krfkeith
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Making Fused Silica
Making fused silica has been one of my goals for awhile, impractical as it may be. The major hangup for me has been finding a proper source of sand to
make it with. I could buy reagent grade SiO2 from Sigma, but I'm sure it's obvious why I chose not to do that. Anyhow, I was doing some
research on how silica is purified, and I came across a paper by some Japanese researchers on purifying diatomaceous earth to 5N. The process is
really simple and just uses NaOH and HCl.
So I've got my silica (well, I will anyhow), what do I do from there? Obviously I need something to melt it with. I have a fairly good idea on how to
construct the furnace, but there are a few things I'm not sure on. I posted this question here because it related to the chemistry of molten silica,
not the mechanics of the apparatus itself.
First of all, what kind of crucible can I use? The idea I had was to use an inductive heating design, with the crucible as a susceptor, so it would
need to be conductive. Alternatively, I have read that zirconia conducts at higher temperatures, so perhaps I could use a resistive pre-heating
element (SiC or something). The problem here is that I'm not sure what is suitable for keeping the glass from being contaminated. A quick review of
the literature makes reference to contaminants from refractory materials. Platinum is cleary useful, but also way outof my price range. Graphite and
glassy carbon would work. I am unsure how contaminating graphite would be, though I know glassy carbon is probably second only to platinum in its
intertness. However, both would need to be under vacuum or inert atmosphere at the required temperatures.
This leads me to my second question, does fused silica need to be melted under a vacuum or inert gas? If so, this would obviate my concerns about
carbon, since I'd need it anyway. Would nitrogen work (for the melting or the carbon crucibles)?
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AndersHoveland
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Do you really need to make pure fused silica? Adding just a small ammount of boric oxide will significantly help to reduce the melting point, making
it easier to work with.
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krfkeith
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Hmm, I suppose I wouldn't. However, how difficult is it purify boric oxide? What sort
of impurities would it commonly have in it?
[Edited on 9-3-2013 by krfkeith]
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daragh8008
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A simple device for melting glass that I came across was a microkiln. Google it. It seems like a cheap simple alternative to constructing a furnace.
It a carbide coated device you stick in a microwave and is used for fusing glass. As for the source. Is getting ultra pure sand is hard I would
probably start with tetraethyl ortho silica (TEOS) and for a polymeric sol via acid catalysed method. You can then dehydrated and dry into a film or
powder as you see fit and fuse from there. Although special considerations such as temperature ramps are important if it is films your after.
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plante1999
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I don't know if you need perfectly clear fused silica, but I have a process for fused silica that can be found in lab scale industrial synthesis
volume 2. It use calcium hydroxide based binding agent, and then fusing.
I never asked for this.
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