[Amps] FW: Filament Growth Question

[Matt]

Thanks John.    I can share what I learned from another ham who emailed me
direct and described that the thorium migrates slowly to the surface of the
tungsten element over time, causing the change in microstructure which
increases the brittleness.  He described the process which is the same as
what contributes to the depredation of incandescent light bulb filaments.

73
Matt
KM5VI

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of John Lyles
Sent: Saturday, April 26, 2014 11:19 PM
To: amps at contesting.com
Subject: [Amps] Filament Growth Question

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.
>
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