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In cryogenics, liquid nitrogen and liquid helium are often used as coolants. Other than their low boiling point of 77K and 4.2K respectively, what properties make them suitable as coolants? Are there any other possible candidates around the same temperature range?

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Liquid nitrogen is used because nitrogen is extremely abundant on earth. Nitrogen makes up approximately 78% of the atmosphere by volume. Hence, liquid nitrogen is rather easy to make (and consequently cheap). I've heard for instance that Fermilab buys liquid nitrogen for cheaper than what you pay for water.

Liquid helium is useful for things that must go to much lower temperatures than 77K. It remains liquid (though it does change into a superfluid) even at absolute zero at atmospheric pressure, while essentially everything else is a solid at those temperatures. Solids aren't useful as coolants for obvious reasons, so liquid helium really is the only option.

There are other cold liquids that could be used, like liquid oxygen (BP at 90K) or hydrogen (BP at 23K), but these don't offer many advantages over nitrogen and helium. They are more reactive as well, which may or may not be a concern depending on what it is you are cooling. For most things nitrogen is sufficient, and for most things it doesn't work for helium does, so that's why they're almost universal.

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Yep. Nitrogen is cheap and slighlty cooler than oxygen or argon, helium goes down forever, argon (which is the third most abundant component of the atmosphere) has a narrow range where it is liquid, and the safety guys will throttle you if you propose using more than a little LO2 or LH2 anywhere you don't absolutely need it. That about covers it. –  dmckee Jun 20 '12 at 23:42

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