Sciencemadness Discussion Board - Coating glass with silver to a specifc thickness?

Coating glass with silver to a specifc thickness?

Athiril - 8-5-2013 at 17:28

So I was reading here about different coating methods of silver http://www.sciencemadness.org/talk/viewthread.php?tid=1422

What interested me was the silver nitrate + dextrose method.

I was wondering if it is possible to calculate coating thickness.

For example. I have glass (a lens) I would like to coat with a 35nm layer of silver. Is it possible to achieve this by having a specific amount of silver nitrate in solution calculated from the surface area to be coated and reacting it out until completion?

Have calculated I need 5.136972157x10^-7 grams of Silver Nitrate (100%, would have to adjust for purity) per mm^2 of area to be coated for 35nm thickness.. assuming that only what I want to be coated gets coated, and that absolutely all the silver nitrate is used to and coated the material with no loss.

[Edited on 9-5-2013 by Athiril]

[Edited on 9-5-2013 by Athiril]

Endimion17 - 9-5-2013 at 09:11

Are you trying to chemically deposit a metallic surface on your lens, so that the very surface is reflective? That won't be possible.
The reflectivity is at the border between adhering surface and the metal. The part exposed to the solution (and air, after the drying) is dirty yellow, matte surface.

blogfast25 - 9-5-2013 at 10:48

Quote: Originally posted by Endimion17

Are you trying to chemically deposit a metallic surface on your lens, so that the very surface is reflective? That won't be possible.
The reflectivity is at the border between adhering surface and the metal. .

Not sure what you're assuming here. Seems to me he wants to produce a reflective lens (or refractive mirror, if you will). That should be possible. And thickness control should work but it could take a few runs to get it right.

You could maybe control thickness by controlling reaction time, assuming you have a means of measuring it.

[Edited on 9-5-2013 by blogfast25]

watson.fawkes - 9-5-2013 at 13:08

Quote: Originally posted by Athiril

SWhat interested me was the silver nitrate + dextrose method.

I was wondering if it is possible to calculate coating thickness. [...] a specific amount of silver nitrate in solution calculated from the surface area to be coated and reacting it out until completion?

This method is called the Brashear process, and it seems to be the most commonly used way in small shops for silvering glass such as dewar flasks. It's also good enough for lens mirrors. See, for example, Tutorial Lesson 17, Silvering Glass. There's lots that's been written about it, with many variations.

I would not expect the reaction to go do completion. On the other hand, if you are worried about Ag loss, you can easily recover the silver remaining in spent plating solution by acidifying with HCl and precipitating silver chloride.

If you need precise control over layer thickness, you should probably sputter the silver on. While that sounds way too fancy, sputtering goes back to Thomas Edison and is within the realm of anybody with a halfway-decent vacuum pump and a modicum of shop skills and electrical knowledge.

Athiril - 9-5-2013 at 16:54

Thanks I'll look into the sputtering method then. Not worried about silver loss, as the amount used is so tiny. Just wanted to be able hit 35nm thickness.

It's not for making a reflective surface. Rather than a thin lens coating.

I read a paper a while back about research on imaging beyond the standard diffraction limit, apart from using negative refractive index materials in a lens, the other way was to prevent evanescent wave decay. They first tried 50nm silver coating which didn't work for them, as they were trying to do it in the visible spectrum, the 50nm was a figure from another bit of research but in a different wavelength of imaging (non-visible spectrum), they found 35nm worked in the visible spectrum.

That is what I would like to try.

Endimion17 - 10-5-2013 at 01:27

Quote: Originally posted by blogfast25

Quote: Originally posted by Endimion17

When silver is being deposited chemically on a piece of glass, you get this:

*A* glass - silver(specular reflectivity) - silver+junk(yellowish matte surface) *B*

So the reflectivity is on the *A* side, therefore the light comes through the glass, refracts, impacts the reflective surface, bounces back, refracts through the glass and exits out. Given the fact that a blown glass bubble will never have a uniform surface (just look at any round flask), the whole process will yield terrible results. Yes, you'll be able to light a piece of paper on fire if you focus Sun's rays like that, but for any optical purposes, that's pretty much a complete failure.
The *B* side is obviously not useable.

Concave telescopic mirrors are made by vacuum deposition of aluminium on a concave/parabolic piece of glass. You get this:

*A* glass - aluminium(specular reflectivity) *B*

*A* side is not used. *B* side is uniform aluminium, highly reflective and bright. The light bounces from the metal itself, it never refracts through any layer of glass.

Quote: Originally posted by Athiril

Thanks I'll look into the sputtering method then. Not worried about silver loss, as the amount used is so tiny. Just wanted to be able hit 35nm thickness.

It's not for making a reflective surface. Rather than a thin lens coating.

I read a paper a while back about research on imaging beyond the standard diffraction limit, apart from using negative refractive index materials in a lens, the other way was to prevent evanescent wave decay. They first tried 50nm silver coating which didn't work for them, as they were trying to do it in the visible spectrum, the 50nm was a figure from another bit of research but in a different wavelength of imaging (non-visible spectrum), they found 35nm worked in the visible spectrum.

That is what I would like to try.

When you say lens coating, what exactly do you mean? A first surface mirror or a second surface mirror? You are aware that the only reflective surface is trapped between the glass and the junk matte surface? It is not possible to use wet chemical process to deposit specular-reflective surface on anything. The inside of Christmas tree ball is not reflective, and neither are the insides of test tubes where silver has been chemically deposited.
Only the outside is reflective, but the light is being refracted through the glass in order to reach the metallic surface.

If metallization was easy, amateur telescope builders would not care about vacuum chambers. They would just mix few chemicals and enjoy the view. Unfortunatelly, that's not how it goes.

[Edited on 10-5-2013 by Endimion17]

blogfast25 - 10-5-2013 at 05:42

Endi:

You're using a very demanding application to make your point. Good quality silver deposits on glass can be obtained in aqueous conditions, see classic mirrors and Dewar flasks for instance.

Endimion17 - 10-5-2013 at 06:09

Quote: Originally posted by blogfast25

Endi:

You're using a very demanding application to make your point. Good quality silver deposits on glass can be obtained in aqueous conditions, see classic mirrors and Dewar flasks for instance.

Those are second surface mirrors. It's impossible to make a first surface mirror using a wet process.

Eddygp - 12-5-2013 at 12:11

http://www.youtube.com/watch?v=6vduHWV47Dg
This forms a mirror coating inside the container (might work OK too on a lens). However, the other surface (the one that doesn't touch the glass) would probably not be shiny.

Athiril - 12-5-2013 at 20:55

Just to point out again, this isn't about making a mirror, or about reflective surfaces, it is about thin metal films on refractive materials (in this case silver on a refractive lens), specifically to take advantage of surface plasmons. I can't find the original paper atm that got me interested in it late last year.

[Edited on 13-5-2013 by Athiril]

blogfast25 - 13-5-2013 at 05:16

Quote: Originally posted by Athiril

Just to point out again, this isn't about making a mirror, or about reflective surfaces, it is about thin metal films on refractive materials (in this case silver on a refractive lens), specifically to take advantage of surface plasmons. I can't find the original paper atm that got me interested in it late last year.

[Edited on 13-5-2013 by Athiril]

In that case sputtering is probably your best bet. The non-glass side or aqueously deposited silver films is indeed of poor quality.