"Ian Iveson" wrote in message
. uk...
: "Ruud Broens" wrote
:
: ...Even lower, say below 10 V or
: so, the cathode will no longer be uniformly heated, resulting in
: increased
: noise and the risk of cathode sputtering - this *does* decrease
: lifespan!
:
: Aha, right, cathode sputtering, er, what's that?
:
: Searched and got a zillion hits. Seems sputtering involves
: disintegration of the cathode coating by ion bombardment (what we
: call stripping) and subsequent deposition elsewhere. You got me
: interested because I thought of it as a deposition process and
: hadn't considered the stripping part.
:
: So where does the stripped material get deposited? I thought it
: ended up as crumbs and powder that just fall to the bottom of the
: envelope. My chemistry is hopeless.
:
: If part of the cathode is not protected by space charge, but the
: valve is operating, then I suppose that part could get bombarded?
: But where does the continuing supply of ions come from?
:
: Also, the most common cathode material replenishes its surface by
: leaching, er, I thought, dimly. I had in mind some kind of dynamic
: equilibrium.
:
: Cathode poisoning has the opposite cause: leaving heaters on with no
: HT. Is that sputtering, with the heater acting as cathode?
: Presumably poisoning stops the leaching.
:
: cheers, Ian
:
Cathode failure mechanisms:
if heated above 850-900 C, Ba will evaporate from the monatomic layer
the 'work function', the escape energy required for electrons, increases;
the emission is lowered.

at even higher temperatures, the current density will enable sputtering,
that is macroscopic particles from the cathode get 'ripped loose'.
....so, better not go much if at all above rated filament power ;-)

If the cathode temperature is too low, the brittle oxide layer becomes locally
overheated, chips 'fall off' - hot spots.

all tubes contain some air molecules. when the accelerating force for the
electrons
is large enough, anode voltage above 20 Volts or so, collisions with air
molecules
result in ionization - these ions love to meet the cathode , hot date ;-)
there, they poison the cathode
so, running indirectly heated tubes at high voltages shortens the usuable
cathode's lifespan.

cheers,
Rudy