Polymer-based Micro Cryogenic Coolers

摘要

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 u3c 10% for the cases studies at temperatures of 295 K, 265 K and252 K.