Material Selection, Insulator, Cryo Temp

[dfly9891]

Hi,

I've got an applications where I need an insulating washer/disk in order to insulate stainless from stainless at 50 degrees Kelvin. I need help in selecting a material to do the job. I.e. low thermal conductivity, durable enough at 50 K to take a compressive static load of approx 40 lbf. Performance, rather than cost, is is the most important characteristic in this case. Any Ideas??

Parameters: OD of washer ~2" Thickness ~1/4"(I can vary on the thickness if necessary but not on the OD). OD of footprint of 40lb load (compression spring) ~1.5".




Thanks in Advance.

 

[mgb_phys]

PTFE is pretty good at 77k

 

[dfly9891]

Thanks. Someone on the engineering forum said the same thing. What about polycarbonates though? They can have lower thermal conductivities but I'm not sure of their performance at cryo temps. Thoughts?

 

[Q_Goest]

Micarta is commonly used for inner vessel supports on cryogenic tanks. Also, crygenic G10 is another excellent choice. Both have relatively low thermal conductivity.

Take a look for properties here:
http://cryogenics.nist.gov/MPropsMAY/material%20properties.htm

 

[dfly9891]

Thanks Q_Goest. This helps a lot. I'll look up the properties and costs. Will let you know...

 

[dfly9891]

Thanks. I'm going with PTFE.

 

[mugaliens]

Uh, have you considered wool? Pretty cheap, and if I'm not mistaken, it http://en.wikipedia.org/wiki/Wool_insulation" .

I've used simple sheepskin for apps involving temps to below -100 deg F. Get's a bit crisp, but it still works well!

 

[dfly9891]

Wool? Nice. Now that's thinking outside the box. I like people like you.

I'm going to be putting the insulator in -370 F conditions so I'm sticking with PTFE for now. There is a seal area with critical dimensions so I'm using PCTFE (NeoFlon) for that as PCTFE is good to -400F while PTFE is good to about -325F

 

[Q_Goest]

Hi dfly. Neoflon and PTFE are commonly used for applications down to -452F. Valve seats used in LHe service for example, are commonly made from Neoflon.

Of the two, Neoflon is your better structural material due to the crosslinking. As I'd mentioned at Eng-tips, Teflon needs to be reinforced when used as a structural material.

Note also that simply finding the lowest thermal conductivity isn't necessarily going to give you the best material for structural supports. Teflon has a much lower strength than Neoflon, so the Neoflon support can be made with a smaller cross section. Similarly, Micarta and G10 have an even higher strength, so supports made from that material can be made with an even smaller cross section.

The best material takes into consideration what can be done by minimizing not just the thermal conductivity of the material, but limiting the cross sectional area to minimize the overall heat transfer.