MadSci Network: Chemistry
Query:

Re: Are there any materials that STAY SOLID at 8500 degrees C?

Date: Thu Dec 26 18:47:28 2002
Posted By: Aaron J. Redd, Post-doc/Fellow, Plasma Physics and Controlled Nuclear Fusion, University of Washington
Area of science: Chemistry
ID: 1039212969.Ch
Message:

Tommy,

I should first point out that a blackbody radiation spectrum extends over
the entire range, from a wavelength of "zero" up to "infinity".  The idea
is simply that the peak of the curve shifts to lower wavelengths as the
temperature increases.  The overall amount of radiated power also
increases, so that (for example) the amount of UV light emitted by an
object increases dramatically as the temperature increases, up to the point
that the UV emission can become a safety issue long before reaching a
temperature like 8500 C.  Note that since UV light is invisible, UV
emissions can significantly damage or destroy your eyesight very quickly,
since you don't instinctively look away, blink or contract your pupils.  So
please be careful before you start messing around with UV emitters.

Now, as far as everyday materials are concerned, 8500 C is an astoundingly
high temperature.  As far as I know, even very durable materials (e.g.,
diamond, metallic tungsten or alumina (a ceramic)) melt or sublimate long
before reaching 4000 C.  So a material that is solid at 8500 C seems
improbable, and perhaps flatly impossible, to me.

That said, there are other ways of generating UV radiation.  Are you
interested in 8500 C because you need to generate broadband radiation in
that range, or do you just want to generate SOME light SOMEWHERE within
that particular wavelength band?

If all you want is to generate some UV light, and you don't really care
about whether it is broadband emission or just a few sharp emission lines,
then I suggest using a mercury-vapor discharge tube.  These are rather easy
to obtain from a scientific supply company, and shouldn't be very expensive
($10-$20, if I recall correctly).  Some higher-end units will come with
their own power supply, so that you won't have to purchase a separate
supply to get the discharge going.  Please also keep in mind the fact that
mercury (and especially mercury vapor) is poisonous, and so you need to be
very careful with the glass tube.

If you are insistent upon generating broadband UV radiation in the 300-400
nanomenter wavelength range, then I'm not certain how you should proceed. 
  As it turns out, 8500 C is tremendously hot for ordinary materials, but
is actually a very cold temperature in plasma physics -- note that the
photosphere of the sun (its cold (!) outer edge) has a temperature in the
neighborhood of 6000 C.  Experimentally, the biggest problem in generating
(and sustaining) a plasma at a temperature of thousands or even low
ten-thousands of degrees Celsius is that the plasma will be very
susceptible to impurity radiation.  In other words, rather than getting the
atoms/ions in the plasma to collectively radiate as a blackbody, you'll
most likely end up with lots of low-charge-state atomic and ion radiation.
 Consider the discharge in a fluorescent tube, for example -- the
electrical discharge in the tube excites a few high-energy emission lines,
and those photons then excite the fluorescent material lining the tube,
producing a broad-band spectrum we perceive as nearly-white light.

Well, I'm not sure what else to say right now.  If you have further
questions or would like clarifications, please feel free to ask.  Good luck!

Sincerely,
Aaron J. Redd



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