In Reply to: How To Control The Parameters In Al Vacuum Evaporation posted by rshong on 04/29/02 at 3:43 AM:
If you are re-using the filaments - Tungsten filaments crystalize and crack on repeatded heating to high temperatures. On subsequent heating the cracks can open up a give high resistances.
Check your clamping arrangement - On heating the ohmic contact can become bad leading to high resistance.
When melting aluminum - When the aluminum melts and wets the filament the resistance goes down and current goes up essentially shorting the other filaments. Uneven melting between filaments can be a problem. The solution may be to pre-melt the aluminum on the tungsten fialment. Aluminum does nt evaporate very much at its melting point.
Rate of heating - Aluminum has to be heated several hundred 0C above its melting point before it begins to evaporate rapidly. If you heat slowly the non-wetted filaments are shorted out longer.
Try - Heating the filaments without anf aluminum and see if the urrent remains equal in all the legs. Don Mattox