Does there exists a sort of shrink tubing with good thermal conductivity?

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exchanger.

Hi John, Jan is doing something where the thermocouple is not welded to the piece of metal. The thermal voltage in generated right at the junction. But if the junction is not in good thermal contact with the DUT then heat can be conducted via the leads to the air.... and thus the junction is at a different temperature than the DUT. The actual temperature of the junction is a balance between the thermal conductivity to the DUT and that of the leads to the air. 100' doesn't matter it's the few inches right near the junction.

No matter what the thermometer it's always a good practice to think about heat leaks via the leads. (Essential if you are doing low temperature stuff in vacuum. A lessen learned years ago when I had to redo most of the electrical wiring on a probe I was making.) Low temperture stuff can be a PITA. A day to cool things down, a day to see that something isn't working, A day to warm up. Fix and repeat. The weeks pass pretty fast that way...

George H.

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exchanger.

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That's great Fred. We make little thermistor probes, (with 'big' thermistor heads), part of the lead hole is filled with an aluminum tube. Tube OD a bit bigger than thermistor head. We fill the gaps with thermal grease. Any suggestions for the glue?

(silly question?) Metals are such great thermal conductors are super conductors also super-thermal conductors?

George H.

I think it's an awesome question! I think there's something called Helium-3, which is a heat superconductor - it doesn't boil, the atoms just leave the surface since there are no hot spots.

Cheers! Rich

ning.

hanger.

I don't think thermal conductivity is an issue. It will stabilize in a few seconds.

I often just dab enamel on it, then slip on shrink leaving the tip exposed.

greg

Hi Rich, Yeah super fluid helium is made with He4. Very cool. I did research back in grad school with a glass helium dewar. When we pumped the helium bath to go to lower temperatures I would always watch the super fluid transistion. Lots of bubbling and then... it suddenly stops.

Well it turns out that when something goes superconducting the thermal conductivity drops! The electrons in the superconducting state don't carry any entropy (heat) and you are left with thermal conductivity only from those electrons *not* in the super conducting state. (As well as the phonon contribution.)

George H.

Deep question, actually. Superconductors have zero resistivity because the electrons don't scatter from thermal (phonon) fluctuations, thus do not add energy to the heat in the conducting medium. That decoupling makes the electrons completely ineffective in moving heat inside the material, and superconductors are thus TERRIBLE conductors of heat. Most superconducting wiring, thus, has to be filaments of superconductor in a metal matrix, or the first quench-while-carrying-current would melt spots. The superconductor transition has been used as a thermal 'switch', as well.

AFAIUI, the heat capacity also drops dramatically below the critical temperature (at in the case of Nb, it apparently increases right at the critical temperature then drops exponentially towards zero as the temperature is lowered).

Yup that's what the data show in Kittel "Intro to Solid State Phys" second edition. (Mind you this is an early version of Kittel and a bit long in the tooth.)

George H.

By the third law of thermodynamics, we know that all materials' heat capacities are zero at zero temperature.

George Herold a écrit :

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opening.

exchanger.

We used high thermal conductivity epoxy: STYCAST 2850 FT

Don't know if it was required or not (didn't do the calc) but we already had it so it didn't cost anything.