With increased focus on miniaturization and high performance in electronics, thermal management is a very important area of research today. In multiple applications such as portable electronics, consumer electronics, military applications, automobile, power electronics, high performance computing, etc. innovative thermal management strategies are necessary. In this work, two novel approaches to dissipate redundant heat better- first by novel carbonaceous-nanoparticle additives to develop thermal interface materials with superior performance and the second by using advanced metal additive manufacturing techniques to design and analyze metal-lattice based heat sinks are presented. Thermal Interface Materials with multiple carbon-based nanoparticle fillers such as coal-derived Multi Layered Graphene (MLG), standard reduced Graphene Oxide (rGO), Multi-Walled Carbon Nano Tubes (MWCNTs), and Graphene Nano-Platelets (GNPs) in thermal paste were synthesized and seen to have superior heat dissipation properties. Also, graphene was synthesized from coal through an in-house, facile, scalable and cost-effective process. The enhancement in thermal conductance varies from ~70% in the coal-MLG to ~14% in MWCNTs-based TIMs. Noteworthy is ~3.5 times larger enhancement in thermal performance with the in-house coal-derived-MLG as compared to the commercially available g-MLG. At a 3% wt. fraction of coal-MLG, enhancement in thermal conductance was almost 120% higher compared to the base thermal grease. In the second part, metal lattice-based heat sinks are designed for additive manufacturing for use in passive cooling of high-flux thermal management. A parametric optimization based on the lattice geometry, thickness, and height subject to additive manufacturing constraints is conducted. Intricate metal lattices with low mass based on the Simple Cubic, Octet, and Voronoi structures were generated by implicit modelling in nTopology® and their thermal performance was analyzed through numerical analysis using commercial CFD packages. The Voronoi lattice performed best with a significant improvement in thermal performance (~18% reduction in junction temperature difference with respect to ambient) as compared to a standard baseline Longitudinal heat Sink (LHS), while reducing the mass of the heat sink by ~2.1 times. Such optimized metal lattice-based heat sinks can lead to significant downsizing, reduction in overall mass and cost in applications where thermal management is critical with a need for low mass. We believe that such novel scalable materials and processes suited for mass production could be critical in meeting the material, design and product development needs to tackle the thermal management challenges of the future. / Master of Science / With increase in demand of high power and performance in electronics, there is a concurrent increase in redundant heat that needs to be dissipated. With enhanced focus and push towards electric vehicles, defense, consumer electronics, datacenter and supercomputing applications, electronics cooling is a critical area of research today. There are two primary resistances to heat- as it is removed from electronics package to the surrounding atmosphere – due to the thin layer of a material called Thermal Interface Material (TIM) at the interface between the heat sink and the package, and the resistance offered by the heat sink itself. In this work, a two-pronged approach for better cooling in electronics is presented. Firstly, carbon-based nano-sized particles are used to synthesize novel TIMs that provide superior heat transport capabilities as compared to a standard baseline. In the second approach, complex metal-lattice based heat sinks are designed for manufacturing with advanced techniques such as metal 3D printing. Multiple carbon-based nano-particle additives such as Multi Layered Graphene synthesized from coal (MLG), standard commercially available reduced Graphene Oxide (rGO), Multi-Walled Carbon Nano Tubes (MWCNTs), and Graphene Nano-Platelets (GNPs) are dispersed in thermal paste and all of the resulting composites were found to remove heat better from electronics packages. The improvement in this ability varies from ~70% in the coal-MLG to ~14% in MWCNTs-based TIMs. Noteworthy is ~3.5 times larger enhancement in the heat transport ability with the use of in-house coal-derived-MLG as compared to the commercially available g-MLG. At an 3% wt. fraction of coal-MLG, there was a 1.2x increase in thermal performance as compared to the base thermal grease. Also, it is significant to mention that MLG was synthesized from coal through an in-house, facile scalable and cost-effective process. In the second part, metal lattice-based heat sinks designed for metal 3D printing for use in passive cooling of electronics was investigated. Multiple geometric parameters such as the lattice type, thickness, and height subject to additive manufacturing constraints were studied. Intricate metal lattices with low mass based on three structures- Simple Cubic, Octet, and Voronoi were generated by implicit modelling, and their thermal performance was predicted by computer based-simulations using commercial CFD packages. The Voronoi lattice performed best with a significant reduction (~18%) in junction temperature difference with the surrounding atmosphere- as compared to a standard baseline rectangular heat sink design, while simultaneously reducing the mass of the heat sink by ~2.1 times. Such optimized metal lattice-based heat sinks can lead to significant reduction in overall mass, size, and cost in weight sensitive applications. We believe that such novel scalable materials, designs, and processes suited for mass production could be critical in meeting the material, design and product development needs to tackle the thermal management challenges of the near future.

<p>The aim of this study is to synthesize a material with high thermal conductivity and a low coefficient of thermal expansion (CTE), useful as a heat sink. Carbon nanofibres (CNF) are first coated with copper by an electroless plating technique and then sintered to a solid sample by either spark plasma sintering (SPS) or hot pressing (HP). The final product is a carbon nanofibre reinforced copper composite. Two different fibre structures are considered: platlet (PL) and herringbone (HB). The influence of the amount of CNF reinforcement (6-24 %wt), on the thermal conductivity and CTE is studied. CNF has an excellent thermal conductivity in the direction along the fibre while it is poor in the transverse direction. The CTE is close to zero in the temperature range of interest. The adhesion of Cu to the CNF surface is in general poor and thus improving the the wetting of the copper by surface modifications of the fibres are of interest such that thermal gaps in the microstructure can be avoided. The poor wetting results in CNF agglomerates, resulting in an inhomogeneous microstructure. In this report a combination of three different types of surface modifications has been tested: (1) electroless deposition of copper was used to improve Cu impregnation of CNF; (2) heat treatment of CNF to improve wetting; and (3) introduction of a Cr buffer layer to further enhance wetting. The obtained composite microstructures are characterized in terms of chemical composition, grain size and degree of agglomeration. In addition their densities are also reported. The thermal properties were evaluated in terms of thermal diffusivity, thermal conductivity and CTE. Cr/Cu coated platelet fibres (6wt% of CNF reinforcement) sintered by SPS is the sample with the highest thermal conductivity, ~200 W/Km. The thermal conductivity is found to decrease with increasing content of CNFs.</p>

A stacked microchannel heat sink was developed to provide efficient cooling for microelectronics devices at a relatively low pressure drop while maintaining chip temperature uniformity. Microfabrication techniques were employed to fabricate the stacked microchannel structure, and experiments were conducted to study its thermal performance. A total thermal resistance of less than 0.1 K/W was demonstrated for both counter flow and parallel flow configurations. The effects of flow direction and interlayer flow rate ratio were investigated. It was found that for the low flow rate range the parallel flow arrangement results in a better overall thermal performance than the counter flow arrangement; whereas, for the large flow rate range, the total thermal resistances for both the counter flow and parallel flow configurations are indistinguishable. On the other hand, the counter flow arrangement provides better temperature uniformity for the entire flow rate range tested. The effects of localized heating on the overall thermal performance were examined by selectively applying electrical power to the heaters. Numerical simulations were conducted to study the conjugate heat transfer inside the stacked microchannels. Negative heat flux conditions were found near the outlets of the microchannels for the counter flow arrangement. This is particularly evident for small flow rates. The numerical results clearly explain why the total thermal resistance for counter flow arrangement is larger than that for the parallel flow at low flow rates. In addition, laminar flow inside the microchannels were characterized using Micro-PIV techniques. Microchannels of different width were fabricated in silicon, the smallest channel measuring 34 mm in width. Measurements were conducted at various channel depths. Measured velocity profiles at these depths were found to be in reasonable agreement with laminar flow theory. Micro-PIV measurement found that the maximum velocity is shifted significantly towards the top of the microchannels due to the sidewall slope, a common issue faced with DRIE etching. Numerical simulations were conducted to investigate the effects of the sidewall slope on the flow and heat transfer. The results show that the effects of large sidewall slope on heat transfer are significant; whereas, the effects on pressure drop are not as pronounced.

The work reported in this thesis is an attempt to enhance heat transfer in electronic devices with the use of impinging air jets on pin-finned heat sinks. The cooling per-formance of electronic devices has attracted increased attention owing to the demand of compact size, higher power densities and demands on system performance and re-liability. Although the technology of cooling has greatly advanced, the main cause of malfunction of the electronic devices remains overheating. The problem arises due to restriction of space and also due to high heat dissipation rates, which have increased from a fraction of a W/cm2to 100s of W /cm2. Although several researchers have at-tempted to address this at the design stage, unfortunately the speed of invention of cooling mechanism has not kept pace with the ever-increasing requirement of heat re- moval from electronic chips. As a result, efficient cooling of electronic chip remains a challenge in thermal engineering. Heat transfer can be enhanced by several ways like air cooling, liquid cooling, phase change cooling etc. However, in certain applications due to limitations on cost and weight, eg. air borne application, air cooling is imperative. The heat transfer can be increased by two ways. First, increasing the heat transfer coefficient (forced convec- tion), and second, increasing the surface area of heat transfer (finned heat sinks). From previous literature it was established that for a given volumetric air flow rate, jet im-pingement is the best option for enhancing heat transfer coefficient and for a given volume of heat sink material pin-finned heat sinks are the best option because of their high surface area to volume ratio. There are certain applications where very high jet velocities cannot be used because of limitations of noise and presence of delicate components. This process can further be improved by pulsating the jet. A steady jet often stabilizes the boundary layer on the surface to be cooled. Enhancement in the convective heat transfer can be achieved if the boundary layer is broken. Disruptions in the boundary layer can be caused by pulsating the impinging jet, i.e., making the jet unsteady. Besides, the pulsations lead to chaotic mixing, i.e., the fluid particles no more follow well defined streamlines but move unpredictably through the stagnation region. Thus the flow mimics turbulence at low Reynolds number. The pulsation should be done in such a way that the boundary layer can be disturbed periodically and yet adequate coolant is made available. So, that there is not much variation in temperature during one pulse cycle. From previous literature it was found that square waveform is most effective in enhancing heat transfer. In the present study the combined effect of pin-finned heat sink and impinging slot jet, both steady and unsteady, has been investigated for both laminar and turbulent flows. The effect of fin height and height of impingement has been studied. The jets have been pulsated in square waveform to study the effect of frequency and duty cycle. This thesis attempts to increase our understanding of the slot jet impingement on pin-finned heat sinks through numerical investigations. A systematic study is carried out using the finite-volume code FLUENT (Version 6.2) to solve the thermal and flow fields. The standard k-ε model for turbulence equations and two layer zonal model in wall function are used in the problem Pressure-velocity coupling is handled using the SIMPLE algorithm with a staggered grid. The parameters that affect the heat transfer coefficient are: height of the fins, total height of impingement, jet exit Reynolds number, frequency of the jet and duty cycle (percentage time the jet is flowing during one complete cycle of the pulse). From the studies carried out it was found that: a) beyond a certain height of the fin the rate of enhancement of heat transfer becomes very low with further increase in height, b) the heat transfer enhancement is much more sensitive to any changes at low Reynolds number than compared to high Reynolds number, c) for a given total height of impingement the use of fins and pulsated jet, increases the effective heat transfer coefficient by almost 200% for the same average Reynolds number, d) for all the cases it was observed that the optimum frequency of impingement is around 50 − 100 Hz and optimum duty cycle around 25-33.33%, e) in the case of turbulent jets the enhancement in heat transfer due to pulsations is very less compared to the enhancement in case of laminar jets.

The principal goal of this thesis is to develop a systematic method for the design of composite heat sinks (CHSs) that serve as passive and transient cooling devices for microelectronics. This is accomplished by posing the CHS design problem as a topology optimization problem wherein a phase-change material and a high-conductivity material are to be optimally distributed. Two different types of formulations are proposed. The first one aims to maximize the time of operation before a tolerable temperature is reached at the interface between a heat source and the CHS. The second one aims to minimize the maximum temperature across the heating interface for a given time of operation. The two materials are interpolated in topology optimization using the usual mixture law with penalty. The phase-change is modeled using the apparent heat capacity method in which the specific heat is taken as a nonlinear function of the temperature so that the latent heat absorption is accounted for at the melting point. The ensuing new transient topology optimization problem involving an interpolated material property that depends on the state variable is solved using continuous optimization algorithm. The validity of the phase-change modeling is verified with a one dimensional model as well as experimentation. Analytical sensitivity analysis is derived and verified with the finite difference derivatives. Several examples are solved to illustrate the intricacies of the problem and the effectiveness and the limitations of the proposed design method. Prototypes of an intuitively conceived CHS and optimized one are made. An experimental setup is devised to test the two prototypes. Based on the insight gained from the experiments, an improved conduction model is studied to also incorporate convective heat transfer also into the model.

The aim of this study is to synthesize a material with high thermal conductivity and a low coefficient of thermal expansion (CTE), useful as a heat sink. Carbon nanofibres (CNF) are first coated with copper by an electroless plating technique and then sintered to a solid sample by either spark plasma sintering (SPS) or hot pressing (HP). The final product is a carbon nanofibre reinforced copper composite. Two different fibre structures are considered: platlet (PL) and herringbone (HB). The influence of the amount of CNF reinforcement (6-24 %wt), on the thermal conductivity and CTE is studied. CNF has an excellent thermal conductivity in the direction along the fibre while it is poor in the transverse direction. The CTE is close to zero in the temperature range of interest. The adhesion of Cu to the CNF surface is in general poor and thus improving the the wetting of the copper by surface modifications of the fibres are of interest such that thermal gaps in the microstructure can be avoided. The poor wetting results in CNF agglomerates, resulting in an inhomogeneous microstructure. In this report a combination of three different types of surface modifications has been tested: (1) electroless deposition of copper was used to improve Cu impregnation of CNF; (2) heat treatment of CNF to improve wetting; and (3) introduction of a Cr buffer layer to further enhance wetting. The obtained composite microstructures are characterized in terms of chemical composition, grain size and degree of agglomeration. In addition their densities are also reported. The thermal properties were evaluated in terms of thermal diffusivity, thermal conductivity and CTE. Cr/Cu coated platelet fibres (6wt% of CNF reinforcement) sintered by SPS is the sample with the highest thermal conductivity, ~200 W/Km. The thermal conductivity is found to decrease with increasing content of CNFs.

Orientador: Carlos Alberto Carrasco Altemani / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-06T09:32:44Z (GMT). No. of bitstreams: 1 Souza_ValterCesarde_D.pdf: 4532174 bytes, checksum: 07cf885bb91505cb080fc4a067621191 (MD5) Previous issue date: 2005 / Resumo: Os dissipadores térmicos de placas paralelas com entrada de topo e saída lateral constituem uma alternativa para intensificar a remoção da potência elétrica dissipada em microprocessadores. Neste trabalho, três dissipadores térmicos de placas paralelas foram construídos e testados com escoamento variável de ar sob condições de entrada de topo e saída lateral. Os resultados experimentais foram comparados com correlações da literatura e com resultados de simulações numéricas tridimensionais. Após a validação com os resultados experimentais, o modelo numérico foi utilizado num procedimento para obter o número de aletas do dissipador para a máxima troca térmica convectiva. Dois casos foram considerados, um deles baseado numa velocidade média do ar constante na entrada do dissipador, e o outro, numa relação linear da curva de operação de um ventilador / Abstract: The parallel plates heat sinks with top inlet and side exit constitute an enhanced heat transfer alternative for the local removal of electric power dissipation in microprocessors. In the present work, three parallel plates heat sinks were built and tested with variable airflow under the conditions of top inlet and side exit. The experimental results were compared with correlations from the literature and with results from three-dimensional numerical simulations. After the validation with the experimental results, the numeric model was used in an optimization procedure to obtain the heat sink number of fins for the maximum convective heat transfer. Two cases were considered, one based on a constant average inlet air velocity, and the other, on an assumed linear fan curve / Doutorado / Termica e Fluidos / Doutor em Engenharia Mecânica