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Re: Filament -Thoriated Tunsten



There is another issue regarding the choice of alloy for the filament.
It happens that the common failure mode for filaments was
recrystallization on rapid cooling when the lamp was turned off. The
preferred orientation along the direction of the wire is the 110, a low
index plane, having therefore a low slip energy. The result was that the
filament would take on a bamboo like structure of crystallines separated
by low angle grain boundaries which could easily slip just due to the
elastic forces on the coiled wire. Once slip occur, that section of the
wire was much thinner, and the filament failed. The W as alloyed to
prevent the recrystallization, although I don't off hand recall what the
alloying element was.

cheers,

joe

ps, years ago, in another life, I made vacuum tubes and studied electron
emission, so all this comes from my long term memory which seems to work
much better than my short term one...why was I writing this anyway?