Novel carbon-nanotube-based interface materials and two-phase microchannel cold plates for high-density electronics cooling

This dissertation focuses on two aspects of the thermal management for high density electronics: (1) dense vertically aligned multiwalled carbon nanotubes (CNTs) as thermal interface materials to enhance heat conduction across solid surfaces, and (2) pumped-loop two-phase (boiling) microchannel and micro pin-fin array cold plates for removing high heat fluxes.The novel material structure based on vertically aligned multiwalled CNTs bears promises for enhancing heat conduction across two mating solid surfaces due to previously observed very high thermal conductivity of individual CNTs. Synthesized by chemical vapor deposition (CVD) techniques, these multiwalled CNT arrays have typical tube diameters ranging from 20 to 30 nm and aerial densities on the order of 1010-1011 tubes/cm 2. The vertical height (tube length) was controlled by growth time and can vary anywhere from a few micrometers to half a millimeter. Grown on silicon substrates, the CNT arrays were brought into contact with a glass target surface and held together by van der Waals (vdW) adhesion. The thermal properties of the heterostructured interface were then characterized using an optical phase-sensitive transient thereto-reflectance (PSTTR) method, which has the capability to differentiate component thermal resistances at the two CNT-solid contacts as well as the thermal resistance through the CNT array itself. Our measurement results show that while the chemically bonded interface (through CVD catalyst particles) between CNTs and silicon has a thermal resistance &sim0.01°·cm 2/W (or conductance of &sim106 W/m2·K), the physically bonded interface (by van der Waals attractions) between CNTs and glass has a thermal resistance &sim0.1°C·cm2/W (or a conductance &sim105 W/m2·K), which is the dominant thermal resistance. By introducing an additional wetting layer of indium between the CNTs and glass, we show that the thermal contact resistance between CNTs and glass can be reduced by almost one order of magnitude making CNTs attractive as a thermal interface material.In addition to the excellent thermal transport properties, CNTs enjoy unprecedented mechanical properties as well. For applications as a thermal interface material, the mechanical properties of the CNT arrays are relevant in terms of: (1) adhesive properties at the free CNT tip surface (van der Waals adhesion), such that these CNT arrays may be applied as reversible and thermally and electrically conducting switches and (2) mechanical compliance of the array, i.e., compressibility, such that they can better conform to loading conditions and absorb shock. The dry adhesive and compressive properties of the CNT arrays were characterized in this study.For the second focus of this dissertation, we have systematically studied two-phase (boiling) flow in microchannel and micro pin-fin array flow passages. Two-phase microchannel flow has been actively studied recently due to promises of even better thermal performances than single-phase microchannels. However, most previous studies involved only one specific channel design and one type of working fluid, making cross comparison difficult. Furthermore, understanding of the phase-change processes in microchannels has been inconclusive so far. In the current work, three microchannel cold plate designs with hydraulic diameters ranging from &sim100 mum to 337 mum and a micro pin-fin array cold plate with feature size &sim160 mum were studied with two different working fluids: deionized water at sub-atmospheric pressures (&sim25 to 45 kPa) and HFE-7100 (highly-wetting characteristics) under ambient pressure conditions. Hydraulic (pressure drop) and thermal performances are discussed from single-phase regime to two-phase regime. Cold plate design concerns and technical recommendations are suggested accordingly. With the help of high-speed macro imaging facilities, boiling flow patterns/mechanisms as well as fluctuations were extensively studied and discussed. A compilation of detailed visualization results is also presented. Finally, a scaling model predicting the departing nucleating vapor bubble sizes is presented and matches well with our experimental observations. These experimental and modeling studies should provide some insight to the phase-change processes in micro flow passage systems.