Heat routing with liquid-vapor phase change phenomena in microscale porous media
Keywords: Mechanical Engineering.

As electronic devices continue to miniaturize in size and grow in system complexity, conventional thermal management techniques must evolve to gradually include the concept of not merely heat dissipation, but controllable heat routing. To optimize lifetime reliability and overall device performance, many applications benefit from isothermalization within a narrow temperature window. Such devices often experience heat generation and environmental conditions that are transient and/or spatially variant, however, creating undesirable thermal profiles that cannot be addressed by constant heat removal strategies. Proper thermal management of such systems therefore benefits from the introduction of nonlinear thermal components that can exhibit properties such as heat flow transformation, thermal switching, and thermal isolation. This work examines the use of liquid-vapor phase change phenomena as a mechanism for controllable heat routing. First, we examine the use of liquid-vapor phase change as a heat spreading mechanism. Vapor chambers or flat plate heat pipes have long existed as effective heat flow transformers used to reduce highly concentrated local heat fluxes before heat rejection to an external sink. Conventional vapor chamber materials are not coefficient of thermal expansion matched to semiconductor device materials, however, necessitating the use of intermediary thermal interface materials that act as bottlenecks to the overall system performance. We present the concept of a miniature silicon-based vapor chamber for die-matched heat spreading, and demonstrate that the vapor transport can effectively improve the die-level temperature uniformity even over relatively small (~1 x 1 cm2) spreading areas. Due to the miniature volume of the device, we also develop an analytical model to predict the effect of liquid charge on the thermal resistance, and find the performance to be sensitive to be within ± 2 μL. Second, we purposefully introduce a non-condensable gas (NCG) to act as a diffusion barrier to the vapor transport. In a one-dimensional transport scenario, the binary vapor/NCG diffusion process creates a highly nonlinear, temperature dependent thermal resistance that can act as a passive thermal regulator. We perform measurements of the steady state thermal characteristics with varying amounts of NCG charge and find it to be an effective mechanism for tuning the regulatory properties of the device. We present an analytical model that captures the temperature dependent behavior of the binary diffusion process, and perform a parametric optimization to a resistance switching ratio of up to 14 in response to varying levels of heat input. Third, we examine the implications of thermal capacitance when assessing the effectiveness of various thermal regulatory schemes. We perform transient measurements of the binary diffusion based regulator in response to pulsed heating inputs to characterize the thermal response time. We find that the diffusion process creates a phenomena where the temperature difference across the device becomes clamped to a constant, relatively heat flow independent value above a threshold temperature. When subjected to transient heat loads, this manifests in an asymmetric device response time, where the low resistance state approaches the steady state value rapidly, and the high resistance state is amplified over the steady state value. Finally, we examine the more fundamental aspects of heat transfer during liquid to vapor phase change in microscale porous media. We leverage the highly ordered structure of inverse opal metal films to examine the limitations of capillary-fed evaporation and boiling in the porous wicks that drive the heat and mass transfer in device level heat routing applications. We find that maintaining the contact angle stability of the metal wicks is crucial for long term reliability and stable performance, and present results from different surface treatment schemes to generate various hydrophilic nano/microstructures