Cooling of Bulk Material

Cooling of bulk material has been achieved with a solid-state refrigerator. At the heart of the NIST-Boulder device is a tiny sandwich-shaped diode whose layers are successively a normal metal, an insulator, and a superconductor. The stack has the effect of pulling the hottest electrons out of the normal-metal layer. This no-moving-parts refrigerator is not the first to achieve 100 mK temperatures but it is the first to do so with technologically useful cooling powers.

The NIST micro-fridge chilled a cube of germanium about 250 microns on a side and with a mass of 80 micrograms. This sounds like a small speck of matter, but it was enormous compared to the size of the refrigerating junctions (see figure at www.aip.org/png ). Indeed, the ratio of the volume of the cube to the volume of the junctions is 11,000. This is equivalent to a refrigerator the size of a person chilling something the size of the Statue of Liberty. In preliminary tests, the cube was cooled from 320 mK down to 240 mK.

Future improvements should lower the base temperature to near 100 mK. According to NIST physicist Joel Ullom (ullom@boulder.nist.gov), their refrigerator works best at temperatures below 1 K, so it won't be used to cool foods. But it will be very useful for chilling circuitry on chips and maybe samples as large as the centimeter size. (Clark et al., Applied Physics Letters, upcoming article)