On Mon, 8 Dec 2003, James Lerch wrote:
Perhaps someone knows there is in fact a reason to not use these
other options?
Several things to consider:
#1 Wetting Action
If your going to use a Boat / Box thermal source, then wetting action isn't
important anymore.
I was thinking about car cigarette light in these terms actually. I had
idea to use a nozzle arrangement as an outlet above the heat source, such
that the vapor would be forced to rise upward before encountering the
nozzle which points downward in my tentative design.
#2 Chemistry
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.
Great! That was the kind of value (821 C) I knew existed somewhere.
NiCr has a melting point of 1395C, while at the same time it appears to
sublimate at 987C @ 10-6 torr.
This sublimation temp is indeed inferior to tungsten.
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?
Exactly! And this might even be highly reflective and less prone to
oxidation.
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 :)
Kanthal = Swedish product, so we we hear about it alot up here.
There are different grades of this wire. I recall some very high temp
versions when I was interested in higher melting point glass and
ceramic materials years ago. The "Kanthal D" is likely the most famous
(max temp 1300 C), but there is also "Kanthal Super", which has max temp
1700-1800 C. I'll attach the Kanthal Handbook in a private post (to
James).
BTW, I did find this interesting, Tungsten appears to sublimate at 2407C @ 10-6
torr,
Immediate thought: If you need such high temperature, there is something
seriously wrong with either the temperature reading or the pressure
reading or the material you think is (reduced) aluminum. I see no
hope for tungsten on the mirror to contribute positively. I am not totally
committed on this statement, either. I am suspicious that rate of
vaporization could be a factor, in which case your heat excess is an
unavoidable consequence of achieving a desirable instant vaporization.
To that end, perhaps the lower pressures I am getting, like below 10e-7
could be favorable.
Any idea if there is a pressure that is too low (within reason for a
diffusion pump)?
Dominic-Luc Webb
