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Copper and/or gold for infrared reflectors
Current question and answers:November 28, 2020 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread
Q. Hello to all. We have a project that needs a large (400 X 400 mm square shape) infrared reflector. This reflector is situated under (at 10 mm distance) five 500W ruby halogens / 2500 W total power. We think that we can make this reflector from highly polished copper sheet due to its reflectance proprieties on infrared. The only problem that we have is that the copper will oxidize rapidly over time and loose the reflectance proprieties. Please advise me about coatings options that are affordable and withstand high temperature. Thank you in advance, DanDan Badea
- Lugo /Spain
A. Hi Dan. I am having problems helping with your posting because 'high temperature' is vague and I am not a heat transfer expert able to estimate the temperature of this reflector. Further, I'm not understanding quite what 'affordable' means. Is this a one of a kind apparatus for scientific experiments, or a mass-produced infrared heater for paint curing and similar ovens?
The ideal coating would probably be nickel plating followed by gold plating per the discussion above, which would not be prohibitive for an important instrument, but probably would be prohibitive for a mass-produced reflector.
A dip in a proprietary copper preservative based on sodium benzotriazole would be the first step for a cheaper reflector, followed by a clear coating. Although we can't suggest a particular brand or source for the clear coat (why?), I would probably investigate the UV/radiation cured coatings used on auto headlights.
Luck & Regards,
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Aloha -- an idea worth spreading
November 28, 2020
Q. Thank you kindly Ted for your reply. High temperature can be about 500-600 °C (gold halogen reflectors work under 900 °C I think... because gold withstand about this temperature...) if this reflector is a 10 mm away from the halogens, I think that is no more than 500-600 °C (my ruby halogens do not have gold plated reflector).
The nickel / gold plating I know it, but it has same problem... needs a transparent high temperature coating to protect the gold. A direct copper coating, that have more or less same transmittance of gold. I think that is cheaper than nickel / gold coating followed by a transparent coating.
The project is for heat a electronic board, I don't know if I can say mass produced. I think that can be similar with paint heat curing system. But the infrared curing systems or have gold reflector directly on the halogens tube or have aluminium reflector, but aluminium is not as good as copper in infrared spectrum. I discovered a commercial "Cerakote" that says it withstands high temperature of 800 °C. Somebody know this? Can it be used for my purpose? Thank you.Dan Badea [returning]
- Lugo Spain
November 29, 2020
A. A few years ago we did yellow, gold color anodizing on stainless steel. Not all alloy types can be color anodized, but 420, 430, 304 did well. Anodized layer stands high temperature test. I recall testing it up to 500 °C.
November 30, 2020
Kindly thank you Anna for your attention, the infrared high power reflectors is a tricky business; for example, 0.25 microns of gold plated over a quartz plate is transparent. It is transparent for a "VIS" spectrum (visible spectrum) and for infrared spectrum it's more transparent.
Gold reflectors for infrared spectrum are adequate for more than 1 micron, above this 1 micron the gold are opaque / 100% reflective (95 to 99% in theory) and has comportment as a real infrared reflector. The technology on quartz / high temperature glass is Heraeus "real gold 24K glass paint". for a copper / aluminium reflector the solutions (classical) are Nickel / Gold plating above 1 micron. but this is a high price option. The lowest price can be a copper reflector (copper is more or less the same reflectance as gold in infrared) (please do not correct again reflectance to transmittance please)... but I need a protection layer against oxidation ... the only one that I find on internet is this... eastwood.com/exoarmour.html. "Sodium benzotriazole" as Ted suggested ... well, do not withstand the temperature the melting point at 100 °C and boiling point at 350 °C. Another thank you for your attention.
- Lugo Spain
Ed. note: "(please do not correct again reflectance to transmittance please)".
Sorry Dan, but what you see is what you posted. If you're implying that the word 'tranmittance' in your posting of Nov. 28 should be 'reflectance' we'll be pleased to edit it, and remove your note and this one to reduce confusion :-)
Previous closely related Q&A's starting in:2004
Q. How can I make a machine part with a mirror-like surface of gold, that is stable for thousands of hours at a temperature of 400 °C?
I need a part that behaves as an infrared reflector that will operate in a clean oven at up to around 400 °C, in an air atmosphere. The surface does not need to be optically flat and perfect, but does need to be very shiny, and not absorb or emit or scatter thermal infrared around 8 or 9 micrometers in wavelength. The purpose of the part's finish is to redirect infrared heat flux rather than accept or donate any.
This part will be a foot or so in size. I will probably make a dozen parts a year.
Gold should be a good surface because it is highly reflective in the IR and it does not oxidise. I think making the parts from 50 pound blocks of solid gold would be ideal, but the people paying for them disagree.
Can I gold plate some substrate metal? Do I need intermediate layers of other metals? I hoped for gold plated copper but other posts describe diffusion and oxidation problems discoloring the finish at this high temperature. Substrate metal could be copper, aluminum, even steel or silver or nickel or iron or others. I have mild preference for high thermal conductivity and perhaps coefficient of thermal expansion to match either steel or aluminum, but could design around that. Sorry, but I'm inexperienced in plating and metallurgy - I'm a physicist.
Thanks for any help!Chris Bailey
Industrial process design - Elkton, Maryland, USA
A. Maybe you can use platinum (1.) and gold (2.) plated titanium!
Good luck!Goran Budija
- Zagreb , Croatia
My company is the supplier to NIST of the Infrared Reflectance Standard. The coating is a proprietary gold plating process called Laser Gold. It is a pure hard gold that can be plated thickly enough to withstand migration of thin deposits into the substrates at these temperatures.David Epner
- NYC, New York
A. In the past we have used a Cobalt Hardened Gold for these applications.
As to Brightness, it is dependent on the brightness of the under-plate. With the temperature requirements you mentioned you should use a bright nickel under plate as a diffusion barrier.
When we were involved in mirrors for B-52's the final gold plating was plated to 10 microns and polished to the required reflectivity.Lou Hirbour
- Anaheim, California, USA
A. Long time ago, twenty years or so, I have seen a gold paint, at RR-DERBY-ENGLAND, painting aircraft engine exhaust, to disarray IR.
aircraft maintenance - Al Ain, UAE