Graphene Composites and Foams: Synthesis, Properties, and Applications

Graphene is an attractive two-dimensional material and it has been intensively studied for the past decade due to its excellent and unique properties. In particular, graphene is known to have high thermal and electrical conductivity, as well as excellent mechanical and material properties (such as light weight, high strength and flexibility, and large specific surface area). Thus, it has been believed to be a promising material for not only electronic device applications but also sensing and composite applications and thermal management applications such as thermal interface materials (TIMs). However, in order to develop graphene-based electronic devices or composites in real applications, mass production or scalability is required. Top-down approaches such as chemical reduction, liquid phase exfoliation, or intercalation-exfoliation method are suitable for mass production of graphene flakes. In addition, a large-area graphene-based structure can be made by a bottom-up approach such as chemical vapor deposition (CVD). In my work, I have developed graphene-based composites and three-dimensional (3D) graphene structures (so-called graphene foam (GF)) using the top-down approaches (chemical reduction and intercalation-exfoliation) and the bottom-up approach (CVD), and explored their electrical, thermal, mechanical, and electrochemical (biosensing) applications. Firstly, I report an experimental study of electrical and thermal transport in reduced graphene oxide (RGO)/polystyrene (PS) composites, where RGO was prepared by the chemical reduction method. The electrical conductivity of RGO/PS composites with different RGO concentrations at room temperature shows a percolation behavior and their temperature-dependent electrical conductivity follows Efros-Shklovskii (ES) variable range hopping (VRH) in the temperature range of 30-300 K. I also observe that the thermal conductivity of RGO composites is enhanced by ~90% as the concentration is increased from 0 to 10 vol.%. In addition, I have investigated high-performance thermal interface materials (TIMs) based on few-layer graphene (FLG) composites, where FLG was prepared by the interlayer catalytic exfoliation (ICE) method. I have experimentally demonstrated feasibility of FLG composites as TIMs by studying their thermal and mechanical properties, and reliability. I have measured the thermal interface resistance between FLG composite TIMs (FLGTs) and copper (Cu), and the measured thermal interface resistance is comparable to or even lower than that of many commercial TIMs. Furthermore, the thermal conductivity of FLGTs is increased by an enhancement factor of ~17 as the FLG concentration increases from 0 to 10 vol.%. I have also characterized Vickers hardness and glass transition temperature of my FLGTs. I find that my FLGTs are thermally and mechanically reliable within practical operating temperature and pressure ranges. Instead of using the top-down approaches, I have also used the bottom-up approach to synthesize 3D graphene structures. I have investigated mechanical, thermal, and electrochemical (in particular, glucose-sensing) applications of 3D GFs, where the GFs were prepared by CVD. I have studied the compressive mechanical response of the GFs and the thermal resistance at the interface between Cu and the GFs to evaluate usefulness of the GFs for thermal interface or packaging applications. I observe Young\u27s modulus and compressive strength of GFs have a power law dependence on the density of the GFs and the efficiency of absorbed energy of the GFs can be comparable to or higher than that of many polymeric materials. In addition, the measured thermal resistance at the interface between Cu and the GFs can be as low as that of a commercial graphite paper. Lastly, I have demonstrated a non-enzymatic glucose sensor based on a copper oxide (CuO) and GF hybrid structure (CuO/GF), where the CuO is utilized as an electrocatalytic medium to oxidize glucose and the GF is used as a 3D electrode platform. I have observed a unique and fluffy morphology of nanocrystalline CuO clusters, and demonstrated that the CuO/GF has an excellent electrocatalytic activity toward the glucose oxidation, giving rise to a high sensitivity for detecting glucose. I find that the measured sensitivity of the CuO/GF is among the highest in many metal oxide (or hydroxide) non-enzymatic glucose sensors