Fabrication, assembly, and characterization of a hollow-core fiber-based micro cryogenic cooler

One of the world smallest Joule-Thomson (JT) micro cryogenic cooler (MCC) aimed at cooling low power-consumption electronics or high temperature superconductors, is demonstrated. The MCC is composed of a hollow-core fiber-based micro heat exchanger (HX) and silicon micro machined JT expansion valve. The heat exchanger is 25 mm long and 0.61 mm in outer diameter, with six hollow-core fibers of 125 mum O.D./76 mum I.D. bundled internally; the cold head is built with a stack of 2 mm square silicon chip.;To fabricate and assemble the MCC, a controlled etching process has been established to fabricate a multi-layer silicon structure for a heat exchanger-to-cold head assembly. Metals are selectively coated on the silicon structure and ends of fibers to facilitate fluxless solder jointing and bonding process to avoid adhesives clogging in the micro channels. One of the main features of the assembly was the 700 nm gap between the silicon structure and a glass cover; the gap was the expansion valve for the cooler. Another feature was the hermetic coupling for the high and low pressure (e.g. 16:1 atm ratio) micro-fluidic channels using 6 hollow fibers enclosed by a glass capillary tube. The capillary tube was covered by a segmental metal coating for minimizing radiation and conduction heat leakage from the room temperature surrounding to the cooler. The heat exchanger-to-cold head assembly was connected to an existing macro-scaled compressor through a coupler, which consisted of three micro-machined silicon structures bonded by a self-patterned SU8 film.;To achieve the temperature at which we aimed, and to provide required refrigeration power, the MCC is well thermally isolated to minimize heat loads from the environment. Thermal isolation analysis, designs, and measurements of MCCs are investigated. In a 77 K cold head under a 300 K shielding temperature, the glass capillary based test vehicle, the hollow-core fiber-based MCC with segmental low emissivity metal coating, and the hollow-core fiber-based MCC without segmental coating enclosed by an enhanced low emissivity shielding are tested to achieve a heat leak of 5.1 mW, 9.6 mW, and 3.8 mW, respectively. By cooling the enhanced low emissivity shielding to 240 K from 300 K, the MCC without metal coating reached a heat leak of 1.9 mW, and a thermal isolation up to 85,000 K/W is achieved. The enhanced low emissivity shielding has been proven to provide a better thermal isolation by reducing the radiation around the cold head and minimize the conduction by avoiding metal coatings on the heat exchanger.;Two different optimized mixed refrigerants (three-component mixture and five-component mixture) are designed to enhance the efficiency in the MCC, to minimize required flow and pressure ratio. With 1.6:0.087 MPa pressure ratio and 38 mumol/s flow, the cold head achieved a 112 K temperature by using three-component mixture. With 1.4:0.07 MPa and 11 imol/s flow, a 140 K stable temperature is achieved by using five-component mixture. A 76 K temperature in transient can also be observed in the five-component mixture cooling. In this thesis, the concept, designs, fabrication, assembly and test results are reported and discussed in detail.