Gilhwan Cha

Reversible thermal interfaces based on microscale dielectric liquid layers

We present a reversible thermal interface that can circumvent limitations of direct solid-solid contacts. A thin continuous layer of a dielectric liquid is formed between two solid substrates to provide a low-resistance heat conduction path. The liquid is initially confined in an array of discrete microchannels and undergoes reversible morphological transition into a continuous film as the loading pressure is increased. We theoretically and experimentally determine the relationship between loading pressure and liquid morphology. The interfaces can achieve thermal resistance comparable to that of solid-solid contacts but at loading pressures orders of magnitude smaller.

Thermal switches based on coplanar EWOD for satellite thermal control

We propose and demonstrate a novel liquid-droplet-based thermal switch aimed for satellite thermal control by using coplanar electro-wetting-on-dielectric (EWOD) configuration. By fabricating and testing the proof-of-concept devices, we confirm the mechanism of droplet detaching and attaching for the thermal switches. Thermal tests have shown the reasonable thermal performance with water and glycerin. Further development with novel liquid with low vapor pressure and high thermal conductivity is expected to improve the performance.

Experimental characterization of thermal conductance switching in magnetorheological fluids

We experimentally investigate thermal conductance switching in Fe-based magnetorheological fluids (MRFs). The transient hot-square technique is employed to directly measure enhancement in the thermal conductivity of bulk samples with volume concentrations up to 33% along the field direction. The ratio of the thermal conductivities of bulk MRFs under no and strong (∼290 kA/m) field is approximately 1.3, nearly independent of particle concentration. Significantly higher on-off conductance ratios can be achieved at a device level by exploiting the normal field instability to form columns of MRFs across an air gap. We experimentally demonstrate reversible switching in one implementation of this device concept.

Electric field dependence of the Curie temperature of ferroelectric poly(vinylidenefluoride-trifluoroethylene) co-polymers for pyroelectric energy harvesting

We experimentally study pyroelectric energy harvesting in a ferroelectric co-polymer thin film, with a particular focus on the electric field dependence of its Curie temperature. Liquid-based switchable thermal interfaces are employed to achieve fast thermal cycling and reduce potential measurement errors resulting from leakage currents. Our work confirms that pyroelectric energy harvesting is most effective around the Curie temperature and that the Curie temperature does not depend strongly on electric fields, at least up to 500 kV cm 1 . These results are consistent with a recent study of the electrocaloric effect in co-polymer films of a similar composition. (Some figures may appear in colour only in the online journal)

Switchable Thermal Interfaces Based on Discrete Liquid Droplets

We present a switchable thermal interface based on an array of discrete liquid droplets initially confined on hydrophilic islands on a substrate. The droplets undergo reversible morphological transition into a continuous liquid film when they are mechanically compressed by an opposing substrate to create low-thermal resistance heat conduction path. We investigate a criterion for reversible switching in terms of hydrophilic pattern size and liquid volume. The dependence of the liquid morphology and rupture distance on the diameter and areal fraction of hydrophilic islands, liquid volumes, as well as loading pressure is also characterized both theoretically and experimentally. The thermal resistance in the on-state is experimentally characterized for ionic liquids, which are promising for practical applications due to their negligible vapor pressure. A life testing setup is constructed to evaluate the reliability of the interface under continued switching conditions at relatively high switching frequencies.

Liquid-Mediated Reversible Thermal Interface for Satellite Thermal Management

Ability to establish and break thermal contacts in a reversible manner is important in a wide range of applications. These include active thermal conductance control for bolometers [1], pulsed thermoelectric cooling [2], chip scale atomic clocks, and thermal energy harvesting, and thermally reconfigurable networks for satellite thermal management [3]. The last is particularly interesting as it potentially has significant near- as well as long-term technical impact.Copyright

High-power density pyroelectric energy harvesters incorporating switchable liquid-based thermal interfaces

Pyroelectric thermal energy harvesters are intriguing alternatives to thermoelectric devices due to their high thermodynamic efficiency and reduced heat sink requirements. We report a concept for pyroelectric energy harvesters that utilize liquid-based switchable thermal interfaces to achieve thermodynamic cycling frequencies and hence high-power densities. Pyroelectric energy harvesting in thin films of 56/44 P(VDF-TrFE) copolymer is demonstrated at thermodynamic cycle frequencies of the order of 1 Hz and material-level power densities of the order of 100 mW/cm3. The present work demonstrates the viability of high-power density pyroelectric thermal energy harvesters based on liquid-based switchable interfaces and identifies a need for optimized electrode arrays to maximize their potential.

Microscale liquid-based mechanical elements for multifunctional integration

We report a novel strategy that exploits the unique mechanical characteristics of microscale liquid elements to enable versatile multifunctional integration of brittle components into load-bearing structures. The preliminary feasibility of this integration strategy is demonstrated experimentally using chemically patterned 500 -µm-thick silicon chips to emulate brittle functional components. A systematic study of the mechanical characteristics of microscale liquid bridges, including the force-gap relations under axial loading and the control of rupture distances, is discussed to help guide the systematic design of liquid-based mechanical elements for multifunctional integration.

Impact of Micro/Nanoscale Heat Transfer in Silicon Substrates on Thermal Interface Resistance Characterization

We consider different sources of potential systematic errors in the thermal interface resistance measurements. Sub-continuum heat conduction and spatial non-equilibrium between electrons and phonons in a metal may lead to overestimation of the thermal interface resistance. The use of an erroneous substrate thermal conductivity can also cause significant systematic errors in steady- and quasi-steady state measurements of the thermal interface resistance. Our results highlight an urgent need for new systematic experimental studies to confirm the magnitude of intrinsic thermal interface resistance.Copyright