Cryo Material Selection, Insulation, 50 Kelvin

[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".

 

[Compositepro]

Teflon (solid).

 

[dfly9891]

Someone from the physics forum also recommended Teflon. Teflon has a coeff. of therm. conductivity of .2 W/mK. There are polycarbonates out there with lower therm. cond. but i'm not sure how polycarbs would perform at cryo temps.

 

[itascot]

Try Kel-F material (part of Telfon family) good properties down to around 34° Kelvin

 

[jedward]

Polycarbonate (Lexan) is an excellent thermal insulator and is used at even lower temperatures.
Very difficult to find a better insulator of any material.

 

[dfly9891]

Thanks itascot, I'll look up Kel-F and check out cost, and thermal conductivity.

Thanks jedward, wow, I would not have thought.... and lexan is relatively inexpensive. I'll look up the specs.

Before I read these last two posts, I was homing in on PTFE, Virgin Electrical Grade (ASTM D 1710). Operating temp range -328 deg F to 500 deg F (73.1K to 533K). I'll re-think my inks based on this new info. Thanks. I'll let ya know...

 

[dfly9891]

jedward, what grade of lexan do you recommend?

 

[Compositepro]

In my experience Lexan is prone to cracking due to solvent/chemical exposure and clamping stresses.

 

[jedward]

All reference I have seen do not specify a grade of Lexan

 

[desertfox]

hi dlfly9891

Have a look at this link:-

http://cryogenics.nist.gov/Papers/Cryo_Materials.pdf

personally I think Teflon is possibly your best choice.

desertfox

 

[itascot]

Hope this link can help on Kel-f properties (fact is that Teflon or part of its family seems to be the answer).

http://www.boedeker.com/pctfe_p.htm

This material is what we use as Safety Relief Valve seats, dealing with LN2 containing vessels.
Itascot

 

[dfly9891]

Thanks to you all for your time and your comments/knowledge/sharing. I'm going with PTFE, virgin electrical grade. McMaster Carr PN: 8596K173

 

[dfly9891]

sorry, I meant PN: 2905K18. Good temp range. Check it out.

 

[iainuts]

Note that virgin grade Teflon may cold flow a bit.

 

[dfly9891]

Please define cold flow.

 

[iainuts]

Teflon is not unlike a very thick grease. It's more like silly putty than plastic. Although it seems relatively hard, if it is exposed to a load, it will slowly deform. Metals in comparison, must be exposed to a stress in excess of their yield strength before they permenantly deform. Virgin Teflon has no specific yield strength, and will deform very gradually when under stress. So if you want to use Teflon as a structural material to support a load, it should have some kind of filler in it, such as 25% glass fiber, in order to help stop it from deforming under load.

 

[dfly9891]

Hmmm.... that's good to know. Thanks alot. 25% filler is common. I'll got with that to be on the safe side.

 

[dfly9891]

I'm going with PCTFE, Neoflon.

It was called Kel-F when it was owned by 3M. Today it is owned by Daikin and is called Neoflon.

Due to some crosslinking, Neoflon is more rigid with higher yield strength. Also, PBY in Ontario Ca is the best place to get it. Compression molded is ideal over other forms (i.e. extrude.) Heat causes the material to degrade faster.

 

[itascot]

A pleasure to be of assistance.

Ciao,

 

[dfly9891]

By the way, I leared a few things about PCTFE. The extruded stuff creeps quite a bit while the compression molded stuff does not.

After talking with someone at PBY plastics, they said it has something to do with crystalinity. The more slowly they cool it down, the more crystaline it becomes during curing. I would guess that more crystaline = more rigid = better dimensional stability = less creep. NO?