Date of Award

Spring 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Y.C. Lee

Second Advisor

Ray Radebaugh

Third Advisor

Victor Bright

Fourth Advisor

Scott Bunch

Fifth Advisor

James Booth

Abstract

This dissertation studies the design and fabrication of polymer-based planar, Joule-Thomson (J-T) micro cryogenic coolers (MCCs). The polyimide layers are used for fluid channels defined by copper sacrificial layers. The first planar MCC consists of a micro machined polyimide counter-flow heat exchanger and a silicon/glass anodic-bonded coldhead with a J-T expansion valve. Main features of the MCC demonstrated are: 1) the J-T valve with a size of 1.2 mm by 1.7 mm and a 3 µm gap ; 2) the high pressure fluid channel with a size of 12 mm by 2 mm and a 20 µm gap ; 3) the low pressure fluid channel with a size of 12m by 2mm and 10 µm gap ; 4) the DRIE-etched opening for the fluid coupling between the heat exchanger and a compressor with a size of 1.6mm by 300 µm and a through-wafer depth of 550 µm; 5) the staggered posts with a diameter of 60 µm inside the fluid channels for withstanding high pressure 6) the O-ring like trenches with a depth of 5 µm for the fluid coupling between the heat exchanger and the substrate. This planar MCC is functional with the coldhead temperatures reaching 233K; however, it suffers a leakage problem at the soldered-interface between the heat exchanger and the coldhead. This assembly problem is solved by an improved wafer-level processing for a monolithic polyimde MCC. The new cold stage including the heat exchanger and the J-T valve is fabricated using copper-polyimide processes, monolithically on a wafer . Improved features are: 1) the polymer J-R valve with a size of 1.2 mm by 1.4 mm and a 3.2 µm gap ; 2) the polyimide tethers to support the suspended heat exchanger; 3) the 3-dimensional fluid interconnects in different layers. This monolithic polyimide MCC does not encounter the mechanical leakage problem since the soldered-interface is removed. It also enhances the manufacturability and scalability of the MCC through the wafer-level processing. The coldhead temperatures improve from 233 K to 190 K with a flow rate reduced from more than 260 sccm to about 60 sccm. The cryogenic demonstration is accomplished by using a custom-designed 5 components refrigerant (8% methane, 46% ethane, 14% propane, 4% butane and 26% pentane) optimized by scientists in NIST. During the demonstration studies, an accurate model to design a polymer J-T valve is identified as a critical need. Therefore, this thesis experimentally measures flow characteristics of different polymer J-T valves in order to establish the design model. Specifically, an apparatus is built to measure pressure drop vs. flow rate corresponding to pure nitrogen and a gas mixture consisting of methane 34%, 22% ethane, 20%, ethylene 12% isobutane and 12% isopentane. A valve resistance prediction model is established with a homogeneous assumption for a multiphase flow assisted by the calculation of fluid properties using NIST-REFPRO. The model is proven accurate with a mean deviation < 10% for the cases studies at temperatures of 295 K, 265 K and252 K.

Share

COinS