Yunda Wang

The Development of Polymer-Based Planar Microcryogenic Coolers

In this paper, polymer-based microcryogenic cooler (MCC) cold stages, each with a footprint of 17 mm × 5 mm, have been successfully developed and tested. The MCC cold stage consists of a polyimide heat exchanger (HX) which is monolithically manufactured on a silicon substrate by using a polyimide/copper microelectromechanical systems process and a silicon/glass Joule-Thomson (J-T) valve which is formed with anodic bonding. Specifically, PI-2574 (HD MicroSystems) was used to make the HX in this work. The two parts are assembled together, forming a cooler which uses the J-T cycle. A cooling test with an optimized five-component mixture as the refrigerant was conducted to examine the performance of the cooler. The cooling temperature was able to reach 233 K under an operating pressure ratio of 0.7:0.15 MPa. It is one of the worlds smallest J-T cold stages and the first one fabricated and assembled based on wafer-level processes.

Demonstration of an Integrated Micro Cryogenic Cooler and Miniature Compressor for Cooling to 200 K

Joule-Thompson (J-T) based micro cryogenic coolers (MCCs) are attractive because they can provide the cryogenic temperatures needed for small electronic devices while having a low cost and small volumetric footprint. A compressor is a major part of a cryogenic system, but so far J-T based MCCs have not used miniature or micro scale compressors. This work demonstrates a J-T based MCC coupled with a miniature compressor for cooling to 200 K, using a custom hydrocarbon mixture as refrigerant. The compressor is formed by coupling a miniature piston oscillator built for Stirling coolers with a micromachined check valve assembly. The MCC is formed by glass fibers within a capillary forming a counter flow heat exchanger, and a silicon and glass chip forming a J-T valve. Minimum temperatures of 166 K have been observed in transient, and stable temperatures of 200 ±1 K have been observed for >1 hour. Some insight is given into the unstable performance in terms of intermittent liquid accumulation. The coefficient of performance is analyzed for the system, and it is found that most of the inefficiencies arise at the compressor.Copyright

Wafer-level processing for polymer-based planar micro cryogenic coolers

In this paper, we present the fabrication and testing of a novel polymer-based Joule-Thomson (J-T) micro cryogenic cooler (MCC). Techniques including monolithic fabrication of high pressure polymer channels and 3-D interconnect for fluid channels including a flow resistance are developed to fabricate a planar MCC. The MCC contains a polymer heat exchanger and a silicon/glass J-T valve. A low temperature of 233 K was achieved under an operation pressure ratio of 0.7:0.15 MPa by using a custom designed mixed refrigerant. It is the first demonstration ever reported for J-T MCCs fabricated and assembled based on wafer-level processes.

Silicon Heat Switches for Electrocaloric Cooling

This paper presents two versions of a silicon mechanical heat switch designed for electrocaloric cooling. The first design, which consists of two 10-mm-by-12.8-mm micromachined silicon parts, allows investigation of the performance of a reciprocating solid thermal shunt device. This heat switch has a measured thermal contrast ratio in the range of 34-59. The second design adds self-alignment features that constrain the motion of the switch to facilitate fabrication and integration. The self-aligned heat switch has a thermal contrast ratio >28. It has been successfully operated for >18 000 cycles and employed in an electrocaloric cooler. Design, fabrication, and characterization of both heat switches are reported. [2016-0218]

A Monolithic Polyimide Micro Cryogenic Cooler: Design, Fabrication, and Test

In this paper, we present the design, fabrication, and testing of a monolithic polymer Joule-Thomson microcryogenic cooler (MCC) cold stage. The MCC cold stages were fabricated monolithically on a wafer out of polyimide. The fabrication was based on surface micromachining technologies using electroplated copper as the sacrificial layers and polyimide as the structural material. The process consisted of multilayers of metallization, coating of polyimide, and the patterning on each layer. One of the key techniques enabling this monolithic approach was the development of the wafer-level 3-D interconnect for making high pressure (e.g., 10 atm) polymer fluid microchannels. To evaluate the performance of the MCC, a five-component fluid mixture designed for cooling from 300 to 200 K was used as a refrigerant. The cold tip reached 190 K under a refrigerant pressure ratio of 6.2:1.1 bar. The heat-lift at 200 K was measured to be 5.2 mW.

Micro Cryogenic Coolers With Mixed Refrigerants

A number of small electronic devices benefit from micro-scale low temperature operation. Recently we have developed micro cryogenic coolers (MCCs) using a low-pressure, mixed-refrigerant Joule-Thomson cycle. The cryocoolers utilizes a MEMS-enabled gas compressor coupled to a micro cold stage. Two cold stages have been developed: one which uses a fiber-enabled heat exchanger assembled to a micro-machined throttling valve, and another which uses a MEMS-based heat exchanger. A microcompressor has been developed which uses MEMS-based check valves coupled to a membrane, which is actuated with a mechanically amplified piezoelectric amplifier. The compressor measures a volume 15 mL, can generate a pressure ratio of 6:1 and a maximum flow-rate of 60 standard mL/min. The complete cryocooler has reached low temperatures of 177 K, although temperature instability has been an issue, due to 2-phase flow through the micro-channels. This paper will cover the development and testing of the micro cryogenic cooler, as well as an analysis of the micro channel flow. A proper understanding of the micro-channel flow allows us to design refrigerant mixtures to improve the cooling power, and modify the cooler to eliminate temperature instabilities.Copyright

A Polyimide Micro Heat Exchanger With a Suspended 3D Parallel Channel Structure for Cryogenic Application

A heat exchanger (HX) plays an important role in many cryogenic systems. This paper demonstrates a fully integrated fabrication process to make a novel monolithic polyimide micro heat exchanger. This heat exchanger consists of two parallel rectangular polyimide channels stacked on top of each other with a typical dimension of 15 mm × 4 mm × 90 μm. The stacked channels were suspended on top of a silicon substrate as one of the ends anchored. The fabrication is based on surface micro-machining technology using electroplated copper as the sacrificial layers and polyimide as the structural material. Preliminary testing results demonstrated that the polyimide HX can sustain a cryogenic temperature lower than 77 K and is able to hold a pressure larger than 1 MPa. The pressure drop across the channel was found to increase parabolically with the flow-rate through it, as assessed with standard nitrogen at room temperature.© 2011 ASME