Re: FW: ATM RE: Gluing Blanks Together

["Wm. D. Hanagan, Jr." <hanaganw@attglobal.net>]

"Kestly, J David" wrote:
> 
> I have been very interested in the possible slumping and gluing or fusing of
> Astrosital blanks since I have several 0.4" thick plates that are 12" square
> made of Astrosital.  I have heard that there are possible toxic materials
> which might outgas from the material when heated on tha downside, but, the
> low thermal expansion of the Astrosital is a big plus for fusability from
> what I have read.  I am wondering if fusing and slumping these blanks is
> feasible for a 12" mirror.  I have also wondered about making a square
> mirror since I read an article recently saying that square mirrors have some
> advantages in controlling the diffraction patterns of the aperture.  Any
> comments or suggestions?
> Dave K.
> 

Dave, 

I was looking at four 13" Astrosital blanks from ISP some time back and found
that all had significant strain using the crossed polarizer test.  I asked Monty
MaGill whether he could re-anneal these blanks and he indicated that Astrosital,
like Zerodur, should NOT be re-annealed, because the material could become
"structurally unstable".  I sent the blanks back to ISP, who graciously refunded
my money.  

As a side note, there is some question as to whether strain in a low CTE
material like this is actually a problem or not.  At least one well respected
professional mirror maker ignores the strain in Astrosital, though I myself am
uncomfortable doing that.   

Astrosital and Zerodur are structurally similar in that they are both a
composite of a glass and a ceramic phase.  When these materials are made, the
starting materials are melted at a very high temperature and then cooled
somewhat and held for a period of time at a temperature which favors the
formation of microscopic crystals of a ceramic suspended within the molten
glass.  When enough microscopic ceramic crystals are formed in the melt and the
crystals are the right size, the glass is cooled further so that the ceramic
crystals stop growing.  The ceramic has a negative coefficient of thermal
expansion (CTE) in the temperature range of interest, i.e. it shrinks at
slightly warmer temperatures instead of expanding.  The amount of ceramic that
is present in crystal form almost exactly compensates for the positive CTE of
the glass in which it is suspended.  

Some of the raw material that forms the ceramic still remains dissolved in the
glass, so re-heating could cause the formation of additional ceramic crystals
and destruction of the glass's low CTE.  Further, if these crystals grow large
enough to settle or rise, depending on their density relative to the glass, and
the glass is molten long enough for that to happen, the resulting material could
end up highly strained when cooled, possibly shattering with considerable
force.  

I should also mention that it is possible that the ceramic material is extremely
high-melting and is formed by chemical reaction from multiple starting materials
and not simply melted and re-solidified.  As a result, it may be impossible to
get back to a glass melt having no or fewer ceramic crystals simply by
reheating.  

Since reheating could wreck the low CTE of this material and may actually cause
the material to break up forcefully, I don't think that you want to try to fuse
this type of material.  

I am still looking for a 13" unstrained Astrositall blank, if you have any of
those you would be willing to part with.  


Clear Skies,  

Bill Hanagan