Thermo-physical characterization of Hexadecane during the solid/liquid phase change

Abstract

In this study we provide a multi-physical and multi-scale characterization of an organic Phase Change Material (PCM), i.e. hexadecane, for both its liquid and solid phase and during the phase transition. Macroscopic thermal and physical properties provided are density and viscosity in the liquid phase and thermal conductivity and heat capacity in each phase. Further macroscopic measurements were done by differential scanning calorimetry (DSC), which was used to obtain a first estimation of temperatures at which the solid/liquid phase transition occurs. DSC results present a thermal hysteresis between melting (Tm) and solidification (Ts) temperature. A similar hysteresis was also collected during rotational and oscillatory rheometry at the phase change, where Ts is found to depend on the applied cooling rate. Moreover, near Ts the forming solid structure is continuously affected by conditions imposed by the rheometer (i.e. applied shear rate or stress) and the breakage of crystals takes place even at the weak imposed stress tested (0.001 Pa). Beside the bulk behavior, the local melting and solidification were studied at microscopic scale through Raman spectroscopy. The local melting temperature is very close to what found by DSC and by rheometry. On the other hand, crystallization onset is found at higher temperatures for long waiting times, thus considerably reducing the thermal hysteresis. Finally, we highlight a key influence of interfaces on the phase transition. Variations in boundary conditions (thermal and/or kinematical conditions) are found responsible for the way hexadecane’s solidification occurs.