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Title: A molecular dynamics study of the effect of thermal boundary conductance on thermal transport of ideal crystal of n-alkanes with different number of carbon atoms

Phase change materials such as n-alkanes that exhibit desirable characteristics such as high latent heat, chemical stability, and negligible supercooling are widely used in thermal energy storage applications. However, n-alkanes have the drawback of low thermal conductivity values. The low thermal conductivity of n-alkanes is linked to formation of randomly oriented nano-domains of molecules in their solid structure that is responsible for excessive phonon scattering at the grain boundaries. Thus, understanding the thermal boundary conductance at the grain boundaries can be crucial for improving the effectiveness of thermal storage systems. The concept of the ideal crystal is proposed in this paper, which describes a simplified model such that all the nano-domains of long-chain n-alkanes are artificially aligned perfectly in one direction. In order to study thermal transport of the ideal crystal of long-chain n-alkanes, four (4) systems (C 20H 42, C 24H 50, C 26H 54, and C 30H 62) are investigated by the molecular dynamics simulations. Thermal boundary conductance between the layers of ideal crystals is determined using both non-equilibrium molecular dynamics (NEMD) and equilibrium molecular dynamics (EMD) simulations. Both NEMD and EMD simulations exhibit no significant change in thermal conductance with the molecular length. However, the values obtainedmore » from the EMD simulations are less than the values from NEMD simulations with the ratio being nearly three (3) in most cases. This difference is due to the nature of EMD simulations where all the phonons are assumed to be in equilibrium at the interface. Thermal conductivity of the n-alkanes in three structures including liquid, solid, and ideal crystal is investigated utilizing NEMD simulations. Here, our results exhibit a very slight rise in thermal conductivity values as the number of carbon atoms of the chain increases. The key understanding is that thermal transport can be significantly altered by how the molecules and the nano-domains are oriented in the structure rather than by the length of the n-alkane molecules.« less
Authors:
ORCiD logo [1] ;  [1] ;  [1]
  1. Auburn Univ., AL (United States). Dept. of Mechanical Engineering
Publication Date:
Grant/Contract Number:
SC0002470
Type:
Accepted Manuscript
Journal Name:
Journal of Applied Physics
Additional Journal Information:
Journal Volume: 119; Journal Issue: 20; Journal ID: ISSN 0021-8979
Publisher:
American Institute of Physics (AIP)
Research Org:
Univ. of Alabama, Tuscaloosa, AL (United States)
Sponsoring Org:
USDOE Office of Science (SC)
Country of Publication:
United States
Language:
English
Subject:
25 ENERGY STORAGE
OSTI Identifier:
1471499
Alternate Identifier(s):
OSTI ID: 1254367

Rastgarkafshgarkolaei, Rouzbeh, Zeng, Yi, and Khodadadi, J. M.. A molecular dynamics study of the effect of thermal boundary conductance on thermal transport of ideal crystal of n-alkanes with different number of carbon atoms. United States: N. p., Web. doi:10.1063/1.4952411.
Rastgarkafshgarkolaei, Rouzbeh, Zeng, Yi, & Khodadadi, J. M.. A molecular dynamics study of the effect of thermal boundary conductance on thermal transport of ideal crystal of n-alkanes with different number of carbon atoms. United States. doi:10.1063/1.4952411.
Rastgarkafshgarkolaei, Rouzbeh, Zeng, Yi, and Khodadadi, J. M.. 2016. "A molecular dynamics study of the effect of thermal boundary conductance on thermal transport of ideal crystal of n-alkanes with different number of carbon atoms". United States. doi:10.1063/1.4952411. https://www.osti.gov/servlets/purl/1471499.
@article{osti_1471499,
title = {A molecular dynamics study of the effect of thermal boundary conductance on thermal transport of ideal crystal of n-alkanes with different number of carbon atoms},
author = {Rastgarkafshgarkolaei, Rouzbeh and Zeng, Yi and Khodadadi, J. M.},
abstractNote = {Phase change materials such as n-alkanes that exhibit desirable characteristics such as high latent heat, chemical stability, and negligible supercooling are widely used in thermal energy storage applications. However, n-alkanes have the drawback of low thermal conductivity values. The low thermal conductivity of n-alkanes is linked to formation of randomly oriented nano-domains of molecules in their solid structure that is responsible for excessive phonon scattering at the grain boundaries. Thus, understanding the thermal boundary conductance at the grain boundaries can be crucial for improving the effectiveness of thermal storage systems. The concept of the ideal crystal is proposed in this paper, which describes a simplified model such that all the nano-domains of long-chain n-alkanes are artificially aligned perfectly in one direction. In order to study thermal transport of the ideal crystal of long-chain n-alkanes, four (4) systems (C20H42, C24H50, C26H54, and C30H62) are investigated by the molecular dynamics simulations. Thermal boundary conductance between the layers of ideal crystals is determined using both non-equilibrium molecular dynamics (NEMD) and equilibrium molecular dynamics (EMD) simulations. Both NEMD and EMD simulations exhibit no significant change in thermal conductance with the molecular length. However, the values obtained from the EMD simulations are less than the values from NEMD simulations with the ratio being nearly three (3) in most cases. This difference is due to the nature of EMD simulations where all the phonons are assumed to be in equilibrium at the interface. Thermal conductivity of the n-alkanes in three structures including liquid, solid, and ideal crystal is investigated utilizing NEMD simulations. Here, our results exhibit a very slight rise in thermal conductivity values as the number of carbon atoms of the chain increases. The key understanding is that thermal transport can be significantly altered by how the molecules and the nano-domains are oriented in the structure rather than by the length of the n-alkane molecules.},
doi = {10.1063/1.4952411},
journal = {Journal of Applied Physics},
number = 20,
volume = 119,
place = {United States},
year = {2016},
month = {5}
}