Laboratory of Integrated Bio Medical Micro/Nanotechnology & Applications

Design, Fabrication, and Heat Flow Analysis of a Mesoscopic Pulse Tube for Integrated Micro-Refrigeration Systems:

Ninad Shinde1, Rashid Bashir2, Eckhard Groll1, George Chiu1
1Ray W. Herrick Laboratories, School of Mechanical Engineering, Purdue University
2School of Electrical and Computer Engineering, Purdue University

Key words: Pulse tube, Surface Heat Pumping, MEMS Sensors, Quartz Micro channels

The phenomenon of surface heat pumping has been widely used for refrigeration in macro-scale devices such as pulse-tubes and thermo-acoustic devices. Pulse tube refrigerators are typically used to lift heat at cryogenic temperatures. The advantage of a pulse tube as a no moving part device makes it amenable to miniaturization at dimensional scales where areal effects, such as friction and viscosity, dominate.

The fabrication and characterization of a device to investigate the effect of surface heat pumping in mesoscopic glass channels is described in this paper (see Figure 1). The device is operationally parallel to a basic pulse tube refrigerator. However, taking necessary measures concerning the inlet conditions of the refrigerant (which could be either air or Helium) precludes the necessity for a regenerator. The miniature channels are etched in glass and are closed at one end with miniature heat exchangers incorporated at the ends. The need to measure pressure pulsations and temperature gradients along the length of a closed channel provides opportunity for the integration of Silicon MEMS based sensors (see Figure 2) as well as process control into a truly integrated system.

An innovative approach to designing and fabricating bulk micromachined piezoresistive pressure sensors for pressures in the MPa range with an effort to improve sensitivity is described. The pressure sensor is modeled as a thin square diaphragm clamped at the edges. The pressures to be measured are in the MPa range. While higher sensitivity for pressure measurements is desired, the maximum pressure of interest must not exceed the burst pressure of the sensor diaphragms. Stress concentration at the diaphragm edges can results in lower burst pressure. This increased stress results from the intersection of the <111> and <100> planes due to anisotropic etching of silicon. By performing a short isotropic etch the sharp edges can be rounded in order to reduce the stress concentration factor and thus permit higher burst pressures and hence higher sensitivities. The miniature heat exchangers are diaphragms fabricated on the silicon die using the same process as the sensor diaphragms, though they have a much larger surface area. Heat transfer to ambient air occurs through these diaphragms while the other areas of the die are thermally isolated by an oxide film. Thin film Pt temperature sensors are patterned adjacent to the pressure sensors. The etched glass channels are anodically bonded to the silicon dies so that the resulting device has a semi-circular profile. The sensors are arrayed along the length of the tube to monitor temperature and pressure. Fabrication process flow and device heat pumping characteristics will be presented. By pulsing the open end of the device with He or air, a temperature gradient is set up along the length of the tube with the closed end being the hot end. A device of this sort could also be used to study compression and expansion of gas on a microscale. These devices can also be used for biomedical sample storage, micro-sample refrigeration, cooling in integrated circuits, and infrared sensors.

References:
[1] Ninad Shinde, Rashid Bashir, Eckhard Groll, George Chiu, "Design, Fabrication, and Heat Flow Analysis of a Mesoscopic Pulse Tube for Integrated Micro-Refrigeration Systems", 2000 International Mechanical Engineering Congress and Exposition, American Society of Mechanical Engineers, November 5th –10th, 2000, Orlando, Florida.