Re: [ATM] [Fwd: [atm_free] Effect of pH on polishing fused silica was Re. Session 24...]

[Mark Cowan <toolontop@yahoo.com>]

Jeff, et al,

I'd discussed pH (for CeOx on Pyrex primarily) with
Randy at Salem some time back.  Indeed he has gone
elsewhere, but it sounded to me like it was a better
opportunity. :) I don't know where he went off to, but
he did love to chat glass. ;)

Anyway, the upshot was that pH wasn't that big of
factor so long as it was close to the design range.

What piqued my interest for quartz was Herbert's query
to me about pH and the follow up of the copy of the
paper that he sent me.  As I said on the other list,
the results were not encouraging, but obviously
/something/ was happening.  I'm including the rest of
what I posted there, that Guy left out, to avoid any
confusion.

BTW if anybody would like a copy of the copy of this
paper I've cited, let me know at < mec @ raddobs . com
>.

Best,
Mark

PS the first copy of this never seemed to get through
to the list...so this is a resend
===========================================
AlOx isn't a polishing agent, per say, as it must
leave scratches.  In
super fine grades (down to 50nm particle size) it is
used on a lot of
materials though, according to Salem Distributing
literature.  It will
burnish the glass but it can't dissolve it as a
polishing agent does,
SFAIK.  Compounds that polish have to be able to form
suspended
silicates over a time scale of fractional seconds, at
least.  I don't
know what drives the process but I /suspect/ it's
local differences in
pH, as pH plays a role in polishing efficacy.

Here's an interesting experiment that demonstrates
that.

H. Highstone set me a copy of an Applied Optics paper
from '92 (pp
7164-7172, Vol 31, no 34) that explored the use of
various agents
(CeO2, ZrO2, Al2O3, Y2O3, YF3) on fused silica at
varying pH ratios.
Al2O3 has by far the highest roughness of any of these
compounds.
Anyway, lowering the pH from 7 to 4 with CeO2 on fused
silica lowers
the roughness by a factor of almost 4x under
controlled conditions.
You can use citric acid to buffer the pH of a slurry
down to these
levels.  I've tried it.  The lap drag immediately
becomes /enormous/,
to the point that the lap can't be moved smoothly. 
Although it may
result in lower roughness under machine control, by
hand the result
was horrible, with roughness quite visible under
Foucault testing.

So my conclusion is that just changing that one factor
accelerates the
chemistry going on considerably.

---------------------------------------------

I was looking through the paper I cited in the last
post and noted the
following about hydration layers and silicate
redeposition:

Buffering the CeO2 slurry to pH 4 increased the drag
greatly, but
according to the paper and the assorted micrographic
and AFM imagery
the resulting surface is much smoother - /because/
(they say) the
redeposition from the slurry is greatly reduced.  Thus
the polishing
action is surface removal primarily.  They also say
the lap lasts much
longer because the "ceria particles are suspended in
the slurry rather
than embedded in the lap."  The lack of the
redeposited layer means
that the final surface is made from the pure
substrate, and so might
bond to the coating better, improving coating life.

Perhaps you need to start with a fresh lap for this
experiment. I plan
to test some of this with the robotic figuring
machine, which is much
more capable than I am of maintaining constant lap
speed against high
resistance.

=============================================


--- Jeff Rowe <jeffrowe@austin.rr.com> wrote:

> Friends;
> Salem Distributing's old catalog discribed pH and
> Baum'e in their 
> sections on CeO2 supplies ( pgs. 30-35)  In
> commercial optic production 
> it is a significant concern. In Salem's catalog they
> stated various pH 
> specs for different CeO2 products, as well as
> materials being polished, 
> these varied between ~6 and 9, and were in a narrow
> range for each 
> product. A short summary of why is: glass is
> alkaline and it increases 
> alkalinity of the slurry when it leaches into the
> polishing slurry. 
> Stock removal rates are affected  by pH, so it must
> be controlled, some 
> polishing compounds lose their suspension properties
> and settle if the 
> pH is not maintained.
>  The Baum'e was directly proportional to the oz of
> Cerium per gal. H2O, 
> Baum'e 1 at 1.6 oz/gal. to Baum'e 12 at 17.6 oz/gal.
> I just went to 
> Salem's website and they don't have their old ATM
> section anymore.
> 
>  I had read on the list that Randy Rains (sp?) had
> moved to another 
> company a while ago. I looked for other pH links in
> Google, here are a 
> few. There were more that sent you to, you pay/
> patent abstracts
> 



       
____________________________________________________________________________________
Take the Internet to Go: Yahoo!Go puts the Internet in your pocket: mail, news, photos & more. 
http://mobile.yahoo.com/go?refer=1GNXIC
_______________________________________________
ATM mailing list http://www.atmlist.net/