Micro Miniature Refrigerators are small, cryogenic refrigerators that derive their cooling power from the Joule-Thomson expansion of a high-pressure gas. This effect is amplified by using the cooled gas to pre-cool incoming gas in a counter-current heat exchanger. Temperatures down to 70 K can be achieved in devices a little larger than a matchbox in size. Typically, the cold stage is a ceramic pad 14 mm x 10 mm in size supplied with a temperature sensor and resistive heater.
Micro miniature refrigerators provide many advantages in scientific applications:
MMR provides many different micro miniature refrigerators for use in chemistry, physics, and materials science — including ones that are tailored for specific applications...
The drawing illustrates a typical configuration of an MMR micro miniature refrigerator system. In addition to the refrigerator and vacuum chamber, a temperature controller is needed, either manual or programmable; a source of dry nitrogen (99.998% purity); a filter dryer; and vacuum pump capable of maintaining a vacuum of <5 mTorr in the chamber. MMR also provides other accessories that can add to the convenience of operating the system.
Joule-Thomson cooling occurs when a non-ideal gas expands from high to low pressure at constant enthalpy. The effect can be amplified by using the cooled gas to pre-cool the incoming gas in a heat exchanger.
This can be understood by referring to the Temperature vs. Entropy graph for nitrogen. Nitrogen at 100 atm passes down a counter-current heat exchanger from A to B. At B it is allowed to expand through a valve or restricting capillary where it cools by the Joule-Thomson effect. The cold gas then passes back up the exchanger from D to E.
This can be used for refrigeration as illustrated in the Hampson Cycle diagram. A compressor compresses the gas to high pressure, forcing it through the filter-dryer, down the counter-current heat exchanger, then through the JT valve where it expands and cools. Eventually the gas liquefies and collects in the reservoir. The evaporated vapor passes back up the heat exchanger, pre-cooling the incoming gas, then returns to the compressor, and the cycle is repeated.
Miniature JT refrigerators have been used for the past several decades, primarily for the cooling of infrared detectors for night vision. Micro-miniature refrigerators were developed at MMR Technologies. These are miniature coolers further reduced in size by two orders of magnitude. This was made possible by the photolithographic techniques developed at MMR for the fabrication of the micron size channels needed for the tiny heat exchangers for these devices. (See, W.A. Little, Review of Scientific Instruments, 55, 661 (1984)).
The low gas consumption, low cost of operation, and absence of maintenance of the MMR micro miniature refrigerators has made them attractive for incorporation in a wide range of laboratory instruments.