Re: Gas discharge lamps was Re: Blue Bulb Physics

[Robert Myers <rmyers@inetone.net>]

Hi Paul,

Good question.

To some extent you do get crystal growth.  Associated with that, other
portions of the element get smaller.  Eventually a portion of the filament
gets so thin that it can't carry the entire current and it breaks and the
bulb burns out.  

However, the system is operating pretty much under kinetic control rather
than thermodynamic control.  Thus whenever a WI3 molecule strikes the
extremely hot filament (a more or less random process) the molecule
decomposes and deposits a W atom where it hit.  This reduces the tendency
toward crystal growth which is favored under thermodynamic control conditions.

Under thermodynamic control conditions the WI3 molecule will be permitted
to be more selective about where it reacts.  This allows time for the
selection process to do its thing and produce thermodynamically more stable
crystals.  This process would take place better at lower filament
temperatures.  The filament would need to be hot enough so that at least
some W vapor would be produced but not so hot that a WI3 molecule would
react instantly whenever it strikes the filament.

But...  Crystal growth is probably the main reason halogen bulbs don't last
(figuratively) forever.

At 09:57 AM 4/17/98 -0700, you wrote:
>
>Thanks for the details once again Bob.  What you described sounds
>like a classic chemical vapor deposition inside the bulb.  What I 
>want to know is this, why dont you end up with a single crystal of
>tungstun inside the cell with this constant redeposition occuring?
>Must have something to do with the source and target being one in 
>the same.
>
>paul timmerman
>
>
>
___
   Bob
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