Although tremendous efforts have been devoted to enhance thermal conductivity in polymer fibers, correlation between the thermal-drawing conditions and the resulting chain alignment, crystallinity, and phonon transport properties have remained obscure. Using a carefully trained coarse-grained force field, we systematically interrogate the thermal-drawing conditions of bulk polyethylene samples using large-scale molecular dynamics simulations. An optimal combination of moderate drawing temperature and strain rate is found to achieve highest degrees of chain alignment, crystallinity, and the resulting thermal conductivity. Such combination is rationalized by competing effects in viscoelastic relaxation and condensed to the Deborah number, a predictive metric for the thermal-drawing protocols, showing a delicate balance between stress localizations and chain diffusions. Upon tensile deformation, the thermal conductivity of amorphous polyethylene is enhanced to 80% of the theoretical limit, that is, its pure crystalline counterpart. An effective-medium-theory model, based on the serial-parallel heat conducting nature of semicrystalline polymers, is developed here to predict the impacts from both chain alignment and crystallinity on thermal conductivity. The enhancement in thermal conductivity is mainly attributed to the increases in the intrinsic phonon mean free path and the longitudinal group velocity. This work provides fundamental insights into the polymer thermal-drawing process and establishes a complete process-structure-property relationship for enhanced phonon transport in all-organic electronic devices and efficiency of polymeric heat dissipaters.

To harvest and reuse low-temperature waste heat, we propose and realize an emergent concept—barocaloric thermal batteries based on the large inverse barocaloric effect of ammonium thiocyanate (NH 4 SCN). Thermal charging is initialized upon pressurization through an order-to-disorder phase transition, and the discharging of 43 J g −1 takes place at depressurization, which is 11 times more than the input mechanical energy. The thermodynamic equilibrium nature of the pressure-restrained heat-carrying phase guarantees stable long-duration storage. The barocaloric thermal batteries reinforced by their solid microscopic mechanism are expected to substantially advance the ability to take advantage of waste heat.

Strain engineering of metal halide perovskites shows promise for better stability and device performance, but the impact on thermoelectric performance remains elusive. We demonstrate that the electronic structures and carrier transport properties in halide perovskites CsPb­(I1–x Br x )3 can be tailored synergetically through the practical biaxial strain-engineering strategies. For the pure halide perovskite CsPbI3, the lattice geometry and electronic structures are basically retained under strains (from −6 to 8%), leading to moderately varied transport properties. Interestingly, under a −8% compressive strain, sharp changes in the carrier transport properties are observed in CsPbI3 because of the dramatically increased contribution of iodine electrons to the conduction band minimum. For the mixed halide perovskites, we find that CsPbI3/2Br3/2 is the thermodynamically most stable CsPb­(I1–x Br x )3 as determined by the generalized quasi-chemical approximation method. The band gap, carrier effective mass, and other carrier transport properties of CsPbI3/2Br3/2 change dramatically in response to high external strains (≤−6 or ≥6%), accompanied by the ultralow thermal conductivities. Such abnormal phenomena originate from the distorted lattice geometry that is caused by the non-uniform internal stress distribution under high external strains. In addition, external strains can also tailor the optimal carrier concentration needed to achieve the maximum figure of merit (ZT), providing a new avenue to tackle the longstanding challenge in heavy-doping perovskites. Finally, the ZT values are very sensitive to the magnitude of strains, especially for mixed halide perovskites, showing enhanced ZT from ∼0.1 without strain to ∼0.9 under a −6% compressive strain at 300 K. This work provides practical biaxial strain-engineering strategies to enhance the thermoelectric performance and also to optimize the doping process in mixed halide perovskites.