Determination of phase change temperature of materials from adiabatic scanning calorimetry data

Abstract

Melting and other first-order phase changes usually occur in phase change materials (PCMs) within a noticeable temperature range rather than at a unique phase change temperature ($T_{\mathrm{pc}}$). Then the enthalpy and heat capacity have rather wide jumps and peaks, respectively, spread over such ranges of temperatures. Surprisingly, wide jumps and peaks are observed even in plain and simple cases when PCMs are pure substances with negligible hysteresis and/or supercooling and the measurements are quasi-equilibrium using very slow heating/cooling rates, as in adiabatic scanning calorimetry (ASC). We show that in such cases a unique $T_{\mathrm{pc}}$ can be identified and calculated from the measured heat capacity peaks. It suffices to take into account that PCM samples do not have an ideal microstructure but are rather composed of many micro- to nano-sized domains. The heat capacity peak is then an average of individual peaks that (a) come from all domains and (b) have different shifts from $T_{\mathrm{pc}}$ for different domain sizes. Interpreting a heat capacity peak measured by ASC in this way, we present a procedure from which $T_{\mathrm{pc}}$ can be evaluated. We apply the procedure to three examples of materials using available ASC data and point out the importance of the size distribution of domains.