Archival Journals Publications

J. Buongiorno, Cahill, D. G., Hidrovo, C. H., Moghaddam, S., Schmidt, A. J., and Shi, L., “Micro- and Nanoscale Measurement Methods for Phase Change Heat Transfer on Planar and Structured Surfaces,” Nanoscale and Microscale Thermophysical Engineering, vol. 18, pp. 270-287, 2014. Publisher's VersionPDF icon phasechangemeasurements.pdf
H. Fateh, Baker, C. A., Hall, M. J., and Shi, L., “High fidelity finite difference model for exploring multi-parameter thermoelectric generator design space,” Applied Energy, vol. 129, pp. 373 - 383, 2014. Publisher's VersionAbstract
Abstract Thermoelectric generators (TEGs) are being studied and developed for applications in which waste heat, for example, from the exhaust of motor vehicles is converted into usable electricity. \{TEGs\} consisting of \{TE\} elements integrated with an exhaust heat exchanger require optimization to produce the maximum possible power output. Important optimization parameters include \{TE\} element leg length, fill fraction, leg area ratio between n- and p-type legs, and load resistance. A finite difference model was developed to study the interdependencies among these optimization parameters for thermoelectric elements integrated with an exhaust gas heat exchanger. The present study was carried out for \{TE\} devices made from n-type Mg2Si and p-type MnSi1.8 based silicides, which are promising \{TE\} materials for use at high temperatures associated with some exhaust heat recovery systems. The model uses specified convection boundary conditions instead of specified temperature boundary conditions to duplicate realistic operating conditions for a waste heat recovery system installed in the exhaust of a vehicle. The 1st generation, and an improved 2nd generation \{TEG\} module using Mg2Si and p-type MnSi1.8 based silicides were fabricated and tested to compare \{TE\} power generation with the numerical model. Important results include parameter values for maximum power output per unit area and the interdependencies among those parameters. Heat transfer through the void areas was neglected in the numerical model. When thermal contact resistance between the \{TE\} element and the heat exchangers is considered negligible, the numerical model predicts that any volume of \{TE\} material can produce the same power per unit area, given the parameters are accurately optimized. Incorporating the thermal contact resistance, the numerical model predicts that the peak power output is greater for longer \{TE\} elements with larger leg areas. The optimization results present strategies to improve the performance of \{TEG\} modules used for waste heat recovery systems.
X. Chen, Girard, S. N., Meng, F., Lara-Curzio, E., Jin, S., Goodenough, J. B., Zhou, J., and Shi, L., “

Approaching the Minimum Thermal Conductivity in Rhenium-Substituted Higher Manganese Silicides

,” Advanced Energy Materials, vol. 4, no. 14, pp. 1400452 , 2014. Publisher's VersionPDF icon re_substituted_hms_author_version.pdf
I. Jo, Pettes, M. T., Ou, E., Wu, W., and Shi, L., “Basal-plane thermal conductivity of few-layer molybdenum disulfide,” Applied Physics Letters, vol. 104, pp. 201902, 2014. Publisher's Version
V. Singh, Bougher, T. L., Weathers, A., Cai, Y., Bi, K., Pettes, M. T., McMenamin, S. A., Lv, W., Resler, D. P., Gattuso, T. R., Altman, D. H., Sandhage, K. H., Shi, L., Henry, A., and Cola, B. A., “High thermal conductivity of chain-oriented amorphous polythiophene,” Nature Nanotechnology, vol. 9, pp. 384-390, 2014. Publisher's VersionAbstract
Polymers are usually considered thermal insulators, because the amorphous arrangement of the molecular chains reduces the mean free path of heat-conducting phonons. The most common method to increase thermal conductivity is to draw polymeric fibres, which increases chain alignment and crystallinity, but creates a material that currently has limited thermal applications. Here we show that pure polythiophene nanofibres can have a thermal conductivity up to [sim]4.4 W m-1 K-1 (more than 20 times higher than the bulk polymer value) while remaining amorphous. This enhancement results from significant molecular chain orientation along the fibre axis that is obtained during electropolymerization using nanoscale templates. Thermal conductivity data suggest that, unlike in drawn crystalline fibres, in our fibres the dominant phonon-scattering process at room temperature is still related to structural disorder. Using vertically aligned arrays of nanofibres, we demonstrate effective heat transfer at critical contacts in electronic devices operating under high-power conditions at 200 [deg]C over numerous cycles.
A. L. Moore and Shi, L., “Emerging challenges and materials for thermal management of electronics,” Materials Today, vol. 17, pp. 163 - 174, 2014. Publisher's Version
C. A. Baker, Osman Emiroglu, A., Mallick, R., Ezekoye, O. A., Shi, L., and Hall, M. J., “Development of an Analytical Design Tool for Monolithic Emission Control Catalysts and Application to Nano-Textured Substrate System,” Journal of Thermal Science and Engineering Applications , vol. 6, no. 3, pp. 031014 - 031014, 2014. Publisher's VersionAbstract
An analytical transport/reaction model was developed to simulate the catalytic performance of ZnO nanowires as a catalyst support. ZnO nanowires were chosen because they have easily characterized, controllable features and a spatially uniform morphology. The analytical model couples convection in the catalyst flow channel with reaction and diffusion in the porous substrate material; it was developed to show that a simple analytical model with physics-based mass transport and empirical kinetics can be used to capture the essential physics involved in catalytic conversion of hydrocarbons. The model was effective at predicting species conversion efficiency over a range of temperature and flow rate. The model clarifies the relationship between advection, bulk diffusion, pore diffusion, and kinetics. The model was used to optimize the geometry of the experimental catalyst for which it predicted that maximum species conversion density for fixed catalyst surface occurred at a channel height of 520 μm.
C. - C. Chen, Li, Z., Shi, L., and Cronin, S. B., “Thermal interface conductance across a graphene/hexagonal boron nitride heterojunction,” Applied Physics Letters, vol. 104, pp. 081908, 2014. Publisher's Version
S. Wen, Fleming, E., Shi, L., and da Silva, A. K., “Numerical Optimization and Power Output Control of a Hot Thermal Battery with Phase Change Material,” Numerical Heat Transfer, Part A: Applications, vol. 65, pp. 825-843, 2014. Publisher's VersionAbstract
Numerical simulations were conducted to investigate the release of heat from a thermal storage unit, which we refer to as a hot thermal battery. The battery is composed of a hexagonal arrangement of parallel tubes through which a heat absorbing fluid flows, surrounded by phase change material (PCM) that fills spaces between adjacent tubes. The simulations implemented, aimed to optimize the battery such that it meets a total volume constraint while ensuring a critical power output before the phase change fronts of the PCM surrounding two adjacent heat transfer tubes merge—indicating the PCM is completely solidified—after which, only sensible heat can be released by the battery. It was found that the PCM latent heat has negligible impact on the optimal heat exchanger (HEx) design. In comparison, increasing either the flow velocity of the heat transfer fluid in the tubes or PCM thermal conductivity can significantly reduce the needed volume of heat exchanger. Additionally, a novel closed loop control modeling approach is proposed to dynamically tune the heat transfer fluid flow rate such that the thermal battery yields a constant power output. The flow tuning results indicate the optimal dynamic HTF velocity curve shape, obtained from closed-loop method, is unique and this optimal flow velocity is dependent on the location of the phase change front. Numerical results were also compared against hot battery discharge experiments, using both constant and dynamically tuned flow rates, indicating a good agreement for both cases.
D. G. Cahill, Braun, P. V., Chen, G., Fan, S. H., Goodson, K. E., Keblinski, P., King, W. P., Mahan, G. D., Majumdar, A., Maris, H. J., Phillpot, S. R., Pop, E., and Shi, L., “Nanoscale Thermal Transport II: 2003-2012,” Applied Physics Reviews, vol. 1, pp. 011305, 2014. LinkAbstract
A diverse spectrum of technology drivers such as improved thermal barriers, higher efficiency thermoelectric energy conversion, phase-change memory, heat-assisted magnetic recording, thermal management of nanoscale electronics, and nanoparticles for thermal medical therapies are motivating studies of the applied physics of thermal transport at the nanoscale. This review emphasizes developments in experiment, theory, and computation in the past ten years and summarizes the present status of the field. Interfaces become increasingly important on small length scales. Research during the past decade has extended studies of interfaces between simple metals and inorganic crystals to interfaces with molecular materials and liquids with systematic control of interface chemistry and physics. At separations on the order of , the science of radiative transport through nanoscale gaps overlaps with thermal conduction by the coupling of electronic and vibrational excitations across weakly bonded or rough interfaces between materials. Major advances in the physics of phonons include first principles calculation of the phonon lifetimes of simple crystals and application of the predicted scattering rates in parameter-free calculations of the thermal conductivity. Progress in the control of thermal transport at the nanoscale is critical to continued advances in the density of information that can be stored in phase change memory devices and new generations of magnetic storage that will use highly localized heat sources to reduce the coercivity of magnetic media. Ultralow thermal conductivity—thermal conductivity below the conventionally predicted minimum thermal conductivity—has been observed in nanolaminates and disordered crystals with strong anisotropy. Advances in metrology by time-domain thermoreflectance have made measurements of the thermal conductivity of a thin layer with micron-scale spatial resolution relatively routine. Scanning thermal microscopy and thermal analysis using proximal probes has achieved spatial resolution of 10 nm, temperature precision of 50 mK, sensitivity to heat flows of 10 pW, and the capability for thermal analysis of sub-femtogram samples.
H. X. Ji, Sellan, D. P., Pettes, M. T., Kong, X. H., Ji, J. Y., Shi, L., and Ruoff, R. S., “Enhanced Thermal Conductivity of Phase Change Materials with Ultrathin-Graphite Foams for Thermal Energy Storage,” Energy & Environmental Science, vol. 7, pp. 1185–1192, 2014. LinkAbstract
For thermophysical energy storage with phase change materials (PCMs), the power capacity is often limited by the low PCM thermal conductivity (κPCM). Though dispersing high-thermal conductivity nanotubes and graphene flakes increases κPCM, the enhancement is limited by interface thermal resistance between the nanofillers, among other factors such as detrimental surface scattering of phonons. Here, we demonstrate that embedding continuous ultrathin-graphite foams (UGFs) with volume fractions as low as 0.8–1.2 vol% in a PCM can increase κPCM by up to 18 times, with negligible change in the PCM melting temperature or mass specific heat of fusion. The increase in κPCM, thermal cycling stability, and applicability to a diverse range of PCMs suggests that UGF composites are a promising route to achieving the high power capacity targets of a number of thermal storage applications, including building and vehicle heating and cooling, solar thermal harvesting, and thermal management of electrochemical energy storage and electronic devices.
M. T. Pettes and Shi, L., “A Reexamination of Phonon Transport through a Nanoscale Point Contact in Vacuum,” Journal of Heat Transfer, vol. 136, no. 3, pp. 032401, 2014. LinkAbstract
Using a silicon nitride cantilever with an integral silicon tip and a microfabricated platinum–carbon resistance thermometer located close to the tip, a method is developed to concurrently measure both the heat transfer through and adhesion energy of a nanoscale point contact formed between the sharp silicon tip and a silicon substrate in an ultrahigh vacuum atomic force microscope at near room temperature. Several models are used to evaluate the contact area critical for interpreting the interfacial resistance. Near field-thermal radiation conductance was found to be negligible compared to the measured interface thermal conductance determined based on the possible contact area range. If the largest possible contact area is assumed, the obtained thermal interface contact resistance can be explained by a nanoconstriction model that allows the transmission of phonons from the whole Brillouin zone of bulk Si with an average finite transmissivity larger than 0.125. In addition, an examination of the quantum thermal conductance expression suggests the inaccuracy of such a model for explaining measurement results obtained at above room temperature.
B. A. Cola, Daiguji, H., Dames, C., Fang, N., Fushinobu, K., Inoue, S., Kikugawa, G., Kohno, M., Kumar, S., Li, D. Y., Lukes, J. R., Malen, J. A., McGaughey, A. J. H., Nakabeppu, O., Pipe, K., Reddy, P., Shen, S., Shi, L., Shibahara, M., Taguchi, Y., Takahashi, K., Yamamoto, T., and Zolotoukhina, T., “Report on the Seventh U.S. –Japan Joint Seminar on Nanoscale Transport Phenomena — Science and Engineering,” Nanoscale and Microscale Thermophysical Engineering , vol. 17, pp. 25-49, 2013. LinkAbstract
The seventh U.S.–Japan Joint Seminar on Nanoscale Transport Phenomena was held in Shima, Japan, from December 11 to 14, 2011. The goals of this joint seminar were to provide a critical assessment of the state of the art and future directions in the field of nanoscale transport phenomena and energy conversion processes, to foster U.S.–Japan collaborations, and to provide international exposure to a new generation of scientists in this field. Issues discussed in the joint seminar were organized in 10 topical sessions, including (1) nanoscale thermophysical measurements; (2) optical characterization; (3) thermal and molecular transport; (4) phonon transport modeling; (5) energy storage and conversion; (6) nanoscale fluidics and phase change phenomena; (7) biological and organic systems; (8) interfacial thermal transport; (9) novel thermoelectric and thermal management materials; and (10) nanocarbon materials and devices. In addition to these topical sessions, the joint seminar featured an opening plenary session and a closing plenary session as well as an expert panel, where leading experts provided critical assessment of the past progress and addressed future directions in the field. In addition, an evening poster session provided opportunities for graduate and postdoc students to present their latest research results. About 35 researchers from Japan and 31 researchers from the United States participated in the meeting. The meeting was organized by S. Maruyama, K. Fushinobu, L. Shi, and J. Lukes together with about 20 other participants who served as session chairs. Summaries of different sessions of the seminar were prepared by the session and conference chairs and are collected into this report.
D. R. Birt, An, K. M., Weathers, A., Shi, L., Tsoi, M., and Li, X. Q., “"Brillouin Light Scattering Spectra as Local Temperature Sensors for Thermal Magnons and Acoustic Phonons,” Appl. Phys. Lett., vol. 102, pp. 082401, 2013. LinkAbstract
We demonstrate the use of the micro-Brillouin light scattering (micro-BLS) technique as a local temperature sensor for magnons in a permalloy (Py) thin film and phonons in the glass substrate. When the Py film is uniformly heated, we observe a systematic shift in the frequencies of two thermally excited perpendicular standing spin wave modes. Fitting the temperature dependent magnon spectra allows us to achieve a temperature resolution better than 2.5 K. In addition, we demonstrate that the micro-BLS spectra can be used to measure the local temperature of magnons and the relative temperature shift of phonons across a thermal gradient. Such local temperature sensors are useful for investigating spin caloritronic and thermal transport phenomena in general.
S. Z. Butler, Hollen, S. M., Cao, L. Y., Cui, Y., Gupta, J. A., Gutiérrez, H. R., Heinz, T. F., Hong, S. S., Huang, J. X., Ismach, A. F., Johnston-Halperin, E., Kuno, M., Plashnitsa, V. V., Robinson, R. D., Ruoff, R. S., Salahuddin, S., Shan, J., Shi, L., Spencer, M. G., Terrones, M., Windl, W., and Goldberger, J. E., “Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene,” ACS Nano, vol. 7, pp. 2898–2926, 2013. LinkAbstract
Graphene’s success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in field-effect transistors, spin- and valley-tronics, thermoelectrics, and topological insulators, among many other applications.
I. Jo, Pettes, M. T., Kim, J. H., Watanabe, K., Taniguchi, T., Yao, Z., and Shi, L., “Thermal Conductivity and Phonon Transport in Suspended Few-Layer Hexagonal Boron Nitride,” Nano Letters, vol. 13, pp. 550–554, 2013. LinkAbstract
The thermal conductivity of suspended few-layer hexagonal boron nitride (h-BN) was measured using a microbridge device with built-in resistance thermometers. Based on the measured thermal resistance values of 11–12 atomic layer h-BN samples with suspended lengths ranging between 3 and 7.5 μm, the room-temperature thermal conductivity of a 11-layer sample was found to be about 360 W m–1 K–1, approaching the basal plane value reported for bulk h-BN. The presence of a polymer residue layer on the sample surface was found to decrease the thermal conductivity of a 5-layer h-BN sample to be about 250 W m–1 K–1 at 300 K. Thermal conductivities for both the 5-layer and the 11-layer samples are suppressed at low temperatures, suggesting increasing scattering of low frequency phonons in thin h-BN samples by polymer residue.
L. A. Valentín, Betancourt, J., Fonseca, L. F., Pettes, M. T., Shi, L., Soszyński, M., and Huczko, A., “A Comprehensive Study of Thermoelectric and Transport Properties of β-Silicon Carbide Nanowires,” Journal of Applied Physics, vol. 114, pp. 184301, 2013. LinkAbstract
The temperature dependence of the Seebeck coefficient, the electrical and thermal conductivities of individual β-silicon carbide nanowires produced by combustion in a calorimetric bomb were studied using a suspended micro-resistance thermometry device that allows four-point probe measurements to be conducted on each nanowire. Additionally, crystal structure and growth direction for each measured nanowire was directly obtained by transmission electron microscopy analysis. The Fermi level, the carrier concentration, and mobility of each nanostructure were determined using a combination of Seebeck coefficient and electrical conductivity measurements, energy band structure and transport theory calculations. The temperature dependence of the thermal and electrical conductivities of the nanowires was explained in terms of contributions from boundary, impurity, and defect scattering.
X. Chen, Weathers, A., Salta, D., Zhang, L. B., Zhou, J. S., Goodenough, J. B., and Shi, L., “Effects of (Al,Ge) Double Doping on the Thermoelectric Properties of Higher Manganese Silicides,” Journal of Applied Physics, vol. 114, pp. 173705, 2013. LinkAbstract
Experiments and analysis have been carried out to investigate the effects of Al and (Al,Ge) doping on the microstructure and thermoelectric properties of polycrystalline higher manganese silicide (HMS) samples, which were prepared by solid-state reaction, ball milling, and followed by spark plasma sintering. It has been found that Al doping effectively increases the hole concentration, which leads to an increase in the electrical conductivity and power factor. By introducing the second dopant Ge into Al-doped HMS, the electrical conductivity is increased, and the Seebeck coefficient is decreased as a result of further increased hole concentration. The peak power factor is found to occur at a hole concentration between 1.8 × 1021 and 2.2 × 1021 cm−3 measured at room temperature. The (Al,Ge)-doped HMS samples show lower power factors owing to their higher hole concentrations. The mobility of Mn(Al0.0035 Ge ySi0.9965-y)1.8 with y = 0.035 varies approximately as T−3/2 above 200 K, suggesting acoustic phonon scattering is the dominant scattering mechanism. The thermal conductivity of HMS does not change appreciably by Al or (Al,Ge) doping. The maximum ZT of (Al,Ge)-doped HMS is 0.57 at 823 K, which is similar to the highest value found in the Al-doped HMS samples. The ZT values were reduced in the Mn(Al0.0035 Ge ySi0.9965-y)1.8 samples with high Ge concentration of y = 0.025 and 0.035, because of reduced power factor. In addition, a two-band model was employed to show that the hole contribution to the thermal conductivity dominates the bipolar and electron contributions for all samples from 300 to 823 K and accounts for about 12% of the total thermal conductivity at about 800 K.
M. T. Pettes, Maassen, J., Jo, I., Lundstrom, M., and Shi, L., “Effects of Surface Band Bending and Scattering on Thermoelectric Transport in Suspended Bismuth Telluride Nanoplates,” Nano Letters, vol. 13, no. 11, pp. 5316–5322, 2013. LinkAbstract
A microdevice was used to measure the in-plane thermoelectric properties of suspended bismuth telluride nanoplates from 9 to 25 nm thick. The results reveal a suppressed Seebeck coefficient together with a general trend of decreasing electrical conductivity and thermal conductivity with decreasing thickness. While the electrical conductivity of the nanoplates is still within the range reported for bulk Bi2Te3, the total thermal conductivity for nanoplates less than 20 nm thick is well below the reported bulk range. These results are explained by the presence of surface band bending and diffuse surface scattering of electrons and phonons in the nanoplates, where pronounced n-type surface band bending can yield suppressed and even negative Seebeck coefficient in unintentionally p-type doped nanoplates.
K. D. Bi, Weathers, A., Matsushita, S., Pettes, M. T., Goh, M. J., Akagi, K., and Shi, L., “Iodine Doping Effects on the Lattice Thermal Conductivity of Oxidized Polyacetylene Nanofibers,” Journal of Applied Physics, vol. 114, pp. 194302, 2013. LinkAbstract
Thermal transport in oxidized polyacetylene (PA) nanofibers with diameters in the range between 74 and 126 nm is measured with the use of a suspended micro heater device. With the error due to both radiation and contact thermal resistance corrected via a differential measurement procedure, the obtained thermal conductivity of oxidized PA nanofibers varies in the range between 0.84 and 1.24 W m−1 K−1 near room temperature, and decreases by 40%–70% after iodine doping. It is also found that the thermal conductivity of oxidized PA nanofibers increases with temperature between 100 and 350 K. Because of exposure to oxygen during sample preparation, the PA nanofibers are oxidized to be electrically insulating before and after iodine doping. The measurement results reveal that iodine doping can result in enhanced lattice disorder and reduced lattice thermal conductivity of PA nanofibers. If the oxidation issue can be addressed via further research to increase the electrical conductivity via doping, the observed suppressed lattice thermal conductivity in doped polymer nanofibers can be useful for the development of such conducting polymer nanostructures for thermoelectric energy conversion.