Cross-plane thermal transport in graphene-based structures

Han, Meng
Major Professor
Xinwei Wang
Committee Member
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Mechanical Engineering
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Mechanical Engineering

Thermal transport properties in graphene and graphene-based materials have been extensively studied due to the extremely high intrinsic thermal conductivity of graphene and its broad applications. However, most of the research is focused on the mass production and thermal transport improvement, while the layered structure and the corresponding large anisotropy in thermal transport are lack of attention. In this work, several graphene-based structures are investigated to uncover the energy coupling between graphene and substrate and between graphene layers. The thermal conductance induced by few-layered graphene (Gr) sandwiched between β-phase tungsten (β-W) films is first studied. Our differential method is able to distinguish the thermal conductance induced by the β-W film and the β-W/Gr interface. The β-W/Gr interface thermal conductance (GW/Gr) features strong variation from sample to sample and has a lower-limit of 84 MWm-2K-1 at room temperature (RT). This is attributed to possible graphene structure damage and variation during graphene transfer and W sputtering. Compared to up-to-date reported graphene interface thermal conductance, the β-W/Gr interface is at the high end in terms of local energy coupling. Then the cross-plane thermal conductivity (kc) of highly reduced and ordered graphene paper (GP) is characterized from 295 K down to 12.3 K. kc is 6.08 à ± 0.6 Wm-1K-1 at RT, close to the well-accepted value of graphite along the c-axis. An anisotropic specific heat model is developed to identify the specific heat that sustains heat conduction along the c-axis, based on the phonon propagation direction. This model predicts a c-axis mean free path (c-MFP) of 165 nm for graphite at RT, very close to the value of 146 nm from molecular dynamics (MD) modeling. For widely studied normal graphite materials, this model combined with the residual thermal reffusivity theory, predicts a structural domain size of 375 nm, close to the 404 nm grain size uncovered by transmission electron microscopy. The c-MFP induced by defect in the GP sample is evaluated at 234 nm based on the low-momentum phonon scattering uncovered by the 0 K limit residual thermal reffusivity. This structural domain size significantly exceeds the graphene flake thickness (1.68~2.01 nm) in our GP, uncovering excellent c-direction atomic structure order. By subtracting the residual thermal reffusivity, the defect-free kc and c-MFP of GP are obtained. At RT, the defect-free kc is 9.67 Wm-1K-1 at RT, close to 11.6 Wm-1K-1 of graphite from the recent MD simulations. The thermal transport properties of partially reduced graphene paper (PRGP) and graphene oxide paper (GOP) are then studied and compared to that of GP. For PRGP, the determined kc varies from 0.14 Wm-1K-1 at 295 K to 1.2 à  10-3 Wm-1K-1 at 12 K. For GOP, kc decreases from 0.16 Wm-1K-1 at 295 K down to 9.6 à  10-3 Wm-1K-1 at 12.5 K. We eliminate the influence of heat capacity of different structures, and further study the thermal diffusivity (αc) of these two structures. In contrary to kc, αc of PRGP increases from (1.02 à ± 0.09) à  10-7 m2/s at 295 K to (2.31 à ±0.18) à  10-7 m2/s at 12 K. Such small αc is mainly attributed to the small crystallite size (4.8 nm from XRD) in the cross-plane direction and the relatively larger interlayer spacing (3.68 à  compared with 3.35à  of GP and graphite). For GOP, αc varies from (1.52 à ± 0.05) à  10-7 m2/s at 295 K to (2.28 à ±0.08) à  10-7 m2/s at 12.5 K. The cross-plane thermal transport of GOP is attributed to the high density of functional groups between carbon layers which provide weak thermal transport tunnels across the layers. This work sheds light on the understanding and optimizing of nanostructure of graphene-based materials for desired thermal performance.