Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!timbuk!kilian
From: kilian@cray.com (Alan Kilian)
Newsgroups: sci.electronics
Subject: Re: Do Halogen lamps need ballasts?
Summary: Some real quartz specs.
Message-ID: <153447.28605@timbuk.cray.com>
Date: 2 Dec 90 21:45:41 GMT
References: <714@skipper.dfrf.nasa.gov> <1147@inews.intel.com> <1197@inews.intel.com>
Distribution: sci.electronics
Lines: 56

In article <1197@inews.intel.com>, bhoughto@cmdnfs.intel.com (Blair P. Houghton) writes:
> In article <1990Dec1.032426@envy.bellcore.com> karn@thumper.bellcore.com writes:
> >In article <1147@inews.intel.com>, bhoughto@cmdnfs.intel.com (Blair P.
> >Houghton) writes:
> [...deformation ==> rupture...]
> >|> The oil doesn't "weaken the structure."  You can
> >|> wipe it off.
> >
> >I don't think that's quite right. Quartz is used precisely
> >because of its very low temperature expansion coefficient.
> 
> Very low != nonexistant.  Also, quartz is very brittle.
> Any deformation causes catastrophic displacement
> (cleaving).  When there's a uniformity of heat
> distribution, things expand normally.  When there isn't,
> they don't.
> 
> >I think the problem is that the heat will decompose the oil, producing
> >elemental carbon which will then absorb the light hitting it from the
> >filament.  This causes a hot spot on the surface of the envelope which
> >could well cause it to soften and fail.
> 
> Take a halogen lamp.  Turn it on.  Allow a drop of water to
> fall on it.  Run like hell.
> 
> You don't need any chemistry.  Temperature differences
> are all you need to cause one of these to blow.
> 
> I don't think you can "soften" quartz without _extreme_
> temperatures (thousands and thousands of K).
> 
> 				--Blair
> 				  "Billions and billions of Big Macs..."


From the CRC handbook od chemistry and physics 63 edition 1982-1983
PP F65:

Coefficient of thermal expansion 5.5 X 10^-7 cm/cm/degree C From 20 to 320 C
   (That's way low)
Tensile strength 7,000 PSI Compressive strength > 160,000 PSI
   (That's very strong)
Softening point (Approx) 1665 Degrees C (1938 degrees K)
   (That's way up there)

So, If I place some oil on the quartz with the lamp off and let the 
temperatures equalize and then turn on the lamp WHY does the quartz
break (If it really does as so many have said).

I guess I'll have to buy some bulbs and try this at home because I don't
buy any of these theorys. (Except the carbon one maybe)

-Alan Kilian
 Cray Research, Inc.
 655 F Lone Oak Drive
 Eagan, MN 55121       kilian@cray.com


Brought to you by Super Global Mega Corp .com