Calorimetry of phase change materials (PCM) using the three-layer version of the T-history calorimeter

Research on phase change materials (PCM) frequently requires calorimetric measurements with sample sizes larger than those typically used in differential scanning calorimetry, particularly for heterogeneous or phase-separating materials and PCM with pronounced supercooling. The insulated three-layer version of the T-history method (“three-layer calorimeter”) is suitable for such measurements, but its operating principle, calibration, and measurement accuracy have not yet been analyzed in detail. The primary objective of this work is therefore to develop and assess a traceable calibration and evaluation methodology for three-layer calorimetry. A custom-built three-layer calorimeter equipped with high-precision temperature sensors and an integrated foil heater was investigated. The internal temperature was found to be homogeneous within approximately 0.2 K under typical operating conditions. A lumped-parameter thermal node model accounting for transient heat conduction in the insulation was developed and calibrated using Joule heating and empty runs instead of reference samples. The method was validated using the well-characterized reference materials water, n-hexadecane, and n-octadecane. In addition, gelled calcium chloride hexahydrate containing a nucleating agent (CCH-NA) was investigated as an application-relevant salt-hydrate PCM. The mean measured phase-change enthalpies deviated from the literature values by less than 0.5% for n-octadecane, by 0.3% during melting and 0.9% during crystallization for n-hexadecane, and by approximately 0.5% during melting and 1.5% during crystallization for water. The measured melting temperatures were systematically overestimated and the crystallization temperatures underestimated by approximately 0.3–0.8 K. This shift is attributed primarily to the thermal contact between the foil bag and the measuring cell because the sample temperature is not measured directly. The results demonstrate that the proposed calibration and evaluation procedure enables precise phase-change-enthalpy measurements on comparatively large PCM samples. The method is particularly useful for investigating supercooling and heterogeneous materials, while direct sample-temperature measurement and a more complete uncertainty assessment remain subjects for future improvement.