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Re: Filament -Thoriated Tunsten



At 17:36 4/8/98 -0700, you wrote:
... Someone else in the group gave the recipe for light
bulb filaments, a ductile tungsten copper alloy which was then
purged of its copper by heating.
...
Leigh

Hehehe...I was unwise enough to uncritically quote this sinter/copper
or nickel coat process in connection with filament production on a
news group, having read something to that effect on this list.

This particular "copper" stage occurs much earlier apparently, and with
continued swageing or drawing the tungsten wire becomes moderately ductile
unless alloyed with unfavorable metals - and so I was quite properly
castigated.

Could it be that the Brits got their recipes mixed up?

I am disappointed that Leigh should pursue the stereotyped response to
the "Brit" part of Enc Brit. I thought it was quite generally known which
National resources provide a large part of this excellent first reference,
these days?

Thoriated tungsten was used for vacuum tube cathodes. The thorium
lowered the work function of the metal, enhancing emission of
electrons.

While it was true that thoriated tunsten was used for some directly heated
tubes (other choices being straight tungsten or tantalum), the great majority
of tubes during the hey-day of vacuum-tubes were indirectly heated. In
this case, the tungsten heater was placed inside a nickel tube coated with
oxides of barium, calcium, or strontium. These had superior work-functions.

Leigh's familiarity stems perhaps from the (Radio) Amateur use of
thoriated tungsten direct heated cathodes in transmitter tubes despite
their poorer work function, necessitating higher temperatures over 1950K,
compared with the efficient indirect cathode coatings which gave
satisfactory service below 1150K.
High transmitter anode voltages tended to disintegrate the more delicate
cathode coatings, while thoriated tugsten held up better.

I expect I had better reassure folks that I used a text published in
West Nyack for technical support in this response. :-)


Whatcott Altus OK