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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: Re: I'm sorry I forgot about the drains... posted by Dave-A on July 7, 2006 at 21:09:14:
You're talking about the metal structure upon which the cathode material is deposited rather than the cathode material itself. If that is so then yes nickel was often the material of choice, especially for indirectly heated cathodes.When you said "WE made some 10's that had nickel filaments, which was a departure from the normal TT found in all the others." I took you too literally. I can now see you meant to say that WE made some 10s with oxide coated direct heated cathodes instead of thoriated tungsten .
The thorium is indeed alloyed with the tungsten but because the cathode is operated at such a high temperaure the thorium migrates to the surface to form a monoatomic surface layer. This layer slowly evaporates but is replenished by diffusion of thorium from the remaining alloy.
BTW alloying virtually always results in a reduction of melting point as the mixture heads towards the eutectic point. This is because mixing atomic sizes reduces the energy of crystallisation due to the mechanical disruption of the lattice, reducing the energy input required to completely disrupt (melt) the crystal. The exceptions to this occur when one of the alloying elements has a very high crystallisation energy and thus high melting point in which case the eutectic point still exists but it is relative to a heavily sloped gradient from one metal to the other so the melting point of the alloy is above that of the (lower MP) base metal.
Mark Kelly
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Follow Ups
- Now I see the problem - Mark Kelly 23:06:23 07/07/06 (3)
- Re: Another example - Dave-A 06:27:29 07/08/06 (2)
- Just for consistency - Mark Kelly 16:11:40 07/08/06 (1)
- Re: Just for my understanding... - Dave-A 09:34:21 07/09/06 (0)
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