## Making an aluminum cold finger with LN2

Hi, I've looked everywhere for some advice on this, I hope someone can help. We are making a cooling system for cooling thin films, and part of that setup is a liquid nitrogen-cooled cold finger. This cold finger will be inside an aluminum tube, and will move closer and farther away from the thin film, which is mounted on the tube.

We could buy a cold finger, but they are all Pyrex or glass, which could scratch, and they mostly have inconvenient shapes. If we could make our own out of aluminum (because it needs to be nonmagnetic and conduct heat), that should work great.

Does anybody have design ideas for making this? The basic idea is just to circulate the liquid nitrogen in and out of the cold finger, but we're not sure how to seal the aluminum and have the in/out tubes fit properly. Thanks!
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 Recognitions: Gold Member Science Advisor Why would Pyrex scratch where aluminium wouldn't?
 Well, Pyrex scratching on aluminum could break the Pyrex, right? Whereas it doesn't really matter if an inner aluminum rod gets scratched from the outer one. I could be completely wrong, but our professor doesn't like the idea of having it made of Pyrex for some reason...

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## Making an aluminum cold finger with LN2

I suspect your professor is concerned about the Pyrex getting extremely brittle and possibly cracking. I've never heard of Pyrex being used at cryogenic temperature and I don't think I've ever seen material properties at low temp, so I would agree with your professor that Pyrex probably isn't a good material.

Is your cold finger going to look like the one on the left in this picture?

You could certainly use aluminum which has very good cryogenic properties, but I would suggest using copper tube/pipe. Copper also remains very ductile at low temperature and there are lots of fittings that can be used to make the assembly.
 Yeah, it will look like the one on the left. We don't want to use copper, because we are doing this under a magnetic field, and aluminum will interfere with the results less. The problem we're having is how to make those "nozzle" things that the tubes (with liquid nitrogen) attach to. If we could even just find a place to buy aluminum pipe with those on it, that would also work great. But it's not looking like we can find that.

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 Quote by mjjaques Yeah, it will look like the one on the left. We don't want to use copper, because we are doing this under a magnetic field, and aluminum will interfere with the results less.
Both copper and aluminum are non-magnetic and used in MRI machines for example where they have very strong magnetic fields. I don't understand why copper wouldn't work for you, but certainly you can use aluminum.

 Quote by mjjaques The problem we're having is how to make those "nozzle" things that the tubes (with liquid nitrogen) attach to. If we could even just find a place to buy aluminum pipe with those on it, that would also work great. But it's not looking like we can find that.
If you're using pipe, you can use NPT threaded connections. I'd recommend using tube however, along with compression fittings such as Swagelok or equivalent which are commonly used in cryogenic applications.
 Well, we would want to use pipe for the actual cold finger because it's solid, and then tubing for the liquid nitrogen to go in and out. What about a hose barb like this? http://www.westmarine.com/webapp/wcs...2#.T-sPwrU7X4s Would it work to screw one side into the pipe, with the tubing attached to the barbed side?
 Copper is routinely used on the sample holders of superconducting magnets with fields of 10T and more and temperatures down to 2K (below liquide He). Al and stainless steel are commonly used for baths and so on. Stainless has very bad thermal conductivity, so it is always used to mount things to the (warm) outside. Don't use Cu or Al if you plan to use pulsed magnetic fields, as you will get lots of eddy currents. I agree with Goest that your best bet is probably to use Swagelok parts. On the LN2 supply side you need some kind of flow control, e.g. a needle valve. If you can open/close that in a controlled way, that is best way to regulate the temperature, together with a heating element on the Cu sample holder. For the "nozzle" you probably need some kind of capillary coiled up over some length to create an impedance. On the exit side you probably want a small vacuum pump.

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