Mechanical Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2795
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Item Microencapsulated Phase Change Materials for Energy Storage and Thermal Management(2014) Cao, Fangyu; Yang, Bao; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The continuous increase of greenhouse gas emission, the climb in fuel prices, and the limited natural resources drive human beings to utilize energy more effectively. Changes are required in energy storage and thermal management systems, particularly through the advanced technologies and systems of thermal energy storage and heat dissipation. Phase change materials (PCMs) have received considerable attention for these applications. As a novel technology to utilize PCMs, microencapsulated phase change materials (microPCMs) have drawn great interest due to their high heat capacity and easy manipulating and operating, and thus are potentially applicable in various industries. This dissertation provides results of a systematic investigation on the design, synthesis, characterization, and applications of microPCMs. With either solid-solid PCM or liquid-solid PCM as the core material, microPCMs have been synthesized with wet-chemical methods using colloidal solutions as the reaction media. To begin with, the thermophysical properties of colloidal systems were investigated, especially the change of thermal conductivity with the concentration of surfactant. Two types of microPCMs were then synthesized using emulsion techniques, and the synthesis parameters were manipulated to enhance the thermophysical properties of the microPCMs and suppress the supercooling of encapsulated PCMs. To enhance their thermal conductivity, microPCMs with large latent heat capacity and suppressed supercooling were coated with a metal layer. The as-synthesized phase changeable and thermal conductive microPCMs were applied in a heat transfer fluid to enhance the heat transfer performance. This work was focused on the following aspects. The first aspect is the thermophysical properties of colloidal solutions, such as thermal conductivity, at low surfactant concentrations around the critical micelle concentration (CMC). The second aspect is the synthesis of microPCMs in the colloidal systems with solid-solid PCM neopentyl glycol and liquid-solid PCM n-octadecane as the core material. The third aspect is the enhancement of thermophysical properties (e.g., heat capacity, supercooling,) of the microPCMs, which was achieved by manipulating the parameters of the environment of chemical synthesis. The fourth aspect is the elevation of the thermophysical properties of the microPCMs, such as thermal conductivity, after the microPCMs were produced. The fifth and final aspect is the applications of the as-produced microPCMs, e.g., to enhance the heat transfer in bulk solid materials for latent heat storage and heat transfer fluids for heat dissipation with the aid of microPCMs, with or without coating with thermal conductive silver layer.Item EVALUATION OF THERMAL INTERFACE MATERIALS AND THE LASER FLASH METHOD(2009) Khuu, Vinh; Khuu, Vinh P; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Thermal interface materials (TIMs) are used to reduce the interfacial thermal resistance between the chip and the heat sink, which has become a bottleneck to heat removal in a variety of electronic applications. Degradation in thermal performance of the TIM can contribute to unacceptably high chip temperatures, which can significantly impact device or system performance during operation. While progress has been made in recent years in the development of tools to measure beginning-of-life thermal performance, characterizing the long-term performance of the TIM can be crucial from a life cycle stand point since TIMs may experience harsh operating conditions, including high temperature and high humidity, for extended periods of time in typical applications. The laser flash method is one approach for measuring thermal conductivity that has an advantage over more commonly used techniques because of the non-contact nature of the measurement. This technique was applied to 3-layer structures to investigate the effects of thermal cycling and elevated temperature/humidity on the thermal performance of select polymer TIMs in pad form, as well as an adhesive and a gel. While most samples showed little change (less than 10% in thermal resistance) or slight improvement in the thermal performance, one thermal putty material showed degradation due to temperature cycling resulting from bulk material changes near the glass transition temperature. Scanning acoustic microscope images revealed delamination in one group of gap pad samples and cracking in some putty samples due to temperature cycling. Finite element simulations and laser flash measurements performed to validate the laser flash data indicated that sample holder plate heating, an effect previously unexamined in the literature, can lead to inaccurately high TIM thermal conductivity values due to suppression of the sample temperature rise during the laser flash measurement. This study proposed a semi-empirical methodology to correct for these effects. Simulated laser flash test specimens had bondlines that showed little thickness variation (usually within the measurement error) due to clamping by the sample holder plates. Future work was proposed to refine the laser flash sample holder design and perform additional validation studies using thermal test vehicles based on nonfunctional packages.Item Characterization, Modeling, and Optimization of Polymer Composite Pin Fins(2005-08-24) Bahadur, Raj; Bar Cohen, Avram; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Increasing electronic product manufacturing volumes and cooling requirements necessitate the use of new materials and innovative techniques to meet the thermal management challenges and to contribute towards sustainable development in the electronic industry. Thermally conductive polymer composites, using high thermal conductivity fillers such as carbon fibers, are becoming commercially available and provide favorable attributes for electronic heat sinks, such as low density and fabrication energy requirements. These polymer composites are inherently anisotropic but can be designed to provide high thermal conductivity values in particular directions to address application-specific thermal requirements. This Thesis presents a systematic approach to the characterization, analysis, design, and optimization of orthotropic polymer composite fins used in electronic heat sinks. Morphological characterization and thermal conductivity measurements of thermally conductive Poly-Phenylene Sulphide composites are used to determine the significant directional thermal conductivity in such composites. An axisymmetric orthotropic thermal conductivity pin fin equation is derived to study the orthotropic thermal conductivity effects on pin fin heat transfer rate and temperature distribution. FEM simulation and water cooled experiments, focusing on the radial temperature variations in single pin fins, are used to validate the analytical model. Theoretical models, CFD modeling, and experiments are used to characterize the thermal performance of heat sinks, fabricated of PPS composite pin fins, in air natural convection and forced convection modes. Simplified solutions, for the orthotropic fin heat transfer rate that are easy to use and can be easily implemented in a heat sink design and optimization scheme, are presented.