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----- Original Message -----
From: "Dominic-Luc Webb" <dlwebb@...>

Tungsten is strong for its thickness, but generally is very, very
thin, as in light bulbs. However, the nichrome and kanthal wires
are commonly much thick so are quite strong. Reduced form of
aluminum goes to melts at slightly above 500 degrees. It is
not clear to me that the temperature ratings you mention are
the determining factor in choice. I rather like the strength of
highly re-usable things like car cigarette lighters and kanthal
wires.

Perhaps there are specific tungsten electrodes one would suggest?

Well, if you want pre-finished ready to go Tungsten, try :



Alternativly, I cut and form my own tungsten using bulk 0.032" wire from here:



Another easy to scrounge source of Tungsten is from the Tig Welding Industry.
(however it only comes in 150mm lengths, and can be somewhat expensive.)

If you go with either the bulk Tungsten wire, or Tig Rod, before you try to
Cut/Form the wire, you'll need to heat it past the "Ductile to Brittle
Transition Tempature" At room temps, Tungsten is surprisingly brittle and any
attempts to cut/bend will just snap it like a lead pencil.

Fortunatly the ductile transition tempature starts at around 100c, once the wire
is above this tempature it will bend / cut easily. I use a 2400watt hot air gun
to pre-heat the tungsten before inserting into my filament mold.

Perhaps someone knows there is in fact a reason to not use these
other options?

Several things to consider:

#1 Wetting Action

If you plan on using a wire filament, if the Al won't wet onto the filament
material, it will just melt and fall off, and if the blob of liquid Al lands on
a glass surface, it WILL FRACTURE the glass (been there, done that, makes for a
VERY BAD DAY!)

If your going to use a Boat / Box thermal source, then wetting action isn't
important anymore.

#2 Chemistry

Al is amazingly reactive stuff, even when used with Tungsten, some of the
Tungsten will disolve into the molten Al (cause of the wetting action?) As the
Al evaporates, the Tungsten is re-deposited back onto the filament. If the
filament is too small in diameter (compared to the amount of Al) dissolution of
the tungsten into the Al will cause the filament to separate into two pieces,
and evaporation quickly stops :-0

I tried using a heating element material used for HVAC (no idea what it was but
probably not NiCr) and as soon as I applied power, the filament went "poof" into
two rather small pieces.

Based on the Al deposition data chart from Lesker's ( )
for a coil /wire thermal source, they only recomend Tungsten, for a boat they
recomend Tungsten or Titanium-Boride.

If you try NiCr or Kanthal, here are few thinkgs to consider.

Al has a melting point of 660C, and a evaporation temp of 821C @ 10-6 torr.

NiCr has a melting point of 1395C, while at the same time it appears to
sublimate at 987C @ 10-6 torr.

So if your chamber is at 10-6 torr, you'll need to keep the NiCr wire below 987C
and above 660C, or you will end up with some form of NiCrAl film?

Kanthal is another interesting species, its made from FeCrAl, the Al forms an
Aluminum oxide which protects the FeCr from oxidation when used as a heating
element. No data is listed for Kanthal's melting point or vapor pressure, so
who knows what will happen :)

BTW, I did find this interesting, Tungsten appears to sublimate at 2407C @ 10-6
torr, and of late, after the Al has evaporated off the Tungsten, I've been
running the filament power up untill the Tungsten was white hot (why, no reason
other than it's entertaining in an odd way). I mention this as I suddenly
wonder if perhaps I might be sublimating a very thin film of Tungsten over the
Al coat. If that's a good idea or not, I don't know....

James