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Design of electrostatic force assisted, piezoelectrically driven silicon chip-based compressor for micro vapor compression refrigeration cooling
Keywords: refrigeration cooling, MEMS, compressors
Applications for highly miniaturized refrigeration cooling systems include thermal management of electronics and sensors [1], an advantage over passive cooling technologies being that refrigeration cooling allows the junction temperature to be lowered below the ambient. Over the last three decades there has been interest in using MEMS technology to build chip- sized refrigeration cooling systems based on the Joule-Thomson (JT) effect. Our research group has over the last decade developed various types of microfabricated JT coldstages and heat exchangers [2-7] and miniaturized compressors [8, 9]. Most recently, we demonstrated an integrated system consisting of a silicon chip-based compressor driving a microcapillary-based JT coldstage [10]. The design, fabrication and standalone characterization of the coldstage is reported in ref [11]. The compressor [10] consisted of a 17mm x 17 mm silicon chip with polyimide diaphragm, polyimide check valves, and was driven by an external commercial piezoelectric (PZT) stack actuator with dimensions 10 mm x 10 mm x 36 mm, and maximum displacement of 40 um at 150V. The total compressor package size was 35 mm x 35 mm x 68 mm. This compressor produced a pressure ratio of 2:1 with flowrate of 2 sccm of N- butane refrigerant, with an experimental adiabatic efficiency of 0.56, resulting in a reduction of the coldstage’s evaporator temperature from 293K to 288 K without any thermal isolation [10]. A major limitation to this compressor’s further miniaturization is assembly-induced dead volume. There have been several studies proposing electrostatically actuated microfabricated diaphragms for making MEMS compressors [12, 13]. However, the predicted pressures were too low for vapor compression refrigeration cycles, and would thus require many such electrostatic compressors be arrayed to produce the required pressures and flow rates. Our approach therefore focused on using commercial low-cost PZT stack actuators because their large forces enabled our single- stage chip-based compressor to produce a high enough pressure, in this case about 151 kPa (22 psig) pressure rise at no flow, and 90 kPa (13 psig) at 2 sccm [10]. However, the PZT actuator’s displacement had to be large enough (40 um) to compensate for assembly- induced micron-level gaps between the diaphragm and substrate. Eliminating these dead volumes would allow the use of shorter PZT stacks, leading to smaller compressor package sizes or higher pressure ratios for the same total size. The proposed design modification is to augment the piezoelectric drive with an electrostatic force. The diaphragm would still be driven primarily by the PZT actuator, but in addition an electrostatic force would also be applied between the diaphragm and substrate at the end of the compression stroke to collapse the residual gaps. Analytical models suggest that for a 1.2 um gap between diaphragm and substrate over an area of 1 mm2, pull-in of the diaphragm may be achieved at about 300 V in the present compressor configuration and at the operating pressure of 203 kPa (2 atm), provided that electrostatic breakdown through the N-butane does not occur.
Li-Anne Liew, senior research associate
University of Colorado at Boulder, and National Institute of Standards and Technology
Boulder, CO

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