A heat exchanger made largely from pieces of single-crystal silicon has been designed for use as a recuperator in a reverse-Brayton-cycle cryocooler. Its design is optimized for transferring heat between counterflows of neon at a mass flow rate of 0.7 g/s, with an inlet temperature of 300 K and pressure of 2.21 atm (0.224 MPa) on the warm side and an inlet temperature of 60 K and pressure of 1.3 atm (0.13 MPa) on the cold side.

The Thermal Conductivity of Silicon exceeds that of copper in the lower part of the intended range of operating temperatures.

Ordinarily, one might be inclined to make a heat exchanger from a highly thermally conductive metallike copper. However, single-crystal silicon offers advantages over copper in this particular application. One advantage pertains to thermal conductivity. The thermal conductivity of single-crystal silicon ranges from less than that of copper at room temperature to greater than that of copper at cryogenic temperatures. Detailed heat-transfer calculations performed with known conductivity-vs.-temperature data (see figure) have shown that on balance, the superior low-temperature thermal conductivity of silicon prevails in the specific intended application. This makes it possible to obtain better heat-transfer performance in a heat exchanger of given size, shape, and complexity; or, alternatively, to obtain equivalent heat-transfer performance in a smaller and/or less complex heat exchanger.

Another advantage of silicon is that established technology for micromachining of single-crystal silicon by photolithography, etching, and related techniques is commercially available. Thus, silicon components with multiple narrow and/or short flow passages can readily be fabricated to obtain flow and thermal-conduction geometries that are defined with high precision to optimize heat-transfer performance.

Yet another advantage of silicon is that its mass density is only 2.33 g/cm³, whereas that of copper is 8.92 g/cm³. As a result, a silicon heat exchanger weighs about 74 percent less than does an identically sized and shaped copper heat exchanger for the intended operating temperatures.

This work was done by Thomas J. Jasinski and William E. Nutt of Creare, Inc., for NASA's Jet Propulsion Laboratory. No further documentation is available.

Inquiries concerning rights for the commercial use of this invention should be addressed to

the Patent Counsel, NASA Resident Office-JPL; (818) 354-5179.

Refer to NPO-30021.

NASA Tech Briefs Magazine

This article first appeared in the March, 1998 issue of NASA Tech Briefs Magazine.

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