Thermal transport and phonon focusing in complex molecular crystals: Ab initio study of polythiophene

Abstract

Thermally conductive molecular crystals are of fundamental interest because they are unlike typical complex crystals, which conduct heat poorly owing to their large phonon scattering phase space. While molecular crystals with high thermal conductivity in the range of tens of Wm −1 K −1 have been known experimentally for decades, their intrinsic upper limits for thermal conductivity are unclear. Ab initio methods that have been successfully applied to simple crystals have proved difficult to adapt to molecular crystals due to quantum nuclear motion and their complex primitive cells. Here, we report the thermal transport properties of crystalline polythiophene with 28 atoms per primitive cell using an ab initio approach that rigorously includes finite-temperature anharmonicity and quantum nuclear effects. The calculated room temperature thermal conductivity is 198 Wm −1 K −1 along the chain axis, a high value that arises from exceptional phonon focusing along the chain for both acoustic and optical branches for nearly all wave vectors and despite short lifetimes in the picosecond range. Our finding, along with other recent ab initio studies of polyethylene, suggests that the intrinsic upper bounds for the chain axis thermal conductivity of polymer crystals may exceed 100 Wm −1 K −1 .

Quantifying anisotropic thermal transport in two-dimensional perovskite (PEA2PbI4) through cross-sectional scanning thermal microscopy

“…As such 𝐶 𝑣 does not have any directional dependence in contrast to 𝜗 and 𝜏 in governing the anisotropic thermal transport in the compound. Along the in-plane direction, the presence of a continuous [PbI4] 2octahedral framework allows uninterrupted pathways for heat transport [58]. In short, phonons would possess relatively large lifetimes and group velocities (𝜗 𝑞,𝑥𝑦 ) within these inorganic layers resulting in a higher thermal conductivity (𝑘 𝑙,𝑥𝑦 = 0.45 Wm -1 K -1 ).…”

Remarkably Weak Anisotropy in Thermal Conductivity of Two-Dimensional Hybrid Perovskite Butylammonium Lead Iodide Crystals

“…7 Here, we pin down the orientational effects to phonon group velocities. Since stronger covalent bonds in one crystal axis result in higher group velocities in that axis, 36 the group velocities along the organic chain direction are higher than those perpendicular to the chain direction. In the out-of-plane direction, the preferentially vertical alignment of organic chains leads to large phonon group velocities along the organic chains, compensating the weak interlayered interaction.…”

State-of-the-art, opportunities, and challenges in bottom-up synthesis of polymers with high thermal conductivity

“…Simulated thermal conductivities of polythiophene that include ∼62.9 ± 2.5 W m −1 K −1 of polythiophene fibers at room temperature, 163 ∼129 ± 30 W m −1 K −1 along the polythiophene chain with 150 unit cells, 54 and ∼198 W m −1 K −1 along the polythiophene chain axis. 164…”