Publications

2017 Journal Articles

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10. Ab initio study of mode-resolved phonon transmission at Si/Ge interfaces using atomistic Green's functions
Benoit Latour, Nina Shulumba, and Austin J. Minnich
Phys. Rev. B 96, 104310

We report an ab initio based study of phonon transmission at Si/Ge interfaces using a recent extension of the atomistic Green's function method that resolves transmission coefficients by mode. These results provide a detailed framework to investigate the precise transmission and reflection processes that lead to thermal resistance and how they depend on phonon frequency as well as incident angle.

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9. Photo-excited hot carrier dynamics in hydrogenated amorphous silicon imaged by 4D electron microscopy
Bolin Liao, Ebrahim Najafi, Heng Li, Austin J. Minnich, and Ahmed H. Zewail
Nature Nano. 12, 871–876

In this work, carrier dynamics in hydrogenated amorphous silicon is imaged in space and time using scanning ultrafast electron microscopy (SUEM). An unexpected regime of fast diffusion immediately after photoexcitation, together with spontaneous electron-hole separation and charge trapping induced by atomic disorder, is observed.

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8. Experimental metrology to obtain thermal phonon transmission coefficients at solid interfaces
Chengyun Hua*, Xiangwen Chen*, Navaneetha K. Ravichandran and Austin J. Minnich
Phys. Rev. B, 95(20)

This work reports the first measurement of the Fresnel transmission coefficients for thermal phonons at a metal-semiconductor interface using ab-initio phonon transport modeling and a thermal characterization technique.

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7. Elastic and thermal properties of free-standing Molybdenum Disulfide Membranes measured using Ultrafast Transient Grating Spectroscopy
Taeyong Kim, Ding Ding, Jong-Hyuk Yim, Young-Dahl Jho, and Austin J. Minnich
APL Materials, 5(086105)

Molybdenum disulfide (MoS2) is of intense interest due to its unique electronic and thermoelectric properties. However, reports of its in-plane thermal conductivity vary due to the difficulty of in-plane thermal conductivity measurements on thin films. Here, we use time-resolved transient grating spectroscopy to simultaneously measure the in-plane elastic and thermal properties of free-standing MoS2 membranes at room temperature. Our measurements provide useful insights into the elastic and thermal properties of MoS2 and demonstrate the capability of transient grating spectroscopy to investigate the in-plane vibrational properties of van der Waals materials.

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6. Propagating elastic vibrations dominate thermal conduction in amorphous silicon
Jaeyun Moon, Benoit Latour, Austin Minnich
arXiv:1704.08360,
In a-Si, prior works concluded that non-propagating waves are the dominant contributors to heat transport, while propagating waves are restricted to frequencies less than a few THz and are scattered by anharmonicity. Here, we present a lattice and molecular dynamics analysis of vibrations in a-Si that supports a qualitatively different picture in which propagating elastic waves dominate the thermal conduction and are scattered by elastic fluctuations rather than anharmonicity. Our work suggests that most heat is carried by propagating elastic waves in a-Si and demonstrates a route to achieve extreme thermal properties in amorphous materials by manipulating elastic fluctuations.

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5. Thermal response of materials to extreme temperature gradients and the role of the spatial frequency
Xiangwen Chen*, Chengyun Hua*, Hang Zhang, Navaneetha K. Ravichandran, and Austin J. Minnich
The thermal response of materials to extreme temperature gradients has recently become of intense interest both scientifically and for applications. Here, we probe the thermal response of crystals to large thermal gradients generated by optical heating of nanoline arrays. We demonstrate that the ballistic thermal resistance associated with nanoscale heat sources can be nearly eliminated by rationally engineering the spatial arrangement of individual heaters, a finding that could impact strategies for heat dissipation in electronics and other applications.

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4. Semiconductor-based Multilayer Selective Solar Absorber for Unconcentrated Solar Thermal Energy Conversion
Nate Thomas*, Zhen Chen, Shanhui Fan, and Austin J. Minnich
Scientific Reports 7, 5362

We report a semiconductor-based multilayer selective absorber that exploits the sharp drop in optical absorption at the band gap energy achieve a measured absorptance of 76% at solar wavelengths and a low emittance of 5% at thermal wavelengths. In field tests we obtain a peak temperature of 225°C. With straightforward optimization, our work will allow unconcentrated solar thermal systems to reach stagnation temperatures exceeding 300°C, thereby eliminating the need for solar concentrators for mid-temperature solar applications.

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3. The role of low energy phonons in nanocrystalline Si and SiGe: an ab-initio based study
Lina Yang and Austin J. Minnich
Scientific Reports 7, 44254,

Using a variance-reduced Monte Carlo method with the full phonon dispersion and intrinsic lifetimes from first-principles as input, we examine phonon transport in nanocrystalline SiGe alloys with ab-initio electronphonon scattering rates. Our calculations show that low energy phonons still transport substantial amounts of heat in these materials, and thus improvements in ZT are still possible by disrupting these modes.

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2. Daytime Radiative Cooling Using Infrared Black Bodies
Jun-long Kou, Zoila Jurado, Zhen Chen, Shanhui Fan, and Austin J. Minnich
ACS Photonics,

We demonstrate that a polymer-coated fused silica mirror, as a near-ideal blackbody in the mid-infrared and near-ideal reflector in the solar spectrum, achieves radiative cooling below ambient air temperature under direct sunlight by 8.2 degrees. Our work demonstrates that abundant materials and straight-forward fabrication can be used to achieve daytime radiative cooling, advancing applications such as dry cooling of thermal power plants.

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1. Intrinsic localized mode and low thermal conductivity of PbSe
Nina Shulumba, Olle Hellman, and Austin J. Minnich
Physical Review B, 95, 014302,

In this work, we use ab-initio calculations that explicitly account for strong anharmonicity to identify the origin of the low thermal conductivity in PbSe as an anomalously large anharmonic interaction, exceeding in strength that in PbTe, between the transverse optic and longitudinal acoustic branches.

2016 Journal Articles

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8. Sub-amorphous thermal conductivity in amorphous heterogeneous nanocomposites
Jaeyun Moon and Austin J. Minnich
RSC Advances, 105154-105160 ,

This work employs equilibrium molecular dynamics to show an amorphous SiGe nanocomposite possesses thermal conductivity substantially lower than those of the amorphous Si and Ge constituents.

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7. Active Thermal Extraction and Temperature Sensing of Near-field Thermal Radiation
Ding Ding, Taeyong Kim, and Austin J. Minnich
Scientific Reports 6, Article number: 32744,

This works reports a theoretical framework for active extraction of near field radiation in the context of laser cooling of solids with the direct application of temperature sensing.

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6. Exploring the extremes of heat conduction in anisotropic materials.
Austin J. Minnich
Nanoscale and Microscale Thermophysical Engineering, Vol. 20, No. 1, 1-21,

Anisotropic solids possess thermal conductivities ranging from among the highest found in nature, as in the in-plane thermal conductivity of graphite, to the lowest, as in the cross-plane thermal conductivity of disordered layered crystals. In this review article, we discuss the recent advances in our understanding of thermal phonon transport in anisotropic solids obtained using new theoretical, computational, and experimental tools.

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5. Thermal Characterization of Substrate Options for High-Power THz Multipliers Over a Broad Temperature Range
Tero Kiuru, Goutam Chattopadhyay, Theodore J. Reck, Austin J. Minnich, Robert Lin, Erich Schlecht, Jose V. Siles, Choonsup Lee, and Imran Mehdi
IEEE Transactions on Terahertz Science and Technology, vol. 6, no. 2, pp. 328-335,

This paper presents thermal characterization results for three high-power THz Schottky frequency multipliers in the temperature range of 20–380 K. At temperatures below 125 K, the GaAs membrane is on par, or even has lower thermal resistance than the membrane on diamond.

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4. Temperature Dependent Mean Free Path Spectra of Thermal Phonons Along the c-axis of Graphite
Hang Zhang, Xiangwen Chen, Young-Dahl Jho, Austin J. Minnich
Nano Letters,

This work reports the first quantitative measurements of c-axis phonon MFP spectra in graphite at a variety of temperatures using time-domain thermoreflectance measurements of graphite flakes with variable thickness. Our results indicate that c-axis phonon MFPs have values of a few hundred nanometers at room temperature and a much narrower distribution than in isotropic crystals. At low temperatures, phonon scattering is dominated by grain boundaries separating crystalline regions of different rotational orientation.

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3. Active Thermal Extraction of Near-field Thermal Radiation
D. Ding, T. Kim, and A. J. Minnich
Physical Review B, 93, 081402(R),

This work proposes and numerically demonstrates an active scheme to extract near-field surface modes to the far-field. This approach exploits the monochromatic nature of near-field thermal radiation to drive a transition in a laser gain medium, which, when coupled with external optical pumping, allows the resonant surface mode to be emitted into the far-field.

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2. The Role of Thermalizing and Non-thermalizing Walls in Phonon Heat Conduction along Thin Films
Navaneetha K. Ravichandran, Austin J. Minnich
Physical Review B, 93, 035314,

Here we examine thermal transport along thin films with both thermalizing and non-thermalizing walls by solving the spectral Boltzmann transport equation (BTE) for steady state and transient transport. Our results provide insights into the boundary scattering process of thermal phonons over a range of heating length scales that are useful for interpreting thermal measurements on nanostructures.

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1. Heat conduction in multifunctional nanotrusses studied using Boltzmann transport equation
Nicholas G. Dou, Austin J. Minnich
Applied Physics Letters, 108, 011902,

Nanotrusses consisting of hollow nanoscale beams architected into a periodic truss structure can potentially achieve low density, low thermal conductivity yet also high stiffness due to their lattice architecture and nanoscale features. In this work, we study heat conduction in the exact nanotruss geometry by solving the frequency-dependent Boltzmann transport equation using a variance-reduced Monte Carlo algorithm.

2015 Journal Articles

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9. Crystalline Polymers with Exceptionally Low Thermal Conductivity Studied using Molecular Dynamics
Andrew B. Robbins, Austin J. Minnich
Applied Physics Letters, 107, 201908,

In this paper, we use molecular dynamics simulations to study heat transport in polynorbornene, a polymer that can be synthesized in semi-crystalline form using solution processing. We demonstrate that even perfectly crystalline polynorbornene has an exceptionally low thermal conductivity near the amorphous limit due to extremely strong anharmonic scattering. This scattering is sufficiently strong to prevent the formation of propagating phonons, with heat being instead carried by non-propagating, delocalized vibrational modes known as diffusons.

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8. Thermal phonon boundary scattering in anisotropic thin films
A. J. Minnich
Applied Physics Letters, Vol. 107, Issue 18,

In this paper, we derive a generalization of the Fuchs-Sondheimer equation for solids with arbitrary dispersion relations and examine its predictions for graphite. We find that the isotropic equation vastly over-estimates the boundary scattering that occurs in thin graphite films due to the highly anisotropic group velocity, and that graphite can maintain its high in-plane thermal conductivity even in thin films with thicknesses as small as ten nanometers.

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7. Multidimensional quasiballistic thermal transport in transient grating spectroscopy
A. J. Minnich
Physical Review B,

This paper analyzes quasiballistic transport in a two-dimensional transient grating experiment, in which heat conduction can occur both in- and cross-plane. Using an analytic Green's function of the Boltzmann equation, we demonstrate a method by which phonon MFPs can be extracted from these measurements, thereby extending the MFP spectroscopy technique using transient grating to opaque bulk materials.

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6. Semi-analytical solution to the frequency-dependent Boltzmann transport equation for cross-plane heat conduction in thin films
C. Hua and A. J. Minnich
J. Appl. Phys.

This work presents a semi-analytical series expansion method to solve the transient frequency-dependent Boltzmann transport equation in a finite domain, which enables us to obtain solutions for the cross-plane heat transport in thin films.

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5. Length dependent thermal conductivity measurements yield phonon mean free path spectra in nanostructures
H. Zhang, C. Hua, D. Ding and A. J. Minnich
Scientific Reports,

This work develops a reconstruction method to obtain mean free path spectra of nanostructures from variable-length thermal conductivity measurements. Using this method, we investigate recently reported length-dependent thermal conductivity measurements on SiGe alloy nanowires and suspended graphene ribbons.

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4. Selective radiative heating of nanostructures using hyperbolic metamaterials
D. Ding, and A. J. Minnich
Optics Express,

This work demonstrates the concept of using hyperbolic metamaterials (HMM) to enable selective heating of a sub-wavelength plasmonic nanowire by controlling the angular mode number of a plasmonic resonance.

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3. The best nanoparticle size distribution for minimum thermal conductivity
H. Zhang and A. J. Minnich
Scientific Reports,

This work demonstrates the best size distribution of nanoparticles embedded in a crystalline solid, which can yield the lowest thermal conductivity. Counterintuitively, nanoparticles with several well-chosen discrete sizes can most effectively scatter the continuous broad thermal phonon spectrum.

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2. Phonon heat conduction in layered anisotropic crystals
A. J. Minnich
Physical Review B,

The work extends an analytical solution of the Boltzmann transport equation to highly anisotropic solids and demonstrates a method to reconstruct the anisotropic mean free path spectrum of crystals with arbitrary dispersion relations without any prior knowledge of their harmonic or anharmonic properties using observations of quasiballistic heat conduction.

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1. Advances in the measurement and computation of thermal phonon transport properties
A. J. Minnich
J. Phys. Cond. Matt., 27, 053202,

In this topical review, the author provides an overview of the methods in both computation and experiment, which enable an unprecedented microscopic view of thermal transport by phonons, as well as the insights they are providing and their impact on the science and engineering of heat conduction.

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2014 Journal Articles

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7. Analytical Green's function of the multidimensional frequency-dependent phonon Boltzmann equation
C. Hua and A. J. Minnich
Physical Review B, 90,214306,

This paper presents an analytical Green's function for the frequency-dependent, multidimensional Boltzmann equation and provides simple closed-form solutions for a number of multidimensional problems for which the only prior solution methods have been computationally expensive numerical simulations.

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6. Phonon blackbody radiation limit for heat dissipation in electronics
J. Schleeh, J. Mateos, I. Iniguez-de-la-Torre, N. Wadefalk, P. A. Nilsson, J. Grahn, and A. J. Minnich
Nature Materials, 14, 187-192,

This paper shows that the origin of the noise saturation in cryogenic amplifiers is due to ballistic thermal phonon transport near the transistor and demonstrates that the lowest noise that can be achieved in these transistors at cryogenic temperatures is largely governed by the heat dissipation in the device.

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5. Heat under the microscope
I. Maasilta and A. J. Minnich
Physics Today (invited, peer-reviewed),

In this article, the authors review recent advances in computational and spectroscopic tools which offer new insights into the nature of thermal conduction at ever-finer length scale and ways to control it.

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4. Importance of frequency-dependent grain boundary scattering in nanocrystalline silicon and silicon-germanium thermoelectrics
C. Hua and A. J. Minnich
Special invited issue of Semiconductor Science and Technology, 29 (2014) 124004,

This work examines the impact of frequency-dependent grain boundary scattering in nanocrystalline silicon and silicon-germanium alloys in a realistic 3D geometry using frequency-dependent variance-reduced Monte Carlo simulations.

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3. Radial quasiballistic transport in time-domain thermoreflectance studied using Monte Carlo simulations
D. Ding, X. Chen and A. J. Minnich
Applied Physics Letters 104, 143104,

This paper presents a numerical study to identify the radial suppression function that allows the mean free path spectrum to be quantitatively reconstructed from variable pump-size measurements in time-domain thermoreflectance.

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2. Coherent and incoherent thermal transport in nanomeshes
N. K. Ravichandran and A. J. Minnich
Physical Review B, 89, 205432,

This paper demonstrates that thermal transport in nanosctructures that can be created with available lithographic techniques is dominated by incoherent boundary scattering at room temperature.

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1. Transport regimes in quasiballistic heat conduction
C. Hua and A. J. Minnich
Physical Review B, 89,094302,

This work presents a new analytical solution of the frequency-dependent Boltzmann transport equation in transient grating decay, which leads to a more accurate measurement of phonon mean free path spectrum.

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2013 Journal Articles

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2. Thermal transport: Naturally glassy crystals
A. J. Minnich
Nature Nanotechnology,Vol. 8 pp. 392-393,
 
This News and Views article describes a recent work (See [Delaire et al]) that reports the origin of a glass-like thermal conductivity in a perfectly crystalline semiconductor.

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1. Direct Measurement of Room-Temperature Nondiffusive Thermal Transport Over Micron Distances in a Silicon Membrane
J. A. Johnson, A. A. Maznev, J. Cuffe, John, J. K. Eliason, A. J. Minnich, T. Kehoe, C. M. Torres, G. Chen and K. A. Nelson.
Physical Review Letter Vol. 110, 025901,
 
This work presents a simple experiment demonstrating that room-temperature thermal transport in Si significantly deviates from the diffusion model already at micron distances.

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2012 Journal Articles

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2. Coherent Phonon Heat Conduction in Superlattices
Maria N. Luckyanova, Jivtesh Garg, Keivan Esfarjani, Adam Jandl, Mayank T. Bulsara, Aaron J. Schmidt, Austin J. Minnich, Shuo Chen, Mildred S. Dresselhaus, Zhifeng Ren, Eugene A. Fitzgerald, Gang Chen
Science,Vol. 338 no. 6109 pp. 936-939,
 
This paper describes the first observation of coherent heat conduction, in which thermal phonons travel as waves rather than particles, in a GaAs/AlAs superlattice.

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1. Determining Phonon Mean Free Paths from Observations of Quasiballistic Thermal Transport
A. J. Minnich
Physical Review Letters
, 109, 205901 (2012)  
This work demonstrates how the phonon mean free path distribution may be reconstructed from quasiballistic thermal measurements by solving a common inverse problem.

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2011 Journal Articles

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3. Power Factor Enhancement by Modulation Doping in Bulk Nanocomposites
Mona Zebarjadi, Giri Joshi, Gaohua Zhu, Bo Yu, Austin Minnich, Yucheng Lan, Xiaowei Wang, Mildred Dresselhaus, Zhifeng Ren, and Gang Chen
Nano Lett., 2011, 11 (6), pp 2225–2230

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2. Quasiballistic heat transfer studied using the frequency-dependent boltzmann transport equation
A. J. Minnich, G. Chen, S. Mansoor, and B. S. Yilbas
Physical Review B
, 84(23):235207, December 2011 
This paper presents a computational study of one-dimensional quasiballistic in a metal/semiconductor structure.

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1. A thermal conductivity spectroscopy technique to measure phonon mean free paths
A. J. Minnich, , J. A. Johnson, A. J. Schmidt, K. Esfarjani, M. S. Dresselhaus, K. A. Nelson, and G. Chen
Physical Review Letters
, 107:095901, Aug 2011. Chosen as Editor's suggestion
This paper introduces the first experimental technique that is able to measure thermal phonon mean free paths over a wide range of length scales and materials.

2011 Conference Papers

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Quasi-ballistic heat transfer from metal nanostructures on sapphire
Austin Minnich and Gang Chen
In Proceedings of the 8th ASME/JSME Thermal Engineering Joint Conference, page 44094, 2011

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2010 Journal Articles

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2. Theoretical studies on the thermoelectric figure of merit of nanograined bulk silicon
Qing Hao, Gaohua Zhu, Giri Joshi, Xiaowei Wang, Austin Minnich, Zhifeng Ren, and Gang Chen
Applied Physics Letters
, 97(6):063109, 2010 

1. Thermal conductance and phonon transmissivity of metal-graphite interfaces
Aaron J. Schmidt, Kimberlee C. Collins, Austin J. Minnich, and Gang Chen
Journal of Applied Physics, 107(10):104907, 2010

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2009 Journal Articles

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4. Modeling study of thermoelectric SiGe nanocomposites
A. J. Minnich, H. Lee, X. W. Wang, G. Joshi, M. S. Dresselhaus, Z. F. Ren, G. Chen, and D. Vashaee
Physical Review B
, 80(15):155327, October 2009

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3. Bulk nanostructured thermoelectric materials: Current research and future prospects
A. J. Minnich, M. S. Dresselhaus, Z. F. Ren, and G. Chen
Energy & Environmental Science
, 2(5):466 - 479, 2009 (Invited, peer-reviewed. Among the top 10 downloaded articles in EES for July, August, December 2009, January-May, July-December 2010, January 2011)

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2. Enhancement of thermoelectric figure-of-merit by a bulk nanostructuring approach
Yucheng Lan, Austin Minnich, Gang Chen, and Zhifeng Ren
Advanced Functional Materials
, 20:357 - 376, 2009

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1. Solubility study of Yb in n-type skutterudites YbxCo4Sb12 and their enhanced thermoelectric properties
J. Yang, Q. Hao, H. Wang, Y. C. Lan, Q. Y. He, A. J. Minnich, D. Z. Wang, J. A. Harriman, V. M. Varki, M. S. Dresselhaus, G. Chen, and Z. F. Ren
Physical Review B
, 80(11):115329, 2009

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2008 Journal Papers

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High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys
Bed Poudel, Qing Hao, Yi Ma, Yucheng Lan, Austin Minnich, Bo Yu, Xiao Yan, Dezhi Wang, Andrew Muto, Daryoosh Vashaee, Xiaoyuan Chen, Junming Liu, Mildred S. Dresselhaus, Gang Chen, and Zhifeng Ren
Science
, 320(5876):634-638, 2008

2008 Conference Papers

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2. Modeling grain boundary scattering in nanocomposites
A. Minnich, D. Vashaee, and G. Chen
In Proceedings of the 2008 ASME International Mechanical Engineering Congress & Exposition, pages IMECE2008-67385, Boston, November 2008

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1. Modeling the thermoelectric properties of nanocomposites
A. Minnich and G. Chen
In Proceedings of the 3rd ASME Energy Nanotechnology International Conference, Jacksonville, August 2008

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2007 Journal Articles

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Modified effective medium formulation for the thermal conductivity of nanocomposites
Austin Minnich and Gang Chen
Applied Physics Letters
, 91(7):073105, 2007 (also in August 27, 2007 issue of Virtual Journal of Nanoscale Science & Technology)

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Book Chapters

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Measuring phonon mean free paths using thermal conductivity spectroscopy
Austin J. Minnich
Annual Review of Heat Transfer, Belsevere 2012.

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