Heat transfer analysis of polymer composites for thermal insulation and heat sink applications

Abstract

The heat transfer phenomena through polymer composites is the main topic of this study. This study dealt with diverse processing techniques, morphological analysis, experimental and numerical results of thermal, mechanical behavior. The findings showed that the thermal conductivity of polymer composite decreases with increasing the interfacial thermal resistance, resulting in variation of surface structure. It was also found that nanostructures due to the generation of zinc oxide (ZnO) nanorods were more critical for the heat dissipation than the minimization of surface. This study will contribute to understanding of the underlying physics behind heat transfer phenomena, such as thermal insulation and thermal dissipation. In chapter 2, the thermal conductivity of aerogel/epoxy composite based on the inexpensive powder form of silica aerogels by using water glass under ambient drying conditions was evaluated to investigate the relationship between the internal structure and the thermal conductivity of the composite. A processing method for preserving the aerogel pores was then developed using ethanol evaporation, which lowered the thermal conductivity of the composite. To enhance the morphostasis of silica aerogel composite, a fabrication method was designed by applying the thermal characteristics of silica aerogels with preservation of pores in the aerogel to achieve extremely low thermal conductivity of the composites. A new process was proposed to generate interfaces between superhydrophobic silica aerogels and a hydrophilic polyvinyl alcohol (PVA) solution and to fabricate the silica aerogel/PVA composite forcibly while PVA is precipitated over the interfaces by making the solvent vaporize at a slow rate during stirring. In chapter 3, we demonstrated the synergistic effect of dual scale shape memory polyurethane (SMPU) foams that recovered from the compressed shape to their initial dimension above the transition temperature. Such a recovery leads to an enhancement in the thermal features, especially thermal resistance. Dual scale shape memory foams were fabricated by using the salt leaching method and their internal structure was analyzed morphologically. The porosity and interfacial thermal resistance of the dual scale foam were enhanced significantly, which leads to the excellent thermal resistance compared with other single scale foams. The thermal behavior of foamed materials was modelled analytically. The synergistic effect on mechanical properties, which was induced by the dual size pores, was explained by the result of numerical simulation. In addition, thermo-mechanical properties of the shape memory foam such as shape recovery, shape fixity, and shape repeatability were characterized. SMPU filled with carbon nanotubes (CNTs) has been investigated to actuate by thermo-response. Although the pure SMPU foam was not responsive to microwave radiation, the microwave absorption dosage in the SMPU/CNT foams considerably could be increased with adding amount of CNTs. When exposed to microwave radiation, the embedded CNTs were absorbed the external electromagnetic energy and reacted as heat sources in the SMPU/CNT foamed material were heated volumetrically and led to response fast. The influences of the CNTs on the mechanical and thermal properties of the SMPU foams with 0.01 wt%, 0.05 wt% and 0.1 wt% CNTs were investigated. The shape recovery behaviors of the SMPU foams were also characterized by microwave radiation. In chapter 4, we studied a new approach where structurally gradient nanostructures were fabricated by means of hydrodynamics. ZnO nanorods were synthesized in a drag-driven rotational flow in a controlled manner. The structural characteristics of nanorods such as orientation and diameter were determined by momentum and mass transfer at the substrate surface. The nucleation of ZnO was induced by shear stress that plays a key role in determining the orientation of ZnO nanorods. The nucleation and growth of such nanostructures were modelled theoretically and analyzed numerically to understand the underlying physics of the fabrication of nanostructures controlled by hydrodynamics. The findings demonstrated that the precise control of momentum and mass transfer enabled the formation of ZnO nanorods with a structural gradient in diameter and orientation. The study also describes the hydrothermal growth of ZnO nanostructures on graphene/polyethylene phthalate (PET) films and their thermal properties. The ZnO nanostructures were grown on graphene sheets of a few layers thick with seed layer. The dimensions and density of the ZnO nanorods could be easily controlled by changing the hydrothermal growth conditions such as temperature, number of spin coating and concentration of growth solution. Moreover, the interfacial effect induced by ZnO nanostructures was investigated to analyze the thermal behavior of ZnO/graphene/PET film such as heat dissipation, heat flux.