Polishing copper, and the purity of the copper surface

Factual Questions
1

I’m trying to figure out how to maintain a pure shiny copper surface.

I have a big block of copper, several hundred pounds. It’s for laboratory experiments in which its flat top surface, a square somewhat over 1 foot in size, conducts the heat out of soft conformable objects while we precisely measure temperatures in those soft objects, and the intent is for the copper to be as good a contact heat sink as possible. This is all around room temperature. Therefore, I want a very smooth shiny pure copper surface, as nearly as I can maintain it. The copper is well documented commercially pure elemental copper, “alloy C10100”, at least 99.99% Cu, with the worst impurity measured as Ag (silver) at 0.0013% and all others roughly an order of magnitude below that. I can’t have any laquer or other coating on the copper; it has to be metal right at the surface, or as close as possible. In use there will typically be a fine coating of glycerine on the copper as a heat transfer medium (like the heat sink compound you’d use on a microprocessor heat sink, only without the powder component). The soft conformable objects will be drawn against the copper surface with a moderately hard vacuum so that the glycerine escapes from any would-be gaps and only fills the microscopic surface texture to displace air, which is much more insulative. The intent is to minimize the thermal contact resistance in accordance with mainstream surface roughness and compliance models such as the model developed in Chakravarti Madhusudana’s mechanical engineering specialty textbook “Thermal Contact Conductance”.

I had a local metal polishing shop finish the top surface. Polishing shops seem oriented toward a visual result, and it was a pretty good mirror when completed. However, it sat in the shop with the polished surface facing upward unprotected, and a metal polishing shop might be the worst environment in the world for airborn abrasive grit, so it was visibly dusty when we retrieved it. Transporting it generated a number of fingerprints on the polished face near the edges, and for some reason the human species is apparently completely unable to resist touching anything shiny, so the fingerprint population has grown to include some big fat ones in the critical area around the center of the polished face. In my growing experience with contract metal polishing, not only do these mishaps interfere with the perfection of a surface, but polishing operations typically leave substances on the surface, such as whatever binder holds abrasive particles together in the polishing compounds used. A visual mirror finish isn’t the same thing as a pure surface.

After locating the block in its final laboratory setting, I used some nonabrasive laboratory wipers and 91% isopropyl alcohol to wipe it off with a rolling, lifting motion to avoid dragging abrasive around on its surface, so that the salty and oily fingerprints did not sit there chewing away over the Labor Day holiday weekend. Now its shine is more uniform and there are no visible fingerprints or dust layer, though I think I can see the mirror finish darkening with tarnish. I wish I had a workable maintenance procedure that would keep it close to a mathematically perfect surface directly on the copper.

But how? Bear in mind that this is practically an immobile object so I can’t dip it. Perhaps it would be best to have some purely chemical means of removing copper oxide and other copper compounds, without abrasives. It would be pretty difficult to use coarse abrasives and step through grades to the finest, as a routine measure. I got some inexpensive test sheets of copper and have been experimenting with Tarnex and isopropanol and glycerine, as a nonabrasive minimalist chemical assortment, but so far am not seeing the bright clean surface I’m hoping for.

What do I need to understand about pure copper surfaces and polishing and finishing to do well with this?

Thanks!!

2

Copper oxidizes in the air and much faster if there is moisture present. I’d think there is almost certainly moisture present in your lab’s air. The typical greenish hue copper takes on is usually Cu[sub]4[/sub]SO[sub]4/sub[sub]6[/sub] or
Cu[sub]2[/sub]CO[sub]3/sub[sub]3[/sub]

It’s usually cleaned with acetic acid or citric acid, but I’ve only ever seen that done mixed with salt or baking soda, which I’d guess you don’t want to do.

I’ve heard that acetone by itself may work

3

Why does the surface need to be bare copper? I’d think an electroless nickel coating would have negligible effect on thermal conduction and adequately protect the surface from oxidation.

4

I’ve read about acetic acid plus salt. Some references say the salt is there for abrasion, and some say it’s there for chemical action so a surplus of salt in solid particles is not necessary. I could be perfectly happy with either acid with some salt in solution. Do you know if surplus particles are called for?

What’s the deal with the baking soda? Is it a chemical or an abrasive in this context? I hadn’t heard of that.

The acetone tip is very, very intriguing. I’ll have to try that on one of my test sheets!

5

This might be good, especially if it can be mirror like. I’m not sure how smooth nickel plating can get. The nickel plating (both electroplating and electroless plating) that I’ve had done so far has not been very smooth but we haven’t been doing it on smooth substrates. I’ve also thought about gold plating, which is pretty unreactive and might not oxidize, or if it does, only does a very little. My experience with gold on copper is that if it gets hot the copper diffuses through the gold and then blackens the surface, but I think this may take forever at room temperature. Or perhaps we should consider rhodium plating, which would be harder, if that proves useful – but I’ve never used that before.

Actually, doing this with a piece of solid pure gold would be ideal…

6

There are many options depending on how much you are willing to experiment :

  1. Coat the surface with Tin - very common for electrical applications but tin has a lower thermal conductivity but again it’s a thin film

  2. Eliminate moisture (humidity) and possibly CO2 from the environment you work in. CO2 can be removed by mol-sieves or scrubbers.

  3. Provide cathodic protection with Zinc

  4. Use 100% ethyl alcohol to clean surface. It gets oxidized to aldehyde.

7

Another possibility is to apply a somewhat thicker nickel coating and then polish the nickel surface.

8

More research – nickel electroplating deposits nickel, electroless nickel apparently deposits a nickel phosphorus alloy. Either can be made mirror smooth on a smooth substrate. Apparently, copper plating fills scratches, so perhaps adding some electroplated copper to my slab, then plating it with nickel or whatever, could be even smoother. The more I think about it, the thinness of plating means the conductivity of the plating metal will not matter.

9

We get a cupricsulfate percipitant on our copper at work along with finger prints and oxyditation which are all implrtant to clean off in place especially before pictures of the stills. We’ve got other contaminants as well and so the typical CIP proceudre is 15 min of 20% caustic spray followed by 5 min 120F water rinse followed by 15 min 20% citric acid spry followed by room temp water spray. I typically use R/O water for the final rinse to eliminate spots.

10

You might also look at electropolishing your part. Copper electropolishes very well, and the process preferentially removes asperities that stand proud.

11

If I were working with something whose purity were that essential, I’d put it in a glove box and keep it in a nitrogen or argon atmosphere.

12

Second that. Since glycerin (which is hygroscopic) is already being used to minimize boundary layer thermal resistance, the polished metal maybe just for cosmetic reasons.

If reducing the thermal resistance of the boundary is the main concern then flatness of the surface matters more than its polish. Similarly using a different heat transfer cement/grease will give better dividends.

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13

Can you cover the entire block with something even somewhat airtight? If you can, I’m wondering if doing that, then purging the oxygen by pumping in something like nitrogen or argon (or some other inert gas you have around the lab) would protect it…and also keep people from touching it.

14

I think that would be a good idea, even with nickel coating.

15

How about occasionally polishing it like this:
[ul][li]Start with a thick piece of glass (extremely flat)[/li][li]Wrap the glass with 2000-grit (or finer) abrasive paper[/li][li]Lightly wet-sand the copper surface[/li][li]Flood with water to remove residue[/li][li]Dry with isopropanol and a soft lint-free cloth[/li][/ul]

16

I’m no expert on machining, but if you need your surface to be really flat you should not be randomly polishing it, instead use a more deliberate method.

17

Exclude air from the surface by coating it with an inert oil (silicone oil?) in between uses? The oil can be washed off with a solvent; silicone oil is itself a good thermal conductor so a thin film left behind may not be a big deal anyway.

18

Could you set something nonreactive on top of the block, that’s removed only while actively using it?

I pictured a sheet of glass, a bit bigger than the block so it can be picked up without anyone touching the copper.

How big a block are we talkin’ about here, anyways?

19

I’ve read some interesting articles about electrodeposition of diamond-like carbon films. Diamond is an excellent thermal conductor, and a diamond film (assuming it’s good enough at the start) should be fairly hardwearing.

20

I’d really like a protocol like you describe. I’m looking for something I can repeat frequently. Since it’s frequent, there should be little new corrosion to remove. I think the surface should settle into a sort of equilibrium, getting the same treatment again and again, with very little removal. However, I just don’t know much about how fine an abrasive paper I can get, and what its result would look like. I do have a field trip to a source of very fine abrasives soon, so perhaps that will enlighten me. Do you know how fine a ready-made abrasive paper is available, and how glossy it would make copper?

Yeah, I know. Earlier in this project I decided to use only conformable samples to press against the block, so the flatness does not matter in this case. In other projects I have really struggled to find a shop that can give flatness and surface finish at the same time, especially in highly conductive metals like copper, silver, and aluminum. I haven’t even asked about gold, because it’s so expensive in large sizes.

I like this! But isn’t oxygen pretty soluble in silicone? If I have a metal foil or a metallized plastic film held against the surface with an oil in between that would be better…

Yes, I like this too. The block is a 12.5" square, 3" thick.