Calorimetric evaluation of phase change materials for use as thermal interface materials

The phase change behavior of organic and inorganic phase change materials, namely paraffin wax, microcrystalline wax, Na₂SO₄·10H₂O and CaCl₂·6H₂O, with melting temperatures close to room temperature, was evaluated by differential scanning calorimetry. The melting and solidification temperatures, supercooling, heat of fusion and thermal cycling stability of these materials, with and without additives, were determined. Paraffin was, with or without α-Al₂O₃ or BN particles, are potentially good thermal interface materials, because of the negative supercooling (down to -7°C), large heat of fusion (up to 142 J/g) and excellent thermal cycling stability. Microcrystalline wax is not suitable, due to its unclear endothermic and exothermic peaks and wide melting temperature range. The addition of 20-60 wt.% α-Al₂O₃ to paraffin wax decreases the melting temperature by 7°C. Beyond 60 wt.% α-Al₂O₃, the melting temperature of paraffin wax increases toward the value without α-Al₂O₃. The heat of fusion of paraffin wax attains a minimum at 20 wt.% α-Al₂O₃. The addition of BN has little effect on the phase change behavior of paraffin wax. The inorganic materials Na₂SO₄·10H₂O and CaCl₂·6H₂O, with and without nucleating additives, are not suitable for use as thermal interface materials, due to the incongruent melting and decomposition behavior, large supercooling (8°C or more) and thermal cycling instability.