Thermal and Electrical Performance Control and Lifespan Progression of Graphene-Based Polymer Composites
Power dissipation rates of modern semiconductor devices continually increases year after year. Despite this, safe operating temperatures of these devices remain moderately low, typically below 90 °C, after which point devices performance deviates from design and lifespan significantly diminishes. As a result, ever-increasing thermal dissipation capacity is required to achieve acceptable operating temperature. Polymer composites are often applied between two imperfect surfaces to replace air gaps and act as a better thermal bridge to facilitate heat flow between solids or as chip encapsulants to protect from environmental contamination. In each application, the overall thermal dissipation, and thus operating temperature, can be improved by using more thermally conductive polymers. The thermal conductivity of most polymers, certainly all of those in common general use in industry are quite low, below 0.3 W/mK. It is a common tactic to composite the polymers with other, highly thermally conductive materials to then raise the overall composite thermal conductivity. The extraordinary thermal conductivity of graphene has prompted significant study into composites composed of it for these applications. However, since graphene is also electrically conductive, composites filled to a sufficient level exhibit substantial electrical conductivity, which can be problematic in the electronics industry. The composite may make immediate contact or over the course of a realistic lifespan may make unavoidable, inadvertent contact with active circuit elements resulting from migration phenomena, which may threaten circuit stability depending on the composite’s electrical conductivity. This dissertation research developed a methodology for the independent control of thermal and electrical conductivity of graphene composites through hybridization by finely controlling filler levels of graphene and electrically insulating hexagonal boron nitride. Additionally, the lifespan performance of graphene polymer composites was studied in a custom-built accelerated aging instrument which revealed an unexpected enhancement of thermal conductivity in up to 500 power cycling treatments. The improved performance was attributed to an increased coupling between graphene and epoxy polymer resultant from increased cross-linking density, resulting in more firm contact and lower Kapitza resistance. The obtained results are important for future thermal management technologies that leverage the unique heat conduction properties of graphene.