Atomistic Insights into the Anisotropic and Low Thermal Conductivity in Neopentyl Glycol Crystals: A Molecular Dynamics Study
SM Wang and L Sun and B Li and L Dai, JOURNAL OF PHYSICAL CHEMISTRY C, 125, 15853-15862 (2021).
Neopentyl glycol (NPG) is a promising next-generation environment- friendly refrigerant, because NPG can release huge latent heat during a solid-phase transition from a plastic crystal phase to a true crystal phase. However, NPG has a very low thermal conductivity, which restricts its applications. In this paper, we investigated the mechanisms of thermal transport of an NPG crystal by performing atomistic molecular dynamics (MD) simulations. Our simulation results obtained the thermal conductivities of 0.50, 0.32, and 0.33 W m(-1) K-1 at 298.15 K along the a*, b*, and c* directions, respectively, which agree with the experimental results ranging from 0.15 to 0.42 W m(-1) K-1. The anisotropy of the thermal conductivity along the three directions is caused by the hydrogen-bond network in NPG. We reveal the reasons for the low thermal conductivity: the large gap between the low-frequency region and the high-frequency region in the phonon spectrum and the ultrashort phonon mean free path (MFP). The effective MFPs are only 1.28, 5.47, and 2.22 nm along the a*, b*, and c* directions, respectively. In addition, we find that the thermal conductivity is insensitive to the temperature from 218.15 to 298.15 K, probably because the ultrashort MFP is insensitive to the temperature. Furthermore, we find that vacancy defects affect the thermal conductivity in an intriguing manner. When the defect concentrations are 2 and 4%, the thermal conductivities along the b* and c* directions increase abnormally with the increase in temperature, which is related to the reorientation of hydroxyl groups upon the change in temperature. Overall, this work reveals the molecular mechanism of the thermal transport of NPG, which should provide valuable insights in enhancing the thermal conductivity of NPG for the application as an environment- friendly refrigerant.
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