Silica Glass vs Pyrex
Take off UrPants to reply
dialectric for the reason that its conductivity does not increase significantly
with temperature. "Great", I thought, but why fused silica. To my understanding
that is just sintered powder. It probably has microscopic pores that might
allow a plasma stream to form and lead to a catastrophic collapse.
OK, well the obvious answer I thought would be pure silica glass. No holes. So
I thought ... might be expensive. I wonder if I could just get some beach sand,
wash it with concentrated HCl and then lots of of water and then melt it into a
plate. Ok, this does sound like a lot of work to save a little money, but give
me a break, I have already said that I love doing new little projects and i
would have become an MD long ago if I had any practical sense.
Anyway, There were two major problems I saw with this project.
1) Pure silica glass probably has a much higher melting point than regular
glass, so god knows if I could improvise a furnace that could melt sand.
2) Even if it didn't breakdown electrically, the damn stuff yould probably just
crack due to thermal expansion.
Well of course I opened up the Merck to start thinking about the possibilities
and I found two things that really surprised me.
1) The damn book does not have the melting point of Silica! I give a hearty
polish salute to the editors who have otherwise done a fine job. Off to the HB
of P and C.
2) That very same entry says "silica glass has he lowest thermal coefficient of
expansion of any substance known."
Now this raises a couple of questions.
1) What is the point of borosilicate glass if pure silica glass has the lowest
TCE. Is it just that the melting point of silica glass is to high to make it
practical for manufacture? Does BS glass have other physical properties beyond
a low TCE that makes it siutable for various high temp app?
2) Why does silica glass have the lowest TCE? What are the molecular mechanisms
behind variability in TCE? I think I understand the general mechanisms.
Mark Thorson
> Several of you suggested that I look at fused silica insteda of pyrex as my
> dialectric for the reason that its conductivity does not increase significantly
> with temperature. "Great", I thought, but why fused silica. To my understanding
> that is just sintered powder. It probably has microscopic pores that might
> allow a plasma stream to form and lead to a catastrophic collapse.
The reason I suggested fused silica is that glass is an ionic
conductor, and fused silica doesn't have much ions. It's
like comparing distilled water with tap water.
Fused silica products are optically clear -- not sintered.
However, mica is probably a better choice. They make
capacitors out of mica, but not fused silica.
Repeating Decimal
wrote on 5/6/02 6:55 AM:
IIRC Vycor is a Corning trade name for sintered silica. It is cheaper to
produce than true fused silica. It does not require reaching as high a
temperature. I am fairly sure that if you can heat silica with oxy-hydrogen
torches to where if flows, it would conduct ionicly. It is just difficult to
reach such temperatures.
Someone asked the question why fused silicon has low thermal expansivity. It
has to do with the the change in bond angles compensating for linear
expansivity.
Bill
Tom Walz
Ah grasshopper,
There are many things to consider. Toughness is important in lab
ware especially as it applies to thermal shock. Isn't Pyrex tougher?
I would bet that melting silica into silica glass is pretty complex to
get good results. We melt silver braze alloy onto tungsten carbide
saw tips. You can do it with a torch but it won't stay on the saw.
In fact the silver will burn rather than melting so you need a flux.
(Potassium salts of Boron and Fluorine.) And so on. Good, consistent
manufacturing processes depend on good conditions. To get the kind of
quality you want will take you a great deal of work. It will be fun
Check American Science & Surplus for lab ware. We've bought some and
it is pretty good stuff at a pretty good price.
Tom
Take off UrPants to reply
>capacitors out of mica, but not fused silica.
>
Hmmm, ... where would one go looking for a mica plate? Are such things
expensive?
Take off UrPants to reply
>torches to where if flows, it would conduct ionicly. It is just difficult to
>reach such temperatures.
Hmmmm... I have access to an oxy-acetylene torch. Any problems with using that?
Also, why would a pure silica glass be an Ionic conductor? Isn't it just SiO2?
Take off UrPants to reply
>it is pretty good stuff at a pretty good price.
>
Ahh! Good idea.I hadn't thought of looking with for this sort of stuff. I just
usually buy novelty science toys for my son from them.
Ian Gay
micha...@aol.comUrPants (Take off UrPants to reply) wrote:
>> I am fairly sure that if you can heat silica with oxy-hydrogen
>>torches to where if flows, it would conduct ionicly. It is just difficult to
>>reach such temperatures.
>
>Hmmmm... I have access to an oxy-acetylene torch. Any problems with using that?
Not if you maintain good ventilation. SiO2 vapour is bad news for the lungs.
You'll also need welders' goggles - SiO2 near its melting point is _bright_.
*** To reply by e-mail, make double u single in address ***
Joshua Halpern
Take off UrPants to reply wrote:
>
> Several of you suggested that I look at fused silica insteda of pyrex as my
> dialectric for the reason that its conductivity does not increase significantly
> with temperature. "Great", I thought, but why fused silica. To my understanding
> that is just sintered powder. It probably has microscopic pores that might
> allow a plasma stream to form and lead to a catastrophic collapse.
Fused silica and artificial quartz are interchangeable
terms. Fused silica is optically clear. There are several
grades, depending on impurities and bubble inclusions
http://www.sico.at/products/quartz.html
http://www.gequartz.com/en/properties.htm
Your questions about properties are all answered at
these sites
>
> OK, well the obvious answer I thought would be pure silica glass. No holes. So
> I thought ... might be expensive.
More expensive than glass, but not out of reach, for example:
http://www.escoproducts.com/ for optical components or
http://www.quartz.com/ for process tubing
> I wonder if I could just get some beach sand,
> wash it with concentrated HCl and then lots of of water and then melt it into a
> plate. Ok, this does sound like a lot of work to save a little money, but give
> me a break, I have already said that I love doing new little projects and i
> would have become an MD long ago if I had any practical sense.
This is essentially beating your head against a wall when you could
walk around it at less cost.
> Anyway, There were two major problems I saw with this project.
>
> 1) Pure silica glass probably has a much higher melting point than regular
> glass, so god knows if I could improvise a furnace that could melt sand.
The term you want is FUSED silica. The softening point is about
1050 C, and it pretty much stays in place as you heat it quite a
ways above that. At much higher temperature it will flow, but it
also "devitrifies" (ie goes back to sand). I've never seen it flow
using an oxy-hydrogen torch. I have seen it devitrify....ugly, ugly.
>
> 2) Even if it didn't breakdown electrically, the damn stuff yould probably just
> crack due to thermal expansion.
It has nice thermal properties
http://www.gequartz.com/en/properties.htm
http://www.janostech.com/techinfo/uvfs.html
http://www.janostech.com/techinfo/irfs.html
> Well of course I opened up the Merck to start thinking about the possibilities
> and I found two things that really surprised me.
>
> 1) The damn book does not have the melting point of Silica! I give a hearty
> polish salute to the editors who have otherwise done a fine job. Off to the HB
> of P and C.
>
> 2) That very same entry says "silica glass has he lowest thermal coefficient of
> expansion of any substance known."
Yep.
>
> Now this raises a couple of questions.
>
> 1) What is the point of borosilicate glass if pure silica glass has the lowest
> TCE. Is it just that the melting point of silica glass is to high to make it
> practical for manufacture? Does BS glass have other physical properties beyond
> a low TCE that makes it siutable for various high temp app?
The temperature is one thing, the cost of manufacture the other,
You have to pay for that higher temperature. Regular glass is
dirt cheap, easy to pour, mold, and blow.
> 2) Why does silica glass have the lowest TCE? What are the molecular mechanisms
> behind variability in TCE? I think I understand the general mechanisms.
WAGNER: More long range order than other amorphous glasses, smaller nearest
neighbor distances.
josh halpern
donald haarmann
> Several of you suggested that I look at fused silica insteda of pyrex as my
> dialectric for the reason that its conductivity does not increase significantly
> with temperature. "Great", I thought, but why fused silica. To my understanding
> that is just sintered powder. It probably has microscopic pores that might
> allow a plasma stream to form and lead to a catastrophic collapse.
>
[snip]
Some years back in the Corning catalog was a picture of molten lead being
poured into a Vycor dish sitting a top a block of dry ice! That's stability!
Vycor's price results from a difficult manufacturing process:—
Another substitute for vitreous silica can be produced by melting a suitable
borosilicate glass and then heating it at around 600oC until it separates into
two phases. The alkali-borate phase may be leached out with acids, [HF?]
leaving a 96% silica phase with open pores of controllable size which can be
converted into clear glass. Porous glasses of this kind, commonly known as
Vycor, from the first commercial version produced by Corning Glass Works Ltd,
may be used as membranes for filtration purposes and for certain biological
applications.
[Vycor is re-melted after leaching into its final size.]
http://www.britglass.co.uk/publications/mglass/making2.html
See also - http://www.chem.fsu.edu/gelb/papers/langmuir1998.pdf
&c.
--
donald j haarmann
----------------------------
Art and science have their
meeting point in method.
Bulwer
Repeating Decimal
reply at micha...@aol.comUrPants wrote on 5/6/02 9:27 AM:
I must admit not knowing. Do fixed bond angles imply covalent bonds?
Bill
Repeating Decimal
5/6/02 11:36 AM:
> In article <20020506122715...@mb-mj.aol.com>,
> micha...@aol.comUrPants (Take off UrPants to reply) wrote:
>>> I am fairly sure that if you can heat silica with oxy-hydrogen
>>> torches to where if flows, it would conduct ionicly. It is just difficult to
>>> reach such temperatures.
>>
>> Hmmmm... I have access to an oxy-acetylene torch. Any problems with using
>> that?
>
> Not if you maintain good ventilation. SiO2 vapour is bad news for the lungs.
> You'll also need welders' goggles - SiO2 near its melting point is _bright_.
There is a problem with quartz blowers disease. Little wisps of clean and
zctive SiO2 are formed. Inhaling them leads to silicosis.
Bill
Repeating Decimal
jhal...@neteze.com wrote on 5/6/02 12:54 PM:
> Fused silica and artificial quartz are interchangeable
> terms. Fused silica is optically clear. There are several
> grades, depending on impurities and bubble inclusions
Artificial quartz is not the same as fused silica. Artificial crystalline
quartz is grown in large quantity for optical and electronic use, especially
for frequency control crystals.
Bill
Mark Thorson
> Also, why would a pure silica glass be an Ionic conductor? Isn't it just SiO2?
Fused silica is pure, hence it would not be an ionic conductor.
Glass, such as soda-lime glass, has stuff dissolved in the SiO2,
which makes it a conductor when hot.