All-Micromachined Joule-Thomson Cold Stage

A micro Joule-Thomson cold stage was designed, built and tested, as part of the micro cooling research project at the University of Twente. The cold stage consists of a stack of three glass wafers. In the top wafer, the high-pressure line is etched as a rectangular channel with supporting pillars. The high-pressure line ends in a flow restriction and an evaporator volume that crosses the center wafer into the bottom wafer. The bottom wafer contains the low-pressure line, again etched as a rectangular channel containing supporting pillars, thus forming a counter-flow heat exchanger. A design aimed at a net cooling power of 10 mW at 96 K and operating with nitrogen as the working fluid was optimized based on the minimization of entropy production. The optimum cold finger measures 28 mm x 2.2 mm x 0.8 mm (max. dimensions). It should be able to generate a net cooling power of 10 mW at 96 K at a nitrogen flow of 1 mg/s at a high pressure of 80 bar and a low pressure of 6 bar. A batch of 14 prototype coolers were made for 8 different designs, including a design of the theoretical optimum. Liquid nitrogen is collected in the evaporator, and since the low pressure is 6 bar, the temperature should be 96 K. However, due to some thermal resistance in the thermocouple attached to the cold tip, a temperature of 105 K was achieved. A net cooling power of 5 mW was measured. In the paper, the design of the coolers will be discussed along with experimental results.