A friend is making a spacer to go between a tab on a device and a heat sink. The total thickness will be about 0.25 inch (6 mm?). Of course thermal grease will be used at both matings.
I've been asked what kind of aluminum.
What (in descending order of preference) are the aluminum types that would be good raw material for this?
6061 aluminum alloy - 166 w/m-k, if I correctly translated from
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6061 aluminum alloy - 180 w/m-k
(There are heat-treated versions of 6061 including T4 and T6, which have mildly increased electrical resistivity, and I suspect also slightly decreased thermal conductivity.)
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If this spacer has to conduct several 10's of watts or more and every degree matters, I say make it out of copper or silver.
Otherwise, extruded aluminum, poured aluminum and 6061 are close enough to each other for "best aluminum", and I expect ordinary aluminum plate and bar stock (to be machined into shape) to be similar.
All aluminum alloys have approximately the same conductivity (thermal and electrical). Pure aluminum (1199 is the purest) is maybe 20% higher conductivity than 356, 6061, 7075, etc.
Addendum: all metallic alloys have higher resistivity (thermal and electrical) than either pure metal. For instance, sterling silver is more resistive than either silver or copper. I suppose aluminum alloys are fairly surprising in that regard, as they are still fairly conductive.
You could thermally epoxy one side if possible. Fine mating surfaces and compression during the face to face cure. Then build up a small bead of the epoxy around that fixed end, since those finely mated faces won't have much epoxy between them... at all. Silver filled IC chip epoxy is best for that, but many others are also suitable... if you decide to "fix" one side. Other wise, managing the two (four) faces that will be mating is possible, just a tad more difficult sometimes.
Aluminum alloys are poorer thermal conductors than pure Al is. Pure is like 210 W/m-K,and 6061-T6 is like 167 W/m-K.
Looks like the closer to pure, the better, but all searches nowadays point one toward Copper and alloys thereof.
copper is better. aluminium is cheaper. pure is better than alloy.
the less gap you get at each join the better, so a flat mirror finish on all surfaces is good.
Metal is much much better than heatsink paste so if you can solder or weld the spacer to the heatsink (with the apropriate solder and tools) so much the better.
'Ya know, if you can clamp the assembly quite tightly (or even apply some persuation as a part of assembly), you could use pure aluminum to fill the space, no grease. It's soft stuff, about like lead. Oh, and use springy washers too, I suppose...
The shiny things are copper heat spreaders, nickel plated. I did one DSRD-based [1] HV pulser that was built on a gold-plated copper block, water cooled; it looked great. You can get a peek of it here:
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