Electron and phonon transport in nanostructured exfoliated graphene nanoplatelets and its potential in thermal energy conversion

ELECTRON AND PHONON TRANSPORT IN NANOSTRUCTURED EXFOLIATED GRAPHENE NANOPLATELETS AND ITS POTENTIAL IN THERMAL ENERGY CONVERSION By Jinglei Xiang Graphene nanoplatelets (GNP), produced from microwave assisted thermal exfoliation of graphite intercalated compounds, possess exceptional electrical and thermal properties thanks to the well preserved conjugation in the graphene basal plane as a result of this production method. The potential of these intriguing physical properties can only be realized if these nanoparticles are assembled on a macroscopic scale. However, numerous interfaces thus created between nanoparticles could have a large impact on the physical properties of GNP based nanocomposites. This research is dedicated to the investigation of the difference of electron and phonon transport at the interfaces and how to make use of this unique distinction to nanostructure and assemble GNP for thermoelectric application which requires a high electrical to thermal conductivity ratio σ/κ as well as high Seebeck coefficient S of the material. In the first part of the discussion, the effect of incorporating GNP into a polymer to improve its electrical and thermal conductivity is discussed. It is found that there are distinct property enhancements around percolation threshold in the nanocomposites originating from the different mechanisms of electron and phonon transport across the GNP/polymer interface. In order to take full advantage of the excellent in-plane physical properties of GNP, a highly ordered, highly flexible binder free GNP paper was prepared by a simple filtration technology and annealing that shows much enhanced electrical and thermal conductivity. The second part of this research is dedicated to separation of electron and phonon transport in GNP paper by various nanostructuring techniques in order to enhance σ/κ ratio. In the first approach, monodispersed metal nanoparticle spacers were synthesized on the surface of GNP particles. The Au/GNP hybrid film prepared by filtration shows decoupled electron and phonon transport. As a result, the in-plane electrical conductivity increased by 70% while a 7% reduction in thermal conductivity was observed in comparison to a neat GNP paper. In the second approach, polyaniline (PANi) was synthesized by in-situ chemical oxidative polymerization in the presence of GNP where GNP served as a template for nucleation and growth of PANi. Depending on the composition and protonation ratio of PANi, σ/κ increased from 2.5 for neat GNP paper to 4.7 for the as-made PANi/GNP and further to 11 for the reprontonated PANi/GNP film. The optimal thermoelectric properties was achieved at a protonation of 0.2 with approximately 40wt% of PANi in the nanocomposite, reaching an electrical conductivity of 59 S/cm, a thermal conductivity of 12W/mK and a Seebeck coefficient of 33 μV/K at 300K. In the third approach, three dimensional folding of a core-shell nanostructure based on PANI/GNP paper was adopted to take advantage of the high anisotropic conduction in a GNP paper, its high flexibility and foldability, but more importantly the strong phonon scattering at the corner of the folds as compared to a much lower disruption in the electron flow. Taken together, the ability to produce and nanostructure these inexpensive graphene nanoplatelets opens up numerous opportunities for the development of plastic thermoelectrics in the future. Copyright by Jinglei Xiang 2012