Alan McGaughey Carnegie Mellon University
Ming Su University of Central Florida
Shawn Putnam Wright-Patterson AFB
Jun Shiomi University of Tokyo
BB3: Fluid-Solid Interfaces
Wednesday AM, April 27, 2011
Room 3010 (Moscone West)
9:30 AM - **BB3.1
Spray and Jet Impingement with Nano-PCM Slurry.
Louis Chow 1 , Wei Wu 1 , Shujiang Ding 1 2 , Yan Hong 1 2 , Ming Su 1 2 , John Kizito 3 , Lois Gschwender 4 , Ed Snyder 4 Show Abstract
1 Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, Florida, United States, 2 NanoScience Technology Center, University of Central Florida, Orlando, Florida, United States, 3 Mechanical Engineering, North Carolina A&T State University, Greensboro, North Carolina, United States, 4 Materials and Manufacturing Directorate, Air Force Research Lab, WPAFB, Ohio, United States
Polymer encapsulated nano phase change materials (paraffin) in particulate form (nano PCMs) are added in water to enhance the heat transfer properties of jet impingement and spray cooling processes. The nano PCM particles absorb heat during a phase change process when the paraffin changes from solid to liquid phase. Polymer encapsulated paraffin (wax) nanoparticles, which are 100 nm in size, are prepared using colloid method by trapping paraffin wax (melting at 28oC) into the polymer shell. The shells prevent leakage and agglomeration of wax in the low temperature applications. While both jet impingement and spray show heat transfer enhancement of nano PCM slurry, sprays provide a higher average heat transfer coefficient. Compared to water, slurry with 28% particle volume fraction improved heat transfer coefficient up to 50% and 70%, for jet impingement and spray cooling cases, respectively. This significant improvement in heat transfer performance can be explained by the large increase in effective specific heat of the nano-PCM slurry over water and the very small melting time (~1.5 microseconds) of the nano-PCM. Repeated tests performed in the closed loop test section demonstrated the structural integrity of encapsulation. Nanoparticles particle volume fraction have a strong effect on the pressure drop and heat transfer performance. Nano PCM slurry provides a very effective heat transfer mechanism which enhances the thermal capacity of carrying fluid (water).
10:00 AM - BB3.2
Multiplexed Biomarker Detection Using Solid-liquid Phase Change Nanoparticles.
Ming Su 1 , Chaoming Wang 1 , Liyuan Ma 1 Show Abstract
1 NanoScience Technology Center, University of Central Florida, Orlando, Florida, United States
We will describe a new broad-range biomarker assay using solid to liquid phase change nanoparticles, where a panel of metallic nanoparticles (metals and eutectic alloys) are modified with a panel of ligands to establish a one-to-one correspondence, and attached on ligand-modified substrates by forming sandwiched complexes. The melting peak and fusion enthalpy of nanoparticles in thermal analysis reflect the type and concentration of biomarker, respectively. The thermal readout conditions have been adjusted in such a way that multiple biomarkers that have concentration difference over three-to-five orders of magnitude have been detected simultaneously under the same condition. The proposed thermal readout is immune to salt, pH, colored and electrochemically-active species, and can be used to detect cancer biomarkers in body fluids with minimal sample preparation.
10:15 AM - BB3.3
Enhanced Thermoreflectance Sensor for Studying Heat Transfer in Boiling in Microchannels.
Scott Parker 1 , Chang-Ki Min 1 , David Cahill 1 , Steve Granick 1 Show Abstract
1 Materials Science and Engineering, University of Illinois, Urbana, Illinois, United States
By creating an optical cavity of 100 nm amorphous silicon, we have designed an enhanced thermoreflectance sensor with sensitivity up to an order of magnitude higher than most thermoreflectance materials. Because of the increased sensitivity we do not need to rely on standard lock-in techniques and are therefore able to perform thermoreflectance measurements on fast stochastic events using a CCD camera capable of recording at 10,000 fps. We have applied this technique to understand thermal transport during the growth and departure of individual vapor bubbles at the TiO2 – water and the Au – water interfaces when the surface temperature is locally raised above the boiling point. Side images of the nucleating bubble have been simultaneously recorded to compare fluid behavior to interfacial temperatures. Using focused ion beam (FIB) and a custom built laser-patterning system surface patterning and roughness are controlled.
10:30 AM - BB3.4
Preparation of Nano-textured Surfaces Capable of Achieving Cooling Rates of the Order 1 KW/cm2 in Spray Cooling.
Suman Sinha Ray 1 , Yiyun Zhang 1 , Alexander Yarin 1 2 Show Abstract
1 Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, Illinois, United States, 2 Center for Smart Interfaces, Technische Universität Darmstadt, Darmstadt Germany
In the present work high-heat flux surfaces were covered with electrospun polymer nanofiber mats of thicknesses of about 20-30 μm. Then, different metals (namely, copper, silver, nickel and gold) were electroplated on these polymer nanofibers, When water drops impacted onto the the metal-plated nanofiber mats they stuck in the spread-out state to their surfaces and rapidly fully evaporated removing significant amount of heat in the form of latent heat of evaporation. The phenomena observed on nano-textured surfaces were unlike those on the bare substrate (copper) surface, where significant drop receding motion was accompanied by its bouncing from the surface, especially at high temperatures. Therefore, the nano-textured surfaces completely eliminated the detrimental Leidenfrost effect. Of the different metals it was found that copper-plated nanofiber mats proved to be the most suitable candidates for enhanced cooling applications from the viewpoint of their stability and high heat removal rates. It was observed that surfaces of individual copper-plated nanofibers resembled to some extent a small Australian thorny devil lizard, i.e. became very rough (on the nano-scale), as well as possessed high thermal diffusivity. Silver-plated nanofiber mats also became very rough due to the dendrite-like and cactus-like nano-structures on their surfaces and possessed high thermal diffusivity. On the other hand, nickel-plated nanofibers were only partially rough, while their mats incorporated large domains of smooth nickel-plated fibers. Gold-plate nanofibers were practically smooth and could not remove heat at high rate, even though they possessed high thermal diffusivity comparable to those of copper and silver. Drop impacts on the hot surfaces nano-textured with copper-plated and silver-plated nanofibers revealed tremendously high values of heat removal rates up to 0.6 kW/cm2. Such high values of heat flux are an order of magnitude higher than those available at present, and probably can be increased even more using the same technique. They open up a completely new perpective to spray cooling of high heat flux surfaces, which can facilitate further miniaturization of microelectronic devices.
10:45 AM - BB3.5
Jet Impingement Heat Transfer with Air-borne Nanoencapsulated Phase Change Materials.
Wei Wu 1 , Huseyin Bostanci 1 , Louis Chow 1 , Yan Hong 1 2 , Ming Su 1 2 , John Kizito 3 Show Abstract
1 Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, Florida, United States, 2 NanoScience Technology Center, University of Central Florida, Orlando, Florida, United States, 3 Mechanical Engineering, North Carolina A&T State University, Greensboro, North Carolina, United States
Polymer encapsulated paraffin nanoparticles (nano-PCMs) are suspended in air to enhance jet impingement heat transfer performance. Paraffin nanoparticles undergo solid to liquid phase change by absorbing heat from a heat source, while the polymer shells prevent paraffin leakage, agglomeration and attachment to the heat source. Nano-PCMs, approximately 100 nm in size, are prepared using colloid method by trapping paraffin wax (melting point ~28oC) inside polymer shells. Air with suspended nano-PCMs is used as a working fluid for jet impingement using circular nozzles. The nozzle diameter is 2 mm, the distance between the nozzle orifice to the heater surface varies between 20 to 40 mm, and the heat source is a 30-mm diameter aluminum cylinder with a cartridge heater inside. The pressure drop and convective heat transfer coefficient were measured at different flow velocities. At a velocity of 15 m/s at the orifice, air with 2.5% nano-PCMs can increase the heat transfer coefficient by about 50 times compared to air in a closed loop. At the same condition, the pressure drop across the orifice increases by approximately 15 times. Repeated tests demonstrated the structural integrity of the polymer encapsulation. Air-borne nano-PCMs provides a very effective heat transfer enhancement by increasing the effective thermal capacity of carrying fluid (air ).
11:30 AM - BB3.6
Nano to Micro Scale Water Droplet Growth Dynamics during Condensation on Surperhydrophobic Surfaces.
Konrad Rykaczewski 1 , John Henry Scott 1 , Andrei Fedorov 2 Show Abstract
1 Materials Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 G. W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Eighty years ago, Schmidt and coworkers  demonstrated that the heat transfer rate during dropwise condensation is an order of magnitude higher than during filmwise condensation.Unfortunately, degradation of coatings promoting dropwise condensation has prevented any practical use of this process .Due to their water shedding characteristics, superhydrophobic surfaces (SHS) have generated a lot of interest in their application as promoters for dropwise condensation.Most of the research effort in this area has been dedicated to the design of the SHS and characterization of their wetting behavior , and only a few studies have paid attention to the condensation dynamics [4-6].All of those studies focused on growth of drops with diameters ranging from ~10 µm to a few millimeters.However, as demonstrated by Graham and Griffith , droplets with diameters below ~10 µm account for the majority of the heat transferred during dropwise condensation.Furthermore, in our recent work  we observed a significant increase in the number of droplets with diameters below 10 µm during condensation on a nanostructured SHS.Therefore in this work we focus on visualization of the droplet growth dynamics on SHS in the nano-to-microscale regime using in-situ ESEM and wet-STEM for bulk and cross-sectional samples, respectively. We demonstrate that under high magnification electron beam heating causes rapid evaporation of the condensed water droplets and limits the viewing field on the bulk samples to ~5x5 µm areas.Next we show that when imaged in a non-invasive way, the initially condensed droplets combine filling in and wetting the spaces in between the nanostructures until forming a liquid bridge to a flat external surface with characteristic dimension of ~2-4 µm.Next, a drop begins to emerge from the liquid spot with most of the growth due to significant contact angle increase with a small base area increase.When enlarged to a diameter of ~4-6 µm, the drop reaches a near spherical shape and switches the growth mode from near constant base area mode to near constant contact angle mode.While it qualitatively corresponds to larger scale behavior of droplets with diameters above ~10 µm growing on SHS with micro-roughness, the observed droplet growth behavior on nanostructured SHS should have a significantly higher impact on the overall heat transfer rate due to the smaller range of droplet diameters.1. Schmidt, E., et al. Forschung im Ingenieurwesen, 1930. 1(2): p. 53-63.2. Carey, V.P., Liquid-Vaport Phase-Change Phenomena. 2nd ed. 2008, New York: Taylor and Francis.3. Dorrer, C. and J. Ruhe, Soft Matter, 2009. 5(1): p. 51-61.4. Narhe, R.D. and D.A. Beysens, Langmuir, 2007. 23(12): p. 6486-6489.5. Nosonovsky, M. and B. Bhushan, Langmuir, 2007. 24(4): p. 1525-1533.6. Zheng, Y.M., et al., APL, 2008. 92(8): p. 3.7. Graham, C. and P. Griffith, Inter J. Heat and Mass Transfer, 1973. 16(2): p. 337-346.8. Dietz, C., et al., APL. 97(3): p. 033104-3.
11:45 AM - BB3.7
Wettability Controls Thermal Accomodation at Gas-solid Interfaces.
Chang-Ki Min 1 , Kejia Chen 1 , David Cahill 1 , Steve Granick 1 Show Abstract
1 Departments of Materials Science and Engineering, of Chemical and Biomolecular Engineering, of Chemistry, and of Physics, University of Illinois, Urbana, Illinois, United States
A new photoacoustic method, impulsive heating by femtosecond laser pulses followed by time-resolved ellipsometry at the Brillouin frequency, has been applied to study interfacial heat transfer from solid to gas. Wettability is varied experimentally by depositing self-assembled organic thiol monolayers terminated with functional groups to render them either hydrophobic or hydrophilic. Impulsive heating creates acoustic pulses in nearby gas that propagate and are measured on nanosecond time scales. We find that the amplitude of this acoustic signal increases linearly with the gas pressure, signifying that the signal is predominantly interfacial, but that the dependence on surface wettability depends on the gas. For argon gas, thermal accommodation is independent of surface wettability. But for methanol gas (polar molecule), heat transfer is several-fold larger for the hydrophilic surface. It is remarkable that energy exchange mechanisms (thermal accommodation coefficients) at solid/gas interfaces can be so explicitly controlled. Our interpretations are supported by finite difference solutions of thermal-mechanical coupling with various boundary conditions.
Wednesday PM, April 27, 2011
Room 3010 (Moscone West)
12:00 PM - BB4.1
Contradictory Evidence for the Role of Temperature and Particle Size in Nanofluid Thermal Conductivity.
Rebecca Christianson 1 , Jessica Townsend 1 Show Abstract
1 , Franklin W. Olin College of Engineering, Needham, Massachusetts, United States
The prospects for increased cooling capacity from the use of nanofluid coolants has created a tremendous amount of interest. However, in the years since the initial thermal conductivity measurements of nanoparticle suspensions were reported, there has been a lot of inconsistency in data published in the literature. The International Nanofluids Benchmarking Exercise was a significant step towards creating a reliable set of data on the thermal conductivity enhancement of stable nanofluids, however there remain many unanswered questions. Most significant, perhaps, are the contradictory results on the effects of particle size and temperature. In the past year alone it is possible to find published reports on nominally identical samples claiming precisely opposing trends in thermal conductivity with decreasing particle size at room temperature. Some studies also claim an increasing enhancement at higher temperatures, sometimes linking this to small particle sizes. In this work we review the literature claims for particle size and temperature results, as well as presenting new data with the aim of resolving the dispute and identifying the origins of the evidence for contradictory claims.
12:15 PM - BB4.2
Robustness and Thermal Properties of Nanofluids.
Olivier Poncelet 1 , Daniel Getto 1 , Julien Jouhannaud 1 , Renaud Borlet 1 , Anton Gruss 2 , Andre Bontemps 3 , Sebastien Ferrouillat 3 Show Abstract
1 DRT/LITEN/DTNM/LCSN, CEA, Grenoble France, 2 DRT/LITEN/LTS/LETH, CEA, Grenoble France, 3 LEGI, Universite Joseph Fourier, Grenoble France
The advent of high heat flow processes has created significant demand for newtechnologies to enhance heat transfer. The best example is, microprocessors which have continually become smaller and more powerful, and wherein heat flow demands have steadilyincreased over time leading to new challenges in thermal management. Furthermore,there is also an increasing interest in improving the efficiency of conventional heat transfer processes such like in automotive systems where a better heat transfer could lead to smaller heat exchangers for cooling resulting in reduced weight and footprint. It has been demonstrated, since a few years that the heat transfer coefficient could be greatly improved far from that was expected by the theory (Maxwell) via the addition of solid particles to the liquid coolant. When nanosized particles are used, the resulting dispersions are known as nanofluids. Numerous works have been published in this area since a few years, so we decided to study the robustness (ageing) and the physical performances of metal oxide based aqueous nanofluids (Al2O3, ZnO, SiO2) in thermal labs loops. The evolution of critical parameters such like thermal conductivity, viscosity, colloidal stability depending on the size, the aspect-ratio or the nature (amorphous or crystalline) of nanosized particles across analytical data set (zetasizer measurements, TEM, pH, electrical conductivity) will be discussed.Finally we will present a novel strategy to enhance the thermal conductivity of glycols based nanofluids using soft nanosized particles.
12:30 PM - BB4.3
Performance of Silicone Oil with Carbon Nanotube Inclusions as Thermal Interface Materials.
Michael Rosshirt 1 , Christopher Cardenas 1 , Patrick Wilhite 1 , Drazen Fabris 1 , Cary Yang 1 Show Abstract
1 Center for Nanostructures, Santa Clara University, Santa Clara, California, United States
Thermal interface materials [TIM] are often inserted between contacting surfaces in electronic packages to reduce thermal contact resistance [Rtc] and improve overall heat transport. Traditional particle-laden polymeric TIM are approaching their thermal performance limits and account for a growing share of the total allowable package resistance as heat dissipation demands increase. Multi-walled carbon nanotubes [CNTs], which possess both a high aspect ratio and high thermal conductivity, hold great promise as potential high-conductivity fillers in advanced TIM. Polymeric TIM with CNT inclusions may exhibit lower thermal resistance as a result of an increase in more highly conductive percolation paths across the interface at lower filler concentrations than traditional TIM. To experimentally quantify the thermal performance of different TIM, we have developed an advanced steady-state thermal resistance measurement system based on the ASTM D5470-06 standard. Using the steady-state approach we measure the thermal resistance of TIM samples at a constant heat rate while varying the externally applied pressure. The resistance is evaluated against the optically measured TIM thickness to determine the thermal conductivity and contact resistance with the substrate. High aspect ratio multi-walled CNTs with diameter ~ 50 nm grown using chemical vapor deposition, are mechanically mixed with silicone oil (ν = 1x105 cSt) at varying percent weight loadings up to approximately 1%. The overall thermal resistance, thermal conductivity, and contact resistance of the CNT-based TIM mixtures [CNT-Oil] are measured and evaluated against the measured thermal properties of the pure oil matrix as well as traditional particle-laden TIM. Our thermal resistance measurements indicate that improved TIM performance is achieved with less viscous composites at lower CNT loadings. Overall thermal resistance is optimized near a CNT weight loading of 0.1% with an averaged measured resistance of 0.051 K-cm2/W at 50 psi and 0.031 K-cm2/W at 90 psi, which is comparable to commercially available TIM. The thermal performance diminishes at high CNT loadings near 1%, likely due to the increased viscosity and diminished surface wetting properties of the mixture. Conductivity measurements show that unlike traditional TIM the addition of CNTs at very low concentrations results in significant improvements in conductivity over the matrix material. At a CNT weight loading of less than 0.01% a 22% increase in conductivity over pure silicone oil was observed. This improvement is increased by several factors as the applied pressure is increased. The ultimate objective of this work is to demonstrate the potential of utilizing CNTs as conductive particle fillers in TIM for advanced thermal management in electronic packaging.
12:45 PM - BB4.4
Nanoparticle Heating: Basic Science and Biological Applications.
Pawel Keblinski 1 Show Abstract
1 , Resselaer Polytechnic Institute, troy, New York, United States
Recently a concept of nanoparticle heating localized to nano/micro scale regions have been promoted for cell-level selective hypothermia cancer therapy. However, an analysis of the diffusive heat flow equation shows that heating powers generated by rf magnetic fields and continuous lasers do not lead to a meaningful heating at nanoscale. Biologically meaningful temperature increase can be achieved with a large number of nanoparticle heat sources, however, over macroscopic regions. A realistic approach for producing a significant local temperature rise on nanometer length scales is heating by high-power pulsed lasers. In this context, we demonstrated, consistently with the experiment, that such heating may melt or even “evaporate” nanoparcles into the solution without formation of macroscopic vapor. This vapor formation suppression is explained by the curvature-induced Laplace pressure close to the nanoparticle, which inhibits boiling. We observe similar behavior for water and organic fluids, underscoring generality of the results.
Wednesday PM, April 27, 2011
Room 3010 (Moscone West)
4:30 PM - BB6.1
Thermal Conductivity of Metallic Nanowires near Room Temperature.
Nenad Stojanovic 1 2 , Jordan Berg 1 2 , Sanjeeva Maithripala 3 , Mark Holtz 2 4 Show Abstract
1 Mechanical Engineering, Texas Tech University, Lubbock, Texas, United States, 2 Nano Tech Center, Texas Tech University, Lubbock, Texas, United States, 3 Mechanical Engineering, University of Peradeniya, Peradeniya, Kandy, Sri Lanka, 4 Department of Physics, Texas Tech University, Lubbock, Texas, United States
In metallic structures with nanoscale dimension both electrical, σ, and thermal, κ, conductivities are known to be significantly different from their bulk counterparts. The observed reductions are generally attributed to scattering of electrons and phonons at surfaces and by grain boundaries. While proper understanding of these properties is critical for the device performance and reliability, the value of thermal conductivity is particularly important to establish in densely packed devices since dissipation of self-heating is critical in maintaining reliable performance. The total thermal conductivity κ is generally the sum of the electronic thermal conductivity, κe, and the phonon thermal conductivity, κph. In this talk we focus on electron and phonon heat transport in metals, in the so-called high temperature range near to or above the Debye temperature. In this range it is generally assumed that phonon component is negligible for metals, an assumption that has not been subjected to rigorous experimental verification, particularly at the nanoscale, due to difficulties in direct measurement of κph.There is also growing experimental evidence that the Wiedemann-Franz (W-F) law breaks down at the nanoscale. The neglected phonon component is one factor that has been cited as contributing to the apparent discrepancy in W-F. Another factor is inelastic electron-phonon scattering that influences transport due to a temperature gradient, but not due to an electric field. These hypotheses are plausible, but have not been fully investigated.We develop models based on the Boltzmann Transport Equation for size dependence of electrical and thermal conductivity in nanowires with rectangular cross section, near to and above the Debye temperature. Thermal conductivity contributions from phonons and electrons are considered. The model addresses intrinsic effects of electron-phonon, phonon-phonon, and phonon-electron scattering, and grain boundary and surface interactions. Excellent agreement is found between model results and available data reporting direct measurements of thermal conductivity of nanowires, ribbons, and thin films in Al, Pt, and Cu, respectively. We use the validated model to predict σ, κe, and κph in square nanowires with characteristic dimensions in the 25 to 500 nm range of Au, Ag, Cu, Al, Ni, Pt, and W. The Wiedemann-Franz law and Lorenz factor are examined with decreasing size; their applicability is found to degrade in nanowires due mainly to increased relative phonon contribution. The effect of differences in the electron mean free path for thermal versus electrical forcing is also examined. A modified version of W-F is presented, corrected for these two factors and valid from macro- to nanoscale provided characteristic sizes exceed the phonon mean free path.
4:45 PM - BB6.2
Thermoelectric and Structural Characterization of N-type Silicon Germanium Nanowires.
Liang Yin 1 , Choongho Yu 1 Show Abstract
1 MEEN, Texas A&M University, College station, Texas, United States
Thermoelectric transport in one-dimensional nanostructures is very attractive for thermoelectric energy conversion due to a potential improvement of efficiency of thermoelectric devices, which is typically described as thermoelectric figure of merit. Silicon-Germanium (SiGe) alloy nanowires are promising for thermoelectrics because of their low thermal conductivities reduced by phonon alloy scattering in addition to boundary scattering. In this work, SiGe alloys were heavily doped with phosphorus and made into nanowire shapes. Their thermoelectric transport properties (thermal conductivity, electrical conductivity and the Seebeck coefficient) of different diameter nanowires were characterized as a function of temperature from 60K to 450K. The thermal conductivities strongly depend on the atomic ratio of Ge and diameters of the nanowires. Thermal conductivity of nanowire with 40~60 at% Ge was observed to be ~1 W/m-K at 300K, which is much lower than that of nanowire with 10 at% Ge (~2.5 W/m-K), even lower than amorphous silicon (1.38 W/m-K). With high electrical conductivity and the Seebeck coefficient, the thermoelectric figure of merit is competitive compared to those of bulk state-of-the-art thermoelectric materials at room temperature.
5:00 PM - BB6.3
Thermal Properties of Zinc Oxide Nanowires and Nanowire Films.
Saniya LeBlanc 1 , Sujay Phadke 1 , Takashi Kodama 1 , Alberto Salleo 2 , Kenneth Goodson 1 Show Abstract
1 Mechanical Engineering, Stanford University, Stanford, California, United States, 2 Materials Science and Engineering, Stanford University, Stanford, California, United States
Nanostructured materials are excellent candidates for energy conversion applications [1,2]. Zinc oxide nanostructures are promising as transparent conductors in solar cells and for thermoelectric energy conversion [3-5]. In contrast to other candidates, ZnO is non-toxic, abundant, and compatible with large-area processing techniques. Thermal properties of ZnO nanostructures including films and nanowires need to be determined because they directly affect energy conversion performance.This work measures the effective thermal conductivity of individual ZnO nanowires and ZnO nanowire films. Colloidal synthesis in an organic solvent creates ZnO nanostructures, and 1D wires grow preferentially along the c-axis due to surface energy anisotropy. Purified nanowires are suspended in ethanol for deposition. We drop cast dilute nanowire solutions for single nanowire studies and spray coat high concentration solutions for films.We characterize single nanowires with two types of microfabricated devices. One has an insulating SiO2 substrate; the other has a patterned trench. The microdevices have four electrodes for delivering current and measuring temperature and voltage. Titanium electrodes patterned using electron beam lithography make ohmic contacts with Ga-doped ZnO nanowires. We suspend the wires over trenches to thermally isolate them. On-substrate and suspended nanowire measurements allow us to examine thermal conductance between a wire and the underlying substrate . Preliminary data suggest we can determine thermal conductance of a single wire and the influence of insulating substrates. This comparison is particularly important since most nanowire applications require nanowire placement on/between other device layers. For randomly-oriented nanowire films, the 3-ω method determines the film thermal properties on silicon and glass substrates. A parylene coating minimizes the impact of surface roughness and porosity and provides electrical isolation between the heater and film. Shadow masking is used to generate the surface heating structures because the ZnO films cannot tolerate standard photolithography solvents. We measure the effective thermal conductivity for films of ZnO nanowires and report the variation of conductivity with film thickness and the influence of multiple substrate types. A simple model accounts for coupling between nanowires . M.S. Dresselhaus, Advanced Materials 19 (2007).Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Advanced Materials 15 (2003).L. Goris, R. Noriega, M. Donovan, J. Jokisaari, G. Kusinski, A. Salleo, Journal of Electronic Materials 38 (2009) 586.C.-H. Lee, G.-C. Yi, Y. M. Zuev, P. Kim, Applied Physics Letters 94 (2009).Z. L. Wang, J. Phys.: Condens. Matter 16 (2004) R829.E. Pop, D. Mann, Q. Wang, K. Goodson, H. Dai, Nano Letters 6 (2006).M. Panzer, G. Zhang, D. Mann, X. Hu, E. Pop, H. Dai, K. E. Goodson, Journal of Heat Transfer (2007).
5:15 PM - BB6.4
Quantifying the Effect of Surface Roughness on Phonon Transport in Silicon Nanowires.
Jongwoo Lim 1 2 , Kedar Hipalgaonkar 3 , Sean Andrews 1 , Arun Majumdar 4 , Peidong Yang 1 2 Show Abstract
1 Department of Chemistry, University of California, Berkeley, Berkeley, California, United States, 2 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Department of Mechanical Engineering, University of California, Berkeley, Berkeley, California, United States, 4 Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, Washington DC, District of Columbia, United States
It has been demonstrated that electrolessly etched silicon nanowires (EE NWs) have potentially desirable thermoelectric properties due to highly suppressed thermal conductivity. This result implies that the nature of the rough surface aids in inhibiting phonon transport. However, difficulty in controlling the surface roughness of EE NWs and their various doping levels (~p-type 10-17 ~- 19 cm-3) make it hard to define and experimentally quantify its effect on phonon transport, though there have been several theoretical approaches reported. In this study, we control the surface roughness by directly etching intrinsic silicon nanowires (Etched NWs) grown by vapor-liquid-solid mechanism. We then quantify individual Etched NWs’ surface roughness by transmission electron microscopy and measure the thermal conductivity of each nanowire. A statistically significant correlation between quantified roughness and thermal conductivity is observed. In this study, we study the phonon scattering mechanism at rough surface boundaries with experimental evidence. These results can act as a guide into the nature of surface roughness, and what must be introduced in order to minimize the thermal conductivity of silicon nanowires for use in efficient thermoelectric applications.  Hochbaum, A., Chen, R. et al., Nature, 451, 10(2008) Murphy, P. et al., Phy. Rev. B, 76, 155313 (2007) Moore, A. L. et al., Appl. Phy. Let., 93, 083112 (2008) Martin. P. et al., Phy. Rev. Let., 102, 125503 (2009)
5:30 PM - BB6.5
Molecular Dynamics Simulations of Heat Conduction in Nanostructures.
Jie Chen 1 2 , Gang Zhang 3 4 , Baowen Li 5 1 2 Show Abstract
1 Physics, National University of Singapore, Singapore Singapore, 2 Centre for Computational Science and Engineering, National University of Singapore, Singapore Singapore, 3 Key Laboratory for the Physics and Chemistry of Nanodevices , Peking University, Beijing China, 4 Department of Electronics, Peking University, Beijing China, 5 , NUS Graduate School for Integrative Sciences and Engineering, Singapore Singapore
Heat conduction in nanostrcutures is of great importance from both fundamental and application point of view. Due to the small size, it is still a great challenge to investigate the thermal properties of nanostructures experimentally. Thus in order to understand the heat conduction in systems at nanoscale, people nowadays depend heavily on computer simulations. Atomic level simulations, particularly molecular dynamics (MD) simulations, play an important role in the study of heat conduction at nanoscale. There are two types of MD simulations widely used to calculate thermal conductivity: one is the non-equilibrium molecular dynamics (NEMD) simulations, and another is the equilibrium molecular dynamics (EMD) simulations. In spite of the importance of MD simulations in the study of nanoscale heat conduction, there are still some ambiguities in both NEMD and EMD simulations which may limit their applications. We systematically investigate the impacts of heat bath used in NEMD simulations on heat conduction in nanostructures exemplified by Silicon nanowires and Silicon/Germanium nanojunction1. It is found that multiple layers of Nosé-Hoover heat bath are required to reduce the temperature jump at the boundary, while only a single layer of Langevin heat bath is sufficient to generate a linear temperature profile with small boundary temperature jump. Moreover, an intermediate value of heat bath parameter is recommended for both Nosé-Hoover and Langevin heat bath in order to achieve correct temperature profile and thermal conductivity in homogeneous materials. Furthermore, the thermal rectification ratio in Si/Ge thermal diode depends on the choice of Nosé-Hoover heat bath parameter remarkably, which may lead to non-physical results. In contrast, Langevin heat bath is recommended because it can produce consistent results with experiment in large heat bath parameter range. EMD simulations through Green-Kubo formula have been widely used in the study of thermal conductivity of various materials. We demonstrate that the fluctuation in calculated thermal conductivity is due to the uncertainty in determination of the truncation time, which is related to the ensemble and size dependent phonon relaxation time. We thus propose a new scheme in the direct integration of heat current autocorrelation function (HCACF) and a nonzero correction in the double-exponential-fitting of HCACF to correctly describe the contribution to thermal conductivity from low frequency phonons. By using crystalline Si and Ge as examples, we demonstrate that our method can give rise to the values of thermal conductivity in an excellent agreement with experimental ones2. Reference1. Chen, J., Zhang, G. & Li, B. J. Phys. Soc. Jpn 79, 074604 (2010). 2. Chen, J., Zhang, G. & Li, B. Phys. Lett. A 374, 2392-2396 (2010).
5:45 PM - BB6.6
Thermal Interaction between Dielectric Nanomaterials and Planck Radiation.
Yann Chalopin 1 , Hichem Dammak 1 , Marc Hayoun 3 , Jean-Jacques Greffet 2 Show Abstract
1 , Ecole Centrale Paris, Chatenay Malabry France, 3 , Ecole Polytechnique, Palaiseau France, 2 , Institut d'Optique, Palaiseau France
The study of thermal properties and phonon relaxation processes in condensed materials has been intensively investigated by classical Molecular dynamics (MD) simulations. MD is known to be restricted to large temperature regimes where classical approximation remains valid. This talk will discuss the possibility to overcome this limitation and demonstrates that quantum statistics can be included in MD simulations if we reproduce the effect of Planck radiation on the dynamics of ions . We applied this formalism to model a dielectric nanoparticle in thermal interaction with a blackbody cavity. We observed how confinement allows the trapping of heat into selected vibrational modes, leading to the apparent violation of energy equipartition at high temperatures.
Alan McGaughey Carnegie Mellon University
Ming Su University of Central Florida
Shawn Putnam Wright-Patterson AFB
Jun Shiomi University of Tokyo
BB7: Characterizing Nanostructures Using Lasers and Light
Thursday AM, April 28, 2011
Room 3010 (Moscone West)
9:00 AM - **BB7.1
Modeling Silicon Excited by Femtosecond Laser Pulses.
Patrick Schelling 1 , Lalit Shokeen 1 Show Abstract
1 AMPAC and Department of Physics, University of Central Florida, Orlando, Florida, United States
For ultrashort laser pulses interacting with semiconductor surfaces, electron-hole excitations lead to weakening of the interatomic bonds and lattice instabilities. This mechanism can lead to melting or ablation on very short time scales. To elucidate the dynamical aspects of this problem, we have developed a empirical potential where the interactions depend on the local carrier temperature. The carriers are modeled at the continuum level, and are allowed to exchange energy with the lattice. We will present results showing how the kinetics and thermodynamics predicted by the empirical potential are linked closely to behavior predicted in far-from-equilibrium regime.
9:30 AM - BB7.2
Testing the Minimum Thermal Conductivity Model for Amorphous Polymers Using High Pressure.
Wen-Pin Hsieh 1 , David Cahill 1 , Mark Losego 1 , Paul Braun 1 , Sergei Shenogin 2 , Pawel Keblinski 2 Show Abstract
1 , University of Illinois, Urbana, Illinois, United States, 2 , Rensselaer Polytechnic Institute, Troy, New York, United States
Pressure dependence of thermal conductivity provides a critical test of the validity of the model of the minimum thermal conductivity for describing heat transport by molecular vibrations of an amorphous polymer. We measure the pressure dependence of the thermal conductivity of poly(methyl methacrylate) (PMMA) brushes grafted from SiC substrates using a combination of time-domain thermoreflectance and SiC anvil cell techniques. The maximum pressure reached is 10 GPa. We also determine pressure dependence of the thermal conductivity from a computational model of amorphous polystyrene. In both cases, the thermal conductivity is accurately predicted by the minimum thermal conductivity model via the pressure dependence of the elastic constants and density.
9:45 AM - BB7.3
Local Temperature Determination of Gold Nanostructures Using a Novel Optical Thermal Sensor.
Michael Carlson 1 , Hugh Richardson 1 Show Abstract
1 Chemistry and Biochemistry, Ohio University, Athens, Ohio, United States
Nanoscale heat generation and dissipation impacts many fields of current research, including applications in medical therapies and the semiconductor industry, where device dimensions continue to be reduced as number densities increase. As device sizes approach the nanoscale, they become close in scale to the phonon mean-free path, where certain pathways for heat dissipation are less efficient. In this regime, interfacial properties begin to dominate and limit the heat transfer away from the nanostructure. Characterizing these interfacial effects, especially the interfacial conductance, becomes increasingly important to the understanding of nanoscale heat transport. We show that a novel optical thermal sensor can be used to determine the local temperature of an optically-excited gold nanostructures. A thin film of Al0.94Ga0.06N incorporated with Er3+ is used as the thermal sensor to determine the temperature surrounding a single 40 nm gold nanoparticle (NP) or a single lithographically-prepared nanostructure during laser excitation. We determine the temperature of the thermal sensor film using the relative photoluminescence intensities of Er3+ ions.1, 2. We use a temperature transfer parameter to relate the measured temperature to the local temperature at the NP. This temperature transfer parameter allows us to predict the size of our lithographically-prepared nanodot (~ 70 nm high and 120 nm diameter) and the melting temperature. Finally, we are able to observe an enhancement in the Er3+ photoluminescence due to an interaction with the 40 nm gold NP. We use this enhancement to determine the laser intensity that melts the NP and find that there is a positive discontinuous temperature. We use this discontinuous temperature to obtain an interface conductance of ~10 MW/m2-K for a 40 nm gold NP immobilized on our thermal sensor surface. 1.Gurumurugan, K.; Chen, H.; Harp, G. R.; Jadwisienczak, W. M.; Lozykowski, H. J., Applied Physics Letters 1999, 74, (20), 3008-3010.2.Garter, M. J.; Steckl, A. J., Ieee Transactions On Electron Devices 2002, 49, (1), 48-54.
10:00 AM - BB7.4
Utilizing Micro-raman and Micro-photoluminescence to Characterize Stress and Temperature in GaN Devices.
Sukwon Choi 1 , Samuel Graham 1 Show Abstract
1 Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
The high power density in AlGaN/GaN HFETs leads intense heating which is detrimental to the operation of these wide band gap semiconductors. The heating is primarily caused by the generation of longitudinal optical phonons which build-up in the channel resulting in the hot phonon effect. Although high thermal conductivity substrates have been used to dissipate heat from these devices (e.g., SiC, diamond), the self heating in AlGaN/GaN heterostructures remains a critical issue in the success of their commercialization. The large phononic band gap provides additional resistance in dissipating the thermal energy from these devices which has received little attention from researchers. In addition, the stress in AlGaN/GaN HFETs which includes residual stresses, thermoelastic stresses, and inverse piezoelectric stresses, also significantly impacts the device reliability. Thus a reasonably accurate estimate of the channel temperature and stress are desired. To investigate the temperature rise and stresses in AlGaN/GaN HFETs, micro-Raman spectroscopy was utilized to obtain a high-spatial-resolution temperature profile within the active region by probing changes in the optical phonons in the channel (lifetime and frequency). Various measurement techniques for retrieving true temperature information utilizing the linewidth and peak position of the E2(high) and A1(LO) phonon modes are demonstrated. A method to deconvolute thermoelastic stress and temperature in the devices based on measuring multiple characteristics of phonon peak positions and linewidths will be presented. A comparative analysis of the stress measured by means of micro-Raman spectroscopy and micro-photoluminescence will also be given. The shift in the E2(high), A1(LO) phonon modes and the band edge luminescent peak are utilized to extract stress information throughout the GaN layer and near the AlGaN/GaN interface where the 2-DEG channel resides in these devices.
10:15 AM - BB7.5
Capturing the Cumulative Effect in the Time Domain Thermoreflectance Technique Using Network Identification by Deconvolution Method.
Younes Ezzahri 1 , Gilles Pernot 2 , Karl Joulain 1 , Ali Shakouri 2 Show Abstract
1 Fluides, Thermique et Combustion, Institut Pprime, CNRS-Universite de Poitiers-ENSMA, Poitiers France, 2 Electrical Engineering, University of California Santa Cruz, Santa Cruz, California, United States
Network Identification by Deconvolution (NID) method is used to study the laser pulse train cumulative effect in the homodyne configuration of the Time Domain Thermoreflectance (TDTR) experiment. We show that this intrinsic behavior can be presented as a cumulative effect weight function that affects the measured time constant spectrum. This weight function depends on the repetition rate of the pulsed laser and the modulation frequency of the pump beam. We show how the cumulative effect weight function may affect the extraction of the thermal properties of the sample under study, particularly the thermal conductivity and the interface thermal resistance. Numerical simulations of the TDTR experiment using thermal quadrupoles method are used as input signals to validate the application of the NID method. Limitation of the NID to the laser experiments will also be discussed.
10:30 AM - BB7.6
Thermal Conductivity Spectroscopy in Bulk and Nanostructured Materials.
Austin Minnich 1 , Jeremy Johnson 2 , Keith Nelson 2 , Gang Chen 1 Show Abstract
1 Department of Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 2 Department of Chemistry, MIT, Cambridge, Massachusetts, United States
Classical size effects in heat transfer, where the characteristic length scales of a system are comparable to the phonon mean free paths (MFPs), have long been of interest. In this talk, we show that quasi-ballistic heat transfer can be observed and controlled using a standard transient thermoreflectance technique. By systematically varying the diameter of the heated region, it is possible to control the fraction of phonons which are ballistic and infer the contribution of these ballistic modes to the thermal conductivity. From this, we can determine the contribution of phonons of various MFPs to the total thermal conductivity in different materials, a technique we call thermal conductivity spectroscopy. We apply this technique to several bulk and nanostructured materials to determine how different phonon modes are affected by nanostructuring.This work is supported by S3TEC, a DOE BES funded EFRC, and NSF-GRFP.
10:45 AM - BB7.7
Frequency-dependent Thermal Conductivity in Time Domain Thermoreflectance Analysis of Thin Films.
Gilles Pernot 1 , Hélène Michel 3 , Jean-Michel Rampnoux 2 , Stefan Dilhaire 2 , Younès Ezzahri 4 , Peter Burke 5 , Hong Lu 5 , Arthur Gossard 5 , Ali Shakouri 1 Show Abstract
1 Electrical Engineering, University of California Santa Cruz, Santa cruz, California, United States, 3 LITEN, CEA-Grenoble, Grenoble France, 2 CPMOH, Université de Bordeaux 1, Bordeaux France, 4 ENSMA, Université de Poitiers, Poitiers France, 5 Materials Science Department, University of California Santa Barbara, Santa Barbara, California, United States
Over the past three decades, ultrashort laser pulses have been demonstrated to be a very powerful tool to investigate materials properties at the nanoscale. A key driving force is the high-time resolution required to study heat transfer across interfaces and in thin films. The Time-Domain Thermoreflectance (TDTR) is now widely used. This optical technique offers an interesting alternative to electrical approaches such as the 3ω method. The TDTR technique measures the variation of reflectivity of a heated sample. When the surface is covered with a thin metallic layer, the incident optical pulse is converted into a heat pulse which propagates into the sample. The variations of reflectivity of the metallic surface, measured by a weaker probe pulse, are related to its temperature changes. The analysis of TDTR signals is not straightforward. Because the time between two consecutive lasers pulses is not long enough to allow the material to return to equilibrium, the measured signals differ from a single pulse response of the system. We present a complete study of the TDTR signals. Theoretical signals are calculated using a three-dimensional heat transfer model including the effects of the modulated pump pulses. We investigate the influence of the modulation frequency on the measured signals and we show how this experimental parameter could be set to enhance or reduce the sensitivity to a specific thermal parameter. The dependence of the measured “apparent” thermal conductivity of the thin film as a function of the modulation frequency is studied and the role of the ballistic phonons is discussed. Results are applied to investigate thermal properties of a series of InGaAs samples with embedded ErAs nanoparticles.
11:30 AM - BB7.8
Low Temperature Measurements of Heat Transport in Superlattices using Time-domain Thermoreflectance.
Maria Luckyanova 1 , Austin Minnich 1 , Jeremy Johnson 2 , Adam Jandl 3 , Mayank Bulsara 3 , Gene Fitzgerald 3 , Keith Nelson 2 , Gang Chen 1 Show Abstract
1 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
In order to improve the performance of nanoscale energy conversion and electrical devices, it is necessary to have a fundamental understanding of heat transport through nanostructures. Most studies show that heat transfer is dominated by incoherent phonon propagation and phase-destroying scattering processes, and the contribution of coherent phonons is unclear. Some modeling, however, suggests that at cryogenic temperatures, the contribution of coherent transport becomes more important, due partially to a lack of scattering. Coherent transport depends on both the thickness and periodicity of a superlattice (SL). We use an optical pump-and-probe technique to measure the thermal properties of SLs. In one experiment, the thermal conductivity of a periodic GaAs/AlAs SL was compared with aperiodic SLs with increasingly randomized layer thicknesses, grown by MOCVD. Randomization localizes certain phonon modes, preventing them from propagating; transfer matrix method (TMM) calculations predict that SLs with increasing randomization should have decreasing thermal conductivities. The aperiodic samples have standard deviations from the mean layer thickness of 5%, 10%, and 20% of the mean. The randomness of the SL layer thicknesses was verified both by TEM and XRD analyses. To compliment these experiments, the thermal conductivities of periodic SLs with small but increasing numbers of periods (1, 3, 5, 7, and 9) are measured. TMM calculations show that phonon stop-bands form within the first several layers of the SL. Thus, we anticipate being able to observe the formation of these stop-bands by measuring the thermal conductivities of the SLs. Both of these experiments lend insight into the role that coherent phonon propagation plays in thermal transport through nanostructures.*This work is supported by S3TEC, a DOE BES funded EFRC, and NSF-GRFP.
11:45 AM - BB7.9
Nanoscale Thermometry Using Point Contact Thermocouples.
Seid Sadat 1 , Aaron Tan 2 , Yi Jie Chua 1 , Pramod Reddy 1 Show Abstract
1 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
We present an atomic force microscope (AFM) based technique capable of mapping temperature fields in metallic films with ~10 mK temperature resolution and <100 nm spatial resolution. Point contact thermocouples are sequentially created on a periodic grid by placing a platinum coated AFM cantilever in soft mechanical contact with a metallic (gold) surface. The local temperature at each point contact is obtained by measuring the thermoelectric voltage of the platinum-gold point contact and relating it to the local temperature. These results demonstrate a direct measurement of the temperature field in a metallic surface without using specially fabricated scanning-temperature probes. Using this technique, the temperature field (created by an integrated micro heater) in the vicinity of a nanometer-sized gap between two metallic electrodes is studied. This nanometer-sized gap device is specifically designed for studying the relationship between the electronic structure and thermoelectric properties of molecular junctions and is expected to provide new insights into molecular scale thermoelectric phenomena.
12:00 PM - BB7.10
The Acoustic Phonon Dispersion of Ultra-thin Silicon Membranes Measured by Inelastic Light Scattering Spectroscopy.
John Cuffe 2 1 , Emigdio Chavez 1 3 , Pierre-Olivier Chapuis 1 , El Houssaine El Boudouti 4 5 , Francesc Alzina 1 , Damian Dudek 1 , Yan Pennec 4 , Bahram Djafari-Rouhani 4 , Andrey Shchepetov 6 , Mika Prunnila 6 , Jouni Ahopelto 6 , John McInerney 2 , Clivia Sotomayor Torres 1 7 Show Abstract
2 Dept. of Physics, University College Cork, Tyndall National Institute, Cork Ireland, 1 Phononic and Photonic Nanostructures Group, , Catalan Institute of Nanotechnology (ICN), Bellaterra, Barcelona, Spain, 3 Dept. of Physics, Universitat Autónoma de Barcelona, Bellaterra, Barcelona, Spain, 4 Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN), Université de Lille 1, Villeneuve d’Ascq, 59665, France, 5 LDOM, Faculté des Sciences,, Université Mohamed 1, Oujda Morocco, 6 , VTT Technical Research Center of Finland, Espoo Finland, 7 , Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona Spain
The acoustic phonon dispersion relation of ultra-thin Si membranes, with thicknesses of 10 and 30nm, is investigated. The goal of the work is to obtain a deeper understanding of phonon propagation in materials with dimensions comparable to thermal phonon wavelengths. Brillouin Light Scattering (BLS) spectroscopy is used, which gives direct access to the dispersion relation in a non-destructive manner. Using an angle-resolved approach out-of-plane confined phonons as well as flexural, dilatational, and shear modes propagating in the plane of the membrane are investigated. Inelastic light scattering data are compared to semi-analytical calculations and simulations using a Green’s function method based on the continuum elasticity model. The Green’s function method was also used to calculate the density of states of the phonon modes.The ultra-thin nature of our membrane results in a slow group velocity for the first order flexural modes (A0) and a correspondingly large phonon density of states, leading to enhanced acousto-optic interaction and therefore a strong scattered signal. Calculations show that mode frequencies exhibit a detectable shift of 8% with changes in the thickness of the membrane of 1nm for the 10nm membrane, which is on the order of the uncertainty in the thickness measurement. We observe for all three membranes that the experimental data agree with the continuum elasticity calculated dispersion curves within experimental error, apart from the dilatational mode of the 10nm membrane. The flexural mode of the 10nm membrane is also shown to have a narrower linewidth than the 30nm membrane, suggesting an increase of phonon lifetime. To explain this effect, the thin native oxide layer and surface roughness are considered.
12:15 PM - BB7.11
Measurement of Thermal Transport Using Time-resolved Thermal Wave Microscopy.
Marat Khafizov 1 , David Hurley 1 Show Abstract
1 Material Science and Engineering, Idaho National Laboratory, Idaho Falls, Idaho, United States
Time domain thermal reflectance (TDTR) is becoming an attractive method to measure thermal transport in nanoscale thin films due well defined thermal coupling conditions. In this technique, the sample is heated by an amplitude modulated ultrashort laser pulse train. Thermal transport in the film is measured by a time delayed probe pulse that senses small temperature induced changes in reflectivity. This method typically employs a configuration with high amplitude modulation frequency and large beam sizes, resulting in 1D thermal transport which greatly reduces the number of unknown experimental parameters. Due to 1D geometry, this technique is primarily sensitive to thermal transport perpendicular to the film surface and has nanometer scale resolution. This approach can also be applied to measure thermal transport in the lateral direction by employing configuration with small beam diameters and low modulation frequency, resulting in a 3D thermal transport. However, this approach is predicated on knowing the laser spot sizes of the pump and probe. In this presentation, we present an alternative method termed time-resolved thermal wave microscopy (TRTWM) that is well suited to measure lateral thermal transport. The utility of this approach is that the pump and probe spot sizes do not have to be known accurately. In addition, it uses the same experimental setup as TDTR. We utilize TRTWM to study thermal transport in polycrystalline thin films having nanoscale columnar grain microstructure.
12:30 PM - BB7.12
Nano-heat-sinks for Thermal Management of Heterogeneous Chemical Reactions.
Yan Hong 1 2 , Minghui Zhang 3 , Shujiang Ding 1 , Ming Su 1 2 Show Abstract
1 NanoScience Technology Center, University of Central Florida, Orlando, Florida, United States, 2 Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, Florida, United States, 3 Chemistry, Nankai University, Tianjin China
The thermal runaway of an exothermic chemical reaction refers to a situation, in which increases in temperature will change reaction condition in such a way that temperature increases further. The thermal runaway affects the yield, selectivity and safety of many reactions including heterogeneous catalytic reaction, free radical polymerization, electrochemical energy conversion, etc. We put forward a new way to control temperatures of heterogeneous exothermic reactions such as heterogeneous catalytic reaction and polymerization by using encapsulated nanoparticles of phase change materials as thermally functional additives. Silica-encapsulated indium nanoparticles and silica encapsulated paraffin nanoparticles are used to absorb heat released in catalytic reaction and to mitigate gel effect of polymerization, respectively. The local hot spots that are induced by non homogenous catalyst packing, reactant concentration fluctuation, and abrupt change of polymerization rate lead to solid to liquid phase change of nanoparticles so as to avoid thermal runaway by converting energy from exothermic reactions to latent heat of fusion. By quenching local hot spots at initial stage, reaction rates do not rise significantly because the thermal energy produced in reaction is isothermally removed. Phase change nanoparticles open a new dimension for thermal management of exothermic reactions to quench local hot spots, prevent thermal runaway of reaction, and change product distribution.
12:45 PM - BB7.13
Direct Measurements of the Mean-free-paths of Phonons in Si1-xGex by Frequency-domain Thermoreflectance (FDTR).
Yee Kan Koh 1 2 , David Cahill 2 Show Abstract
1 Mechanical Engineering, National University of Singapore, Singapore Singapore, 2 Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois, United States
Knowledge of the mean-free-paths (i.e., the distance phonons propagate without being scattered) of heat-carrying phonons is crucial for the design of new materials with desired thermal properties. For examples, to enhance the efficiency of the thermoelectric energy conversion, researchers intentionally incorporate defects into existing thermoelectric materials to scatter a wide spectrum of heat-carrying phonons and thus reduce the thermal conductivity of the thermoelectric materials. This kind of tailoring of the thermal conductivity could only be effectively achieved through detailed understanding of the mean-free-paths of phonons in crystalline materials and nanostructures. Here, we present the implementation of a new technique to directly probe the mean-free-paths of phonons in crystalline solids. The technique, called frequency-domain thermoreflectance (FDTR), is based on our previous observation that the apparent thermal conductivity measured by time-domain thermoreflectance (TDTR) depends on the frequency of the thermal excitation. To facilitate the analysis of FDTR measurements, we developed a nonlocal theory for heat conduction by phonons at high heating frequencies. Calculations of the nonlocal theory confirm our experimental findings that phonons with mean-free-paths longer than two times the penetration depth do not contribute to the apparent thermal conductivity. For an attainable frequency range of 0.1-10 MHz, the corresponding mean-free-paths are 0.1-10 μm, typical for long-wavelength phonons in typical solids. Thus, FDTR is suitable to directly measure the mean-free-paths of long-wavelength phonons in crystalline materials. We employed FDTR to study the mean-free-paths of acoustic phonons in Si1-xGex. We experimentally demonstrate that 40% of heat is carried in Si1-xGex alloys by phonons with mean-free-path 0.5<λ<5 μm, and phonons with λ>2 μm do not contribute to the thermal conductivity of Si.
Alan McGaughey Carnegie Mellon University
Ming Su University of Central Florida
Shawn Putnam Wright-Patterson AFB
Jun Shiomi University of Tokyo
BB11: Low Thermal Conductivity Materials
Friday PM, April 29, 2011
Room 3010 (Moscone West)
2:30 PM - **BB11.1
Reducing Thermal Conductivity of Binary Alloys Below the Alloy Limit via Chemical Ordering.
Pamela Norris 1 , John Duda 1 , Timothy English 1 , Donald Jordan 1 , William Soffa 2 Show Abstract
1 Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia, United States, 2 Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia, United States
Substitutional solid solutions that exist in both ordered and disordered states will exhibit markedly different physical properties depending on their exact crystallographic configuration. Many random substitutional solid solutions (alloys) will display a tendency to order given the appropriate kinetic and thermodynamic conditions. Such order-disorder transitions will result in major crystallographic reconfigurations, where the atomic basis, symmetry, and periodicity of the alloy change dramatically. Consequently, the dominant scattering mechanism in ordered alloys will be different than in disordered alloys. In this study, we present a hypothesis that ordered alloys can exhibit lower thermal conductivities than their disordered counterparts at elevated temperatures. To validate this hypothesis, we investigate the phononic transport properties of disordered and ordered AB Lennard-Jones alloys via non-equilibrium molecular dynamics. It it shown that the thermal conductivity of an ordered alloy is the same as the thermal conductivity of the disordered alloy at approximately 60% of the melting temperature and lower than that of the disordered alloy above 80% of the melting temperature.
3:00 PM - BB11.2
Thermal Conductivity in UO2: From Anharmonicity to Microstructure.
Aleksandr Chernatynskiy 1 , Chan-Woo Lee 1 , Bowen Deng 1 , Susan Sinnott 1 , Simon Phillpot 1 Show Abstract
1 MSE, University of Florida, Gainesville, Florida, United States
Uranium dioxide is widely used as the fuel for nuclear reactors. As such one of its major performance metrics is a thermal thermal conductivity. We use a variety of computational methods to examine the thermal conductivity of UO2 and how it is affected by the microstructural defects. Anharmonicity is explored with the help of the solution of the Boltzmann transport equation for phonons. It is demonstrated, that this technique provides fast and reliable prediction of thermal conductivity over a wide temperature range. Phonon lifetimes are directly compared with the measurements from neutron scattering experiments. The effects of microstructural defects, including dislocations and voids, are characterized using non-equilibrium molecular-dynamics simulations. Voids are found to have a significant effect on the thermal conductivity, while dislocations affect it only weakly. This work was supported by the Materials World Network, NSF DMR-0710523, the DOE-BES Computational Materials Science Network Project on “Multiscale simulation of thermomechanical processes in irradiated fission-reactor materials” and the DOE-EFRC “Center for Materials Science of Nuclear Fuel”.
3:15 PM - BB11.3
Atomistic Study of Heat Transport in SiGe based Materials: Comparison between Nonporous and Bulk Samples.
Yuping He 1 , Ivana Savic 1 , Davide Donadio 2 , Giulia Galli 1 3 Show Abstract
1 Department of Chemistry, University of California at Davis, Davis, California, United States, 2 , Max Planck Institute for Polymer Research, Mainz Germany, 3 Department of Physics, University of California at Davis, Davis, California, United States
Reducing the thermal conductivity of crystalline materials by nanostructuring is a promising strategy for the development of efficient thermoelectric materials . Building on our previous work on nonporous  and amorphous  silicon, here we explore the changes in thermal conductivity between bulk and nonporous SiGe. In particular, we present an investigation of different effects (e.g. film thickness, pore surface structure, anharmonicity) using molecular dynamics , Boltzmann transport  and Green's function approaches . Work supported by DOE-SciDAC, DE-FC02-06ER25794.  M. S. Dresselhaus, G. Chen, M. Tang, R. Yang, H. Lee, D. Wang, Z. Ren, J.-P. Fleurial, and P. Gogna, Adv. Mater. 19, 1043 (2007).  Y. He, D. Donadio, J.-H. Lee, J. C. Grossman, and G. Galli (submitted for publication).  Y. He, D. Donadio, and G. Galli (submitted for publication).  D. Donadio and G. Galli, Phys. Rev. Lett. 102, 195901 (2009); Nano Lett. 10, 847 (2010).  J. E. Turney, E. S. Landry, A. J. J. McGaughey, and C. H. Amon, Phys. Rev. B, 79, 064301 (2009).  I. Savic, N. Mingo, and D. A. Stewart, Phys. Rev. Lett. 101, 165502 (2008).
3:30 PM - BB11.4
Atomistic Analyses on Thermal Conduction in Layered Oxide Thermoelectric Materials by Perturbed Molecular Dynamics.
Masato Yoshiya 1 2 , Masahiro Tada 1 Show Abstract
1 Department of Adaptive Machine Systems, Osaka University, Suita, Osaka, Japan, 2 Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya, Aichi, Japan
Conventional theories of thermal conduction based on metals and semiconductors often fails to predict or interpret thermal conduction in oxides due to different nature of interatomic bonding, mixture of covalent and ionic nature, and wide spectrum of crystal structures. Nevertheless, its control is critically important for variety of practical applications including thermal barrier coating for gas turbine blades to increase operating temperature thereby reducing CO2 emission and thermoelectric conversion for reuse of waste heat as an alternative energy source. Since no analytical theory of thermal conduction is prerequisite, atomistic simulation including classical and quantum molecular dynamics and lattice dynamics play significant roles to predict thermal conductivity of materials with any nanostructures and to reveal underlying physics governing thermal conduction in oxides toward further materials designing.In this study, we focus on layered oxide thermoelectric materials including NaxCoO2 [1-2] and CaxCoO2 of CdI2 structure, of which thermal conductivity is one of three key materials properties that determine heat-to-electricity energy conversion efficiency. In spite of extensive discussion of electronic conterpart which are the other two key properties, little had been understood for phonon thermal conductivity especially of these layered thermoelectric oxides. Through numerical prediction as well as computational experiment of these oxides by perturbed molecular dynamics and subsequent analyses , it is demonstrated that atomic-level stacking of Na and CoO2 layers or, in other words, internal interfaces between these stacking layers significantly provoke phonon scattering even parallel to the interface. Against the expectation based on equipartition theorem, the lightest O and the heaviest Co ions govern overall in-plane thermal conduction to the almost same extent, suggesting highly-coupled vibration. However, although its negligible contribution of Na ions at x < 1, their vacancies which is inevitably introduced at x < 1 significantly modify phonons dominated by Co and O at neighboring layers across internal interfaces, thereby lowering overall thermal conductivity . Computational experiments in which Na is substituted by smaller or larger alkali metal cations  or cations having similar ionic radius but different valence enlighten about underlying physics that governs thermal conduction in this series of oxides with internal interfaces, thereby allowing further materials designing.References: M. Tada, M. Yoshiya, et al., Trans. Mater. Res. Soc. Jpn., 35 (2010) 205. M. Tada, M. Yoshiya, in preparation. M. Yoshiya, et al., Mol. Simulat., 30 (2004) 159. M. Tada, M. Yoshiya, et al., J. Electron. Mater., 39 (2010) 1439.
3:45 PM - BB11.5
Thermal Conductivity of Half-Heusler Compounds from First Principle Calculations.
Junichiro Shiomi 1 2 , Keivan Esfarjani 1 , Gang Chen 1 Show Abstract
1 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Mechanical Engineering, The University of Tokyo, Tokyo Japan
Detailed and accurate understanding of phonon transport in semiconductors and dielectrics gives us access to better device designing. Recently, thermal conductivity calculations from first principle have been performed by extracting anharmonic force constants directly from the density function theory (DFT) calculations, which have successfully reproduced the experimentally measured thermal conductivity of bulk monoatomic crystals . The next non-trivial challenge now is to apply the first-principle-based approach to more complicated structures, such as multi-atomic crystal and alloys. In this study, we demonstrate its successful application to half-Heusler compounds. Taking the case of a p-type HH structure, ZrCoSb, the harmonic and anharmonic force constants were obtained from a set of force-displacement data calculated by DFT calculations. Here, we have expressed the force field based on Taylor expansion of the total potential energy about the equilibrium configuration . The DFT calculations were also used to estimate the power factor through the band structures with constant relaxation time approximation of charge carriers. Thermal conductivity was calculated by two different methods; (1) Boltzmann Peierls formula with phonon relaxation time calculated by Fermi’s golden rule of three-phonon (normal and Umklapp) scattering processes, and (2) Green-Kubo formula for heat current obtained by molecular dynamics simulations. The results of the two methods were compared at the classical limit, in terms of thermal conductivity and phonon relaxation time. Molecular dynamics simulations further allow us to investigate the mass-difference scattering effects in alloyed HH, for instance, where zirconium is substituted by hafnium or titanium. The overall result reveals phonon dependent contribution on thermal conductivity, which is useful for designing thermoelectric materials in forms of alloys and nanocomposites.This work is supported by S3TEC, a DOE BES funded EFRC, and JSPS Excellent Young Researchers Overseas Visit Program.  D. A. Broido, M. Malorny, G. Birner, N. Mingo, and D. A. Stewart, Appl. Phys. Lett. 91, 231922 (2007). K. Esfarjani and H. T. Stokes, Phy. Rev. B 77, 144112 (2008).
Friday PM, April 29, 2011
Room 3010 (Moscone West)
4:30 PM - **BB12.1
Dyadic Green’s Functions, Thermal Radiation, and Van Der Waals Forces.
Yi Zheng 1 , Karthik Sasihithlu 1 , Ning Gu 1 , Arvind Narayanaswamy 1 Show Abstract
1 Mechanical Engineering, Columbia University, New York, New York, United States
Near–field force and energy exchange between two objects due to electrodynamic fluctuations give rise to dispersion forces such as Casimir and van der Waals forces, and thermal radiative transfer exceeding Planck’s theory of blackbody radiation. The two phenomena – dispersion forces and near—field enhancement of thermal radiation – have common origins in the electromagnetic fluctuations. However, dispersion forces have contributions from quantum (zero—point) as well as thermal fluctuations whereas near—field radiative transfer has contributions from thermal fluctuations alone. The forces are manifested through the Maxwell stress tensor of the electromagnetic field and radiative transfer through the Poynting vector. Both phenomena are elegantly described in terms of the Dyadic Green’s function of the vector Helmholtz equation that governs the electromagnetic fields.In this talk, I will focus on the relation between heat transfer at nanoscale and dispersion forces at nanoscale, and the asymptotic scaling laws for each of them. We will show that experimental measurements of nanoscale radiation can shed light on the temperature dependence of nanoscale dispersion forces. Finally, we will talk about how we can use the Green’s function method to obtain dispersion forces between objects separate by dissipative and dispersive media.
5:00 PM - BB12.2
Nanoscale Thermal Transport during Radiation–materials Interaction.
Xiaojun Mei 1 , Sabry Moustafa 1 , Jacob Eapen 1 Show Abstract
1 Nuclear Engineering, North Carolina State University, Raleigh, North Carolina, United States
Radiation includes electromagnetic waves or photons, charged particles such as electrons, protons and ions, and neutral particles such as neutrons. There are several energy sources and materials processing applications where interaction of radiation with materials plays a critical role. For example, neutrons split (fissionable) heavy fuel atoms into lighter atoms that carry a total of 190 MeV per fission in a nuclear reactor. This energy quickly ionizes the host fuel atoms and causes severe local damage that could evolve into larger defects at longer times.In this paper, we analyze the thermal conduction processes in a radiation environment using atomistic simulations. Our simulations on silicon show that energy deposition – of the order of keV – introduces melting and disordering at nanoscales followed by rapid energy dissipation and cooling. Three dimensional heat flux maps show the asymmetric nature of thermal conduction. We further discuss the appropriate thermal boundary conditions that are pertinent for atomistic simulations.
5:15 PM - BB12.3
Nano-scale Thermal Radiation - Opportunities and Challenges.
Shiu-Wing Tam 1 Show Abstract
1 , Argonne National Laboratory, Argonne, Illinois, United States
Thermal radiation behavior in the near-field was first addressed from a theoretical perspective using fluctuation electrodynamics nearly 60 years ago. This process refers to thermal radiation transport over distances between source and sink ranging from ~ nanometers to ~ sub-micrometers (i.e., ~ nanoscale distances). In contrast far-field thermal radiation which is governed by the Stefan-Boltzmann law refers to distances that are greater than many micrometers. Near-field radiation may differ substantially from what the conventional Stefan-Boltzmann law would suggest. But until recently further advances have been relatively slow primarily due to lack of experimental capability. In fact as late as the mid-nineties attempts to measure thermal radiation over sub-micrometer range was inconclusive. However, recent advances in nanoscale instrumentation have enabled near-field thermal radiation to be characterized in more detail experimentally. This has fueled accelerating interest in this phenomenon driven by the scientific and engineering challenges as well as by the potential technological opportunities ranging from next generation thermal management to energy storage and conversion. In this presentation a dipole model of nano-scale radiation would be presented. This model would be utilized to highlight the differences between the "conventional" descriptions of thermal radiation in the far field from that of the near-field. Within this model the far field contribution would be correlated to the Stefan-Boltzmann description. Recent experimental findings on near-field thermal radiation would be discussed in terms of this model. The geometrical configurations adopted in many recent experiments may give rise to a mix of near, intermediate and far-field contributions to the final measurements. A parameter akin to an effective thermal conductivity for near-field process would be extracted. This facilitates comparison to the bulk scale thermal conduction process and enables one to assess quantitatively the relative significance of the near-field heat transfer process. The applicability of the Derjaguin Approximation would be addressed within this model. Near-field thermal radiation would be discussed in the context of complex impedance. For some categories of materials that is capable of supporting surface polariton resonance in the thermal regime near-field thermal radiation effect maybe significantly affected. A quantitative assessment of this impact would be discussed. Technological applications may require judicious choices of materials selection. Impact on thermal contact resistance would be assessed. Potential opportunities and challenges in the application of nanoscale thermal radiation to next generation thermal management, energy storage and conversion would be addressed.* Partial support for this work has been provided by the U. S. Department of Energy,Office of EERE under contract DE-AC02-06CH11357.
5:30 PM - BB12.4
Increasing Heat Transfer and Tailoring Heat Emission with Polaritons.
Karl Joulain 1 , Jeremie Drevillon 1 , Philippe Ben-Abdallah 2 Show Abstract
1 Laboratoire d'etudes thermiques, Universite de Poitiers, Poitiers France, 2 Laboratoire Charles Fabry de l'institut d'optique, CNRS/Insitut d'optique, Palaiseau France
In the last decade, numerous works have shown how the presence of surface waves such as phonon polariton propagating along the surface of a material could drastically change its radiative properties . Tunneling of surface wave between polar materials such as SiC or SiO2 can enhance heat transfer by several orders of magnitude. We present a general formulation where radiative heat transfer at nanoscale is described through a Landauer type modeling. Heat transferred depends on the number of modes per surface unit in the near-field and on a transmission coefficient for the modes involved .Different types of strategy can be used to increase heat transfer : increasing the number of guided waves near a material and the transmission coefficient for the coupled modes. We first consider the so-called metamaterials and show that the presence of a magnetic resonance (a magnetic polariton) opens new channels for near-field heat transfer . One channel is the direct analog of the dielectric polariton whereas another one is a more complex mode. We also show how it is possible to increase heat transfer using structures such a photonic crystals . These structures may exhibit several confined modes propagating along their interface : heat transfer intensity is then directly related to the number of confined modes. Finally, we show that the best solution to increase heat transfer is probably to use polaritons with losses comparable to the Planck function width. K. Joulain, J.-P. Mulet, F. Marquier, R. Carminati and J.-J. Greffet, “Surface electromagnetic waves thermally excited :Radiative heat transfer, coherence properties and Casimir Forces revisited in the near field” Surf. Sci. Rep., 57, 59-112 (2005) P. Ben-Abdallah and K. Joulain, “Fundamental limits for non-contact transfers between two bodies”, Phys. Rev. B, 82, 121419(R), (2010) K. Joulain, J. Drevillon and P. Ben-Abdallah, “Noncontact heat transfer between two metamaterials”, Phys. Rev. B, 81, 165119 (2010) P. Ben-Abdallah, K.Joulain and A. Pryamikov, “Surface Bloch waves mediated heat transfer between two photonic crystals”, Appl. Phys. Lett., 96, 143117 (2010)