Stearic acid/fumed silica/FeO composite phase change materials with low thermal conductivities and magnetically accelerated heating performance for wearable thermotherapy.

Accelerating heating rates while maintaining low thermal conductivities (TCs) of fumed silica (FS)-based composite phase change materials is essential to achieve perfect thermotherapy materials. In this work, FS was combined with stearic acid (SA) and FeO nanoparticles (NPs) to form magnetic composite phase change materials (MCPCMs) with low TCs and effective magnetic heating performance. The MCPCMs were investigated with a fixed content of SA (70%) and varying contents of FS (20-30%) and FeO NPs (0-10%). The prepared MCPCMs exhibited suitable exothermic temperatures (∼51 °C) and high energy storage capacities (∼132 J g) accompanied by high crystallization fractions (>93%). The low TCs of FS and FeO NPs resulted in a decrease in TCs for the prepared MCPCMs of 31.8-45.4% compared to pure SA. In addition, the prepared MCPCMs inherited superparamagnetism from FeO NPs, which led to an effective magnetothermal conversion under an alternating magnetic field. For comparison, MCPCM7 (the prepared MCPCM with 7% FeO NPs) exhibited magnetic heating rates 13.0 and 7.5 times faster than heating by using ovens at 100 and 150 °C, respectively. MCPCM7 was used as a heat pad, which could release heat within 42-52 °C for 35 min to a volunteer's body. Last but not least, MCPCM7 possessed high thermal stability and excellent cycling durability for 500 heat storage/release cycles. The above-mentioned properties make it highly promising for practical thermotherapy applications.