Timothée Robert Sophus Frei
High-energy physics (HEP) experiments and space missions share a common challenge: thermal management in harsh environments. The severe constraints in both fields make cooling of electronic components a major design concern. This thesis aims to develop high thermal conductivity, radiation hard devices for thermal management in HEP and space application with the help of micro-technologies.Continuous advances in micro-engineering have opened the door to the development of smaller and more efficient cooling devices capable of handling increasing power densities with minimum mass and volume penalties. In this respect, previous works have focused on the use of microchannels etched in single crystal silicon (ScSi) wafers to circulate a cooling fluid. However, whilst this technique represents the state-of-the-art for thermal management of silicon detectors, it poses several challenges, particularly those associated with the fluidic interconnections, compatibility with high operating pressures and coverage of large areas. A cooling scheme relying on two independent fluidic loops is proposed to overcome these limitations. The two cooling loops, referred to as the primary and the secondary cooling loops (i.e. PCL and SCL respectively), are connected through thermal and mechanical interfaces but do not share any fluidic interface. Both circuits work together to remove heat and control the temperature of multiple electronic components. The SCL transfers the heat generated at the source to the PCL, which transports it further away and dissipates it to the environment.The present thesis investigates the use of micro-oscillating heat pipes (uOHPs) to implement the secondary cooling loops. These devices offer great potential, particularly if embedded in silicon substrates. However, a better understanding of the mechanisms responsible for their self-actuated, two-phase flow is essential to produce high performance devices which meet the stringent requirements of particle detectors and spacecraft. Two types of dual-diameter uOHPs were microfabricated: the first version used a glass wafer to close trenches etched in a silicon substrate, while an all-silicon construction was adopted in the second type. The thermocompressed interface used to bond the two silicon wafers yielded excellent pressure resistance. The thermal performance of the of uOHPs was studied for different working fluids, charging ratios, orientations, and heat inputs, taking advantage of the transparent cover in the glass devices to monitor the two-phase flow for the different conditions. The results revealed that the performance of the proposed uOHPs was orientation-independent. Furthermore, under certain conditions the equivalent thermal conductivity of acetone-charged devices exceeded that of an equivalent copper strip.Finally, an innovative technique for charging and sealing the micro-cooling heat pipes was developed. It offers a low-volume, small-footprint solution to enclose working fluids in the devices. The proposed technique, which relies on a compressed Indium preform to seal the inlet charging port, yield very good leak-tightness results.
EPFL2021
