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RE: Filament resistance numbers



In view of this discussion, I wrote to an acquaintance I knew in grad
school who went to work for GE designing light bulbs. He responded with
the following:

I have a text book, _The Science of Incandescence_, written by a GE
scientist,
Milan Vukcevich. I have copied portions of the text that are relevant to
your
questions and will snail-mail them to you later today (4-9-98). I have
access
to a scanner and OCR software, but there are so many equations and
illustrations that it is far, far quicker to just take a low-tech approach.

Briefly:

1. Adding thoria to tungsten does lower its work function and improve
emission. If I remember correctly, however, this is more relevant in arc
discharge lights and less important in incandescent lights. I took a tour of
GE's tungsten plant, and I don't remember any copper being used. One of the
greatest challenges in using tungsten was making the sintered and drawn wires
ductile.

2. From _Light Sources_ by W. Elenbaas, "Coolidge found a method of making
more ductile tungsten wire; the process involved compacting tungsten powder,
sintering in a hydrogen atmosphere, swaging, and drawing. A property of
tungsten, namely its non-sag quality, is very important for coiled filaments
and is realized by the addition of special dopants.... Potassium and silicon
oxides proved to be the active agents, and later aluminum was found to be
active too.... These materials are now deliberately added to the tungsten
oxide in quantities of the order of 1/2%, the greater part of which
evaporates
during further processing.... The doping results in long and interlocking
grains roughly parallel to the axis of the wire after recrystallization."

3. Halogen gas (methyl bromide, CH3Br) does "recycle" the evaporated
tungsten
off of the filament wall and redeposits it back onto the hottest part of the
filament (which, fortunately, is also where the fastest evaporation is
occurring). And the bulb wall must reach a certain minimum temperature in
order to activate the halogen cycle, so a halogen bulb that is repeatedly
turned on for only a minute and then allowed to cool will probably fail
prematurely.


Halogen chemistry (from _The Science of Incandescence_):

1. Evaporation from the filament.
W(s) -> W(g)

2. Diffusion to the bulb.
W(g)

3. Condensation on the bulb wall.
W(g) -> W(s)

4. Reaction at the bulb wall.
W(s) +O2 + 2X -> WO2X2(g), where X is the halide (such as Br)

5. Diffusion to the filament.
WO2X2(g)

6. Decomposition close to the filament.
WO2X2(g) -> WO2(g) + 2X(g)
WO2(g) -> WO(g) + 1/2 O2(g)
WO(g) -> W(g) + 1/2 O2(g)

7. Condensation on the filament.
W(g) -> W(s)

Regards,

Lance Kaczorowski
Grote Industries
lkaczoro@grote.com