Two approaches to micromachining si heat exchanger for Joule-Thomson cryosurgical probes

This paper describes results from two types of micromachined recuperative heat exchangers intended for Joule-Thomson (J-T) cryosurgical probes, which require high stream-to-stream thermal conductance while restricting parasitic stream-wise (axial) conduction. In design A, rows of fins composed of high conductivity silicon are bonded onto a 100 mum thick base plate composed of low conductivity Pyrex glass. This planar device has a footprint of 6times1.5 cm² and 2.5 mm thickness, and is fabricated using a 5-mask process. In design B, numerous high-conductivity silicon plates alternating with low-conductivity Pyrex spacers are stacked together. This has a footprint of 1times1cm², a length of 1.4 cm, and is fabricated using a 3-mask process. Preliminary experiments show that the primary performance constraint for design A is imposed by the compromise between mechanical robustness and transverse conductance of the thin glass base plate that separates the high pressure and low pressure streams. Design B enhances the robustness of the device and can sustain higher pressure.