Re: Sodium doping of fused silica
From: Andreas Rutz (enemy_soldier_at_hotmail.com)
Date: 07/13/04
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Date: Wed, 14 Jul 2004 00:44:28 +0200
max wrote:
> I like the idea of mixing SiO2 and NaCO3 (or oxide) in a crucible and
> heating it. A few questions about this method though.
>
> 1. I don't have SiO2 powder, intead its bulk fused silica glass. So,
> how would I use a bulk material and mix it with the carbonate or
oxide
> to form a homogenous melt.
try to crush the silica as fine as possible, or buy some. It doesn't
cost a fortune, although you have to be sure that it is pure enough for
your application. It doesn't work well with bulk material, you had to
wait for a long time to get a good homogenity as fused silica is highly
viscous up to very high temperatures.
> 2. What would the temperature of the furnace have to be in order to
> get a homogenous melt.
You need a very high temperature furnace. Depending on the composition
at least 1600°C. If your intended sodium content is low (doping
concentrations), you need at least a 1800°C furnace. (platinum
crucibles won't survive that :-( , you need something else but i can't
say you what is suitable.)
> 3. Regarding immersing silica in an appropriate salt melt, what
> exactly is a salt melt. I have tried once before to deposit dilute
> salt solution in water on to a bulk silica sample and then use an
> ion-exchange process to sodium dope the silica sample at like 260C.
> Wasn't able to achieve much.
260°C is much too low to get any useful effect. Diffusion just doesn't
work in silica when "cold". I can't suggest you any suitable salts from
experience, but you will find some in literature. The ion exchange
process is used to prestress glass surfaces. Although there are alkali
ions exchanged, not Si.
If there are no alkali ions, the process maybe doesn't work. You could
try to heat your sample in sodium oxide at about 1000°C and see what
happens. If nothing, heat more. Wash off the residual sodium oxide in
water after cooldown (careful, wear glasses). Sodium chloride is
useless for this application, nothing happens.
> More suggestions please.
> Thank you.
> max
>
>
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