A dual-paraffin phase change actuator integrating thermal energy storage and thermoelectric conversion

Stefano Morese, Abhijit Telrandhe, Lakshmi Deepika Vandanapu, Uwe Pelz, Frank Goldschmidtböing, Suman Kundu, Peter Woias

Abstract Smart materials that can sense and react to environmental stimuli are enabling technologies in healthcare devices, soft robotics, aerospace and microelectromechanical systems (MEMS). Among these, paraffin-based phase change material (PCM) actuators have received significant attention due to their high mechanical energy density, affordability and versatility. However, their low thermal conductivity generally limits heat transfer, resulting in slow response times and hindering multifunctional integration. This study presents a dual-paraffin thermoelectric generator-PCM (TEG-PCM) actuator comprising controllable actuation, thermal energy storage and thermoelectric energy harvesting within a leakproof PCM-PDMS matrix. The system uses two paraffins with complementary functions. A low-latent heat paraffin (P1), enhanced with 1 wt.% graphene nanoplatelets (GnP), was filled into a copper cavity and optimized for actuation. A high-latent heat paraffin (P2), embedded in a PDMS matrix, provides thermal stabilization and heat storage. A TEG couples these two functions, enabling Peltier-driven actuation and reset as well as energy harvesting from external temperature gradients via the Seebeck effect. Micro-computed tomography and differential scanning calorimetry revealed that 30 wt.% P2 content offered the optimal balance between latent heat storage and mechanical integrity, with no leakage over multiple thermal cycles. The dual paraffin actuator exhibited maximum displacements and forces of 1400 µm and 150 mN, with response times of 208 s/cm³, representing a fivefold improvement over comparable paraffin-PDMS actuators. Directional infrared heating produced temperature differentials of up to 8 °C and open-circuit voltages of approximately 27 mV, corresponding to an estimated maximum power output of ~228 µW under matched-load conditions. When employed as an autonomous thermal switch, the actuator uses the P1-GnP expansion to temporarily bridge a thermal gap, thus doubling the thermoelectric output voltage. This multifunctional device couples autonomous and controllable actuation with thermoelectric energy conversion, representing a promising building block towards multifunctional systems for high-temperature microfluidic and thermal management applications.