Thermally conductive elastic material?

[Richard Rasker]

Hello,

I'm designing on a thermal control system based on Peltier elements to
create a presettable, highly stable temperature for testing newly developed
semiconductor/optical devices.

The Peltier element is placed between two aluminium surfaces; the top
surface is a disc, slightly larger than the Peltier element, and has a
precision temperature transducer attached to it. The bottom surface is
larger, and is bolted to a big aluminium heatsink. The whole structure is
encased in a very rigid foam, with the aluminium top disc cemented flush
with the surrounding foam surface.

The problem is that different sorts of objects are routinely clamped or
pressed on to the top disc for testing; this has the risk that the Peltier
element inside is exposed to an unevenly distributed mechanical stress,
causing it to break. This has already happened on several occasions, and
with a price of some $50 each (not counting building the "thermo-table" in
the first place), this problem needs solving.

I've though about a springy metal construction which presses the Peltier
element gently against the top disc, but I can't come up with something
both small and thermally conductive enough.

The only other thing I can think of, is to use some sort of compressible,
termally conductive material which will even out the mechanical force
dstribution. I tried the normal silicone pads, but these still have a high
thermal resistance when not very firmly compressed, which plays merry hell
with my temperature control loop, amnong other things.

Does anyone know of any more foamy or soft-rubbery substances out there with
good thermal conductivity? Typically, I need some ~0.2 mm (1/100 of an
inch) compressibility without a huge increase in force.

Thanks in advance for any tips, best regards,

Richard Rasker

 

[Richard Rasker]

....

Sounds like the Sil pad material that is used for electrcially
insulating components from the heat sink might work.

But, I'm confused as to why there is so much movement. I do something
similar and have stainless steel screws connecting one aluminum block
to the other. That keeps everything ridgid. One one side the screws
pass through plastic shoulder washers, and then some wave spring
washers to keep the compression roughly constant.

We haven't got much space for extra screws and other hardware (the top disc
is a mere 20 mm across), and the surface must also be liquid-proof, so
we're a bit reluctant to drill holes in it. But I think we'll try and
construct one this way anyway -- because our current solution isn't robust
enough by far.

Also thanks for the Sil-Pad reference; it these remind me of those very
thick thermal pads used in laptop computers, so I'll search a bit more for
those as well.

Richard Rasker

 

[Rich Grise]

We haven't got much space for extra screws and other hardware (the top
disc is a mere 20 mm across), and the surface must also be liquid-proof,
so we're a bit reluctant to drill holes in it. But I think we'll try and
construct one this way anyway -- because our current solution isn't robust
enough by far.

Drill and tap blind holes in the back of the top disk, and secure it
with screws from the heat sink side.

Also thanks for the Sil-Pad reference;

Sil-Pads are crap.

Hope This Helps!
Rich

 

[nospam]

The only other thing I can think of, is to use some sort of compressible,
termally conductive material which will even out the mechanical force
dstribution. I tried the normal silicone pads,

A large range silicone material is available with various thicknesses and
hardness.

If you don't care about electrical isolation there are also graphite based
materials with better thermal conductivity but they will be at the hard end
of the scale and I'm not sure on how elastic their deformation is.

Look at the products from www.bergquistcompany.com and
http://www.tglobal.com.tw/en/

There are also various silicone sealant and potting compounds some of which
have optimised thermal conductivity and obviously no initial compression.
Look at www.acc-silicones.com

 

[Richard Rasker]

Back in 1993 we used graphite cloth as a compressible thermal contact
medium between our Peltier junction and the surrounding metal-work and
heatsink. It performed a good deal better than silicone pads, which
really aren't much good - we were never able to get them to deliver
the sort of low thermal resistances that the manufacturers claimed

Farnell now stocks something similar made by T-Global - their T62
tape. We got ours from Warth. TT-Global's 0.5mm T62 tape might be
thick enough for your job

http://www.farnell.com/datasheets/74197.pdf

OK, great, I'll order some of their stuff first thing in the morning.
I think I'll put this to the test, as well as the construction suggested by
George. I hope this material does the trick, because it would save us some
mechanical and electrical redesigning.

Thanks once again to all who took the time to reply,

Best regards,

Richard Rasker

 

[Jasen Betts]

Hello,

I'm designing on a thermal control system based on Peltier elements to
create a presettable, highly stable temperature for testing newly developed
semiconductor/optical devices.

The Peltier element is placed between two aluminium surfaces; the top
surface is a disc, slightly larger than the Peltier element, and has a
precision temperature transducer attached to it. The bottom surface is
larger, and is bolted to a big aluminium heatsink. The whole structure is
encased in a very rigid foam, with the aluminium top disc cemented flush
with the surrounding foam surface.

The problem is that different sorts of objects are routinely clamped or
pressed on to the top disc for testing; this has the risk that the Peltier
element inside is exposed to an unevenly distributed mechanical stress,
causing it to break. This has already happened on several occasions, and
with a price of some $50 each (not counting building the "thermo-table" in
the first place), this problem needs solving.

hydronics.