Fumed silica-based composite phase change materials with effective electric and magnetic heating abilities for wearable thermotherapy

In this study, paraffin wax (PW) was combined with fumed silica (FS) as a porous support and Fe₃O₄-incorporated expanded graphite (EG@Fe₃O₄) as a thermal conductivity enhancer and multifunctional thermal conversion agent. This combination resulted in the development of PW/FS/EG@Fe₃O₄ composite phase change materials (CPCMs) with varying PW content (60-80%). FS provided ample space to stabilize a significant amount of PW (up to 75%) without liquid leakage. The crystallization fractions of the confined PW exceeded 97%, outperforming most reported values for other PCMs confined in SiO₂-based materials and enabling high phase change enthalpies (<i>e.g.</i>, 146.1 J g^-1 for the composite with 75% PW). The thermal conductivities of the 60-80% PW CPCMs were significantly enhanced to 2.215-1.395 W (m K)^-1, representing an increase of 9.8-6.2 times compared to pristine PW. Additionally, EG@Fe₃O₄ endowed the CPCMs with electrothermal and magnetothermal conversion capabilities due to the high electrical conductivity of EG and the superparamagnetism of Fe₃O₄. Experimental testing of the 75% PW composite demonstrated its ability to exceed its melting point under the application of either a DC voltage or an alternating magnetic field. When used as a heat pack, the 75% PW CPCM maintained a consistent heat release within the 50-55 °C range for 12 minutes on a volunteer's back, meeting and surpassing the requirements for high-temperature thermotherapy. Overall, the combination of high thermal conductivity, substantial phase change enthalpy, excellent cycling durability, and multifunctional thermal conversion makes PW/FS/EG@Fe₃O₄ CPCMs highly promising for practical thermotherapy applications.