@article{kim_vetter_yan_yang_wang_sun_yang_comstock_li_zhou_et al._2023, title={Chiral-phonon-activated spin Seebeck effect}, volume={2}, ISSN={["1476-4660"]}, url={http://dx.doi.org/10.1038/s41563-023-01473-9}, DOI={10.1038/s41563-023-01473-9}, journal={NATURE MATERIALS}, publisher={Springer Science and Business Media LLC}, author={Kim, Kyunghoon and Vetter, Eric and Yan, Liang and Yang, Cong and Wang, Ziqi and Sun, Rui and Yang, Yu and Comstock, Andrew H. and Li, Xiao and Zhou, Jun and et al.}, year={2023}, month={Feb} }
 @article{negi_kim_hua_timofeeva_zhang_zhu_peters_kumah_jiang_liu_2023, title={Ferroelectric Domain Wall Engineering Enables Thermal Modulation in PMN-PT Single Crystals}, volume={4}, ISSN={["1521-4095"]}, url={https://doi.org/10.1002/adma.202211286}, DOI={10.1002/adma.202211286}, abstractNote={Acting like thermal resistances, ferroelectric domain walls can be manipulated to realize dynamic modulation of thermal conductivity (k), which is essential for developing novel phononic circuits. Despite the interest, little attention has been paid to achieving room-temperature thermal modulation in bulk materials due to challenges in obtaining a high thermal conductivity switching ratio (khigh /klow ), particularly in commercially viable materials. Here, room-temperature thermal modulation in 2.5 mm-thick Pb(Mg1/3 Nb2/3 )O3 -xPbTiO3 (PMN-xPT) single crystals is demonstrated. With the use of advanced poling conditions, assisted by the systematic study on composition and orientation dependence of PMN-xPT, a range of thermal conductivity switching ratios with a maximum of ≈1.27 is observed. Simultaneous measurements of piezoelectric coefficient (d33 ) to characterize the poling state, domain wall density using polarized light microscopy (PLM), and birefringence change using quantitative PLM reveal that compared to the unpoled state, the domain wall density at intermediate poling states (0< d33 <d33,max ) is lower due to the enlargement in domain size. At optimized poling conditions (d33,max ), the domain sizes show increased inhomogeneity that leads to enhancement in the domain wall density. This work highlights the potential of commercially available PMN-xPT single crystals among other relaxor-ferroelectrics for achieving temperature control in solid-state devices.}, journal={ADVANCED MATERIALS}, author={Negi, Ankit and Kim, Hwang Pill and Hua, Zilong and Timofeeva, Anastasia and Zhang, Xuanyi and Zhu, Yong and Peters, Kara and Kumah, Divine and Jiang, Xiaoning and Liu, Jun}, year={2023}, month={Apr} }
 @article{yang_raza_li_liu_2023, title={Thermal Transport in Poly(p-phenylene): Anomalous Dimensionality Dependence and Role of & pi;-& pi; Stacking}, volume={7}, ISSN={["1520-5207"]}, DOI={10.1021/acs.jpcb.3c02947}, abstractNote={For heat conduction along polymer chains, a decrease in the axial thermal conductivity often occurs when the polymer structure changes from one-dimensional (1D) to three-dimensional (3D). For example, a single extended aliphatic chain (e.g., polyethylene or poly(dimethylsiloxane)) usually has a higher axial thermal conductivity than its double-chain or crystal counterparts because coupling between chains induces strong interchain anharmonic scatterings. Intuitively, for chains with an aromatic backbone, the even stronger π–π stacking, once formed between chains, should enhance thermal transport across chains and suppress the thermal conductivity along the chains. However, we show that this trend is the opposite in poly(p-phenylene) (PPP), a typical chain with an aromatic backbone. Using molecular dynamics simulations, we found that the axial thermal conductivity of PPP chains shows an anomalous dimensionality dependence where the thermal conductivity of double-chain and 3D crystal structures is higher than that of a 1D single chain. We analyzed the probability distribution of dihedral angles and found that π–π stacking between phenyl rings restricts the free rotation of phenyl rings and forms a long-range order along the chain, thus enhancing thermal transport along the chain direction. Though possessing a stronger bonding strength and stabilizing the multiple-chain structure, π–π stacking does not lead to a higher interchain thermal conductance between phenyl rings compared with that between aliphatic chains. Our simulation results on the effects of π–π stacking provide insights to engineer thermal transport in polymers at the molecular level.}, journal={JOURNAL OF PHYSICAL CHEMISTRY B}, author={Yang, Cong and Raza, Saqlain and Li, Xiaobo and Liu, Jun}, year={2023}, month={Jul} }
 @article{huang_zhong_raza_niu_fu_yu_nakayama_liu_zhou_2023, title={Thermal resistance network model for thermal conductivity of normal liquid helium-4 and helium-3}, volume={35}, ISSN={["1361-648X"]}, DOI={10.1088/1361-648X/ace412}, abstractNote={Liquid heliums are intriguing substance. Superfluid states below certain critical temperatures, notably liquid helium-4 and helium-3 exhibit ultra-high thermal conductivity ( TC) in the superfluid phase. However, the microscopic origin of the TC of liquid heliums in the normal phase remains unclear. In this work, we employ the thermal resistance network model to calculate the thermal conductivities of normal liquid helium-4 (He I) and helium-3. Predicted values are not only in good agreement with the measurements but also reproduce the experimental trend of TC increasing with temperature and pressure.}, number={41}, journal={JOURNAL OF PHYSICS-CONDENSED MATTER}, author={Huang, Dan and Zhong, Jinxing and Raza, Saqlain and Niu, Ran and Fu, Baishan and Yu, Dapeng and Nakayama, Tsuneyoshi and Liu, Jun and Zhou, Jun}, year={2023}, month={Oct} }
 @article{negi_rodriguez_zhang_comstock_yang_sun_jiang_kumah_hu_liu_2023, title={Thickness-Dependent Thermal Conductivity and Phonon Mean Free Path Distribution in Single-Crystalline Barium Titanate}, volume={4}, ISSN={["2198-3844"]}, DOI={10.1002/advs.202301273}, abstractNote={Abstract Nanosized perovskite ferroelectrics are widely employed in several electromechanical, photonics, and thermoelectric applications. Scaling of ferroelectric materials entails a severe reduction in the lattice (phonon) thermal conductivity, particularly at sub‐100 nm length scales. Such thermal conductivity reduction can be accurately predicted using the information of phonon mean free path (MFP) distribution. The current understanding of phonon MFP distribution in perovskite ferroelectrics is still inconclusive despite the critical thermal management implications. Here, high‐quality single‐crystalline barium titanate (BTO) thin films, a representative perovskite ferroelectric material, are grown at several thicknesses. Using experimental thermal conductivity measurements and first‐principles based modeling (including four‐phonon scattering), the phonon MFP distribution is determined in BTO. The simulation results agree with the measured thickness‐dependent thermal conductivity. The results show that the phonons with sub‐100 nm MFP dominate the thermal transport in BTO, and phonons with MFP exceeding 10 nm contribute ≈35% to the total thermal conductivity, in significant contrast to previously published experimental results. The experimentally validated phonon MFP distribution is consistent with the theoretical predictions of other complex crystals with strong anharmonicity. This work paves the way for thermal management in nanostructured and ferroelectric‐domain‐engineered systems for oxide perovskite‐based functional materials.}, journal={ADVANCED SCIENCE}, author={Negi, Ankit and Rodriguez, Alejandro and Zhang, Xuanyi and Comstock, Andrew H. H. and Yang, Cong and Sun, Dali and Jiang, Xiaoning and Kumah, Divine and Hu, Ming and Liu, Jun}, year={2023}, month={Apr} }
 @article{zhong_xi_jin_wang_ma_liu_zhou_2022, title={Enhancement of thermal percolation in composites: A two-dimensional case study}, volume={121}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0093038}, DOI={10.1063/5.0093038}, abstractNote={We investigated the thermal conductivity (TC) of a two-dimensional composite consisting of matrices with low TC and nanowire fillers with high TC using the Monte Carlo method and the finite element method. Compared to the electrical percolation that requires direct contact between fillers, an additional thermal transport channel through non-contact fillers is found to be important for the thermal percolation in composites. This additional channel can lead to an effective expansion of thermal percolation network by including extra fillers into the network. Thus, it can be utilized to enhance the TC of the composite that is helpful to improve the performance of thermal interface materials.}, number={16}, journal={APPLIED PHYSICS LETTERS}, author={Zhong, Jinxin and Xi, Qing and Jin, Hongjie and Wang, Ziqi and Ma, Dengke and Liu, Jun and Zhou, Jun}, year={2022}, month={Oct} }
 @article{raza_he_tafreshi_liu_2022, title={Molecular dynamics simulation of steady-state droplet condensation on a fiber in direct contact membrane distillation settings}, volume={368}, ISSN={["1873-3166"]}, url={http://dx.doi.org/10.1016/j.molliq.2022.120736}, DOI={10.1016/j.molliq.2022.120736}, abstractNote={• Dynamics of droplet condensation-evaporation was simulated in Direct Contact Membrane Distillation (DCMD) environment. • A novel methodology for achieving steady-state supply of water during an infinitely long period was developed. • The fiber was modelled as a tri-layer structure for appropriate depiction of the actual process. • Droplet formation on fibers can be prevented if the fiber’s Young-Laplace contact angle is greater than a critical value. • Our work provides design guidelines for DCMD membranes based on geometric and operational parameters. Understanding the dynamics of droplet condensation–evaporation behavior on fibers is important for improving the performance of fibrous membranes that are used in water purification applications, e.g., Direct Contact Membrane Distillation (DCMD). DCMD is a promising method of purifying water when low-grade waste heat or renewable energies are available. However, DCMD suffers from low throughput mass flux, and it is also prone to membrane flooding. We used molecular dynamics simulations in this work as the conventional (experimental or computational) methods do not have the required atomistic resolution to reveal the dynamics of water condensation–evaporation on the surface of fibers. Our simulations indicate that vapor flux across the membrane remains constant (with no droplet formation on the fiber) when the fibers’ Young–Laplace Contact Angle (YLCA) is greater than a critical value at which condensation is suppressed. However, mass flux decreases with time at lower YLCAs due to the formation and growth of water droplets on the fibers, which could ultimately lead to membrane flooding. We also studied the impact of feed temperature, permeate temperature, fiber diameter, fiber position, and domain size on the fiber critical YLCA. Optimizing these parameters allows the use of a wide array of materials in membrane fabrication, including even hydrophilic materials, while preventing membrane flooding and also enhancing mass flux. In this work, we also present a novel methodology to simulate steady-state droplet condensation–evaporation process in the framework of molecular dynamics simulation, i.e., simulation times >∼10 ns, in contrast to the quasi-steady-state simulations (simulation time <∼2 ns) reported in most previous studies. Our work demonstrates a simulation platform to study the dynamics of the water condensation–evaporation on fibers and can be used to guide the design of DCMD membranes.}, journal={JOURNAL OF MOLECULAR LIQUIDS}, publisher={Elsevier BV}, author={Raza, Saqlain and He, Jixiong and Tafreshi, Hooman V. and Liu, Jun}, year={2022}, month={Dec} }
 @article{cong_xuhui_jun_jun_xiaobo_2022, title={Thermal Transport across Polyethylene Chains}, volume={31}, ISSN={["1993-033X"]}, url={http://dx.doi.org/10.1007/s11630-022-1640-7}, DOI={10.1007/s11630-022-1640-7}, number={4}, journal={JOURNAL OF THERMAL SCIENCE}, publisher={Springer Science and Business Media LLC}, author={Cong, Yang and Xuhui, Duan and Jun, Zhou and Jun, Liu and Xiaobo, Li}, year={2022}, month={Jun} }
 @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}, 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{booth_subramanyan_liu_lukic_2021, title={Parallel Frameworks for Robust Optimization of Medium-Frequency Transformers}, volume={9}, ISSN={["2168-6785"]}, url={http://dx.doi.org/10.1109/jestpe.2020.3042527}, DOI={10.1109/jestpe.2020.3042527}, abstractNote={Current optimization methods for medium-frequency transformers (MFTs) within power electronic converters yield unrealistic results in the multiphysics framework. Comparing the optimal design to an experimental setup for a 3.5-kW MFT, the core loss is underestimated by 28%, which results in the experimental steady-state temperatures being 10 °C greater than the analytically optimized model. To counteract these disadvantages, an optimization procedure, using the aggressive space mapping (ASM) technique, is experimentally verified and compared with the previous state-of-the-art (SOA) method. It is shown that the ASM design produces more realistic and feasible experimental outcomes than the SOA design. The core losses are accurately predicted to within 10%, which, in turn, vastly improves the thermal modeling accuracy. The ASM method accurately predicts the core hot spot temperature and the average core temperature. This work also introduces a robust optimization method to the MFT design process to handle variations from both converter-level attributes and manufacturing tolerances to create a potential design region, which contains 97.725% of possible design outcomes. This method replaces the nominal design optimization that is used to produce the optimized MFTs in the SOA and ASM methods.}, number={4}, journal={IEEE JOURNAL OF EMERGING AND SELECTED TOPICS IN POWER ELECTRONICS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Booth, Kristen and Subramanyan, Harish and Liu, Jun and Lukic, Srdjan M.}, year={2021}, month={Aug}, pages={5097–5112} }
 @article{zhong_xi_wang_nakayama_li_liu_zhou_2021, title={Thermal boundary conductance across solid-solid interfaces at high temperatures: A microscopic approach}, volume={129}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0047396}, DOI={10.1063/5.0047396}, abstractNote={The existing theories for the thermal boundary conductance (TBC) of solid–solid interfaces lead to diverse values deviating from experimental measurements. In this paper, we propose a simple mechanism to evaluate the TBC contributed by phonons at room temperatures, where the microscopic structure of the interface layers between two dissimilar solids is treated as amorphous by taking into account the mismatches of the elastic modulus, the atomic masses, and the lattice spacing. Our theory explains well the observed magnitude of the TBCs across various solid–solid interfaces in the range from 107 to 109Wm−2K−1. The coordination number density and the energy transfer coefficient across interfaces are key parameters for determining the TBCs.}, number={19}, journal={JOURNAL OF APPLIED PHYSICS}, author={Zhong, Jinxin and Xi, Qing and Wang, Zhiguo and Nakayama, Tsuneyoshi and Li, Xiaobo and Liu, Jun and Zhou, Jun}, year={2021}, month={May} }
 @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{jia_ajayi_ramakrishnan_negi_liu_ekkad_xu_2020, title={A skin layer made of cured polysilazane and yttria stabilized zirconia for enhanced thermal protection of carbon fiber reinforced polymers (CFRPs)}, volume={404}, ISSN={0257-8972}, url={http://dx.doi.org/10.1016/j.surfcoat.2020.126481}, DOI={10.1016/j.surfcoat.2020.126481}, abstractNote={This study reported for the first time using cured preceramic polymer precursor and their composites as thermal protection layer for the carbon fiber reinforced polymer (CFRP). The fabricated composite skin consists of cured polysilazane (PSZ) and yttria stabilized zirconia (YSZ) as the reinforcements and has a low density of 1.67 g/cm3, which can be used for both aerospace and defense applications. The coated composite showed excellent bonding integrity at the interface between the composite skin and the CFRP substrate. Thermal studies revealed the cured PSZ has excellent thermal stability up to its cured temperatures. Using the 3ω method, the room temperature thermal conductivity of PSZ was measured as 0.17 ± 0.02 W/m·K along the through-thickness direction. Thermal conductance of the skin layer is around 940 W/m2·K, which is about 45% reduction in conductance compared to the CFRP. Thermal insulation test shows that the coated composite can be used up to 250 °C. Experiment results demonstrated the skin layer's effectiveness in protecting the CFRP in high-temperature environments.}, journal={Surface and Coatings Technology}, publisher={Elsevier BV}, author={Jia, Yujun and Ajayi, Tosin D. and Ramakrishnan, Kishore Ranganath and Negi, Ankit and Liu, Jun and Ekkad, Srinath and Xu, Chengying}, year={2020}, month={Dec}, pages={126481} }
 @article{efficiency improvement of liquid piston compressor using metal wire mesh for near-isothermal compressed air energy storage application_2020, url={http://dx.doi.org/10.1016/j.est.2020.101226}, DOI={10.1016/j.est.2020.101226}, abstractNote={Intermittent nature of power from renewable energy resources demands a large-scale energy storage system for their optimal utilization. Compressed air energy storage systems have the potential to serve as long-term large-scale energy storage systems. Efficient compressors are needed to realize a high storage efficiency with compressed air energy storage systems. Liquid piston compressor is highly effective in achieving efficient near-isothermal compression. The compression efficiency of the liquid piston can be improved with the use of heat transfer enhancement mechanism inside the compression chamber. In this study, a novel heat transfer enhancement technique using metal wire mesh is experimentally tested in a liquid piston compressor to improve compression efficiency. Metal wire meshes of aluminum and copper materials and different wire diameters along with various stroke times of compression are considered in the experimental design. A distinctive Archimedean spiral form of metal wire mesh is considered to facilitate heat transfer in the axial and radial direction inside the compression chamber. Experiments are conducted for the compression of air from the atmospheric pressure to about 280 kPa pressure at various stroke times of compression. Results show that the peak air temperature was reduced by 26–33 K with the use of metal wire mesh inside the liquid piston compressor. Both the materials are observed to be equally effective for temperature abatement. The use of metal wire mesh in liquid piston shifts the compression process towards the near-isothermal conditions. Furthermore, the isothermal efficiency of compression is evaluated to assess the potential for efficiency improvement with this technique. The metal wire mesh was observed to improve the isothermal efficiency of compression to 88–90% from the base efficiency of 82–84%. A 6–8% improvement in efficiency was observed at faster compression strokes signifying the efficacy of metal wire mesh to accomplish an efficient compression with a high power density.}, journal={Journal of Energy Storage}, year={2020}, month={Apr} }
 @article{chatterjee_negi_kim_liu_ghosh_2020, title={In-Plane Thermoelectric Properties of Flexible and Room-Temperature-Doped Carbon Nanotube Films}, volume={3}, url={http://dx.doi.org/10.1021/acsaem.0c00995}, DOI={10.1021/acsaem.0c00995}, abstractNote={Soft materials with high power factors (PFs) and low thermal conductivity (κ) are critically important for integration of thermoelectric (TE) modules into flexible form factors for energy harvesting or cooling applications. Here, air stable p- and n-type multiwalled carbon nanotube films with high PFs (up to 521 μW/m K2) are reported, with n-type doping carried out in a facile two-step process. The maximum figures of merit (ZTs) of p-type and n-type CNTs are obtained as 0.019 and 0.015 at 300 K, respectively, with all three transport properties—Seebeck coefficient, electrical conductivity, and κ—measured in-plane, providing a more accurate ZT. Using time-domain thermoreflectance, we report a fast and non-contact measurement of κ without complex microfabrication or material processing. Moreover, there is no material mismatch between the p- and n-type legs of the TE module. Such materials have the potential for widespread applications in inexpensive and scalable wearable energy harvesting and localized heating/cooling.}, number={7}, journal={ACS Applied Energy Materials}, publisher={American Chemical Society (ACS)}, author={Chatterjee, Kony and Negi, Ankit and Kim, Kyunghoon and Liu, Jun and Ghosh, Tushar K.}, year={2020}, month={Jul}, pages={6929–6936} }
 @article{size effects in the thermal conductivity of amorphous polymers_2020, url={https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.14.044023}, DOI={https://doi.org/10.1103/PhysRevApplied.14.044023}, abstractNote={Manipulating thermal conductivity through nanoengineering is of critical importance to advance technologies, such as soft robotics, artificial skin, wearable electronics, batteries, thermal insulation, and thermoelectrics. Here, by examining amorphous polymers, including polystyrene, polypropylene, polyethylene, and ethylene vinyl alcohol, using molecular dynamics simulations, we find that the thermal conductivities of amorphous polymers can be reduced below their amorphous limit by size effects. Size-dependent thermal transport in amorphous materials is decomposed into crystalline, crystalline-to-amorphous, and amorphous regimes. In the amorphous regime, the mean free path of propagating heat carriers can range from tens of nanometers to more than 100 nm, contributing 16%--36% of the total thermal conductivity. A two-channel model that combines no size effect (i.e., difusons and locons) and size effect (i.e., propagons) is proposed to account for size-dependent thermal conductivity. We also find that the presence of charged molecules in polymers can significantly affect the thermal conductivity and its size effects due to electrostatic interactions. This work provides insights into the thermal conductivity of amorphous polymers that will have a broad impact on the nano- and chemical engineering of polymers for various energy-related applications.}, journal={Physical Review Applied}, year={2020}, month={Oct} }
 @article{feng_he_rai_hun_liu_shrestha_2020, title={Size Effects in the Thermal Conductivity of Amorphous Polymers}, url={https://doi.org/10.1103/PhysRevApplied.14.044023}, DOI={10.1103/PhysRevApplied.14.044023}, abstractNote={Manipulating thermal conductivity through nanoengineering is of critical importance to advance technologies, such as soft robotics, artificial skin, wearable electronics, batteries, thermal insulation, and thermoelectrics. Here, by examining amorphous polymers, including polystyrene, polypropylene, polyethylene, and ethylene vinyl alcohol, using molecular dynamics simulations, we find that the thermal conductivities of amorphous polymers can be reduced below their amorphous limit by size effects. Size-dependent thermal transport in amorphous materials is decomposed into crystalline, crystalline-to-amorphous, and amorphous regimes. In the amorphous regime, the mean free path of propagating heat carriers can range from tens of nanometers to more than 100 nm, contributing 16%--36% of the total thermal conductivity. A two-channel model that combines no size effect (i.e., difusons and locons) and size effect (i.e., propagons) is proposed to account for size-dependent thermal conductivity. We also find that the presence of charged molecules in polymers can significantly affect the thermal conductivity and its size effects due to electrostatic interactions. This work provides insights into the thermal conductivity of amorphous polymers that will have a broad impact on the nano- and chemical engineering of polymers for various energy-related applications.}, journal={Physical Review Applied}, author={Feng, Tianli and He, Jixiong and Rai, Amit and Hun, Diana and Liu, Jun and Shrestha, Som S.}, year={2020}, month={Oct} }
 @article{strong electron-phonon coupling induced anomalous phonon transport in ultrahigh temperature ceramics zrb2 and tib2_2020, url={http://dx.doi.org/10.1016/j.ijheatmasstransfer.2020.119481}, DOI={10.1016/j.ijheatmasstransfer.2020.119481}, abstractNote={Ultrahigh temperature ZrB2- and TiB2-based ceramics are widely used in extreme thermal environment. Yet, open questions remain pertaining to their lattice thermal conductivity (кph). In this work we investigate the phonon transport of ZrB2 and TiB2 by systematically evaluating the phonon-phonon interaction (PPI), electron-phonon interaction (EPI) and grain boundary scattering (GBS) from the atomistic level using first-principles. Upon including EPI, the room-temperature кph of ZrB2 and TiB2 is significantly reduced by 38.16% and 52.34%, respectively, and agrees excellently with experimental measurement. Such giant reduction arises from the strong EPI for the heat-carrying acoustic phonons due to phonon anomaly and the existence of Fermi nesting vectors along high symmetry line in the Brillouin zone. Following the Casimir model, the GBS further decreases кph of ZrB2 even by 49.27% for small grain boundary spacing of 50 nm and theoretical calculations agree well with experiments. Thus, GBS crucially influences phonon transport, which explains the large deviation of previous experimental measurements on кph for ZrB2-based ceramics. Moreover, the combined influence of EPI and GBS results in the anomalous phonon transport where кph is almost temperature-independent over a large temperature range, consistent with experimental observations. This work directly reveals the phonon transport mechanism for high temperature ceramics ZrB2 and TiB2, gains deep insight into the large variation in the previously reported кph and provides guidance to engineer it for practical applications.}, journal={International Journal of Heat and Mass Transfer}, year={2020}, month={May} }
 @article{zhang_yan_guo_lu_liu_zhou_xu_2020, title={Superior Thermal Dissipation in Graphene Electronic Device Through Novel Heat Path by Electron-Phonon Coupling}, volume={8}, url={http://www.espublisher.com/journals/articledetails/243}, DOI={10.30919/esee8c386}, journal={ES Energy & Environment}, author={Zhang, Ying and Yan, Yaping and Guo, Jie and Lu, Tingyu and Liu, Jun and Zhou, Jun and Xu, Xiangfan}, year={2020}, pages={42–47} }
 @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{zhou_he_xu_nakayama_wang_liu_2020, title={Thermal resistance network model for heat conduction of amorphous polymers}, volume={4}, url={http://dx.doi.org/10.1103/physrevmaterials.4.015601}, DOI={10.1103/physrevmaterials.4.015601}, abstractNote={Thermal conductivities (TCs) of the vast majority of amorphous polymers are in a very narrow range, 0.1 $\sim$ 0.5 Wm$^{-1}$K$^{-1}$, although single polymer chains possess TC of orders-of-magnitude higher. Entanglement of polymer chains plays an important role in determining the TC of bulk polymers. We propose a thermal resistance network (TRN) model for TC in amorphous polymers taking into account the entanglement of molecular chains. Our model explains well the physical origin of universally low TC observed in amorphous polymers. The empirical formulae of pressure and temperature dependence of TC can be successfully reproduced from our model not only in solid polymers but also in polymer melts. We further quantitatively explain the anisotropic TC in oriented polymers.}, number={1}, journal={Physical Review Materials}, publisher={American Physical Society (APS)}, author={Zhou, Jun and He, Jixiong and Xu, Xiangfan and Nakayama, Tsuneyoshi and Wang, Yuanyuan and Liu, Jun}, year={2020}, month={Jan} }
 @article{hu_liu_subramanyan_li_zhou_liu_2019, title={Enhanced thermoelectric properties through minority carriers blocking in nanocomposites}, volume={126}, ISSN={["1089-7550"]}, url={http://dx.doi.org/10.1063/1.5118981}, DOI={10.1063/1.5118981}, abstractNote={We use the Boltzmann transport equation under the relaxation time approximation to investigate the effect of minority blocking on the transport properties of nanocomposites (NCs). Taking p-type Bi0.5Sb1.5Te3 NCs as an example, we find that the thermally excited minority carriers can be strongly scattered by engineered interfacial potential barriers. Such scattering phenomena suppress the bipolar effect, which is helpful to enhance the Seebeck coefficient and reduce the electronic thermal conductivity, especially at high temperatures. Further combining with the majority carriers low-energy filtering effect, the power factor and the figure of merit (ZT) can be significantly enhanced over a large temperature range from 300 K to 500 K. Such an improvement of ZT is attributed to the majority carriers low-energy filtering effect at low temperatures and to the minority carriers blocking effect at high temperatures. A principle that is helpful to provide guidance on the thermoelectric device design is identified: (1) blocking the minority carriers as often as possible and (2) filtering the majority carriers whose energy is lower than 2–3kBT near the cold end.}, number={9}, journal={JOURNAL OF APPLIED PHYSICS}, author={Hu, Jizhu and Liu, Bin and Subramanyan, Harish and Li, Baowen and Zhou, Jun and Liu, Jun}, year={2019}, month={Sep} }
 @article{on the importance of using exact full phonon dispersions for predicting interfacial thermal conductance of layered materials using diffuse mismatch model_2019, url={http://dx.doi.org/10.1063/1.5121727}, DOI={10.1063/1.5121727}, abstractNote={Several models have been employed in the past to estimate interfacial thermal conductance (ITC) for different material interfaces, of which the diffuse mismatch model (DMM) has been generally accepted as reliable for rough material interfaces at high temperature. Even though the DMM has been shown to predict the correct order of magnitude in isotropic material interfaces, it is unable to reproduce the same accuracy for low-dimensional anisotropic layered materials, which have many potential applications. Furthermore, the use of approximated dispersion curves tends to overestimate the ITC. In this work, we propose a new method that utilizes a mode-to-mode comparison within the DMM framework to predict ITC. We employed this model to calculate ITC between layered materials such as MoS2 and graphite and metals such as Al, Au, and Cr. We then compared our values with previous literature data that employ linear dispersion relations and experimental data from time-domain thermoreflectance measurements. This new framework was then used to visualize the phonon focusing effect in anisotropic materials. Further analysis revealed that counting only the three acoustic modes and neglecting the low-frequency optical modes lead to significant underestimation of the ITC using DMM. Our findings indicate that it is imperative to use the exact full phonon dispersion relations in evaluating the ITC for low-dimensional layered materials.}, journal={AIP Advances}, year={2019}, month={Nov} }
 @inproceedings{booth_subramanyan_liang_liu_srdic_lukic_2019, title={Optimization of Medium Frequency Transformers with Practical Considerations}, DOI={10.1109/apec.2019.8722164}, abstractNote={A method to design and optimize a Medium Frequency Transformer (MFT) based on commercially available components and semiconductor and converter constraints is presented. The optimization algorithm is used to redesign the transformer for a scaled-down electric vehicle (EV) fast charger using a three-level resonant circuit topology. The main consideration of this paper is the uncertainty caused by modeling assumptions in optimization algorithms. To reduce this uncertainty, space mapping is used to create an optimized design point. Finally, a comparison of the designs found using the original optimization algorithm and the space mapping technique are compared and analyzed. For simplicity, only a single objective optimization routine is employed.}, booktitle={IEEE Applied Power Electronics Conference and Exposition (APEC)}, publisher={IEEE}, author={Booth, Kristen and Subramanyan, Harish and Liang, Xinyu and Liu, Jun and Srdic, Srdjan and Lukic, Srdjan}, year={2019}, month={Mar} }
 @article{subramanyan_zhang_he_kim_li_liu_2019, title={Role of angular bending freedom in regulating thermal transport in polymers}, volume={125}, ISSN={["1089-7550"]}, url={http://dx.doi.org/10.1063/1.5086176}, DOI={10.1063/1.5086176}, abstractNote={Polymers, despite their desirable structural properties, suffer from low thermal conductivity, which restricts their use. Previous studies have indicated that the strong bond-stretching and angular-bending interactions along the chain are believed to have saturated the maximum achievable thermal conductivity in the along-the-chain direction. Contrary to this belief, our results show an improvement in thermal conductivity. By increasing the bond and angle potential, we studied the effect on the thermal conductivity of polyethylene using non-equilibrium molecular dynamics simulations. In comparison to restricting the bond stretching, we found that restricting angular bending freedom plays a crucial role in improving the thermal transport along the chain. We observed significant changes in the morphology of the polyethylene chains when the angle potential was increased. We also found a remarkable increase in the phonon group velocity accompanied by large shifts in the longitudinal acoustic branch of the dispersion curve. These results when coupled with the structural changes strongly support the argument that thermal conductivity can be controlled by restricting the angular bending freedom.}, number={9}, journal={JOURNAL OF APPLIED PHYSICS}, author={Subramanyan, Harish and Zhang, Weiye and He, Jixiong and Kim, Kyunghoon and Li, Xiaobo and Liu, Jun}, year={2019}, month={Mar} }
 @article{xi_li_zhou_li_liu_2019, title={Role of radiation in heat transfer from nanoparticles to gas media in photothermal measurements}, volume={30}, ISSN={["1793-6586"]}, url={http://dx.doi.org/10.1142/s0129183119500244}, DOI={10.1142/S0129183119500244}, abstractNote={The heat transfer from nanoparticles (NPs) to gas of photothermal effect is investigated by taking into account both conduction and radiation. The steady-state and unsteady-state heat transfer processes are studied analytically and numerically, respectively. In contrast to the photothermal effect in liquid with metal NPs, in which the radiation is negligible, we found that the thermal radiation must be taken into account in the nanoparticle–gas system. The reason is that the thermal boundary conductance (TBC) of gas–solid interface is several orders of magnitude smaller than the TBC of liquid–solid interface, especially when the diameter of nanoparticle is comparable to or smaller than the mean free path of gas molecules. We propose a method to measure the ultra-low TBC of interface between nanoparticle and gas based on our investigations.}, number={4}, journal={INTERNATIONAL JOURNAL OF MODERN PHYSICS C}, author={Xi, Qing and Li, Yunyun and Zhou, Jun and Li, Baowen and Liu, Jun}, year={2019}, month={Apr} }
 @article{duan_li_liu_chen_li_2019, title={Roles of kink on the thermal transport in single polyethylene chains}, volume={125}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.5086453}, DOI={10.1063/1.5086453}, abstractNote={The trans-gauche state transformation commonly exists in polymers. However, the fundamental understanding of the roles of kink (gauche state) on the thermal energy transport in polymer chains is rather limited. From atomic simulations, we show that kinks greatly scatter phonons in single polyethylene chains, and even a single kink can reflect more than half of the phonons. Further studies show that kinks not only add extra thermal resistance to the chain but also break the whole chain into small segments and each with reduced thermal conductivity. A simple series thermal resistance model is proposed to estimate the effective thermal conductivity of single polymer chains with multiple kinks.}, number={16}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Duan, Xuhui and Li, Zehuan and Liu, Jun and Chen, Gang and Li, Xiaobo}, year={2019}, month={Apr}, pages={164303} }
 @article{zhou_hao_clark_kim_zhu_liu_cheng_li_2019, title={Sono-Assisted Surface Energy Driven Assembly of 2D Materials on Flexible Polymer Substrates: A Green Assembly Method Using Water}, volume={11}, ISSN={["1944-8252"]}, url={http://dx.doi.org/10.1021/acsami.9b10469}, DOI={10.1021/acsami.9b10469}, abstractNote={The challenges in achieving a green and scalable integration of two-dimensional (2D) materials with flexible polymer substrates present a major barrier for the application of 2D materials, such as graphene, MoS2, and h-BN for flexible devices. Here, we create a sono-assisted surface energy driven assembly (SASEDA) method that can achieve foot-scale to micrometer-scale assembly of 2D materials, form a conductive network in as short as 10 s, and build hierarchical and hybrid flexible devices such as sensors, resistors, and capacitors by using water as the dispersion solvent. SASEDA highlights two counterintuitive innovations. First, we use an “unfavorable” solvent (i.e., water) for both 2D materials (e.g., graphene, MoS2, and h-BN) and polymer substrates (e.g., polydimethylsiloxane) to drive the assembly process. Second, we use a weak sono-field (0.3 W/cm2) generated by a regular sonication bath cleaner to enhance the assembly efficiency and reorganize and unify the assembly network. This method and its principle pave the way toward affordable large-scale 2D material-based flexible devices.}, number={36}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Zhou, Dong and Hao, Ji and Clark, Andy and Kim, Kyunghoon and Zhu, Long and Liu, Jun and Cheng, Xuemei and Li, Bo}, year={2019}, month={Sep}, pages={33458–33464} }
 @article{kim_he_ganeshan_liu_2018, title={Disorder enhanced thermal conductivity anisotropy in two-dimensional materials and van der Waals heterostructures}, volume={124}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.5031147}, DOI={10.1063/1.5031147}, abstractNote={Two-dimensional (2D) materials and van der Waals heterostructures can naturally function as directional heat spreaders in nanoelectronics due to their intrinsically anisotropic structure. In real nanoelectronic applications, disorders usually appear in those materials where their effects on anisotropic thermal conductivity are not well-understood. We built simple graphite-like material models and systematically incorporated mass disorder or structural disorder into the structures. The anisotropic thermal conductivities calculated by equilibrium molecular dynamics simulations show that mass disorder and stacking disorder can effectively and anisotropically tune the thermal conductivity of 2D materials and van der Waals heterostructures. Compared with pristine graphite, the through-plane thermal conductivity can be reduced up to two orders of magnitude by the through-plane mass disorder, and the thermal anisotropy ratio (i.e., the ratio of in-plane to through-plane thermal conductivity) can be enhanced more than ten times. We attribute this counter-intuitive result to the dramatic decrease in phonon group velocity in the through-plane direction. Our results can shed some light on the thermal management in electronics incorporating 2D materials and van der Waals heterostructures.}, number={5}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Kim, Kyunghoon and He, Jixiong and Ganeshan, Banu and Liu, Jun}, year={2018}, month={Aug}, pages={055104} }
 @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} }
 @article{shi_dong_li_liu_kim_xu_zhou_liu_2018, title={Thermal percolation in composite materials with electrically conductive fillers}, volume={113}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.5039923}, DOI={10.1063/1.5039923}, abstractNote={We measured thermal conductivity and electrical conductivity in organic/inorganic composites with Ag nanowires (NWs) embedded in a poly(vinylidene fluoride) matrix. High thermal and electrical conductivities of 8.43 W/(mK) and 1.02 ×106 S/m are achieved, respectively, when the volume fraction of Ag NWs reaches 28.34%. Both measured electrical and thermal conductivities obey the universal power law commonly described in the percolation theory. The percolation behaviors of thermal and electrical conductivities are clearly observed when the volume fraction of Ag NWs is above the critical volume fraction (2.25%), due to the formation of a percolation spanning cluster. Further calculations on the Lorenz number as a function of Ag NW volume fraction also confirm the percolation behaviors. The power-law exponent for the thermal percolation is slightly smaller than that for the electrical percolation, which is likely due to the “dead-end” structures that do not contribute to electrical percolation. To understand the effect of contact resistance between Ag NWs, we modeled the electron contribution to the electrical and thermal resistance at the contact. The non-ideal contact will cause the interfacial thermal resistance increase much more than the electrical contact resistance. The interfacial Lorenz number will decrease from the Sommerfeld value to a much lower value if the contact is non-ideal. Our work can shed some light on the thermal percolation in composite materials.}, number={4}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Shi, Bo and Dong, Lan and Li, Mingqiang and Liu, Bin and Kim, Kyunghoon and Xu, Xiangfan and Zhou, Jun and Liu, Jun}, year={2018}, month={Jul}, pages={041902} }
 @article{lu_kim_li_zhou_chen_liu_2018, title={Thermal transport in semicrystalline polyethylene by molecular dynamics simulation}, volume={123}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.5006889}, DOI={10.1063/1.5006889}, abstractNote={Recent research has highlighted the potential to achieve high-thermal-conductivity polymers by aligning their molecular chains. Combined with other merits, such as low-cost, corrosion resistance, and light weight, such polymers are attractive for heat transfer applications. Due to their quasi-one-dimensional structural nature, the understanding on the thermal transport in those ultra-drawn semicrystalline polymer fibers or films is still lacking. In this paper, we built the ideal repeating units of semicrystalline polyethylene and studied their dependence of thermal conductivity on different crystallinity and interlamellar topology using the molecular dynamics simulations. We found that the conventional models, such as the Choy-Young's model, the series model, and Takayanagi's model, cannot accurately predict the thermal conductivity of the quasi-one-dimensional semicrystalline polyethylene. A modified Takayanagi's model was proposed to explain the dependence of thermal conductivity on the bridge number a...}, number={1}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Lu, Tingyu and Kim, Kyunghoon and Li, Xiaobo and Zhou, Jun and Chen, Gang and Liu, Jun}, year={2018}, month={Jan}, pages={015107} }
 @article{ming_yang_huang_wu_li_liu_2017, title={Analytical and numerical investigation on a new compact thermoelectric generator}, volume={132}, ISSN={["1879-2227"]}, url={http://dx.doi.org/10.1016/j.enconman.2016.11.043}, DOI={10.1016/j.enconman.2016.11.043}, abstractNote={In order to improve the performance and maximize the efficiency of energy conversion of thermoelectric generator (TEG), a mathematical model to predict the maximum energy conversion efficiency of TEG is developed. Then, a new compact thermoelectric generator (C-TEG) and a dimensional optimized TEG (DO-TEG) are proposed in this article. The compact thermoelectric generator is designed via logical intersection angle selection and layout, thus to improve the electric performance per unit volume. Finally, we compared the output electric performance of C-TEG and traditional thermoelectric generator (T-TEG) and that of DO-TEG under design and off-design conditions via numerical simulations. The results indicate that C-TEG has an excellent electric performance whose voltage, power, and efficiency decrease slightly whereas the output voltage, work, and efficiency compared with that of T-TEG have been significantly improved, with the amplitude increasing with the increase of resistant value of external loads.}, journal={ENERGY CONVERSION AND MANAGEMENT}, author={Ming, Tingzhen and Yang, Wei and Huang, Xiaoming and Wu, Yongjia and Li, Xiaohua and Liu, Jun}, year={2017}, month={Jan}, pages={261–271} }
 @article{forster_lynch_coates_liu_jang_zaia_gordon_szybowski_sahu_cahill_et al._2017, title={Solution-Processed Cu2Se Nanocrystal Films with Bulk-Like Thermoelectric Performance}, volume={7}, ISSN={["2045-2322"]}, url={http://dx.doi.org/10.1038/s41598-017-02944-1}, DOI={10.1038/s41598-017-02944-1}, abstractNote={Abstract Thermoelectric power generation can play a key role in a sustainable energy future by converting waste heat from power plants and other industrial processes into usable electrical power. Current thermoelectric devices, however, require energy intensive manufacturing processes such as alloying and spark plasma sintering. Here, we describe the fabrication of a p-type thermoelectric material, copper selenide (Cu 2 Se), utilizing solution-processing and thermal annealing to produce a thin film that achieves a figure of merit, ZT, which is as high as its traditionally processed counterpart, a value of 0.14 at room temperature. This is the first report of a fully solution-processed nanomaterial achieving performance equivalent to its bulk form and represents a general strategy to reduce the energy required to manufacture advanced energy conversion and harvesting materials.}, journal={SCIENTIFIC REPORTS}, author={Forster, Jason D. and Lynch, Jared J. and Coates, Nelson E. and Liu, Jun and Jang, Hyejin and Zaia, Edmond and Gordon, Madeleine P. and Szybowski, Maxime and Sahu, Ayaskanta and Cahill, David G. and et al.}, year={2017}, month={Jun} }
 @article{liu_lu_wang_liu_nakayama_zhou_li_2017, title={Thermoelectric transport in hybrid materials incorporating metallic nanowires in polymer matrix}, volume={110}, ISSN={["1077-3118"]}, url={http://dx.doi.org/10.1063/1.4978602}, DOI={10.1063/1.4978602}, abstractNote={We propose a type of thermoelectric materials incorporating metallic nanowires in insulating polymers. It is shown that the hybridization of poor thermoelectric materials such as metal and polymer can achieve high performance of thermoelectricity. The electrical conductivity of such hybrid materials is controllable by the volume fraction of metallic nanowires which is above a percolation critical value. Meanwhile, the Seebeck coefficient shows a weak dependence on the volume fraction. Low thermal conductivities required for achieving the high figure of merit can be fulfilled from both the low thermal conductivity of polymer and the interfacial thermal resistance between nanowires and polymer. In this regard, we propose the concept “electron-percolation thermal-insulator,” providing a guide to design efficient hybrid thermoelectric materials.}, number={11}, journal={APPLIED PHYSICS LETTERS}, author={Liu, Bin and Lu, Tingyu and Wang, Biao and Liu, Jun and Nakayama, Tsuneyoshi and Zhou, Jun and Li, Baowen}, year={2017}, month={Mar} }
 @article{mai_liu_evans_segalman_chabinyc_gahill_bazan_2016, title={Anisotropic Thermal Transport in Thermoelectric Composites of Conjugated Polyelectrolytes/Single-Walled Carbon Nanotubes}, volume={49}, ISSN={["1520-5835"]}, url={http://dx.doi.org/10.1021/acs.macromol.6b00546}, DOI={10.1021/acs.macromol.6b00546}, abstractNote={We report a method to determine the thermal conductivities of polymer composites with single-walled carbon nanotubes (SWNTs) using time-domain thermoreflectance. Both through-plane and in-plane thermal conductivities were determined. Two types of CPEs used in these studies are of the same conjugated backbone but with either cationic (CPE-PyrBIm4) or anionic (CPE-Na) pendant functionalities. The CPE-Na/SWNT composites are p-type conductors, whereas the CPE-PyrBIm4/SWNT counterparts exhibit n-type charge transport. The CPE/SWNT films were prepared through a filtration method that preferentially aligns the SWNTs in the in-plane direction. Attaching the composites onto glass substrates with a precoated heat transducer allows one to measure the through-plane thermal conductivity of materials with rough surfaces. The in-plane thermal conductivity can be measured by embedding thick samples into epoxy followed by microtoming to expose the relatively smooth cross sections. The thermal conductivity along the in-pla...}, number={13}, journal={MACROMOLECULES}, author={Mai, Cheng-Kang and Liu, Jun and Evans, Christopher M. and Segalman, Rachel A. and Chabinyc, Michael L. and Gahill, David G. and Bazan, Guillermo C.}, year={2016}, month={Jul}, pages={4957–4963} }
 @article{chang_fang_liu_evans_russ_popere_patel_chabinyc_segalman_2016, title={Electrochemical Effects in Thermoelectric Polymers}, volume={5}, ISSN={["2161-1653"]}, url={http://dx.doi.org/10.1021/acsmacrolett.6b00054}, DOI={10.1021/acsmacrolett.6b00054}, abstractNote={Conductive polymers such as PEDOT:PSS hold great promise as flexible thermoelectric devices. The thermoelectric power factor of PEDOT:PSS is small relative to inorganic materials because the Seebeck coefficient is small. Ion conducting materials have previously been demonstrated to have very large Seebeck coefficients, and a major advantage of polymers over inorganics is the high room temperature ionic conductivity. Notably, PEDOT:PSS demonstrates a significant but short-term increase in Seebeck coefficient which is attributed to a large ionic Seebeck contribution. By controlling whether electrochemistry occurs at the PEDOT:PSS/electrode interface, the duration of the ionic Seebeck enhancement can be controlled, and a material can be designed with long-lived ionic Seebeck enhancements.}, number={4}, journal={ACS MACRO LETTERS}, author={Chang, William B. and Fang, Haiyu and Liu, Jun and Evans, Christopher M. and Russ, Boris and Popere, Bhooshan C. and Patel, Shrayesh N. and Chabinyc, Michael L. and Segalman, Rachel A.}, year={2016}, month={Apr}, pages={455–459} }
 @article{chang w._liu j._russ b._patel s._r._2016, title={Electrochemical effects in thermoelectric polymers}, volume={5}, journal={ACS Macro Letters}, author={Chang W., Fang H. and Liu J., Evans C. and Russ B., Popere B. and Patel S., Chabinyc M. and R., Segalman}, year={2016}, pages={455–459} }
 @article{chang_evans_popere_russ_liu_newman_segalman_2016, title={Harvesting Waste Heat in Unipolar Ion Conducting Polymers}, volume={5}, ISSN={["2161-1653"]}, url={http://dx.doi.org/10.1021/acsmacrolett.5b00829}, DOI={10.1021/acsmacrolett.5b00829}, abstractNote={The Seebeck effect in unipolar ion-conducting, solid-state polymers is characterized. The high Seebeck coefficient and sign in polymer ion conductors is explained via analysis of thermogalvanic multicomponent transport. A solid-state, water-processeable, flexible device based on these materials is demonstrated, showcasing the promise of polymers as thermogalvanic materials. Thermogalvanic materials based on ion-conducting polymer membranes show great promise in the harvesting of waste heat.}, number={1}, journal={ACS MACRO LETTERS}, author={Chang, William B. and Evans, Christopher M. and Popere, Bhooshan C. and Russ, Boris M. and Liu, Jun and Newman, John and Segalman, Rachel A.}, year={2016}, month={Jan}, pages={94–98} }
 @article{lu_liu_xie_cahill_2016, title={Thermal Conductivity in the Radial Direction of Deformed Polymer Fibers}, volume={5}, ISSN={["2161-1653"]}, url={http://dx.doi.org/10.1021/acsmacrolett.6b00048}, DOI={10.1021/acsmacrolett.6b00048}, abstractNote={Thermal conductivity of polymer fibers in the axial direction has been extensively studied while thermal conductivity in the radial direction Λ remains unknown. In this work, polymer fibers with different molecular arrangements (crystalline, liquid crystalline, and amorphous) were plastically deformed. Λ was measured at engineering strains ε = 0.2-2.3 using time-domain thermoreflectance. Λ decreases with increasing strains for polyethylene (PE) and poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers and is independent of strain for poly(methyl methacrylate) (PMMA) fibers. The extrapolated thermal conductivity at zero strain is Λ0 ≈ 0.27 Wm-1 K-1 for crystalline PE, Λ0 ≈ 0.29 Wm-1 K-1 for liquid crystalline PBO, and Λ0 ≈ 0.18 Wm-1 K-1 for amorphous PMMA. Λ of PE drops to Λ ≈ 0.14 Wm-1 K-1 at ε = 1.9; Λ of PBO drops to Λ ≈ 0.12 Wm-1 K-1 at ε = 2.1. We attribute the decrease of Λ with ε in crystalline and liquid crystalline fibers to structural disorder induced by plastic deformation. The combination of structural disorder and phonon focusing effects produces a thermal conductivity in deformed PE and PBO fibers that is lower than amorphous PMMA.}, number={6}, journal={ACS MACRO LETTERS}, author={Lu, Yanfu and Liu, Jun and Xie, Xu and Cahill, David G.}, year={2016}, month={Jun}, pages={646–650} }
 @article{xie_li_tsai_liu_braun_cahill_2016, title={Thermal Conductivity, Heat Capacity, and Elastic Constants of Water Soluble Polymers and Polymer Blends}, volume={49}, ISSN={["1520-5835"]}, url={http://dx.doi.org/10.1021/acs.macromol.5b02477}, DOI={10.1021/acs.macromol.5b02477}, abstractNote={We use time-domain thermoreflectance (TDTR), and the generation and detection of longitudinal and surface acoustic waves, to study the thermal conductivity, heat capacity, and elastic properties of thin films of poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA), polyacrylamide (PAM), poly(vinylpyrrolidone) (PVP), methyl cellulose (MC), poly(4-styrenesulfonic acid) (PSS), poly(N-acryloylpiperidine) (PAP), poly(methyl methacrylate) (PMMA), and a polymer blend of PVA/PAA. The thermal conductivity of six water-soluble polymers in the dry state varies by a factor of ≈2, from 0.21 to 0.38 W m–1 K–1, where the largest values appear among polymers with a high concentration of hydrogen bonding (PAA, PAM, PSS). The longitudinal elastic constants range from 7.4 to 24.5 GPa and scale linearly with the shear elastic constants, suggesting a narrow distribution of Possion’s ratio 0.35 < ν < 0.40. The thermal conductivity increases with the average sound velocity, as expected based on the model of the minimum thermal c...}, number={3}, journal={MACROMOLECULES}, author={Xie, Xu and Li, Dongyao and Tsai, Tsung-Han and Liu, Jun and Braun, Paul V. and Cahill, David G.}, year={2016}, month={Feb}, pages={972–978} }
 @article{zhu_liu_zheng_zhang_li_banerjee_cahill_2016, title={Tuning thermal conductivity in molybdenum disulfide by electrochemical intercalation}, volume={7}, ISSN={["2041-1723"]}, url={http://dx.doi.org/10.1038/ncomms13211}, DOI={10.1038/ncomms13211}, abstractNote={Abstract Thermal conductivity of two-dimensional (2D) materials is of interest for energy storage, nanoelectronics and optoelectronics. Here, we report that the thermal conductivity of molybdenum disulfide can be modified by electrochemical intercalation. We observe distinct behaviour for thin films with vertically aligned basal planes and natural bulk crystals with basal planes aligned parallel to the surface. The thermal conductivity is measured as a function of the degree of lithiation, using time-domain thermoreflectance. The change of thermal conductivity correlates with the lithiation-induced structural and compositional disorder. We further show that the ratio of the in-plane to through-plane thermal conductivity of bulk crystal is enhanced by the disorder. These results suggest that stacking disorder and mixture of phases is an effective mechanism to modify the anisotropic thermal conductivity of 2D materials.}, journal={NATURE COMMUNICATIONS}, author={Zhu, Gaohua and Liu, Jun and Zheng, Qiye and Zhang, Ruigang and Li, Dongyao and Banerjee, Debasish and Cahill, David G.}, year={2016}, month={Oct} }
 @article{liu_wang_li_coates_segalman_cahill_2015, title={Thermal Conductivity and Elastic Constants of PEDOT:PSS with High Electrical Conductivity}, volume={48}, ISSN={0024-9297 1520-5835}, url={http://dx.doi.org/10.1021/MA502099T}, DOI={10.1021/ma502099t}, abstractNote={Mixtures of poly(3,4-ethylenedioxythiophene) and polystyrenesulfonate (PEDOT:PSS) have high electrical conductivity when cast from aqueous suspensions in combination with a high boiling-point cosolvent dimethyl sulfoxide (DMSO). The electronic component of the thermal conductivity of these highly conducting polymers is of interest for evaluating their potential for thermoelectric cooling and power generation. We find, using time-domain thermore- flectance measurements of thermal conductivity along multiple directions of thick (>20 μm) drop-cast PEDOT films, that the thermal conductivity can be highly anisotropic (Λ∥ ≈ 1.0 W m −1 K −1 and Λ⊥ ≈ 0.3 W m −1 K −1 for the in-plane and through- plane directions, respectively) when the electrical conductivity in the in-plane direction is large (σ ≈ 500 S cm −1 ). We relate the increase in thermal conductivity to the estimated electronic component of the thermal conductivity using the Wiedemann−Franz law, and find that our data are consistent with conventional Sommerfeld value of the Lorenz number. We use measurements of the elastic constants (C11 ≈ 11 GPa and C44 ≈ 17 GPa) of spin-cast PEDOT films and through-plane thermal conductivity (Λ⊥ ≈ 0.3 W m −1 K −1 ) of drop-cast and spin-cast films to support our assumption that the phonon contribution to the thermal conductivity does not change significantly with DMSO composition.}, number={3}, journal={Macromolecules}, publisher={American Chemical Society (ACS)}, author={Liu, Jun and Wang, Xiaojia and Li, Dongyao and Coates, Nelson E. and Segalman, Rachel A. and Cahill, David G.}, year={2015}, month={Jan}, pages={585–591} }
 @article{measurement of the anisotropic thermal conductivity of molybdenum disulfide by the time-resolved magneto-optic kerr effect_2014, url={http://dx.doi.org/10.1063/1.4904513}, DOI={10.1063/1.4904513}, abstractNote={We use pump-probe metrology based on the magneto-optic Kerr effect to measure the anisotropic thermal conductivity of (001)-oriented MoS2 crystals. A ≈20 nm thick CoPt multilayer with perpendicular magnetization serves as the heater and thermometer in the experiment. The low thermal conductivity and small thickness of the CoPt transducer improve the sensitivity of the measurement to lateral heat flow in the MoS2 crystal. The thermal conductivity of MoS2 is highly anisotropic with basal-plane thermal conductivity varying between 85–110 W m-1 K-1 as a function of laser spot size. The basal-plane thermal conductivity is a factor of ≈50 larger than the c-axis thermal conductivity, 2.0±0.3 W m-1 K-1.}, journal={Journal of Applied Physics}, year={2014}, month={Dec} }
 @article{pump-probe measurements of the thermal conductivity tensor for materials lacking in-plane symmetry_2014, url={http://dx.doi.org/10.1063/1.4897622}, DOI={10.1063/1.4897622}, abstractNote={We previously demonstrated an extension of time-domain thermoreflectance (TDTR) which utilizes offset pump and probe laser locations to measure in-plane thermal transport properties of multilayers. However, the technique was limited to systems of transversely isotropic materials studied using axisymmetric laser intensities. Here, we extend the mathematics so that data reduction can be performed on non-transversely isotropic systems. An analytic solution of the diffusion equation for an N-layer system is given, where each layer has a homogenous but otherwise arbitrary thermal conductivity tensor and the illuminating spots have arbitrary intensity profiles. As a demonstration, we use both TDTR and time-resolved magneto-optic Kerr effect measurements to obtain thermal conductivity tensor elements of <110> α-SiO2. We show that the out-of-phase beam offset sweep has full-width half-maxima that contains nearly independent sensitivity to the in-plane thermal conductivity corresponding to the scanning direction. Also, we demonstrate a Nb-V alloy as a low thermal conductivity TDTR transducer layer that helps improve the accuracy of in-plane measurements.}, journal={Review of Scientific Instruments}, year={2014}, month={Oct} }
 @article{size effect on the thermal conductivity of ultrathin polystyrene films_2014, url={http://dx.doi.org/10.1063/1.4871737}, DOI={10.1063/1.4871737}, abstractNote={Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation Jun Liu, Shenghong Ju, Yifu Ding, Ronggui Yang; Size effect on the thermal conductivity of ultrathin polystyrene films. Appl. Phys. Lett. 14 April 2014; 104 (15): 153110. https://doi.org/10.1063/1.4871737 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioApplied Physics Letters Search Advanced Search |Citation Search}, journal={Applied Physics Letters}, year={2014}, month={Apr} }
 @article{simultaneous measurement of thermal conductivity and heat capacity of bulk and thin film materials using frequency-dependent transient thermoreflectance method_2013, url={http://dx.doi.org/10.1063/1.4797479}, DOI={10.1063/1.4797479}, abstractNote={The increasing interest in the extraordinary thermal properties of nanostructures has led to the development of various measurement techniques. Transient thermoreflectance method has emerged as a reliable measurement technique for thermal conductivity of thin films. In this method, the determination of thermal conductivity usually relies much on the accuracy of heat capacity input. For new nanoscale materials with unknown or less-understood thermal properties, it is either questionable to assume bulk heat capacity for nanostructures or difficult to obtain the bulk form of those materials for a conventional heat capacity measurement. In this paper, we describe a technique for simultaneous measurement of thermal conductivity κ and volumetric heat capacity C of both bulk and thin film materials using frequency-dependent time-domain thermoreflectance (TDTR) signals. The heat transfer model is analyzed first to find how different combinations of κ and C determine the frequency-dependent TDTR signals. Simultaneous measurement of thermal conductivity and volumetric heat capacity is then demonstrated with bulk Si and thin film SiO2 samples using frequency-dependent TDTR measurement. This method is further testified by measuring both thermal conductivity and volumetric heat capacity of novel hybrid organic-inorganic thin films fabricated using the atomic∕molecular layer deposition. Simultaneous measurement of thermal conductivity and heat capacity can significantly shorten the development∕discovery cycle of novel materials.}, journal={Review of Scientific Instruments}, year={2013}, month={Mar} }
 @article{thermoelectric transport across nanoscale polymer–semiconductor–polymer junctions_2013, url={http://dx.doi.org/10.1021/jp4084019}, DOI={10.1021/jp4084019}, abstractNote={There is an increasing interest in the thermoelectric (TE) properties of hybrid organic–inorganic structures, such as molecular junctions, organic–inorganic multilayers, and nanocomposites, owing to the recent success in nanostructuring of inorganic materials for high-efficiency thermoelectrics and the ability to synthesize hybrid materials in close analog to inorganic counterparts with much lower cost and greater flexibility. Compared to inorganic counterparts, the development of hybrid inorganic-polymer structures for TE applications are in a nascent stage, where theoretical understanding is very much needed and many potential nanoscale structures are yet to be explored. In this work, we study quasi-one-dimensional TE transport in a nanoscale polymer–semiconductor–polymer (PSP) junction, where a semiconductor quantum dot (QD) is trapped between two bulk polymers with aligned polymer chains. The Holstein small polaron model, which can be used for strong electron–phonon interaction in polymers beyond the perturbation theory, is used to model the transport in such a nanoscale PSP junction. We then use the Green’s function method along with the Landauer formula to calculate the TE properties of a nanoscale PSP junction, including the electrical conductance G, the Seebeck coefficient S, and the power factor GS2. Due to the sharp distribution of electron density of states in the polymer leads and the discrete energy levels in a QD, simultaneous enhancement of the Seebeck coefficient and the electrical conductance in nanoscale PSP junctions can be achieved when the energy levels are appropriately aligned, compared to metal–molecule–metal junctions. The theoretical approach to study nanoscale PSP junctions can be readily extended to the study of QD–polymer nanocomposites. The quantitative results obtained in this work can shed some light in material selection for the synthesis of hybrid inorganic–polymer nanocomposites, where theoretical guidance is much in need.}, journal={The Journal of Physical Chemistry C}, year={2013}, month={Nov} }
 @article{ultralow thermal conductivity of atomic/molecular layer-deposited hybrid organic–inorganic zincone thin films_2013, url={http://dx.doi.org/10.1021/nl403244s}, DOI={10.1021/nl403244s}, abstractNote={Atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques with atomic level control enable a new class of hybrid organic-inorganic materials with improved functionality. In this work, the cross-plane thermal conductivity and volumetric heat capacity of three types of hybrid organic-inorganic zincone thin films enabled by MLD processes and alternate ALD-MLD processes were measured using the frequency-dependent time-domain thermoreflectance method. We revealed the critical role of backbone flexibility in the structural morphology and thermal conductivity of MLD zincone thin films by comparing the thermal conductivity of MLD zincone films with an aliphatic backbone to that with aromatic backbone. Much lower thermal conductivity values were obtained in ALD/MLD-enabled hybrid organic-inorganic zincone thin films compared to that of the ALD-enabled W/Al2O3 nanolaminates reported by Costescu et al. [Science 2004, 303, 989-990], which suggests that the dramatic material difference between organic and inorganic materials may provide a route for producing materials with ultralow thermal conductivity.}, journal={Nano Letters}, year={2013}, month={Nov} }
 @article{length-dependent thermal conductivity of single extended polymer chains_2012, url={http://dx.doi.org/10.1103/physrevb.86.104307}, DOI={10.1103/physrevb.86.104307}, abstractNote={The low thermal conductivity of polymers will be one of the major roadblocks for the polymer-based microelectronics and macroelectronics due to the limited heat spreading capability. Despite that the thermal conductivity of bulk polymers is usually low, a single extended polymer chain could have very high thermal conductivity. In this paper we present atomistic simulation studies on the phonon transport in single extended polymer chains of various polymers as a function of polymer chain length. The thermal conductivity of single extended polymer chains can be 1--2 orders of magnitude higher than their bulk counterparts. The thermal conductivity of single extended polymer chains is a strong function of monomer type. For example, the thermal conductivity of the extended polymer chains with aromatic backbone can be up to 5 times as that of a polyethylene chain, while the thermal conductivity of the extended polymer chains with bond-strength or mass disorder can be only 1/25 as that of a polyethylene chain. We analyze the phonon transport mechanisms in single extended polymer chains of various polymers and find that the competition between ballistic phonon transport and diffusive phonon transport in a polymer chain leads to a diverging length-dependent thermal conductivity.}, journal={Physical Review B}, year={2012}, month={Sep} }
 @article{thermal transport across carbon nanotubes connected by molecular linkers_2012, url={http://dx.doi.org/10.1016/j.carbon.2011.10.014}, DOI={10.1016/j.carbon.2011.10.014}, abstractNote={Nonequilibrium molecular dynamics is applied to investigate thermal transport across two CNTs connected longitudinally by molecular linkers, which is a basic building-block for CNT network structures. We show the effect of different numbers, monomer types, and lengths of molecular linkers on the interfacial thermal conductance between CNTs and molecular linkers. We also analyze the density of vibrational normal modes to further understand the interfacial thermal conductance between different molecular linkers and CNTs. For most of the molecular linker type we simulated, the interfacial thermal conductance decreases with the increasing chain length. We find that aromatic-backbone structures are better than aliphatic-backbone structures to obtain higher interfacial thermal conductance with CNTs. Incorporating double bonds, oxygen atoms and amide groups into polymer chains shifts or redistributes of the density of vibrational normal modes, which in turn tunes the interfacial thermal conductance of molecular linker with CNTs. These results provide guidance for choosing molecular linkers to build up large-scale CNT-based network structures with tunable thermal properties.}, journal={Carbon}, year={2012}, month={Mar} }
 @article{heat transfer enhancement by filling metal porous medium in central area of tubes_2010, url={http://dx.doi.org/10.1179/014426010x12592427711911}, DOI={10.1179/014426010x12592427711911}, abstractNote={<title/>Given that the fluid within the tubes of some industrial heat exchangers is under a state of fully developed laminar flow with a constant Nu number, increasing the surface area for heat transfer will significantly increase the flow resistance. In this paper, we filled metal porous medium with high thermal conductivity, high porosity and high filling radius in the central area of fully developed laminar flow within the tube, and established corresponding numerical models for fluid flow and heat transfer. Numerical simulation results indicate that after filling the tube with metal porous medium, the temperature profiles within the porous medium area are very uniform, and the temperature difference between the tube wall and the fluid decreases significantly which correspondingly results in a notable increase of Nu number; meanwhile, the characteristic of flow field redistribution occurs within the enhanced tube, but the total flow resistance composed of the Darcy resistance and inertial resistance of the porous medium area and the shear stress caused by velocity gradient and fluid viscosity of the non-porous medium area near the wall increase; correspondingly, the performance evaluation criteria (PEC) value is thus applied to evaluate the effect of the heat tranfer enhancement method. For a tube of 9 mm in radius, the PEC values are all above 1 when the filling radius of the metal porous medium is larger than 7 mm.}, journal={Journal of the Energy Institute}, year={2010}, month={Mar} }
 @article{tuning the thermal conductivity of polymers with mechanical strains_2010, url={http://dx.doi.org/10.1103/physrevb.81.174122}, DOI={10.1103/physrevb.81.174122}, abstractNote={The low thermal conductivity of polymers limits their heat spreading capability, which is one of the major technical barriers for the polymer-based products, especially electronics, such as organic light emitting diodes. It is highly desirable to enhance the thermal conductivity of polymer materials including polymer composites. Mechanical stretching could align polymer chains which are intrinsically low-dimensional material that could have very high thermal conductivity and thus enhancing the thermal conductivity of polymers. In this work, the all-atom model molecular-dynamics simulation is conducted to investigate the tuning of polymer thermal conductivity using mechanical strains. The simulation results show that the thermal conductivity of polymers increases with the increasing strain and the enhancement is larger when the polymer is stretched slower. Molecular weight also affects the thermal conductivity under the same stretching condition. More importantly, the thermal-conductivity enhancement could be exponentially fitted with the orientational order parameter which describes the chain conformation change. This study could guide the development of advanced reconfigurable and tunable thermal management technologies.}, journal={Physical Review B}, year={2010}, month={May} }
 @article{ultrafast thermoreflectance techniques for measuring thermal conductivity and interface thermal conductance of thin films_2010, url={http://dx.doi.org/10.1063/1.3504213}, DOI={10.1063/1.3504213}, abstractNote={The thermal conductivity of thin films and interface thermal conductance of dissimilar materials play a critical role in the functionality and the reliability of micro/nanomaterials and devices. The ultrafast laser-based thermoreflectance techniques, including the time-domain thermoreflectance (TDTR) and the frequency-domain thermoreflectance (FDTR) techniques are excellent approaches for the challenging measurements of interface thermal conductance of dissimilar materials. Both TDTR and FDTR signals on a trilayer structure which consists of a thin film metal transducer, a target thin film, and a substrate are studied by a thermal conduction model. The sensitivity of TDTR signals to the thermal conductivity of thin films is analyzed to show that the modulation frequency needs to be selected carefully for a high precision TDTR measurement. However, such a frequency selection, which is closely related to the unknown thermal properties and consequently hard to make before TDTR measurement, can be avoided in FDTR measurement. We also found out that in FDTR method, the heat transport in a trilayer structure could be divided into three regimes, and the thermal conductivity of thin films and interface thermal conductance can be obtained subsequently by fitting the data in different frequency range of one FDTR measurement, based on the regime map. Both TDTR and FDTR measurements are then conducted along with the analysis to obtain the thermal conductivity of SiO2 thin films and interface thermal conductance between SiO2 and Si. FDTR measurement results agree well with the TDTR measurements, but promises to be a much easier implementation than TDTR measurements.}, journal={Journal of Applied Physics}, year={2010}, month={Nov} }