Transport properties of graphite-loaded composites in liquid and solid states

Abstract

Composites, either in solid state or liquid state, are widely used to improve the transport properties of the matrix phase in the composites. For instance, high thermal conductivity particles are frequently added into heat transfer liquids, forming colloidal suspensions sometimes also called nanofluids, to increase the liquids thermal conductivity. High electrical and/or thermal conductivity particles are also commonly added into polymers to increase their electrical and thermal conductivities. The properties of composites depend strongly on the microstructures formed. This thesis investigates thermal, rheological, and electrical properties of graphite loaded composites in liquid and solid states. First, we use combined optical microscope and AC impedance spectroscopy technique to study the composite microstructure transformation with respect to the volume fraction to understand the peculiar thermal conductivity behavior in the graphite suspensions we observed: the thermal conductivity increases more rapidly below the percolation threshold than above it, with a sharp kink at the percolation. Our study suggests that the below the percolation, the aggregation of graphite flakes is tighter and interfacial resistance is smaller, while above the percolation threshold, the aggregation of graphite flakes becomes looser and interfacial resistance is larger. Next, we study rheological properties of graphite-loaded liquids. We observed complicate temperature-dependent rheological properties of graphite suspension at different shear rates and temperature. We rationalize the experimental data using ideas from Stokesian dynamics simulations and combined with understandings of the internal structures of suspensions. Our studies show although Brownian motion is not important for thermal transport, it is important for rheological properties of the graphite-loaded suspensions. Chapter 4 studies thermal and electrical properties of graphite flakes loaded into a material going through liquid-to-solid phase transition in the percolated regime and shows that the transport properties can be switched via the phase transition reversibly, especially achieving high contrast in the electrical conductivity. We found that the stress generated during the freezing regulates the contacts among graphite flakes if a crystal-forming liquid is use, thus leading to a dramatically increase in the electrical conductivity in the solid state. In Chapter 5, we study thermal and electrical conductivities of composites with graphite flakes loaded into ultra-high molecular weigh polyethylene that are stretched at different ratios. The mechanical stretching of the composites affect the morphology and transport properties of both the polymer and the fillers, leading to some interesting thermal conductivity behavior. The graphite-loaded composites in liquid and solid states studies in this thesis may have applications in convective heat transfer, resettable fuse, seasonal regulation of building temperature, thermal interface materials, directional heat spreader, etc. The understanding of structure-property relation could also help uS better design composite materials with superior performance..