Kevin Pipe, University of Michigan
Patrick Hopkins, University of Virginia
Yann Chalopin, Ecole Centrale Paris
Baowen Li, National University of Singapore
Tuesday PM, April 02, 2013
Moscone West, Level 3, Room 3002
2:30 AM - V2.01
Hot-phonon Energy Conversion and Efficiency
Seungha Shin 1 Massoud Kaviany 1
1University of Michigan Ann Arbor USAShow Abstract
Optical phonon modes can be overpopulated because of larger generation rate than decay during various energy conversions and resistive processes (e.g., in electric circuits). These overpopulated optical phonons (i.e. hot phonons) hinder the energy relaxation and transport in devices, and they are finally thermalized converting their energy to waste heat by generating entropy. Thus, utilizing the hot phonons before thermalization is expected to improve the efficiency and the performance mitigating heat generation. We suggest the direct energy conversion from hot phonon to electron potential using semiconductor heterobarrier structures.
Here, we employ a heterojunction structure composed of GaAs and Al_xGa_1minus;xAs (x or x_Al is the Al content), which has a band edge discontinuity in the conduction and valence bands depending on x_Al. With electrons as the main charge carriers, conduction band edge discontinuity is regarded as a barrier with a height phi;_b depending x_Al. Potential barriers impose an adverse current by the potential change, and to compensate this adverse effect, large, forward local electric field (e_e,HPAB) formed by x_Al grading is introduced in the barrier. Electrons in the suggested hot-phonon absorbing barrier (HPAB) structure absorb phonon thus increasing their kinetic energy, and this energy is in turn converted to electric potential through the barrier transition.
Hot phonons can be decayed by their downconversion to acoustic phonons in addition to the interactions with electrons. The kinetics of all the interactions (phonon-electron and phonon-phonon) is central in this study, and it can be calculated through the quantum mechanical perturbation theory, i.e. the Fermi golden rule. In order to pass through barriers, electrons require sufficient E_e, or the less energetic electrons need to increase their energy through phonon absorption. Since high-energy carriers are emission-favorable and less populated and from the kinetics multiphonon absorption for low-energy electron transition is not probable enough, then tandem of barrier structures with phi;_b le; E_p,LO (only absorption available) rather than a single barrier with a large height is employed for large potential gain.
Based on the phonon and electron interaction kinetics from the first principle, self-consistent ensemble Monte Carlo (MC) method is employed to show the electric potential/power gain by phonon absorption in HPAB. MC simulations demonstrate that the HPAB produces electric potential gain without current loss through a proper combination of e_e,HPAB, phi;_b and hot phonon population. We found the favorable conditions for unassisted absorption of hot phonons and an optimized GaAs/Al_xGa_1minus;xAs barrier structure which can harvest 19% of hot phonons. By integrating HPAB in electronic devices, reduced heat dissipation by effective removal of excess phonons, as well as additional electric power generation (or recovery) will be achieved.
2:45 AM - V2.02
Thermal Conduction in Aperiodic Superlattices for Quantum Cascade Lasers
Aditya Sood 1 Jeremy A. Rowlette 3 Elah Bozorg-Grayeli 2 Mehdi Asheghi 2 Kenneth E. Goodson 2
1Stanford University Stanford USA2Stanford University Stanford USA3Daylight Solutions San Diego USAShow Abstract
Quantum Cascade Lasers (QCLs) produce coherent radiation in the mid IR to THz frequency range with applications for defense, medicine and communication. QCL operation relies on intersubband electronic transitions within confined quantum wells in superlattices of GaInAs and AlInAs on InP or GaAs substrates. While QCLs offer high efficiency, the associated large levels of heat generation cause thermal conduction within the active region to strongly influence the stability and reliability, particularly for the continuous wave (CW) or high duty-cycle mode at room temperature. With layer thicknesses in the range of 2-6 nm, phonon transport can be strongly influenced by phonon scattering and, potentially, by modifications to phonon dispersion. Previous studies have examined the thermal conductivity of as-prepared QCL structures and some coupled electro-thermal-optical models based on continuum heat conduction have been proposed. In addition, there is a wealth of prior literature on phonon scattering (and even dispersion modification) for periodic nanoscale superlattice structures. However, real QCL cores contain a distribution of well (GaInAs) and barrier (AlInAs) layer thicknesses, typically ranging between 1 nm and 4 nm, with no periodicity on length scales comparable to the phonon mean free path (MFP asymp; 2 - 5 nm; stack period asymp; 50 nm). Therefore, for the better design and optimization of QCL performance, it is crucial to make progress on a comprehensive model that can account for the aperiodic behavior in QCL superlattices. The present work develops an approximate solution to the phonon Boltzmann transport equation (BTE) accounting for both the local interface scattering and the impact of boundaries of one or more interfaces displaced from the local region. A statistical method based on phonon transition at interfaces and survival rates through the material volume is employed to develop a physics-based modeling approach for aperiodic superlattices. The range of phonon transport from ballistic to continuum is studied, using the MFP and layer thicknesses are parameters. Furthermore, a comparison is made with a Matthiessen&’s rule based approach that assumes a distribution of weights assigned to proximate interfaces. The goal is to develop an effective thermal conductivity model that can be applied locally within the aperiodic structure. We compare the predictions of these simulations with out-of-plane thermal conductivity data for QCL superlattice structures obtained using time-domain thermoreflectance.
3:00 AM - V2.03
Phonon Coherence in Semiconductor Superlattices
Benoit Latour 1 Sebastian Volz 1 Yann Chalopin 1
1Laboratoire EM2C - Ecole Centrale Paris Chatenay-Malabry FranceShow Abstract
The thermal conductivity (TC) of superlattices (SLs) has attracted a great attention for the past years for their phonon properties. It has been shown that the TC of SLs decreases and reaches a minimum for a certain period thickness. Phonon coherence effects have been invoked in the literature to explain this trend but no clear evidence of the physics involved has been reported so far. In this work, we propose a mathematical definition of phonon coherence and investigate how it is affected by the period thickness. Semiconductor SLs are studied based on an equilibrium Molecular Dynamics approach. Both perfect and defected SLs are considered in this study.
3:15 AM - V2.04
Measurement of the Thermal Conductivity of Si-Ge-based Nanostructures
Katrin Bertram 1 Bodo Fuhrmann 1 Nadine Geyer 2 Aleksandr Tonkikh 2 Nicole Wollschlaeger 2 1 Peter Werner 2 Hartmut S. Leipner 1
1Martin Luther University Halle Germany2Max Planck Institut for Microstructure Physics Halle GermanyShow Abstract
A reduced thermal conductivity of thermoelectric materials is necessary to obtain an enhanced figure of merit ZT for thermoelectric generators and Peltier coolers. Thin thermoelectric films are suitable for thermoelectric sensor applications. An enhancement in ZT for Si-Ge thin films by nanostructuring offers a new temperature range for Si-based thermoelectric devices. The reduction in the thermal conductivity in superlattices is achieved by diffuse interface scattering of phonons. With a mismatch of phonon dispersion, the scattering of short-wavelength phonons is enhanced due to localized phonon states. Some theoretical work predicted a further decrease in thermal conductivity with nonperiodic superlattice structures, due to a confinement of long-wavelength phonons. The presence of lattice defects such as dislocations can also lead to lower thermal conductivities. In this study, we have investigated the thermal conductivities of Si-Ge based superlattices. Periodic and nonperiodic Sim-Gen superlattices with stacks of m Si and n Ge layers of various thicknesses were grown by molecular beam epitaxy on (111) Si substrates. The influence of periodicity was investigated. Further investigations were carried out on nanowire arrays prepared from these superlattices by catalytic etching. The structures were characterized by cross-section transmission electron microscopy. A comparison between the different approaches for the effective reduction in the thermal conductivity will be presented.
3:30 AM - V2.05
Thermal Conductivity of Amorphous/Crystalline Silicon Superlattices, a Molecular Dynamics Study
Konstantinos Termentzidis 1 Tristan Albaret 2 Samy Merabia 2 Valentin Jean 1 David Lacroix 1
1LEMTA, Universitamp;#233; de Lorraine - CNRS UMR 7563 Vandoeuvre les Nancy France2LPMCN, Un. Claude Bernard Lyon I, CNRS UMR-5586 Villeurbanne FranceShow Abstract
The amorphous/crystalline superlattices (a/c SLs) with large conduction band discontinuities can be used for resonant-tunnelling diodes, modulation doped field effect transistors and quantumwell infrared photodetectors . They are also interesting candidates for low cost thermoelectric power devices . These SLs can be made with materials which can display large lattice mismatch and/or which can have interfaces. The latter ones are essentially defect free and atomically sharp .
To the best of our knowledge the only investigation of the heat transfer through a/c interfaces was made by Von Alfthan et al . He concludes that the thickness of amorphous regions does not affect the thermal conductivity (TC), while the thickness of crystalline regions has a more dramatic effect on the overall conductivity. We propose to extend this study with the prediction of the Kapitza resistance. We will also appraise the effect of the SL&’s parameters such as period and interfacial roughness, as well as the temperature. A recent theoretical work of Donadio and Galli on crystalline silicon nanowires with amorphous surfaces showed that the TC is not affected by the temperature and it is close to the one of amorphous materials . This unusual temperature dependence is explained by the presence of a majority of non-propagating vibrational modes.
There is a lack of information about a/c silicon interfaces, concerning both SLs and nanowires. In our current study SLs of a-Si/c-Si preliminary results will be presented for the TC and Kapitza resistance of the interfaces as a function of the SLs period, the roughness of interfaces and the temperature . Preliminary results show that the Si atoms that belong to the amorphous silicon regions and located close to the interfaces exhibit higher atomic energy than the inner atoms in amorphous regions. This might related with the enhancement of phonon scattering at the interfaces.
1 R.W. Fathauer, “New class of Si based superlattices: Alternating layers of crystalline Si and porous amorphous Si1minus;xGex alloys”, Appl. Phys. Lett. 61, 2350 (1992).
2 H. Ohra, R. Huang and Y. Ikuhara, “Large enhancement of the thermoelectric Seebeck coefficient for amorphous oxide semiconductor superlattices with extremely thin conductive layers”, Phys. Status Solidi (RRL) 2, 105(2008).
3 S.C. Agarwal, “Amorphous silicon-based superlattices ”, Bull. Mater. Sci. 14, 1257 (1991).
4 S. Von Alfthan, A. Juronen, and K. Kaski, Mat. Res. Soc. Symp. Proc. 703, 6.2.1 (2002).
5 D. Donadio and G. Galli, “Temperature dependence of the thermal conductivity of thin silicon nanowires” Nano Lett. 10, 847 (2010)
6 M. Talati, T. Albaret and A. Tanguy “Atomistic simulations of elastic and plastic properties in amorphous silicon”, EPL 86, 66005 (2009)
3:45 AM - V2.06
The Role of Size Effects and Boundary Scattering on the Thermal Conductivity of Silicon-germanium Alloy Thin Films
Ramez Cheaito 1 John C. Duda 1 Thomas E. Beechem 2 Douglas L. Medlin 3 Khalid Hattar 2 Edward S. Piekos 2 Patrick E. Hopkins 1
1University of Virginia Charlottesville USA2Sandia National Laboratories Albuquerque USA3Sandia National Laboratories Livermore USAShow Abstract
We experimentally investigate the role of size effects and boundary scattering on the thermal conductivity of silicon-germanium alloys. The thermal conductivities of a series of epitaxially grown Si1minus;xGex thin films with varying thicknesses and compositions were measured with time-domain thermoreflectance. The resulting conductivities are found to be 3 to 5 times less than bulk values, and vary strongly with film thickness. By examining these measured thermal conductivities in the context of a previously established model, it is shown that long wavelength phonons known to be the dominant heat carriers in alloy films, are strongly scattered by the film boundaries, thereby inducing the observed reductions in heat transport. These results are then generalized to silicon-germanium systems of various thicknesses and compositions; we find that the thermal conductivities of Si1minus;xGex superlattices are ultimately limited by finite size effects and sample size rather than periodicity or alloying. This demonstrates the strong influence of sample size in alloyed nanosystems. Therefore, if a comparison is to be made between the thermal conductivities of superlattices and alloys, the total sample thicknesses of each must be considered.
Tuesday PM, April 02, 2013
Moscone West, Level 3, Room 3002
4:30 AM - V3.01
Micro-Raman Optomechanothermography: A Non-contact Method for Measuring Strain-dependent Thermal Conductivity of Individual Nanowires
Kathryn F. Murphy 1 John P. Sullivan 2 Brian Piccione 1 Daniel S. Gianola 1
1University of Pennsylvania Philadelphia USA2Sandia National Laboratories Albuquerque USAShow Abstract
Silicon nanowires have recently been shown to be promising thermoelectrics due to their reduced thermal conductivity and improved Seebeck coefficient and electrical conductivity relative to bulk. However, they do not have conversion efficiencies comparable with the best bulk thermoelectrics, and further avenues must be explored in order to make them a viable energy harvesting approach. One method of tuning thermoelectric properties which has not been fully developed is that of elastic strain engineering, which is known to affect charge carrier effective mass and band gap as well as phonon dispersion and group velocity. Strain engineering should have enhanced efficacy in nanostructured materials, which exhibit a large range of elastic strain as compared to their bulk counterparts. Although some groups have measured the effect of strain on electrical properties of silicon nanowires, methods for accurate measurement of coupled thermomechanical properties have been elusive.
We present a novel non-contact approach for measuring thermal conductivity of a single suspended nanowire subjected to varied strain levels, which we name µ-Raman optomechanothermography. We employ a microelectromechanical (MEMS) device to apply uniaxial tensile stress to individual silicon nanowires under a confocal µ-Raman spectroscope. Raman maps across the length and width of the nanowire at several different laser intensities enable the deconvolution of the effects of stress and temperature on the Raman spectrum. Fits of the temperature profiles at the various laser intensities along the length of the nanowire combined with estimates of absorbed laser power yield measurements of thermal conductivity corrected for contact resistance. Representative results of thermal conductivity as a function of stress for Si nanowires will be shown and discussed in the context of changes to phonon group velocities and relaxation lifetimes predicted by atomistic simulation.
4:45 AM - *V3.02
Spatially-resolved Profiling of Thermal Resistance within Individual Nanowires
John T.L. Thong 1 3 Rong-Guo Xie 1 2 4 Dan Liu 3 Baowen Li 2 3 4
1National University of Singapore Singapore Singapore2National University of Singapore Singapore Singapore3National University of Singapore Singapore Singapore4National University of Singapore Singapore SingaporeShow Abstract
While the study of thermal transport in low-dimensional nanostructures has been receiving growing attention, the options to conduct experimental measurements on such nanostructures are limited. Certain approaches to measurement, such as the thermal bridge method or 3-omega; method, can be used to derive the thermal conductance of a nanowire as a whole, and scanning probe techniques can be used to conduct spatially-resolved measurements. However, measurement uncertainties associated with the contact thermal resistance between the sample and substrate (or thermal probe) remain a challenge to be addressed. Here we report a new thermal measurement technique that is capable of profiling the thermal conductance of an individual nanowire with a spatial resolution better than 20nm. In this technique, a focused electron beam is employed as a localized heating source to establish a temperature gradient along the nanowire. The heat fluxes from the two ends of the nanowire are measured using platinum resistor loops on two suspended thermally-isolated membranes. We show that, by a single scan of the electron beam along the nanowire, the local thermal conductance altered by non-uniformities in the nanowire, such as local defects, change of surface roughness and variations in diameter, can be discriminated. Moreover, this technique is capable of measuring the thermal boundary resistance across epitaxial material interfaces in the nanowire. By using this technique, the long-standing problem of ill-defined thermal contact resistance between the nanowire and the two membranes, which is a serious drawback in the conventional thermal bridge method, is avoided. This technique thus provides a powerful tool for studying the underlying physics of thermal transport in nanostructures, which will in turn improve the thermal models adopted in the design of nanodevices, and inform the fabrication of nanostructured thermoelectric materials with enhanced performance.
ZQ Wang, R Xie, CT Bui, D Liu, X Ni, B Li, JTL Thong, “Thermal transport in suspended and supported few-layer graphene”, Nano Lett. 11, 113-118 (2011) ;
CT Bui, R Xie, M Zheng, Q Zhang, CH Sow, B Li, JTL Thong, “Diameter-dependent thermal transport in individual ZnO nanowires and its correlation with surface coating and defects”, Small 8, 738-745 (2012).
5:15 AM - V3.03
Reduced Thermal Conductance near Surfaces Roughened by Metal Assisted Chemical Etching
Joseph Patrick Feser 1 David G. Cahill 1
1University of Illinois, Urbana-Champaign Urbana USAShow Abstract
Recently, we have reported that the thermal conductivity of silicon nanowire arrays roughened by metal-assisted chemical etching (MAC-etch) is strongly correlated to both the magnitude of the roughness and a broadening of the one-phonon Raman linewidth. We have hypothesized that microstructural disorder induced by the etching chemistry leads to such changes to the Raman linewidth. Here, we simplify the study of such effects by chemically roughening Si wafers instead of nanowires. We have studied the effects of various previously reported roughening procedures on the near-surface thermal properties using time-domain thermoreflectance. We find that the thermal conductance of the near-surface region is reduced substantially by MAC-etching, despite the expectation that pristine roughened surfaces should have increased conductance due to enhanced surface area. In addition, highly roughened surfaces show strong acoustic echoes with reflection coefficient indicative of a soft interface. These features are consistent with the presence of near-surface disorder and in some cases microporosity.
5:30 AM - V3.04
Joule Heating of a Nanowire with Radiation Through Its Surface: Analytical Series Solution for Temperature Profile in the Wire through Laplace Transforms
K. S. Ravi Chandran 1
1University of Utah Salt Lake City USAShow Abstract
Joule heating in nanowires is a fundamental heat transfer problem in nanoscale science and technology. It is important in thermal management of nanoscale solid-state displays, emitters, thermionic devices and sensors, and, also in new functional applications of nanowires or nanotubes. The resistive heating and the resulting temperature distribution along the wire is a function of several parameters including electrical and thermal properties, wire diameter and circumference, and, more importantly, the boundary conditions at the ends of the wire. Heat transfer in nanowires is also sensitive to the radiation/convection through the circumference because the surface to volume ratio is inversely proportional to wire diameter, potentially creating size effect on the temperature profile. Also, one of the common techniques to determine thermal conductivity of nanowires or films is by Joule heating by applying steady or pulsed electric power. Despite the importance, there is no exact analytical solution yet for temperature distribution in Joule heated nanowires, including the radiative/convective heat transfer and non-zero heat flux boundary conditions. In this presentation, we present an analytical series solution for the temperature distribution along the length of a nanowire. The solution was derived from the modified Fick&’s second law of heat transfer including joule heating and radiation/convection terms. Using Laplace transformation, the time-dependent nonhomogeneous heat transfer equation was converted into the subsidiary equation. The equation was then solved by inverse Laplace transformation, with the solution satisfying the radiative and the flux boundary conditions. This solution can be very useful in determining the temperature distribution as affected by the nanowire parameters and radiative heat transfer conditions as well as in developing chip-based experimental methods to measure size-dependent phase transition temperatures in phase change memory materials.
5:45 AM - V3.05
Using Morphology and Structure to Tune Solid-state Thermal Properties
Kedar Hippalgaonkar 1 Jongwoo Lim 2 Peter Ercius 3 Jia Zhu 1 Renkun Chen 4 Xiang Zhang 1 Peidong Yang 2 Arun Majumdar 1
1UC Berkeley Berkeley USA2UC Berkeley Berkeley USA3Lawrence Berkeley National Lab Berkeley USA4UC San Diego Berkeley USAShow Abstract
Recently, diffusive phonon transport in nanostructured materials has been a subject of intense interest. In this work, we demonstrate how the limits of heat transport can be tested in three novel material systems by extending the thermal measurement platform to probe material structure and provide a direct correlation to their thermal properties.
Phonons are lattice vibrations and their scattering in solids has largely been explained like collision of particles. Since the development of nanostructures, diffusive boundary scattering from large surface-to-volume ratio materials has been studied in nanowires and superlattices. To beat this diffusive scattering limit, we designed nanowires with broadband roughness close to the dominant phonon wavelength (1-10nm) at room temperature. The decrease in thermal conductivity of intrinsic silicon by a factor of ~30 from 140 W/m-K to 5 W/m-K in this sub-diffusive regime opens up the possibility of multiple scattering stemming from coherent phonon wave effects. Transmission Electron Microscopy (TEM) based techniques including 3D tomography were then used to map out the morphology and we find that we can reduce the thermal conductivity to as low as 1 W/m-K, while preserving the single-crystalline core, which is as low as that of amorphous silicon or silica. Correlating the surface roughness and porosity to the measured thermal conductivity opens up a new paradigm to observing wave physics in thermal phonons at room temperature in nanomaterials.
Secondly, the platform developed previously was extended to be compatible with TEM, allowing us to characterize the crystal structure of measured nanowires. While phonon optics experiments in the 1970s showed a crystallographic direction dependent thermal conductivity, we performed the first 1-1 mapping of nanowire growth direction and thermal conductivity in Bismuth Nanowires. In the boundary scattering regime with diameter 100nm, a nanowire in the [-102] direction had k = 8.5 W/m-K, ~6 times higher than a nanowire in the  direction with k = 1.5 W/m-K.
Finally, we manufactured tapered Vanadium Oxide beams to study asymmetric phonon physics that manifested in temperature dependent thermal rectification. The interplay between electrons and phonons and the possibility of asymmetric scattering rates prompted us to look closely at the existence of Metal-Insulator interfaces that could result in thermal rectification. Between 150K and 340K, the VnO2n-1 phases could be either metallic or insulating with nanoscale domains. We performed high resolution Auger spectroscopy on single-crystal Vanadium Oxide beams that showed a stoichiometry variation and measured thermal rectification as high as 22%. The rectification behavior turns off (<4%) once the whole beam reaches the metallic phase, higher than 340K. Our platform thus couples materials characterization, especially TEM, with thermal property measurement to enhance understanding of thermal phonons.
Tuesday AM, April 02, 2013
Moscone West, Level 3, Room 3002
9:30 AM - V1.02
Thermal Transport across Transfer-printed Metal-dielectric and Metal-elastomeric Interfaces
Piyush Kumar Singh 1 Myunghoon Seong 1 Sanjiv Sinha 1 2
1University of Illinois at Urbana-Champaign Urbana USA2University of Illinois at Urbana-Champaign Urbana USAShow Abstract
Transfer printing  integrates dissimilar materials on large-area substrates to enable new functionalities. The interfacial thermal conductance of transfer-printed objects attains importance in applications involving heat dissipation. The first experimental investigation of transport across transfer-printed Au-Si and Au-SiO2 interfaces  indicate a surprisingly high interfacial thermal conductance. We investigate such transport theoretically using a combination of contact mechanics and electron-phonon transport, considering both metal-dielectric and metal-elastomeric interfaces.
Using the theory of Kogut and Etsion , we show that the plastic deformation of Au, coupled with capillary forces due to water bridges formed between asperities lead to significantly large fractional area coverage of ~0.20 for Au-SiO2 interface. We find the value of thermal conductance, G for Au-SiO2 interface to be ~15 MW/m2K which is of the same order of magnitude as the experimental value (~45 MW/m2K) . We hypothesize that the discrepancy between theoretical and experimental data arises due to the heat conduction through water capillary bridges. Using a similar approach for Au-PDMS interface, we get fractional area coverage of ~0.14 and a thermal conductance value of ~1 MW/m2K. We discuss the implications of these results for the use of metallic interconnects in heat dissipating devices.
1. Meitl, M. A.; Zhu, Z.-T.; Kumar, V.; Lee, K. J.; Feng, X.; Huang, Y. Y.; Adesida, I.; Nuzzo, R. G.; Rogers, J. A. Nat Mater 2006, 5, (1), 33-38.
2. D. W. Oh, S. Kim, J. A. Rogers, D. G. Cahill, and S. Sinha, Advanced Materials 23 (43), 5028 (2011).
3. Kogut, L.; Etsion, I. Journal of Colloid and Interface Science 2003, 261, (2), 372-378.
9:45 AM - *V1.03
Thermal Transport through van der Waals Interfaces between Nanostructures
Deyu Li 1
1Vanderbilt University Nashville USAShow Abstract
Nanostructures possess unique thermal transport properties because of nanoconfinement effects. However, nanostructures are usually in contact with other nanostructures, matrix materials or substrates. For example, in various nanostructured microfibers, thin films, and bulk nanocomposites, which have been projected as structural materials for aerospace vehicles and for energy conversion applications, individual nanostructures are often in contact with other nanostructures or host materials through van der Waals (vdW) interactions. Thermal transport through these vdW interfaces could play critical role in the overall thermal transport properties of the nanostructured materials. Therefore, it is important to study thermal transport through vdW contacts between individual nanostructures. We report on experimental and theoretical studies of thermal transport through contacts between individual nanostructures, which includes (1) contact thermal resistance between individual multi-wall carbon nanotubes, and (2) thermal transport through vdW interfaces between individual boron nanoribbons.
10:15 AM - V1.04
Effects of Spatially-Varying Strain on the Thermal Properties of Interfaces
Vahid Rashidi 1 Kevin Patrick Pipe 1 2
1University of Michigan Ann Arbor USA2University of Michigan Ann Arbor USAShow Abstract
Strained semiconductors are widely used in electronic devices due to certain advantages in their electrical and optical properties such as high carrier mobility and bandgap modifications relative to their unstrained crystal structures. Often the devices in which they are used (such as strained-Si transistors or semiconductor lasers with strained active regions) present thermal management challenges due to high heat fluxes. Understanding of heat transfer in such devices is complicated by the facts that 1) the strained layers are often quite thin relative to the phonon mean free path, 2) the “bulk” properties of the strained layers are often themselves not spatially constant if a strain gradient exists within the film, and 3) the effects of strain on the thermal boundary resistance (TBR) to an adjacent material are poorly understood. Furthermore, strain gradients usually occur over relatively long spatial scales (~100 nm) which place a large computational burden (millions of atoms) on atomistic simulations.
In this work we utilize molecular dynamics (MD) to study the thermal properties of strained Si/Ge interfaces. We first impose spatially constant strain within each semiconductor and use non-equilibrium MD (NEMD) to study how the bulk thermal conductivities and TBR vary as a function of this constant strain. We find that increasing compressive strain on both semiconductors results in a reduction in TBR. We then impose selected spatially-varying strain profiles on the structure (for example, exponential reductions in strain normal to the interface with varying relaxation lengths, which mimic relaxed epitaxial interfaces) to understand the effects of these strain profiles on “bulk” film thermal conductivity and TBR. We further study the effect of an intermixed region at the interface of varying thickness, to capture intermixing that happens during epitaxial growth or subsequent thermal annealing. The results are compared with theoretical models for thermal transport in strained semiconductors.
10:30 AM - V1.05
Strain Field and Coherent Domain Wall Effects on Thermal Conductivity and Kapitza Conductance across Internal Boundaries
Brian M. Foley 1 Harlan J. Brown-Shaklee 3 Patrick Hopkins 1 Jon F Ihlefeld 3 Carolina Adamo 2 Linghan Ye 4 Bryan Huey 4 Stephen Lee 3 Darrell G. Schlom 2
1University of Virginia Charlottesville USA2Cornell University Ithaca USA3Sandia National Laboratories Albuquerque USA4University of Connecticut Storrs-Mansfield USAShow Abstract
Phonon scattering has been well studied across material interfaces, grain boundaries, and other incoherent, disordered structures that create resistance to thermal transport. However, phonon interactions with strain fields have been must less rigorously studied. Due to the coherency in strain fields and their common proximity to incoherent phonon scattering sites such as mass impurities or incoherent boundaries, it is often difficult to isolate the specific effects of these coherent strain fields on phonon scattering and thermal resistance. To understand these strain field effects on phonon transport, we study the effects of ferroelectric and ferroelastic domain structures on the thermal transport in BiFeO3 and PbZrxTi1-xO3 thin films. These materials present unique systems to isolate the effects of domain walls and strain fields on phonon scattering phenomena. We measure the thermal conductivity of these films with time domain thermofelctance and characterize the domain structure with piezo force microscopy. We synthesize the BiFeO3 under various conditions to induce different domain variants and wall densities. An increase in the domain wall density leads to a systematic decrease in thermal conductivity of the films. From this data, we determined the Kapitza conductance across the domain walls, which is driven by the strain field induced by the domain variants. This domain wall Kapitza conductance is lower than the Kapitza conductance associated with grain boundaries in all previously measured materials, indicating the strong effects that coherent strain fields may have on phonon scattering and thermal conductance. To study this effect further, we synthesized various PbZrxTi1-xO3 films of different Ti concentrations to yield samples with different phases, and up to 12 different domain variants. We find a strong correlation between the thermal conductivity of single crystalline PbZrxTi1-xO3 films, yet the thermal transport in the PbZrxTi1-xO3 is unaffected by the phase in polycrystalline samples. Our results indicate that the domain structures and strain fields in PZT can scatter phonons more readily than incoherent grain boundaries. In general, this work demonstrates the strong effect of coherent strain fields on phonon scattering, and its comparable effects to typical incoherent phonon scattering mechanisms such as grain boundaries.
10:45 AM - V1.06
Crossover from Phonon to Electron Dominated Thermal Boundary Conductance of Vanadium Dioxide Thin Films across the Metal-insulator-transition
Brian M. Foley 1 Salinporn Kittiwatanakul 2 John C. Duda 1 Jiwei Lu 3 Patrick E. Hopkins 1
1University of Virginia Charlottesville USA2University of Virginia Charlottesville USA3University of Virginia Charlottesville USAShow Abstract
The effects of temperature-induced dynamic changes in phase and structure on the thermal properties of materials have been demonstrated in a wide range of materials. However, the effects of a solids' dynamic properties on the thermal transport across solid interfaces (thermal boundary conductance) have never been observed. Here, we show that the thermal boundary conductance between a metal film and vanadium dioxide (VO2) changes from a phonon mediated phenomenon to an electron dominated process as VO2 crosses over the metal-insulator-transition. VO2 thin films were deposited on Al2O3 substrates using reactive bias target ion beam deposition. We evaporate thin Au:Ti or Al films on the VO2 thin films and measure the thermal boundary conductance across the Au/Ti/VO2 and Al/VO2 interfaces as a function of temperature from 80 to 400K with time-domain thermoreflectance (TDTR). With particular focus on the behavior around the metal-insulator transition (MIT) at 340K, we show that the thermal boundary conductance has a large jump-discontinuity at the MIT, increasing suddenly by about 50% upon VO2 transitioning to its metal state. The temperature trends clearly follow those of phonon- and electron-like thermal boundary conductances in the insulator and metal regimes, respectively. We utilize these results to create thermal device structures that exhibit diode (passive) and switch (active) like behavior intended for nanoscale thermal management applications.
11:30 AM - V1.07
Microscopic Formulation of the Kapitza Transmission
Yann Chalopin 1 Sebastian Volz 1
1CNRS - Ecole Centrale Paris Chatenay Malabry FranceShow Abstract
We present a microscopic formulation of the frequency vs. wave-vector dependent phonon transmission at an interface which reveals the set of mechanisms involved in Kapitza problem. We demonstrate that the complete understanding of interfacial phonon transport relies on the knowledge of the local dynamics of the contact atoms. By illustrating our approach on semiconductors, we unveil the existence of interface modes that play an unnoticed role in transferring thermal energy.
11:45 AM - V1.08
Thermal Boundary Conductance in Metal Matrix Composites: Influence of Interface Condition on Overall Thermal Conductivity
Ludger Weber 1 Christian Monachon 1 Reza Tavangar 1
1Ecole Polytechnique Federale de Lausanne, EPFL Lausanne SwitzerlandShow Abstract
Thermal boundary conductance between the metallic matrix and the reinforcement phase is a crucial parameter for the effective thermal conductivity of MMCs and for the exploitation of the full potential of a materials combination. Its magnitude has further a direct influence on the required particle size of the reinforcement to be effective in improving (or not too strongly reducing) the thermal conductivity of the composites. Along with examples for the influence of alloying elements on composite conductivity (in situ interface modification), the question of chemical surface modification of the particles prior to composite fabrication is adressed. Several phenomena are discussed based on model composites made by gas pressure assisted liquid metal infiltration including the systems Pb/SiC, Pb/TiB2, Al/diamond, Ag-X/diamond and Cu-X/diamond. The comparison of the Pb/SiC and the Pb/TiB2 composites is particularly interesting since it highlights the importance of the electronic contribution to the interface thermal conductance.
12:00 PM - *V1.09
Electronic Thermal Transport in Nanoscale Metal Layers
David Cahill 1 Richard Wilson 1 Wei Wang 1 Joseph Feser 1
1U. Illinois Urbana USAShow Abstract
In most metals, heat transport is dominated by electronic excitations and the Wiedemann-Franz law is typically used to relate the electronic component of the thermal conductivity to the more easily measured electrical conductivity. In this talk, I will highlight three aspects of heat transport in nanoscale metal films and multilayers. i) We combined measurements of the through-thickness thermal conductivity of Pd/Ir multilayers by time-domain thermoreflectance and previously published studies of the through-thickness electrical conductivity of the same samples to validate the interfacial form of the Wiedemann-Franz law. The thermal conductance of Pd/Ir interfaces is asymp;14 GW m-2 K-1, in good agreement with the electronic diffuse mismatch model. ii) By contrast, heat transfer in a Pt/Au or Pt/Cu bilayer is strongly suppressed relative to the prediction of the Wiedemann-Franz law due to the relatively weak electron-phonon coupling in Au and Cu. By measuring the rate of heat transfer between the layers, we measured the strength of the electron-phonon coupling parameter g(T) over a wide range of temperatures, 40-300 K. The temperature dependence of g(T) is in good agreement with the predictions of the 1957 two-tempeature model of Koganov et al. iii) When the metal layer is thin and disordered, the electronic thermal conductivity is not always the dominate channel for heat conduction. We are using a new extension of TDTR measurements that enables quantitative analysis of data collected with a lateral offset between pump and probe. We are using this method with a laser spot radius as small as asymp;1 mu;m to measure the in-plane thermal conductivity of thin metal films. The sensitivity of this method is controlled by the product of the film thickness and thermal conductivity; the current sensitivity limit is on the order of 0.1 mu;W K-1.
12:30 PM - V1.10
Limited Thermal Conductance of Metal - Carbon Interfaces
Jamie Gengler 1 2 Sergei Shenogin 1 3 John Bultman 1 4 Ajit Roy 1 Andrey Voevodin 1 Chris Muratore 1
1Air Force Research Labs Wright-Patterson Air Force Base USA2Spectral Energies, LLC Dayton USA3UES, Inc. Dayton USA4University of Dayton Research Institute Dayton USAShow Abstract
The thermal conductance for a series of metal - graphite interfaces has been experimentally measured with time - domain thermoreflectance (TDTR). For metals with Debye temperatures up to ~ 400 K, a linear relationship exists with the thermal conductance values. For metals with Debye temperatures in excess of ~ 400 K, the measured metal - graphite thermal conductance values remain constant near 60 MW m-2 K-1. Titanium showed slightly higher conductance than aluminum, despite the closeness of atomic mass and Debye temperature for the two metals. Surface analysis was used to identify the presence of titanium carbide at the interface in contrast to the aluminum and gold - carbon interfaces (with no detectable carbide phases). It was also observed that air - cleaved graphite surfaces in contact with metals yielded slightly higher thermal conductance than graphite surfaces cleaved in vacuo. Examination of samples with scanning electron microscopy revealed that the lack of absorbed molecules on the graphite surface resulted in differences in transducer film morphology, thereby altering the interface conductance. Classical molecular dynamic simulations of metal - carbon nanotube thermal conductance values were calculated and compared to the TDTR results. The upper limit of metal - graphite thermal conductance is attributed to the decreased coupling at higher frequencies of the lighter metals studied, and to the decreased heat capacity for higher vibrational frequency modes.
12:45 PM - V1.11
An Evaluation of Energy Transfer Pathways in Thermal Transport across Solid/Solid Interfaces
Yan Wang 1 Xiulin Ruan 1
1Purdue University West Lafayette USAShow Abstract
Thermal transport across solid/solid interface has been extensively studied, but heat transfer pathways other than phonon transmission and electron-phonon nonequilibrium in the metal were usually neglected. In this work, we aim to build a general and unified model including both the above transport channels and others such as electron transmission and electron-interface coupling. We find that for a metallic thin film deposited on a semiconductor substrate, the electron transmission is only important when the semiconductor is heavily doped, or in the case where considerable amount of electrons are excited over the interface potential barrier to dope the substrate. In contrast, the electron-interface channel plays an important role in the intrinsic to low-doped regime, where substrate phonons can remove heat efficiently from the interface. As for transient conditions, high initial temperature and small thickness of the metallic thin film would be beneficial for electron-interface interaction. For metal/metal interface system, an analytical solution to the interfacial thermal resistance is obtained, which shows that the relative contribution from different transport channels depends on both the local condition at the interface and the bulk properties of each side of the interface.
Kevin Pipe, University of Michigan
Patrick Hopkins, University of Virginia
Yann Chalopin, Ecole Centrale Paris
Baowen Li, National University of Singapore
V5: Carbon Nanotubes
Wednesday PM, April 03, 2013
Moscone West, Level 3, Room 3002
2:30 AM - V5.01
Role of Substrate Polar Phonons in Direct Cooling of Current Carrying Multiwall Carbon Nanotube
Norvik Voskanian 1 Kamal H Baloch 1 John Cumings 1
1University of Maryland College Park USAShow Abstract
The one-dimensional nature of carbon nanotubes combined with their high thermal conductivity makes them an excellent candidate for thermal management, thermoelectric power generation and thermal logic devices. We have studied the thermal characteristics of Joule-heated multiwalled CNTs on a polar substrate, using an established high resolution in-situ thermal mapping technique in a TEM that relies on the solid to liquid phase transition of indium islands . We have experimentally observed the remote joule heating of the substrate which serves to demonstrate coupling of hot electrons with the substrate surface polar phonons , supported by theoretical models. The talk will provide a quantitative explanation of the conducted experiment as it probes the mechanism of heat dissipation and its dependence on varying biases.
 T. Brintlinger, et al., Nano Lett. 8, 582 (2008).
 K. H. Baloch, et al., Nature Nano. 7, 316 (2012)
2:45 AM - V5.02
Thermal Conductivity of Multiwalled Carbon Nanotubes for Anisotropic Heat Conduction
Yuan Li 1 2 Nitin Chopra 1 2
1The University of Alabama Tuscaloosa USA2The University of Alabama Tuscaloosa USAShow Abstract
Carbon nanotubes (CNTs) are attractive for applications ranging from thermal management to nanoelectronics due to their unique thermal, mechanical, and electrical properties. The ability of CNTs to result in functional assemblies with interesting surface chemistry makes it necessary to understand thermal conductivity of doped CNTs as well. In addition, thermal conductivity of bulk CNTs is compromised due to the presence of amorphous carbon, defects, impurities, and phonon scattering in 3-D network. Here, we study the thermal properties of undoped multiwalled CNT, purified CNTs, and N-doped CNTs using Raman microscopy method. The results showed that the thermal conductivity of pristine CNT films was around 70 W/m-K and that of N-doped CNT film is around 20 W/m-K to 30 W/m-K. This fundamental study also focuses on the thermal transport behavior of undoped and doped CNT films. Finally, we demonstrate the applications of such CNTs and their functionalized assemblies as an anisotropic heat dissipation material.
3:00 AM - V5.03
Experimental Characterization of Heat Conduction in Multi-walled Carbon Nanotubes
Hiroyuki Hayashi 1 Tatsuya Ikuta 1 Takashi Nishiyama 1 3 Koji Takahashi 1 2 3
1Kyushu University Fukuoka Japan2International Institute for Carbon-Neutral Energy Research (WPI-I2CNER) Fukuoka Japan3JST, CREST Fukuoka JapanShow Abstract
Anisotropy of heat conduction in multi-walled carbon nanotubes (MWNTs) is investigated by measuring axial heat flows in a pristine MWNT and in a MWNT with defects. Since the discovery of carbon nanotubes (CNT), much effort has been devoted for the realization of thermal management devices such as thermal interface materials (TIMs) using their excellent thermal conductivity. Especially, MWNTs can be synthesized at much lower cost than single-walled carbon nanotubes (SWNTs). However, the detailed mechanism of heat conduction in MWNTs, which is very confusing because of their dependence of diameter and length, has been scarcely investigated. The measurements are conducted by using high-sensitive nano-hot-film sensor developed by electron beam lithography. The thermal conductivities are 391 W/mK for a pristine MWNT and 103 W/mK for a MWNT with defects. The in- and out-of-shell thermal conductivities of each MWNT graphite shell are determined numerically with diffusive assumption, and differences of more than four orders of magnitude are obtained. We elucidate that the large anisotropy of thermal conductivity induces the huge reduction of nanotube thermal conductance by 74% compared with that of the pristine MWNT because of the presence of outer shell defects, which comprise only 2.8% volume ratio. Namely, phonons have to first transfer into inner layers at large defects and then transfer in the axial direction, which leads to low thermal conductivity of defected MWCNTs due to the very low out-of-plane thermal conductivity. By detailed transmission electron microscope (TEM) observation, the very low out of plane thermal conductivity is due to not only weak inter-shell interactions but also the gaps induced in the shells during the synthesis process. Furthermore, the hypothesis that the huge anisotropy induces not only diameter dependence but also length dependence of thermal conductivity is demonstrated, even though there is no ballistic phonon transport.
3:15 AM - V5.04
The Effects of Local Structure on Thermal Transport in Carbon Nanotube Materials
Richard N. Salaway 1 Alexey N. Volkov 2 Leonid V. Zhigilei 2
1University of Virginia Charlottesville USA2University of Virginia Charlottesville USAShow Abstract
The exceptionally high values for thermal conductivity reported for individual carbon nanotubes (CNTs) have generated considerable interest in utilization of CNTs in thermal management applications. Measurements on CNT based materials (buckypaper, CNT films and mats), however, have yielded conductivity values that are orders of magnitude lower than those of the constituent CNTs. Two major factors have been proposed as being responsible for the low conductivity of the CNT materials: The reduction of the intrinsic conductivity of individual CNTs due to inter-tube interactions and low thermal conductance at CNT-CNT interfaces that may be sensitive to the surrounding environment. To investigate the effects of local structure on the intrinsic conductivity of individual CNTs and the thermal conductance across CNT-CNT interfaces, we employ non-equilibrium molecular dynamics (MD) simulations performed for different junction properties (contact area, contact angle, etc.) and local structural environments characteristic of CNT network materials (CNT length, distance to nearest neighboring junction, etc.). The results of MD simulations suggest that (1) contrary to the widespread notion of strongly reduced conductivity of individual CNTs in bundles as compared to conductivity of isolated CNTs, the van der Waals interactions between defect-free well-aligned CNTs in a bundle have negligible effect on the intrinsic conductivity of the CNTs, (2) conductance at CNT-CNT junctions is weakly affected by neighboring junctions and only when the junction separation distance is within the range of direct van der Waals interactions, and (3) for sufficiently long parallel nanotubes the conductance through the overlap region between the neighboring CNTs is proportional to the length of the overlap. A general description of the conductance at CNT-CNT interfaces is formulated in a form suitable for implementation in a mesoscopic model for thermal transport in CNT networks.
KEYWORDS: carbon nanotubes, molecular dynamics, interfacial conductance, nanoscale heat transfer
3:30 AM - V5.05
Wave Driven Efficient Energy Transport in Carbon Nanotubes at High Heat Fluxes: Beyond the Heat Flux Chocking Limit
Ming Hu 1 Dimos Poulikakos 1
1ETH Zurich Zurich SwitzerlandShow Abstract
Efficiency energy use is intertwined with many aspects of the efficient performance of many technological applications. Heat dissipation limits the performance of electronics from handheld devices to massive data centers. Carbon nanotubes (CNTs) have attracted significant attention in recent years for thermal management applications utilizing their favorable properties, such as high thermal and electronic conductivities, stemming from their unique atomic structure. So far, a considerable amount of work has been dedicated to energy (electrical and thermal) transport in carbon nanotubes, either measured experimentally [Pop, et al., Phys. Rev. Lett. 95, 155505 (2005); Kim, et al., Phys. Rev. Lett. 87, 215502 (2001); Fujii, et al., Phys. Rev. Lett. 95, 065502 (2005); Choi, et al., Nano Lett. 6, 1589 (2006)] or predicted from theory [Che, et al., Nanotechnology 11, 65 (2000); Wang, et al., Appl. Phys. Lett. 88, 111909 (2006); Lukes, et al., J. Heat Transfer 129, 705 (2007); Donadio, et al, Phys. Rev. Lett. 99, 255502 (2007); Hu, et al, J. Appl. Phys. 104, 083503 (2008)], where electric fields accelerate charge carriers or a temperature gradient drives heat flow through phonon transport. However, to date almost all of the existing investigations of thermal transport of carbon nanotubes have focused on relatively low heat fluxes, i.e. the regime where the Fourier heat conduction law is valid. As transistors are continuously further miniaturized and more heat needs to be dissipated, undesired high heat flux and local hot spot regions unavoidably occur.
Here we present results [Hu, et al., Nano Lett. 12, 3410 (2012)] from non-equilibrium molecular dynamics (NEMD) simulations on a new mechanism responsible for energy transport in a carbon nanotube at high heat fluxes, beyond Fourier conduction: to this end, a wave-dominated energy transport mechanism is identified for high heat fluxes in carbon nanotubes, originated from excited low frequency transverse acoustic waves. The results show that the low frequency wave actually overtakes (more than 60% of the total for some cases) traditional Fourier heat conduction and efficiently transports the energy at high heat fluxes. This mechanism, unlike phonon gas modeling [Wang, et al., Int. J. Heat Mass Transfer 53, 1796 (2010)] which predicts that the heat flow will be choked and temperature jumps will be observed at the tube ends when the phonon gas velocity reaches the thermal speed, is responsible for a different, energy transport scenario: as the heat flux increases, the heat flow will not be chocked. Instead, low frequency waves (not accounted for in phonon gas modeling) will be excited in carbon nanotube and through them the carbon nanotube becomes an energy conduit since the mechanical wave can subsequently transfer the energy more efficiently than the traditional Fourier conduction. Our findings reveal an important new mechanism for high heat flux energy transport in low-dimensional nanostructures.
3:45 AM - V5.06
Breakdown of Fourierrsquo;s Law and Ultrahigh Thermal Conductivity in Millimeter Long Nanotubes
Victor Lee 1 Zong-Xing Lou 1 Wei-Li Lee 2 Chih-Wei Chang 1
1National Taiwan University Taipei Taiwan2Academia Sinica Taipei TaiwanShow Abstract
The law of heat transfer in a solid was discovered by Joseph Fourier in 1811. Since then, Fourier&’s law of thermal conduction has been the empirical foundation of virtually all thermal conductors. Based on Fourier&’s law, high thermal conductivity (κ) is always achieved in crystalline solids exhibiting strong chemical bonds and is independent of sample geometries. Since it is unlikely to synthesize new materials stronger than covalently-bonded carbon materials, the room temperature κ ~ 3000 W/m-K found in diamond, graphene, and single-wall carbon nanotubes (SWCNTs) is generally considered as an insurmountable limit for all materials. Recently, theoretical works have suggested that low dimensional materials may disobey Fourier&’s law and exceed the current limit of κ for samples of very high aspect ratios. However, previous measurements on κ&’s of micrometer-long SWCNTs or graphene samples have yielded no conclusive results. Here we report the observation of non-Fourier heat conduction phenomenon in ultralong SWCNTs whose κ increases with lengths without signs of saturation and reaches an unprecedented high value at room temperature for millimeter-long SWCNTs. Remarkably, the κ&’s of the investigated SWCNTs display power-law divergences even with the presence of isotopic disorders, defects, impurities, or surface absorbents. The unlimited ultrahigh κ discovered in non-Fourier thermal conductors will open an unbounded domain for novel applications for phononics.
V6: CNT Networks and Composites
Wednesday PM, April 03, 2013
Moscone West, Level 3, Room 3002
4:30 AM - V6.01
Effect of Conductivity of Individual Nanotubes and Structural Parameters of Nanotube Networks on the Heat Transfer in Carbon Nanotube Films and Vertically Aligned Arrays
Alexey N. Volkov 1 Bernard K. Wittmaack 1 Richard N. Salaway 1 Leonid V. Zhigilei 1
1University of Virginia Charlottesville USAShow Abstract
The effective thermal conductivity of materials composed of carbon nanotubes (CNTs) is investigated analytically and in mesoscopic simulations. The mesoscopic simulations of CNT materials are performed with a model that represents individual nanotubes as chains of stretchable cylindrical segments and describes van der Waals interaction between nanotubes by the mesoscopic tubular potentials. The description of the heat transfer in the model accounts for the contact heat transfer between nanotubes, the finite thermal conductivity of individual CNTs, and the thermal resistance of bending buckling kinks. The mesoscopic model of heat transfer is parameterized based on results of atomistic simulations of heat transfer performed for several representative systems composed of (10,10) CNTs. The results of mesoscopic simulations of thermal conductivity of interconnected networks of bundles in carbon nanotube (CNT) films and vertically aligned arrays (VAAs) containing tens of thousands of individual nanotubes reveal a strong effect of the finite thermal conductivity of individual nanotubes on the conductivity of the CNT materials. The physical origin of this effect is explained in a theoretical analysis of systems composed of straight randomly dispersed CNTs. An analytical equation for quantitative description of the effect of finite thermal conductivity of individual CNT on the effective conductivity of CNT materials is obtained and adopted for continuous networks of bundles characteristic of CNT films and buckypaper. Contrary to the common assumption of the dominant effect of the contact conductance, the contribution of the finite conductivity of individual nanotubes is found to control the value of conductivity of CNT films and VAAs at material densities and CNT lengths typical of real materials. The correlations between the structural characteristics of the CNT networks, such as CNT bundle size and effective pore diameter, and the conductivity of the CNT materials are discussed.
4:45 AM - V6.02
Thermal Imaging and Analysis of Carbon Nanotube Composites
Feifei Lian 1 David Estrada 1 Hongxiang Tian 2 Alicia Hoag 1 Juan Llinas 1 Marina Timmermans 3 Albert Nasibulin 3 Esko Kauppinen 3 Sanjiv Sinha 2 Eric Pop 1
1University of Illinois at Urbana-Champaign Urbana USA2University of Illinois at Urbana-Champaign Urbana USA3Aalto University School of Science Espoo FinlandShow Abstract
Manipulating the thermal conductivity of composites and nanomaterials is highly desirable not only for thermal insulation or conduction, but also for thermoelectric (TE) applications . In naturally occurring materials, the thermal conductivity is primarily determined by atomic structure and interatomic forces. By contrast, nanomaterial composites could be tuned for either high or low thermal conductivity by manipulating the orientation, density, doping or functionalization of their nanoscale components.
In this study, we develop a suspended infrared (IR) thermometry platform combined with a computational model for rapid analysis of nanomaterial thermal properties. In particular, we use aerosol synthesized single-wall carbon nanotubes (SWCNT) to fabricate networks (CNNs) by a dry deposition technique . Such CNN samples are transferred to our measurement platform, between two large Cu blocks serving both as electrical contacts and heat sinks, separated by an adjustable gap to vary the length of the suspended CNN. We also measure the thermal properties of similar samples embedded in a dielectric (AlOx) using a 3-omega method .
For the suspended CNN samples, we extract the in-plane thermal conductivity from the imaged temperature profile obtained by IR microscopy with the sample under Joule heating. Temperature profiles peak in the center of the film (ΔTpeak ~10 K) with insignificant heating at the Cu contacts, indicating good thermal contact resistance. We simultaneously obtain the electrical characteristics of the samples, including their electrical contact resistance (RC = 2 kOmega;-mm) through a transfer length method (TLM) on samples with varying lengths (0.7-2.3 mm). IR thermometry and 3-omega measurements reveal thermal conductivity of CNN composites ranging from 0.5-50 W/m/K, which is limited by the high thermal resistances of SWCNT junctions . By comparison with scanning electron microscopy (SEM) cross-sections, we uncover how the thermal conductivity of such CNNs correlates with microstructure, alignment, bundling, density of SWCNT junctions and functionalization (e.g. by incorporating in AlOx or PEDOT:PSS polymer matrix). Our comprehensive approach h