UC Berkeley

Throughout the history of electronics, technology has been driven by shrinking devices and increasing performance. This progress has hit a wall that is limiting everyone, and this surprising limitation is thermal management and overheating. Cooling systems for electronics used to be one of the last design elements considered, fins, fans and graphite sheets were sufficient solutions, but that is no longer the case. The technology landscape has changed, and a new wave of solutions must be created and implemented quickly, or innovation will come to a standstill. The solution proposed in this research is the micro hybrid loop heat pipe (HLHP). This device fits in with the recent innovations in using phase change devices, such as micro heat pipes and micro vapor chambers, but uses cutting edge silicon heat transfer technology to improve upon these other approaches.

The system is a three layer stack with an interline, optimized evaporator surface and a coherent porous silicon wick, sometimes referred to as a multi-scale pore membrane (MPM) [1]. Connecting the enhanced silicon components is an aluminum housing to act as the piping. The total stack thickness is less than 3mm, and has the potential to be slimmed to 1-2 mm. The hybrid system is tested at fluxes of 25 W/cm^2 and higher. The evaporator structure has heat transfer coefficients as high at 50,000 W/m^2 K. The work in this dissertation is, to the authors knowledge, the first loop heat pipe that is able to dissipate this amount of heat with such a thin form factor and utilizes a never before tested evaporator arrangement. Data showing a functioning system, the heat transfer rates of that system, the thermal resistance, and other metrics of this cutting edge device are presented in this work.

The path of the research from an initial design to a final system, gives rise to an important discussion of how this device was created. On this path, background research was done that lead to increased focus on each component of the final system. The wick, the evaporator, new possibilities in laser fabrication techniques, and the sealing of the device were all thoroughly explored before the system was built. Once the groundwork of the component research was laid, modeling of the system was conducted to try and determine the limitations and room for improvement in future iterations of this design. Finally, the system was built, assembled and tested, creating the first micro loop heat pipe (uLHP) of this size scale. In addition, this device, shown here in its 3mm version, could be built out of silicon (or other materials) and shrunk to under 1mm without changing any of the channel depths or geometries. The structure was designed to be less than 1mm thick and was made thicker for ease of fabrication. Information on all of the component advancements and the process for creating a system that can handle up to 12W with 50W/cm^2 heat flux is presented here.