Graphene-based Thermal Interface Materials (TIM)

1. Graphene-based Thermal Interface Materials (TIM) A proposal submitted to CTRC (Cooling Technologies Research Center) Principle investigators: Yong P. Chen (Physics, ECE and Birck Nanotechnology Center) Xiulin Ruan (ME) Tim S Fisher (ME and Birck Nanotechnology Center) Purdue University

2. Why Graphene • Graphene: building block • for most carbon materials • ---incl. graphite and • carbon nanotubes(CNT) • Recently, carbon materials • (incl. both graphite and CNT) • investigated as attractive • thermal interface material • (TIM) motivated by their • high thermal conductivity c b a (discovered 2004) • Other advantages of graphene: • High packing density [due to 2D] • rich shapes/geometry • Easily functionalized • Possibilities to bond to surface Graphene: extraordinary thermal conductivity ~ 3000-5000 W/mK [Nano Lett. 8, 902–907, 2008] (highest among materials – responsible for the high thermal conductivity of graphite (ab-plane) and CNT!

3. Research Objectives • Develop high performance TIM based on graphene Approach 1 (focus): Vertically grown (CVD) graphene sheets between (and bonded to) substrates Approach 2 (reference): Graphite micro platelets/powder between substrates • Components: • material (TIM) design • synthesis/fabrication • thermal measurements • modeling Some key issues: -Bonding of filler material (graphene) to surface -Adhesion between filler materials • Metrics to Achieve: • Material thermal conductivity > 1000 W/mK • Interface thermal resistance <1 mm2K/W

4. substrate Gas precursor (eg. CH4) Chemical Reaction on surface Carbon deposition substrate Approach 1: Vertically Grown Graphene Sheets by CVD • Microwave plasma enhanced (PE) • chemical vapor deposition (CVD) grows • vertically aligned graphene sheets • No catalyst needed • Works on almost any substrate • graphene bonded to substrate surface • can have very high filling/packing density Malesevic et al., Nanotechnology’2008 Key idea: CVD grow vertical graphene between two substrates as TIM substrate graphene interface substrate Carbon PECVD apparatus available in Purdue/Birck (Fisher)

5. c b a a c cut b Approach 2: Graphite Platelets, Powders and Graphene Composites • Graphite ab-plane has extraordinary thermal conductivity (due to graphene thermal conductivity) • Make graphite (highly ordered pyrolytic graphite) platelets with thin (vertical) dimension along ab • Fill such graphite platelets as filler material between two substrates as TIM • Alternative: graphite powders (a fraction with vertical along ab) • low cost, low tech, field-applicable • Will investigate geometric factors (size, aspect ratio etc) of filler blocks • Will investigate various bonding glues/epoxy to promote adhesion between fillers and to the • substrate surface • This is a reference approach that will be compared with the CVD grown graphene based TIM • to investigate roles of filler materials and interface bonding • Will also investigate graphene composites (graphene-polysterene composite, courtesy D.Dikin, NWU)

6. Measurement Methods Well established methods developed at Purdue for thermal conductivity/interface thermal resistance measurements, for example: • Electrical • eg., 3-omega: Hu et al., J. Heat Transfer 128, 1109 (2006) Z. Huang et al., presentation at CTRC 10/28/08 • Optical • eg., photoacoustic: Cola et al., J. Appl. Phys. 101, 054303 (2006) • Photoreflectance …

7. Hot Substrates q TIM Cold A General Molecular Dynamics Tool for Thermal Conductance Prediction • The tool is based on LAMMPS to perform non-equilibrium molecular dynamics simulations • Parallel simulation • Various types of interatomic potentials incorporated • 1D, 2D, or 3D arbitrary simulation geometry • Easy to extend with new features and functionality

8. Thermal Conductivity Prediction of Graphene • Non-equilibrium molecular dynamics simulations • Impose a heat flux and calculate the temperature gradient, so the thermal conductivity is derived from Fourier law. Fourier Law • T. Chonan and S. Katayama, J. Phys. Soc. Japn. • graphene nanoribbon: calculated k~1500W/mK • (take thickness =0.35nm)

9. Thermal Conductance Prediction of the Graphene Based TIM • Development of the interatomic potentials between the carbon atoms and the substrate atoms • Non-equilibrium molecular dynamics to calculate the thermal conductance of the TIM. • Atomistic Green’s function will also be used to calculate the phonon transmission, and the results will be compared to the MD simulations.

10. An interesting MD example: thermal rectification in asymmetric graphene nanoribbon: Rectification factor ~3! (largest reported so far) Jiuning Hu et al., in preparation (2008)

11. Deliverables/Benefits • Optimized recipes and procedures to fabricate graphene based thermal interface materials. • Experimentally validated software simulation tool to predict the performance of thermal interface materials.

12. Budget and Program Plan • 2 years: 01/2009-12/2010 • Each year $45K include: • 1.5 student support, leveraged by fellowship and TA to support 2 students on this project • $5000 materials and supplies • Student 1 will work on material fabrication and thermal measurements • Student 2 will develop simulation tool and data analysis • 1 ECE and 1 ME grad students have been identified and ready to perform this research • Start TRL=3, aim TRL=5 at end of program