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**Is there a ‘Nano’ Revolution **

in Thermal Management and Energy Conversion? Advances in Nanostructure based Thermal Interface and Thermoelectric Materials Sebastian Volz Laboratoire EM2C UPR CNRS 288, Ecole Centrale Paris Thermal Nanosciences Group - EUROTHERM 2012 – Poitiers, France – September 5th 2012

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**European CNRS Network on Thermal Nanosciences and NanoEngineering**

Heat Transfer Laws at Small Scales Deviate from Classical Ones Ballistic conduction in air Kn>>1 L NEAR FIELD Radiation When L < the Predominant Photon Wavelength, coupling of Evanescent Surface Waves increases heat flux. Transition from a regime of propagative waves emitted by charges motions to a direct electrostatic interaction. STEFAN-BOLTZMANN « Nano » wire d Conduction Convection If Nu<1, heat conduction in air predominates. If Kn>>1 heat conduction becomes ballistic. Diffusive to Ballistic transition is well-known in gases and radiation. European CNRS Network on Thermal Nanosciences and NanoEngineering

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**Heat carries in solids are SOUND PARTICLES or PHONONs, **

the quanta of lattice vibrational energy i-1 i i+1 a Fij = K.(uj- ui) un=u.expi(kna-wt) k w Periodic Boundary Conditions: k = n . 2/L Density of states

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**Phonons form a GAS of particles to propagate heat !**

Acoustics: Coherent Phonons Continuous limit k=>0 k w Heat Flux: the Phonon Gas Knudsen Transport Applies Phonon Wien’s Wavelength: 3nm (300K) Mean free path: nm

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**Kn>1: Boundary scattering predominates over diffusive scattering**

L p(, , pol, ) =1/3 C v

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**Confinement: Cavity modes appear if L< Wavelength**

Periodicity: e ik(L+x)=e ikx e ikL=e ika=0 un ~ expi(kna-wt)+ expi(-kna-wt) ~ cos(kna)e-iwt a STEADY WAVE has ZERO group velocity =1/3 C v

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**The number of phonon modes depends on Dimensionnality**

3D (bulk) D(k) ~ k2 k-space 1D (wire) D(k) ~ 1 2D (film/SR) D(k ) ~ k Première zone de Brillouin de la maille cfc avec les points de haute symétrie Dimension: Number of States /dk: =1/3 C v

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**At nanoscales, thermal resistance arises from boundaries**

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**At nanoscales, thermal resistance arises from boundaries**

The KEY is to understand phonon transfer at the surface and between two systems

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**Thermal Resistance is an Ambiguous Concept Relating Equilibrium and Non-Equilibrium Quantities**

R= (T1-T2)/Q N1 N2 Q ‘Cheating’ Seems Unavoidable Heat Bath T1 Heat Bath T2

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**Atomic Simulations involve dubious Non-Equilibrium Conditions**

NEMD - STEADY -Thermostats Parameters -Equilibrium Temperatures: coupling with heat bath? NEMD - TRANSIENT -Thermostat Parameters (weaker) -’Short time’ non-equilibrium -Equilibrium Temperatures at each time step? MACRO APPLIED TO MICRO. BUT CAN YOU MEASURE THE TEMPERATURE?

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**Equilibrium Temperature Correlation defines Thermal Resistance**

FLUCTUATIONAL THERMODYNAMICS INTERNAL SCATTERING ACF NEGLECTED (?) DT as small as temperature fluctuations τau JAP, 108, , 2010

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**A Flux based Thermal Conductance can be equivalently derived**

SIMPLE QUESTION TRANSITION = 1 BILLION TIME - 9 ORDERS OF MAGNITUDES, INCREASE -Interfacial Thermal Resistance only depends on Interactions between Atoms of both Sub-Systems -Temperatures involved in the definition of resistance are the Temperatures of the Interacting Atoms

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**Nanostructures have exceptional thermal conductivities**

Carbon Nanotubes Silicon Nanowires 1-3

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**Nanostructures can be used to taylor thermal conductivity**

Thermal Interface Materials: Increase Thermoelectricity: Decrease

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**Use Carbon Nanotube Pellets**

Can Carbon Nanotubes be used as Thermal Interface Materials? Use Carbon Nanotube Pellets J D L Isotropy

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**Isotropic Pellet Thermal Conductivity is promising but….**

Nombre de CNTs est fixé Chalopin, Volz, Mingo, Journal of Applied Physics, 105, , (2009)

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**…Measured Thermal Conductivity is more than disappointing**

Nombre de CNTs est fixé Prasher, Hu, Chalopin, Mingo, Lofgreen, Volz, Cleri, Keblinski, Phys. Rev. Lett.,102, , 2009

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**CNT Orientation is drastically affecting thermal conductivity**

Volkov and Zhigilei Phys. Rev. Lett. 104, (2010)

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**Use of Hybrid Charges Imposes Isotropy**

Bozlar, He, Bai, Chalopin, Mingo and Volz, Advanced Materials, 21, 1, (2009)

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**Vertically aligned CNTs appears as the optimized option**

Applying pressure? CNT-Superstrate contact resistance cancels performances

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**Thermal Conductance is increased when applying Pressure**

Chalopin, Srivastava, Mingo, Volz, submitted to APL

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**Transmission shows the opening of inelastic channels **

when increasing pressure Harmonic Green Functions Fluctuations Anharmonic Green Functions

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**Introducing a polymer layer at contact reduces thermal resistance**

2.5mm2K/W Introducing Covalent Bonds Should Increase Conductance HLK5

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**CNT-HLK5 resistance is three times lower than CNT-PEMA one**

Ni, LeKhahn, Bai, Divay, Chalopin, Lebarny,, Volz Appl. Phys. Lett. 100, (2012)

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**CONCLUSION on Thermal Interface Materials**

Ni, LeKhahn, Bai, Divay, Chalopin, Lebarny,, Volz Appl. Phys. Lett. 100, (2012)

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**Collaborators: THANK YOU FOR YOUR ATTENTION 2007 2010 France: Team:**

Y. Chalopin (CNRS) T. Antoni (Ass. Prof.) T. Dumitrica (Inv. Prof.) Pdocs: J. Ordonez O. Pokropivny PhDs: Y. Ni, S. Xiong, L. Tranchant W. Kassem, J. Jaramillo Ramière, H. Han B. Latour, J. Soussi Abroad G. Chen (MIT) H. Ban (Utah U.) C.W. Chang (National Taiwan Uniiversity) B. Kim (U Tokyo) H. Fujita (U Tokyo) H. Kawakatsu (U. Tokyo) Y. Kosevich (Semenov Inst. Moscow) M. Kazan (U Américaine de Beyrouth) Rajabpour (U Teheran) Y. Ciumakov (Moldova) France: N. Mingo (CEA-LITEN) E. Ollier (CEA-LITEN) A. Ziaei (Thales R&T) L. Divay (Thales R&T) P. Cortona (SPMS, Ecole Centrale Paris) H. Dammak (SPMS, Ecole Centrale Paris) J. Bai (SPMS, Ecole Centrale Paris) L. Aigouy (LPM, ESPCI) B. Palpant (LPQM, ENS Cachan) S. Merabia (LPMNC, U Lyon) P. Chantrenne (MATTEIS, U Lyon) D. Lacroix (LEMTA, U Nancy) J. Amrit (LIMSI, U Orsay) B. LePioufle (SATIE, ENS Cachan) D. Fourmy (Centre de Génétique Mol., Gif) K. Termentzidis (LEMTA, Nancy France) European CNRS Network Thermal Nanosciences and NanoEngineering 2010 THANK YOU FOR YOUR ATTENTION

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**QMNTIA 2013 Quantitative Micro and Nano Thermal Imaging and Analysis**

10-12 July 2013 Reims, France GRESPI Université de Reims-Champagne-Ardenne

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