Hi all, I was going to ask a question about thermal contact resistance, But I figured I should search first and I found this,
Looks like silicon grease is the way to go. I'm wondering if it'll work as well at 77K (LN2) I've got a jar of Dow Corning vacuum grease.
George H.
Vacuum grease is for lubrication, I'd stick to DOW 340 as a thermal filler. It most likely will be fine at 77k. No clues on the DOW site tho, just 'STABLE' at low temps???
Cheers
It's not silicon (the element) but silicone (the family of compounds).
My guess is that it would freeze solid - which won't make much difference t o its thermal conductivity - and get sprung out of intimate contact with th e surfaces it is supposed to couple (which probably will).
Methylcyclohexane has an absurdly low freezing point (146.8 K) which is sti ll well above 77K but if you stirred in a bit of that if might keep the sil icone squidgy at lower temperatures. I've used a methylcyclohexane slush ba th, but I never tried to use it to soften frozen silicone.
What I have used as a thermal contact material - which might well work at 7
7K - is graphite cloth. Unlike silicone grease, it is electrically conduct ive, and it isn't going to freeze (granting that it's - technically speakin g - already "frozen" at room temperature).The graphite pads come at the end of the data sheet ...
Which one?
-- Bill Sloman, Sydney
Silicone greases can outgass and leave drecky coatings on stuff. We're not allowed to use any silicone greases or adhesives on stuff that we sell into the semiconductor industry.
Probably any grease, including vacuum grease, will be almost as good thermally as filled silicone grease.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
"John Larkin"
** Has to get damn hot for than to happen - like 100C or more.Heatsink compound uses filler ( ie Zinc oxide) to improve conductivity and reduce the tendency to migrate.
In many cases I have seen, the filler remains trapped ( and effective) even if the silicone leaves due to evaporation.
** No way.See above.
.... Phil
"John Larkin = Bullshit Artist "
** They help enormously in real practice.But the main advantage occurs in the longer term and for heatsinks than run rather hot - a situation where plain silicone grease slowly departs the scene.
Axle grease is not sold for nor suitable for hot semiconductors.
... Phil
Toothpaste and Vegemite are strong contenders, though.
Matt Roberds
Hi all, I was going to ask a question about thermal contact resistance, But I figured I should search first and I found this,
Looks like silicon grease is the way to go. I'm wondering if it'll work as well at 77K (LN2) I've got a jar of Dow Corning vacuum grease.
George H.
About 2000, when quantum cascade lasers needed to be at LN2 temperatures to operate with continuous current, the standard mounting method was to use a thin layer of Apiezon N vacuum grease between the copper sub-mount and the heatsink in contact with the LN2 reservoir.
Bret Cannon
I was making a connection using regular white thermal paste with added diamond dust. It was working but I have no data.
Greg
If your area is at 77K is it also in a vacuum?
There are several inert materials with reasonably good thermal conductivity that would behave better than the greases in a vacuum.
-- ******************************************************************** Paul E. Bennett IEng MIET..... Forth based HIDECS Consultancy............. Mob: +44 (0)7811-639972 Tel: +44 (0)1235-510979 Going Forth Safely ..... EBA. www.electric-boat-association.org.uk.. ********************************************************************
I have read that silicone grease/oil that migrates onto relay contacts that arc can be converted by the arcing into some glassy insulator that eventually prevents the relay contacts from working. I read it somewhere more reputable but it is also in the Wikipedia article:
Chris
So this is an ancient jar of Dow Corning High Vacuum grease. (no other marking on the jar... does say usefull to pressures of 10^-6 mm) I grad school we used to to make contact from the back side of semi-conductor samples to the metal sample holder. We'd then put a dab of rubber cement on top side to hold things in place.. then cool.. seemed to work.
George H.
to its thermal conductivity - and get sprung out of intimate contact with the surfaces it is supposed to couple (which probably will).
till well above 77K but if you stirred in a bit of that if might keep the s ilicone squidgy at lower temperatures. I've used a methylcyclohexane slush bath, but I never tried to use it to soften frozen silicone.
77K - is graphite cloth. Unlike silicone grease, it is electrically condu ctive, and it isn't going to freeze (granting that it's - technically speak ing - already "frozen" at room temperature). 1372946Hmm is the sheet compliant? I really just want the thinnest coating to tak e up the voids.
Just says high vacuum grease.. it might be ~20-30 years old?
George h.
Fun! thanks.
George h.
Thanks that's what I'm thinking about. Silicone Vacuum grease.
George H.
This isn't the stuff I used back in 1993, but the graphite cloth we used then was very flexible.
In the data sheet, the graphite thicknesses offered range from 0.13mm through o.16mm, 0.2mm. 0.25mm and 0.5mm, and with densities of 1gm/cc and 1.65gm/cc, and a range of z-axis thermal conductivity.
Solid graphite is a bit denser - 2.09 to 2.23 gm/cc
It isn't obvious what's being varied to get the different thermal conductivities - perhaps fibre size and the degree of layering.
Farnell stocks the higher density stuff in the thinner grades - 0.13 and 0.16mm - and the lower density stuff for the thicker grades - 0.25mm and 0.5mm.
-- Bill Sloman, Sydney
This depends greatly on the delta T you can tolerate. Silicone is used because it is thermodynamically a liquid, due to the freedom of movement around the C-Si-C bonds. This helps heat transfer by allowing internal movement. Much of that advantage goes away at 77, as you are below it's glass transition temperature. But it won't hurt, and tight clamping (with the grease) will buy you the most benefit, if you can do it.
Note that in vacuum, clamped metal surfaces have incredibly bad thermal conduction. I was amazed when I did some tests a few hears ago. I clamped a temperature sensor to a liquid-cooled plate, and turned the cooling on in air, and got a time constant of a few seconds. Then, pumped a vacuum and repeated, and the time constant was over 10 MINUTES! Apparently, the air in the interface is the thermal conductor. Pump out the air, no heat flow. Due to the outgassing, I used Bergquist sil-pad material, which works fine without major outgassing. I'm sure the silicone stuff would work, but it continues to wick over surfaces and eventually makes a big mess.
Jon
It's worst with hard surfaces. The contact area is
A ~ (clamping force)/(yield stress)
which can be pretty small.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
The best insulators at low temperature are glasses (liquids) so a supercooled liquid isn't as good as (for instance) a metal. We always used indium for thermal contact (and sealing) at low tempertures; it's nice and squishy at room temperature.
Silicone (not silicon!) grease is OK, of course, when we couldn't use a pressurized indium gasket, or solder, or weld. I thought, though, that best thermal conductivity was with silicone grease filled with some solid material (like the white 'heatsink' goo used to be, before such wonder=goops as Arctic Silver).
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