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.
It remains challenging to achieve both strength and toughness in network materials via crosslinking. The hybridly double-crosslinked carbon nanotube networks designed here nicely achieve cooperative energy dissipation with minimal structural damage.
Azobenzene-doped glassy polyimides (azo-polyimides) offer some of the most efficient optomechanical power densities to date rivaling electrostrictive polymers. Despite such potential attributes, the optomechanical efficiency remains low in comparison to other smart materials. Using high-fidelity coarse-grained molecular dynamics simulations, the authors reconcile both experimental and theoretical challenges to understand the limiting factors for the optomechanical conversion in photostrictive polymers. Interestingly, the ideal optomechanical efficiency of 10-24% for a single-chain azo-imide monomer predicted here is equal to or a little higher than experimental reports, suggesting experimental design space. The time-dependent optomechanical efficiency of bulk azo-polyimide is quantified, for the first time, to be strongly correlated with the initial free volumes, a measure of polymer conformational freedom. This trend is elaborated by conformational order parameters and viscoelastic relaxation moduli. Resembling the role of porosity in azobenzene-contained metal/covalent organic frameworks to enhance the photo-switching efficiency, a larger conformational freedom enables >10 times increase in optomechanical efficiency comparing to existing experiments. This is primarily due to facilitated viscoelastic relaxation after photo-switching which alleviates residual stresses quickly and reduces heat dissipation. These findings suggest opportunities to improve the optomechanical performance through targeted strategies, such as porosity control and thermal annealing.
Superlattices with suppressed thermal conductivity are of great significance in the field of thermoelectricity and can improve the thermoelectric conversion efficiency of materials. Due to Anderson localization of coherent phonons, aperiodic superlattices have lower thermal conductivity than their periodic counterparts. At present, the thermal conductivity of superlattices is mostly predicted through ab initio or molecular dynamics simulations, which is computationally expensive and limits the size of the system. Meanwhile, there are many layered structural combinations for aperiodic superlattices, making it difficult to efficiently screen through all the combinations to search structures with the minimum thermal conductivity. In this work, based on a modified series thermal resistance model (STRM), a new effective medium theory (EMT) is established to predict the thermal conductivity of periodic and aperiodic superlattices. An adjacency factor near the maximum-resistance layers and a correction function, respectively, are introduced to account for the phonon coherence effect and the degree of randomization in the layer thickness. Combined with the genetic algorithm, EMT enables high-throughput screening of millions of aperiodic superlattice structures. This work demonstrates that the thermal conductivities of aperiodic superlattices at a wide range of system size can be constantly reduced to 1.4∼1.8 W/(m·K), which occurs at averaged periodic thicknesses in a stable range of 2.0∼2.5 nm.
In article number 2104414, Shangchao Lin, William Oates, and coworkers model the photo-switching process of bulk azo-polyimide using a novel, high-fidelity coarse-grained molecular dynamics framework. The optomechanical energy conversion efficiency is significantly enhanced by the larger initial free volume fraction due to residual stress alleviation and less heat dissipation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.