High-contrast and reversible polymer thermal regulator by structural phase transition

Abstract

In comparison with the advancement of switchable, nonlinear, and active components in electronics, solid-state thermal components for actively controlling heat flow have been extremely rare. We demonstrate a high-contrast and reversible polymer thermal regulator based on the structural phase transition in crystalline polyethylene nanofibers. This structural phase transition represents a dramatic change in morphology from a highly ordered all-trans conformation to a combined trans and gauche conformation with rotational disorder, leading to an abrupt change in phonon transport along the molecular chains. For five nanofiber samples measured here, we observe an average thermal switching ratio of ~8× and maximum switching ratio of ~10×, which occurs in a narrow temperature range of 10 K across the structural phase transition. To the best of our knowledge, the ~10× switching ratio exceeds any reported experimental values for solid-solid and solid-liquid phase transitions of materials. There is no thermal hysteresis observed upon heating/cooling cycles.

Dual-mode solid-state thermal rectification

“…As shown by the blue circles in Fig. 1d , the measured thermal conductance of a pristine crystalline PE nanofiber first gradually decreases with temperature as explained by the well-known Umklapp scattering of phonons in crystals and then abruptly drops from ~45 nW K −1 to ~9 nW K −1 around 450 K due to the structural phase transition 27 . This sharp change of thermal conductance corresponds to a thermal switching ratio f ~ 5, where f = G on / G off is defined as the ratio of the on-state high thermal conductance G on to the off-state low thermal conductance G off .…”

“…Such a dramatic change in morphology (Supplementary Fig. 2 ) also indicates a structural phase transition from a highly ordered conformation (i.e., high thermal conductivity phase) to a rotationally disordered one (i.e., low thermal conductivity phase) 27 – 29 . As shown by the blue circles in Fig.…”

“…The variation of rectification factors is mainly attributed to the thermal switching ratio of pristine crystalline nanofibers, which usually ranges from 5 to 10. 27 Dynamic responses of HI-P nanofiber junctions upon multiple cycles. To evaluate the dynamic response of the nanoscale thermal diodes, we first measure the temperature-dependent thermal rectification factors of HI-P nanofiber junction #3 by sweeping the environmental temperature until a pronounced thermal rectification effect (rectification factor >10%) is observed at the temperature of 448 K. Then, we anneal and stabilize HI-P nanofiber junction #3 by holding its environmental temperature at 448 K for 6.5 h. After that, we thermally cycle the sample for 20 times by alternately switching its temperature bias from the forward direction to the reverse direction.…”

“…The variation of rectification factors is mainly attributed to the thermal switching ratio of pristine crystalline nanofibers, which usually ranges from 5 to 10. 27

Fig. 3 Measured thermal rectification of HI-P nanofiber junction #1.

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Enhancing the Thermo-Mechanical Property of Polymer by Weaving and Mixing High Length–Diameter Ratio Filler

“…Shen et al fabricated ultra-drawn PE nanofibers with extremely high thermal conductivity [ 9 ], and they later fabricated crystalline PE nanofibers with both high strength and thermal conductivity at low temperature via local heating method [ 10 ]. Recently, Shrestha et al fabricated PE nanofibers with room-temperature thermal conductivity of 50 W m −1 K −1 via two-step drawing method [ 11 ]. Zhang et al found that hard Cu nanowire is more useful than graphene to improve thermal conductivity of silicone [ 12 ].…”