Determination of Thermal Diffusivity of Different Crystal Structures of Phase Change Materials by Means of the Flash Method

Abstract

Paraffin’s are often used as phase change materials (PCM) for thermal energy storage. For the development of storage systems, the thermophysical properties of the used materials have to be known with high accuracy. In this work, the thermal transport properties of a series of specimens are presented. The specimens were all prepared from the same paraffin, RT70HC. However, they differ in the cooling rate during the solidification process. A significant dependence of the resulting thermal transport properties could be observed. Rapidly cooled specimen using liquid nitrogen show about 18% lower thermal diffusivity compared to specimen which are slowly solidified at air temperatures above 30°C. This difference can also be seen in the investigations with scattering experiments on the structure of the specimens. However, the measured values determined for density and specific heat capacity show no direct dependence on the cooling rate during the solidification process. The thermal conductivity calculated from the measurement results shows a comparable cooling rate dependency as the thermal diffusivity. This dependence is several times greater than the expected measurement uncertainty in the determination of the thermal transport quantities. Consequently, when comparing the results of measurements on solid samples of paraffin, care must be taken to ensure that the specimens to be measured have been prepared under conditions which are completely comparable. In addition to these influences from the specimen preparation, it was shown that the pulse energy used in the flash method has a significant influence on the measurement result. It is therefore recommended to correct the measured values with respect to the pulse energy as described in this paper. Should this not be possible for various reasons, the minimum possible pulse energy should be used for the measurement in order to falsify the measurement result as little as possible.