Lithographically Micromachined Si/Glass Heat Exchangers for Joule-Thomson Coolers.

Abstract

Micromachined Joule-Thomson (J-T) coolers have applications ranging from cryosurgery to cooling infrared detectors. With the absence of cold moving parts, the J-T coolers can be implemented with simple structures that are suitable for silicon/glass microfabrication. The investigation proposed in this thesis focuses on the development of micromachined Si/glass heat exchangers used in the J-T coolers that operate at 200-225K when the gas pressure is 1-2MPa. The heat exchangers must maintain good stream-to-stream heat conductance between the high- and low-pressure streams while restricting stream-wise conduction to achieve a high effectiveness. Two heat exchangers were designed, fabricated and tested. The first, a planar design, uses rows of high-conductivity silicon fins bonded onto a 100µm thick low-conductivity glass base plate. It was fabricated using a five-mask process including Si/glass/Si anodic bonding, two-step DRIE, and HF glass etching, etc. The second, a perforated-plate design, uses numerous silicon plates alternated with glass spacers. It was fabricated using a four-mask process including KOH on (110) silicon wafers, HF glass etching and anodic bonding. Platinum resistance temperature detectors were integrated into the heat exchanger for in-situ temperature sensing. Whereas the performance of the planar heat exchanger was limited by its ability to accommodate a pressure differential across the base plate, the perforated-plate heat exchangers demonstrated a high effectiveness (0.912) in at 237-252K in effectiveness tests and good robustness at high pressures (1MPa) in J-T self-cooling tests. The temperature distribution along the heat exchanger was measured by integrated resistance temperature detectors with sensitivities of 0.26-0.30%/K at 205-296K. A J-T system using the perforated-plate heat exchanger achieved 218.7K at steady state and 200.3K in a transient state. The system provided 200mW cooling power at 228K and 1W at 239K with an estimated parasitic heat load of 300-500mW. Finally, a flow-controlled J-T system using a perforated-plate heat exchanger and a piezoelectric microvalve was demonstrated. By modulating the flow, the microvalve could vary the cooling temperature by 5-8K around the operating points, which were 254.5K at 430kPa pressure difference in steady state, and 234K at 710kPa in transient state, without an added heat load.