Liquid-Based Switchable Thermal Interfaces for Thermal Management Systems and Thermal Energy Harvesters

Proper thermal management is necessary for electronic devices to maintain their temperature within acceptable operating and survival limits in variable thermal environments. Thermal switches offer a promising solution for the thermal management system as they provide thermal control by switching between high and low heat transfer modes around a given set point. They can be applied to only select thermal paths, and different set points can be specified for different components. Existing switch concepts rely mainly on solid-solid contacts to establish and break the thermal contacts for switching the heat transfer modes. The solid-conduction based switches, however, have several limitations including high loading pressure requirements for complete thermal contact and failure mechanisms in repeated reversible operations such as fracture and cold-welding.;In the present work, we study liquid-based switchable thermal interfaces that can circumvent those limitations of direct solid-solid contacts. We first propose a liquid-based thermal interface concept that utilizes surface tension-driven morphological transformations of liquid for rapid and reliable thermal switching. We experimentally characterize the interface by measuring two important parameters: thermal interface resistance and loading pressure. We confirm that the liquid mediated interface can achieve thermal resistance comparable to that of direct solid-solid contacts but at loading pressures orders of magnitude smaller.;We then present two device level demonstrations of liquid-based thermal switches. One is a liquid-droplet thermal switch. To make alternating thermal contacts between liquid droplets and hot (cold) surfaces, the contact angle and hence the shape of liquid droplets is dynamically changed by using the electrowetting on dielectric (EWOD) phenomenon. The other concept is a magnetic field-controlled thermal switch which exploits the normal field instability to form columns of magnetorheological fluids (MRFs) across an air gap providing efficient heat conduction paths. We experimentally characterize the device performance and demonstrate reversible operations for both switch concepts.;Lastly, we apply the liquid-based switchable thermal interfaces to a pyroelectric thermal energy harvesting device. Converting spatial temperature gradients into temporal temperature changes is required for harvesting thermal energy from pyroelectric materials. We employ the thermal switch concept to acquire necessary temporal temperature changes. The liquid-based switchable thermal interface enables fast thermal cycling and therefore high power density can be produced. The high frequency operation of the device also allows accurate characterization of the intrinsic energy harvesting capability of pyroelectric materials. We characterize the energy harvesting performance of 56/44 P(VDF-TrFE) co-polymer thin films focusing in particular on its temperature, electric field, and cycle frequency dependence.