[Amps] Filament Growth Question

[John Lyles]

Matt
You are likely correct that grain refinement is leading to brittleness 
in old filaments. I don't understand the metallurgy. 20 years ago I 
heard from RCA that high anode dissipation led to grain growth in the 
copper anode which then would fail to the point where water could leak 
into vacuum. I also have some photos of filaments made with electron 
microscopy and they appear to have enlarged grains.

> Message: 4
> Date: Sat, 26 Apr 2014 02:15:23 -0500
> From: "Matt" <maflukey at gmail.com>
> To: <amps at contesting.com>
> Subject: [Amps] FW:  Filament Voltage Question
> Message-ID: <14f101cf611f$4aff75d0$e0fe6170$@gmail.com>
> Content-Type: text/plain;	charset="us-ascii"
>
> I am perhaps confused.  In my experience, grain growth generally produces
> increased ductility and improved (or restored depending on your point of
> view) yield strength more-or-less according to the Hall-Patch relationship
> (yield strength is proportional to the square root of grain size).  It would
> seem that grain refinement, not growth, would contribute to increased
> brittleness.
>
> Matt
> KM5VI
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of John Lyles
> Sent: Saturday, April 26, 2014 12:39 AM
> To: amps at contesting.com
> Subject: [Amps] Filament Voltage Question
>
> When direct heated filaments of thoriated tungsten (usually 1-2% thoria)
> have a lot of hours of operation (thousands), they become brittle and easy
> to break. Older tubes are much more prone to shipping damage from broken
> filaments for this reason. Grain growth occurs in the microstructure of the
> metal. Thin higher voltage filaments would be even more fragile in this
> condition. Large filament wires or bars can be modified in thickness along
> the length to try and create a more uniform temperature and electron
> emission. Near the bottom, the leads help conduct heat away and reduce the
> temperature. By necking down the wire, the temperature at this point can be
> raised. Lots of tricks like this are possible with fat high current filament
> structures.
>