Message: 4
Date: Sat, 26 Apr 2014 02:15:23 -0500
From: "Matt" <maflukey@gmail.com>
To: <amps@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@contesting.com] On Behalf Of John Lyles
Sent: Saturday, April 26, 2014 12:39 AM
To: amps@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.