Stearic acid/fumed silica/Fe₃O₄ 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 Fe₃O₄ 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 Fe₃O₄ NPs (0-10%). The prepared MCPCMs exhibited suitable exothermic temperatures (∼51 °C) and high energy storage capacities (∼132 J g^-1) accompanied by high crystallization fractions (>93%). The low TCs of FS and Fe₃O₄ 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 Fe₃O₄ NPs, which led to an effective magnetothermal conversion under an alternating magnetic field. For comparison, MCPCM7 (the prepared MCPCM with 7% Fe₃O₄ 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.