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I am running a peltier element, and on the cold side an aluminum heat sink to effectively spread out the cool surface to the maximum possible surface area.

I just bought this new thermal paste: Coolermaster Nano, and whenever it touches the cold side of the peltier (less than 10 Fahrenheit) it freezes into a rock, while thermal paste such as Arctic MX-4 does not.

I had a hunch that this freezing affect was causing problem as temperatures using the Coolermaster Nano thermal paste were far far higher than the Arctic MX-4 (16 Fahrenheit vs 0). So, I ended up removing all thermal paste and saw that even with no thermal paste between cold side and heat sink, the temperature was lower than with using Coolermaster Nano, at 10 Fahrenheit.

I was wondering if someone could explain this to me: are some thermal pastes not designed or suited for low temperatures? Is there something I should look out for when purchasing a thermal paste (carbon based?)

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Yes, of course. A thermal paste intended for CPUs is rarely used below 25C. You need to read their datasheet or search for cryogenic thermal compounds.

It is probably cheaper to select some of the consumer grade ones through their datasheet and test the best for your application. Arctic Silver 5, for example, claims to be stable down to -50C.

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    \$\begingroup\$ "up to -50C"? Or down to -50C? \$\endgroup\$ – Harry Svensson Feb 20 '18 at 1:44
  • \$\begingroup\$ @HarrySvensson Svensson, yes the Coolermaster Nano had the description Nano Diamond particles maintains through a wide range of temperatures (-50 to 150℃). I'm not sure why it hardened like a rock; however, I am going to stick with what I know works. Thoughts? \$\endgroup\$ – Omar Sumadi Feb 20 '18 at 2:44
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    \$\begingroup\$ I wonder if they're just claiming it won't be damaged down to -50℃, and also relying on the fact that CPUs at -50℃ don't need cooling until they've warmed up enough to melt the paste. \$\endgroup\$ – immibis Feb 20 '18 at 3:38
  • \$\begingroup\$ @OmarSumadi My thought's are that you are not understanding my comment. My comment was for gstorto who is saying that it works "up to -50C" => meaning that it will work to something even lower than -50C, and that -50C is the highest temperature. Obviously it's a typo from gstorto, or arctic silver. I Just wanted to point out the misuse of chosen words because it was funny to read. \$\endgroup\$ – Harry Svensson Feb 20 '18 at 5:28
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    \$\begingroup\$ @HarrySvensson I meant down to. \$\endgroup\$ – gstorto Feb 21 '18 at 7:31
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Part of your problem is the expansion and contraction of aluminum, which can be a lot per linear yard/meter. I also had to find a bonding compound to bind an aluminum (5052) bracket to a stainless steel enclosure. Every epoxy and glue I tried failed within 48 hours of heating and freezing twice.

Aluminum contracts a lot when frozen. The solution was to use RTV type compounds, specifically RTV type 262, which had no ammonia to corrode the metals or electronics.

It is only a suggestion that you consider 'rubbery' type compounds that do not freeze per se, but instead shrink with the metal, so it maintains a grip. For Arctic Silver 5 to work, you may need a more rigid metal, but hard steels can be brittle under extreme cold. Copper is an alternative, with Beryllium copper an expensive alternative, but still cheaper than SS.

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