A piezoelectrically actuated cryogenic microvalve with integrated sensors.
Book Description: Future space missions require cooling of large optical structures and cryogenic storage systems. A distributed network of cooling elements, each including actively controlled valves, can provide location specific temperature control. This thesis presents piezoelectrically actuated microvalves for modulating refrigerant flow in a cryogenic cooling system. The first-generation valve consists of a micromachined die fabricated from silicon and glass wafers, a piezoelectric stack actuator, and Macor ceramic encapsulation, having overall dimensions of 1x1x1 cm3. The silicon valve seat is suspended by a crab-leg flexure formation and attached to the piezoelectric stack actuator, which moves in an out-of-plane motion against the glass substrate. To overcome modest displacement provided by piezoelectric actuation, a perimeter augmentation scheme for the valve seat has been implemented to increase flow area and consequently provide high flow modulation. The valve can modulate the flow from 980 mL/min with the valve fully open (0 V) to 0 mL/min with 60 V actuation voltage at a pressure difference of 55 kPa at room temperature. The operation of the valve has been validated at temperatures over 80-380 K, and at pressures up to 130 kPa. The valve has a response time of less than 1 msec and has an operation bandwidth up to 820 Hz. It is used in the Joule-Thomson self-cooling test with a micromachined recuperative heat exchanger, and a temperature decrease of as much as 42 K is presented. For the second-generation design, a similar architecture is used with integrated sensors for inlet pressure and temperature. Implementation of a membrane type suspension substantially decreased the dead volume inside the valve. At room temperature, a normally-open valve achieved gas flow modulation from 200 mL/min to 0 mL/min with 0 V to 40 V actuation. Sensors are strategically positioned at the upstream end of the valve so that the information can be used for closed-loop control. Sensitivities of 356 ppm/kPa for the piezoresistive pressure sensor and 0.29%/K for the platinum resistance temperature detector (RTD) are reported. These valves are compatible with liquids, thus the liquid modulation, using the valve for drug delivery application, is briefly discussed.
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