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 (discovered 2004) c a b 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! 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 Other advantages of graphene: High packing density [due to 2D] rich shapes/geometry Easily functionalized Possibilities to bond to surface

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 mm 2 K/W

4. Approach 1: Vertically Grown Graphene Sheets by CVD Malesevic et al., Nanotechnology’2008 substrate Gas precursor (eg. CH 4 ) Carbon deposition Chemical Reaction on surface Carbon PECVD apparatus available in Purdue/Birck (Fisher) 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 Key idea: CVD grow vertical graphene between two substrates as TIM substrate interface graphene

5. Approach 2: Graphite Platelets, Powders and Graphene Composites cut a b c b c a 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. 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 q Substrates TIM Hot Cold

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

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