Nanoscale Polymer Thermal Regulator and Rectifier Based on Solid-State Phase Transition

Polyethylene nanofibers with high chain alignment and high degree of crystallinity show a high thermal conductivity at room temperature. However, rotational disorder occurs at a high temperature close to melting temperature, in which the nanofibers switch from the orthorhombic phase to the hexagonal phase with a high-contrast and abrupt thermal conductivity change. Such a solid-state phase transition makes the polyethylene nanofiber intrinsically a thermal regulator. Measurements show a thermal switching ratio in average ~8x with maximum ~10x, which is the highest among solid-solid and solid-liquid phase transitions of all existing materials. Based on the sharp and high-contrast phase transition, an unusual dual-mode solid-state thermal rectification effect is demonstrated using a heterogeneous “irradiated-pristine” polyethylene nanofiber junction as a nanoscale thermal diode, in which heat flow can be rectified in both directions by changing the working temperature. For the nanofiber samples measured here, we observe a maximum thermal rectification factor as large as ~ 50 %, which only requires a small temperature bias of <10 K. The nanoscale thermal regulators and rectifiers open up new possibilities for developing advanced thermal management, energy conversion and, potentially thermophononic technologies.