Thus, in order to measure a sample with different size scales, one should use different methods such as DSC for milligram size samples, T-history at the gram scale and pilot plants in the case of testing kilograms of PCMs. According to the measuring principles, DSC machines are divided into heat-flux DSC (hf-DSC) (measurement of exchanged heat between sample and environment) and power compensation (the heat to be measured is compensated by electrical energy) [PERSON] and [PERSON] (2018). In the hf-DSC method, a sample and a reference are subjected to a controlled environment (air, nitrogen, helium, etc.) and a temperature program (heating, cooling and holding isothermally), that lead to a change in both the sample and reference temperatures [PERSON] and [PERSON] (2018). The difference in the heat flow rate to a sample and to a reference, as a result of the sample temperature change, is measured by sensors embedded in the DSC machine [PERSON] and [PERSON] (2018). In another definition, a DSC machine measures the energy change occurring in the sample while it is heated, cooled or held isothermally [PERSON] and [PERSON] (2018). This energy change is connected to a chemical reaction or physical transition, which consumes or generates heat. Thus, measuring this energy change enables us to quantitatively measure the reactions and transitions occurring in the sample [PERSON] and [PERSON] (2018); [PERSON] and [PERSON] (2018). An operator of a DSC machine should always consider some parameters such as DSC calibration, atmospheric conditions, sample  
Figure 1: Solid–liquid transition in a PCM: (**a**) schematic of solid–liquid phase change; (**b**) stored heat as a function of temperature: ideal phase change of adipic acid by NIST (dashed line), actual phase change (continuous line), (personal data). mass and properties, and temperature program. The theoretical principle of hf-DSC will be explained in the following section. As mentioned earlier, in the DSC method one is limited to testing a few milligrams of a sample, which might be not representative for an inhomogeneous material. Larger samples can be measured with the T-history method. The T-history is a method used to determine the stored heat as a function of temperature for larger samples using self-built equipment [22]. This method was firstly proposed by [PERSON] et al. [23] (Figure 2a). Both sample and reference are symmetrically placed in a controlled environment that is subjected to an abrupt temperature change. From the response of the known reference to the temperature change, the heat flow is determined. The heat flow to the sample and subsequent enthalpy curve can be calculated. This method was then modified by [PERSON] et al. [24] and [PERSON] et al. [25]. They modified the heat transfer between (sample/reference) and the ambient by using insulated test tubes for sample and reference in order to establish a more isothermal condition. From Figure 2b, one can see a modified version of the T-history set-up equipped with a fan and heat exchanger to maintain the ambient temperature under isothermal conditions [18]. As stated by [PERSON] and [PERSON] [18] and [PERSON] et al. [10], the main advantages of the T-history method can be listed as follows:  
* The most important one is the ability to measure large samples.