Large Stroke Electrostatic Pump for an Electrocaloric Micro-Cooler

The increasing integration density of electronic components in systems where very high speed and/or power are essential leads to a desire for miniature active cooling technologies. An active micro-cooler that pumps working fluid past an electrocaloric regenerator has potential to realize such a high cooling performance. However, such a micro-cooler requires two key new components – an electrostatic micro-pump subsystem and the electrocaloric regenerator – that are the primary subject of this thesis. The micro-pump system drives the working fluid back and forth through the regenerator. The electrocaloric regenerator provides storage and release of heat under the application and removal of an electric field. Through synchronization of the electrical and mechanical cycling, heat can be extracted from the cold side and released to the hot side to achieve cooling.
A 10 mm-long by 2 mm-wide by 100 μm-deep silicon micro-pump with an electrostatically actuated snap-in action enables a large displacement volume (1 μL). Each pump chamber element comprises an approximately 11 μm-thick polydimethylsiloxane (PDMS) diaphragm with an embedded thin-film aluminum electrode suspended over a silicon chamber that acts as a 3D shaped counter electrode to enable electrostatic “zipper” operation. Both single-pump prototypes and dual-pump test systems confirmed operation predicted by finite-element simulation.
The need for compliant diaphragms in the micro-pump motivated development of a fabrication, release and transfer process for large area (> 5 cm diameter), ultra-thin (1 μm to 20 μm) PDMS membranes having embedded metal mesh electrodes with critical dimensions down to 2 μm. Experimental bulge testing of the ultra-thin PDMS films indicates that Young’s modulus at 1 μm thickness is increased ten-fold over mm-scale PDMS.
A modified grayscale photolithography and silicon etch process enables more grayscale levels than prior approaches, resulting in a smooth etched 3D silicon surface for the micro-pump counter electrode. The combination of the thin PDMS diaphragm and the spline-shaped silicon substrate significantly reduces electrostatic actuation voltage. A maximum snap-in displacement of 100 μm occurs as low as 100 V (dependent on the exact chamber electrode shape), corresponding to a displacement volume of ~1 μL/stroke. This ultra-large stroke PDMS micro-pump opens a promising route for designing and fabricating large electrostatic stroke actuators and soft electronics devices.
Experimental characterization of P(VDF-TrFE-CFE) terpolymer layers using infrared imaging resulted in measurement of an adiabatic temperature change of 5.2°C at an electric field of 90 V/μm, with the material performance acting stable over a long testing period. A prototype regenerator established the feasibility to assemble terpolymer layers interspersed with microfabricated epoxy spacers in order to form microchannels for the working fluid to pass. These results suggest that this polymer is a promising material for micro-scale cooling application.
Multi-physics finite element simulations indicate cooling performance of a 15°C temperature drop with 5 W/cm2 heat rejection and a coefficient of performance (COP) of 4 at 150 V/μm electric field across the electrocaloric regenerator at a 10 Hz operating frequency. One full prototype micro-cooler, comprising two micro-pumps and the electrocaloric regenerator, was assembled into a 3D-printed package; however, it was not functionally tested due to its fragility. Further micro-cooler system-level development is left as potential future work.