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

In this paper, the effect of minor fullerene C-60 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 C-60 were explored in terms of the effective heat capacity and enthalpy of diffuse phase transitions over the (27.65)degrees C. temperature range and C-60 mass fraction up to 0.000746 g.g(-1). The choice of C-60 as PW admixture was based on the hypothesis that a small amount of C-60 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/C-60 in the (40...60)degrees 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 C-60) 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 C-60) during heating. The results obtained indicate the defining influence of C-60 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/C-60. The C-60 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/C-60 samples was found. It was observed that unit cell expansion of PW due to the addition of C-60 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.