Haishan Cao

Micromachined cryogenic cooler for cooling electronic devices down to 30 K

Cryogenic temperatures are required for improving the performance of electronic devices and for operating superconducting sensors and circuits. The broad implementation of cooling these devices has long been constrained by the availability of reliable and low cost cryocoolers. After the successful development of single-stage micromachined coolers able to cool to 100 K, we now present a micromachined two-stage microcooler that cools down to 30 K from an ambient temperature of 295 K. The first stage of the microcooler operates at about 94 K with nitrogen gas and pre-cools the second stage operating with hydrogen gas. The microcooler is made from just three glass wafers and operates with modest high-pressure gases and without moving parts facilitating high yield fabrication of these microcoolers. We have successfully cooled a YBCO film through its superconducting transition state to demonstrate a load on the microcooler at cryogenic temperatures. This work could expedite the application of superconducting and electronic sensors and detectors among others in medical and space applications

Clogging in micromachined Joule-Thomson coolers: Mechanism and preventive measures

Micromachined Joule-Thomson coolers can be used for cooling small electronic devices. However, a critical issue for long-term operation of these microcoolers is the clogging caused by the deposition of water that is present as impurity in the working fluid. We present a model that describes the deposition process considering diffusion and kinetics of water molecules. In addition, the deposition and sublimation process was imaged, and the experimental observation fits well to the modeling predictions. By changing the temperature profile along the microcooler, the operating time of the microcooler under test at 105 K extends from 11 to 52 h.

Cooling a low noise amplifier with a micromachined cryogenic cooler

The sensitivity of antenna systems increases with increasing active area, but decreases at higher noise figure of the low-noise amplifier (LNA). Cooling the LNA locally results in significant improvement in the gain and in lowering the noise figure of the LNA. Micromachined Joule-Thomson (JT) coolers can provide a cryogenic environment to the LNA. They are attractive because they have no cold moving parts and can be scaled down to match the size and the power consumption of LNAs. The performance of a LNA mounted on a JT microcooler with dimensions of 60.0 × 9.5 × 0.72 mm(3) is reported in this paper. The microcooler is operated with nitrogen gas and the cold-end temperature is controlled at 115 K. The measured net cooling power of the microcooler is about 43 mW when the LNA is not operating. The power dissipation of the LNA is 26 mW, with a supply voltage of 2 V. At room temperature the noise figure of the LNA is 0.83 dB and the gain lies between 17.9 and 13.1 dB, in the frequency range of 0.65 and 1.05 GHz. Upon cooling to 115 K, the noise figure drops to 0.50 dB and the increase in gain varies in the range of 0.6-1.5 dB.

Characterization of a two-stage 30 K Joule-Thomson microcooler

Micromachined cryocoolers are attractive tools for cooling electronic chips and devices to cryogenic temperatures. A two-stage 30 K microcooler operating with nitrogen and hydrogen gas is fabricated using micromachining technology. The nitrogen and hydrogen stages cool down to about 94 and 30 K, respectively, using Joule–Thomson expansion in a restriction with a height of 1.10 μm. The nitrogen stage is typically operated between 1.1 bar at the low-pressure side and 85.1 bar at the high-pressure side. The hydrogen stage has a low pressure of 5.7 bar, whereas the high pressure is varied between 45.5 and 60.4 bar. In changing the pressure settings, the cooling power can more or less be exchanged between the two stages. These typically range from 21 to 84 mW at 95 K at the nitrogen stage, corresponding to 30 to 5 mW at 31–32 K at the hydrogen stage. This paper discusses the characterization of this two-stage microcooler. Experimental results on cool down and cooling power are compared to dynamic modeling predictions

Long-life micro vacuum chamber for a micromachined cryogenic cooler

Micromachined cryogenic coolers can be used for cooling small electronic devices to improve their performance. However, for reaching cryogenic temperatures, they require a very good thermal insulation from the warm environment. This is established by a vacuum space that for adequate insulation has to be maintained at a pressure of 0.01 Pa or lower. In this paper, the challenge of maintaining a vacuum chamber with a volume of 3.6 × 10−5 m3 and an inner wall area of 8.1 × 10−3 m2 at a pressure no higher than 0.01 Pa for five years is theoretically analyzed. The possible sources of gas, the mechanisms by which these gases enter the vacuum space and their effects on the pressure in the vacuum chamber are discussed. In a long-duration experiment with four stainless steel chambers of the above dimensions and equipped with a chemical getter, the vacuum pressures were monitored for a period of two years. In that period, the measured pressure increase stayed within 0.01 Pa. This study can be used to guide the design of long-lifetime micro vacuum chambers that operate without continuous mechanical pumping.

Joule-Thomson microcooling developments at University of Twente

The development of Joule-Thomson microcoolers has been an on-going and successful research project at the University of Twente for many years. The aim of the research is to develop small and fully integrated cryogenic cooling systems for cooling small electronic devices such as pre-amplifiers and infrared sensors, in order to improve their performance. In the foregoing years, we have successfully developed single-stage microcoolers (typically cooling to 100 K) and two-stage microcoolers (typically 30 K) using standard micromachining technologies. In the present paper, we emphatically discuss recent developments in the Twente microcooling project among which microcoolers with a double expansion of the high pressure flow (reducing the 100 K to 83 K operating temperature), microcoolers operating with hydrocarbon gas mixtures, and microcoolers with an ejector, the three new developments aiming at lower cold end temperatures, lower operating pressure ratios and/or higher efficiency. Besides, utilization of microcoolers for cooling electronics and clogging phenomenon in microcoolers will also be introduced.