Effect of fullerene C60 on phase transition enthalpy of paraffin wax: Calorimetry and structural analysis

Abstract

In this paper, the effect of minor fullerene С60 admixtures on the phase transitions enthalpy of phase change material (PCM) is studied experimentally. For this purpose, the stable molecular solutions of the paraffin wax (PW) and fullerene С60 were explored in terms of the effective heat capacity and enthalpy of diffuse phase transitions over the (27…65) °С temperature range and C60 mass fraction up to 0.000746 g∙g−1. The choice of C60 as PW admixture was based on the hypothesis that a small amount of C60 contributes to a modification of the PW internal structure and, as a consequence, to a change in its caloric properties. Some factors that may affect the uncertainty for the obtained experimental data were examined. It was shown that the phase transition enthalpy for the PW/C60 in the (40…60) °C temperature range increase from 154.0 J∙g−1 (for pure PW) to 194.7 J∙g−1 (for PW containing 0.000746 g∙g−1 of C60) during cooling and decrease from 233.2 J∙g−1 (pure PW) to 182.3 J∙g−1 (PW containing 0.000746 g∙g−1 of C60) during heating. The results obtained indicate the defining influence of С60 on the PW structure in the liquid and solid states. It was confirmed by an additional structural analysis, which reported that pure PW has a minimal cell volume compared to PW/С60. The C60 addition into PW contributes to the expansion of the cell, indicating that the molecules lose their tight packing. A clear dependence between the enthalpy of melting and unit cell volume for PW/C60 samples was found. It was observed that unit cell expansion of PW due to the addition of C60 reduces the enthalpy hysteresis in the heating-cooling cycle – a key parameter that affects the efficiency of a thermal energy storage material. This positive effect may be due to structural transformation in PW during the melting-solidification cycles.