I’m on the hunt for a material/gel/epoxy that will transfer the cold temperature of a cryo-box to a test article. The cryo-box is curved and the test article is at the same curvature. I’m looking for something to put between them to make the surface of the test article as close to the surface of the cryo-box in temperature. Got any ideas? The test article will be pulled over the cryo-box, so the material can’t be brittle. Temperature is -320°F.
5 Comments
Robert Chave
August 1, 2008Enhancing conduction through a mechanical make-or-break joint like that to a sample is always a little uncertain. Gmelin et al, at a Max Planck Institute did a survey paper on this, working to explain why data in the literature varied by more than an order of magnitude for essentially the same joints prepared by different organizations. It is a first rate treatment of the problem.
“Thermal Boundary Resistance of Mechanical Contacts Between Solids at Sub-Ambient Temperatures”, E. Gmelin, M. Asen-Palmer, M. Reuther and R.Villar, Review Article, Journal of Physics D: Applied Physics 32 =R19-R43, 1999.
Joint pressure, micro-roughness, flatness, oxidation of the contact surfaces all play a role in cryogenic conduction. Pressure is very significant, and many people use Belleville washers to maintain high mechanical joint force independent of thermal contraction during cool down.
For frequently broken joints, and modest gaps with low conductance performance requirements, vacuum grease is used. For larger gaps, the grease is sometimes loaded with powdered, high purity, oxygen-free copper. For no gap the surfaces are left dry and freshly abraded.
Depending on the material, abrading surfaces with dry carborundum paper will remove oxides, and recreate micro peaks in the surface that come into more intimate contact with the mating surface than would a surface with less micro roughness. See Gmelin et al on this.
For longer duration joints, with high required conductance and infrequence make and break some people use indium gaskets. Janis Research, the cryogenic equipment vendor, used to do this, and they may still.
Thermal joint design tends to be an uncertain business. These are some of the more common methods. Also it is not uncommon to suspect the joint when a larger problem lies else where, for instance in shielding.
Jan Patrick Meier
August 1, 2008Since I do not know the detailed setup, it is difficult to give any useful recommendation.
The most simplest and cheapest solution might be vacuum grease (e.g. Apiezon N). It is most probably not brittle at LN2 temperature but will have such a high viscosity that any film will also break up when sheared between two surfaces. Also a very high shearing force is to be expected. There might be other solutions that would be more useful than using a lubrication film between the surfaces, however.
Louis J. Salerno
August 1, 2008I would try a thin layer of Indium foil, if you aren’t concerned with toxicity of materials.
Indium should not become brittle at those temperatures. Apply the layer of indium foil at room temperature and let it cool down. I’m not really certain when you say that the test article will be pulled. Depending upon the clearance, make certain that the indium foil is not pulled off.
Ed Bonnema, Meyer Tool and Mfg.
August 1, 2008Go to Lake Shore Cryotronics’ website: http://www.lakeshore.com/. Look under Cryogenic Accessories. You have a number of choices that could work. (Note some of these things are available elsewhere for less money, but to Lake Shore’s credit, you can get small quantities and the have a variety all shown together.)
Dr. Michael L. Rappaport
August 1, 2008If the movement of the test article is small or cyclic, the usual solution is to use fine copper wires as a thermal link between the test article and the cryo-box.
Copper braid for grounding is often chosen, but if you really want something flexible, strip the insulation from flexible cable such as Flexistrom from Multi-Contact. It comprises thousands of 70 micron diameter, oxygen-free copper wires.
When the above is not possible,
1. Use He exchange gas, if feasible
2. Use graphite powder, whose thermal conductivity is 25 W/(mK) at 77 K
(https://srmors.nist.gov/reports/view_cert2gif.cfm?certificate=3D8424). The graphite also should help prevent galling. The thermal contact will not be good, but it will be better than vacuum.
3. Increase the emissivity of the curved surfaces of both the cryo-box and the test object.