@article{xi_zhong_he_xu_nakayama_wang_liu_zhou_li_2021, title={A Ubiquitous Thermal Conductivity Formula for Liquids, Polymer Glass, and Amorphous Solids (vol 37, 104401, 2020)}, volume={38}, ISSN={["1741-3540"]}, DOI={10.1088/0256-307X/38/3/039901}, abstractNote={Center for Phononics and Thermal Energy Science, China-EU Joint Lab for Nanophononics, School of Physics Science and Engineering, Tongji University, Shanghai 200092, China Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695, USA Hokkaido University, Sapporo, Hokkaido 060-0826, Japan School of Environmental and Materials Engineering, Shanghai Polytechnic University, Shanghai 201209, China Paul M Rady Department of Mechanical Engineering, Department of Physics, University of Colorado, Boulder, CO 80305-0427, USA}, number={3}, journal={CHINESE PHYSICS LETTERS}, author={Xi, Qing and Zhong, Jinxin and He, Jixiong and Xu, Xiangfan and Nakayama, Tsuneyoshi and Wang, Yuanyuan and Liu, Jun and Zhou, Jun and Li, Baowen}, year={2021}, month={Mar} }
 @article{he_liu_2021, title={Evaluating the roles of temperature-dependent eigenvectors in predicting phonon transport properties of anharmonic crystals using normal mode analysis methods}, volume={129}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0053287}, DOI={10.1063/5.0053287}, abstractNote={Theoretical modeling of phonon transport process in strongly anharmonic materials at a finite temperature needs to accurately capture the effects of lattice anharmonicity. The anharmonicity of potential energy surface would result in not only strong phonon scatterings but also shifts of phonon frequencies and eigenvectors. In this work, we evaluated the roles of anharmonicity-renormalized phonon eigenvectors in predicting phonon transport properties of anharmonic crystals at high temperatures using molecular dynamics-based normal mode analysis (NMA) methods in both time domain and frequency domain. Using PbTe as a model of strongly anharmonic crystal, we analyzed the numerical challenges to extract phonon lifetimes using NMA methods when phonon eigenvectors deviate from their harmonic values at high temperatures. To solve these issues, we proposed and verified a better fitting strategy, Sum-up Spectrum Fitting Method (SSFM) than the original frequency-domain NMA method. SSFM is to project the total spectrum energy density data of all phonon modes onto an inaccurate (harmonic or quasi-harmonic) eigenvector base and then manually sum up the peaks that belong to the same phonon mode (at the same frequency). The SSFM relaxes the requirement for accurate temperature-dependent eigenvectors, making it robust for analyzing strongly anharmonic crystals at high temperatures.}, number={21}, journal={JOURNAL OF APPLIED PHYSICS}, author={He, Jixiong and Liu, Jun}, year={2021}, month={Jun} }
 @article{he_liu_2021, title={Molecular dynamics simulation of thermal transport in semicrystalline polyethylene: Roles of strain and the crystalline-amorphous interphase region}, volume={130}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0067999}, DOI={10.1063/5.0067999}, abstractNote={With potential thermal management applications, such as plastic heat exchangers and thermal interface materials, thermally conductive polymers have gained renewed interest in the past decade. Ultradrawn polyethylene fibers and films have been experimentally shown to have thermal conductivities at least two orders of magnitude of these in their amorphous counterparts. However, the theoretical molecular-level understanding of strain effects on the thermal transport in drawn semicrystalline polymers, such as polyethylene, especially the roles of different interlamellar chain topologies in the crystalline-amorphous interphase region, remains elusive. Using molecular dynamics simulations, we investigated the strain effects on the thermal conductivity and vibrational transport in a simplified sandwich semicrystalline structure. We found that the topology of the interlamellar chains determines the dependence of thermal conductivity on strains. Comparing thermal resistances at different regions in the interlamellar structure, thermal resistance at the amorphous region is not necessarily the highest; the interphase region with the transition from the crystalline to amorphous state can have a much higher resistance. We conducted the frequency domain analysis to obtain the heat flux spectrum in the crystalline-amorphous interphase region and found that the vibrational modes at intermediate and high frequencies can contribute more than these at relatively low frequencies to the total heat flux because of the complex interlamellar chain topologies (e.g., loop chains). Our work provides molecular-level understandings of the structural-property relationship in semicrystalline polymers with strains, which could assist the design and development of thermally conductive polymers for thermal management applications.}, number={22}, journal={JOURNAL OF APPLIED PHYSICS}, author={He, Jixiong and Liu, Jun}, year={2021}, month={Dec} }
 @article{zhong_xi_he_liu_zhou_2021, title={Thermal percolation and electrical insulation in composite materials with partially metallic coated fillers}, volume={119}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0067875}, DOI={10.1063/5.0067875}, abstractNote={We propose a type of thermal interface materials incorporating insulating nanowires with partially metallic coating in insulating polymer matrix. Large thermal conductivity can be obtained due to thermal percolation while the electrical insulation is maintained by controlling CMφ<φce and φ>φcth, where φ is the volume fraction of fillers, CM is the metallic coating fraction, and φce and φcth are the electrical and thermal percolation thresholds, respectively. The electrical conductivity of such composite materials can further be regulated by coating configuration. In this regard, we propose the concept of “thermal-percolation electrical-insulation,” providing a guide to design efficient hybrid thermal interface materials.}, number={21}, journal={APPLIED PHYSICS LETTERS}, author={Zhong, Jinxin and Xi, Qing and He, Jixiong and Liu, Jun and Zhou, Jun}, year={2021}, month={Nov} }
 @article{wu_zhang_negi_he_hu_tian_liu_2020, title={Synergistic Effects of Boron Nitride (BN) Nanosheets and Silver (Ag) Nanoparticles on Thermal Conductivity and Electrical Properties of Epoxy Nanocomposites}, volume={12}, ISSN={["2073-4360"]}, url={https://doi.org/10.3390/polym12020426}, DOI={10.3390/polym12020426}, abstractNote={Polymer composites, with both high thermal conductivity and high electrical insulation strength, are desirable for power equipment and electronic devices, to sustain increasingly high power density and heat flux. However, conventional methods to synthesize polymer composites with high thermal conductivity often degrade their insulation strength, or cause a significant increase in dielectric properties. In this work, we demonstrate epoxy nanocomposites embedded with silver nanoparticles (AgNPs), and modified boron nitride nanosheets (BNNSs), which have high thermal conductivity, high insulation strength, low permittivity, and low dielectric loss. Compared with neat epoxy, the composite with 25 vol% of binary nanofillers has a significant enhancement (~10x) in thermal conductivity, which is twice of that filled with BNNSs only (~5x), owing to the continuous heat transfer path among BNNSs enabled by AgNPs. An increase in the breakdown voltage is observed, which is attributed to BNNSs-restricted formation of AgNPs conducting channels that result in a lengthening of the breakdown path. Moreover, the effects of nanofillers on dielectric properties, and thermal simulated current of nanocomposites, are discussed.}, number={2}, journal={Polymers}, publisher={MDPI AG}, author={Wu, Yunjian and Zhang, Xiaoxing and Negi, Ankit and He, Jixiong and Hu, Guoxiong and Tian, Shuangshuang and Liu, Jun}, year={2020}, month={Feb}, pages={426–439} }
 @article{he_kim_wang_liu_2018, title={Strain effects on the anisotropic thermal transport in crystalline polyethylene}, volume={112}, ISSN={["1077-3118"]}, url={http://dx.doi.org/10.1063/1.5010986}, DOI={10.1063/1.5010986}, abstractNote={Thermal transport in the axial direction of polymers has been extensively studied, while the strain effect on the thermal conductivity, especially in the radial direction, remains unknown. In this work, we calculated the thermal conductivity in the radial direction of a crystalline polyethylene model and simulated the uniaxial strain effect on the thermal conductivity tensor by molecular dynamics simulations. We found a strong size effect of the thermal transport in the radial direction and estimated that the phonon mean free path can be much larger than the prediction from the classic kinetic theory. We also found that the thermal conductivity in the axial direction increases dramatically with strain, while the thermal conductivity in the radial direction decreases with uniaxial strain. We attribute the reduction of thermal conductivity in the radial direction to the decreases in inter-chain van der Waals forces with strains. The facts that the chains in the crystalline polyethylene became stiffer and more ordered along the chain direction could be the reasons for the increasing thermal conductivity in the axial direction during stretching. Besides, we observed longer phonon lifetime in acoustic branches and higher group velocity in optical branches after uniaxial stretching. Our work provides fundamental understandings on the phonon transport in crystalline polymers, the structure-property relationship in crystalline polymers, and the strain effect in highly anisotropic materials.}, number={5}, journal={APPLIED PHYSICS LETTERS}, author={He, Jixiong and Kim, Kyunghoon and Wang, Yangchao and Liu, Jun}, year={2018}, month={Jan} }