thoria and tungsten

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

Some research into the whole thoriated tungsten electrodes thing (why 
do they do it in the first place)..

There's some literature (e.g a paper by Lowke, et al, in Journal of 
physics. D, Applied physics ) talks about the difference in heating 
between pure tungsten and thoriated tungsten in arc 
electrodes.  Thoriated electrodes stay cooler, and hence, last longer.

As the papers say this is not well understood, but, it appears that 
addition of rare earths (and thoria has been used for this for 
decades, e.g. in vacuum tube filaments) to tungsten greatly increases 
the electron emissivity when the electrode is hot.  So.. to carry a 
given current, you don't have to have as big a spot on the electrode 
that is hot to generate enough electrons to carry the arc current. 
Less of the electrode hot, less heating, less erosion, etc.

The move to non-thoria appears to be entirely motivated by a fear of 
radioactivity (you have to get pretty contrived before the dose from 
Th232 gets significant). Initially ceriated rods were used (Cerium 
Oxide has been used in carbon arc lamps for centuries, and it's also 
a component in "flints" for those of you who have non-piezo fired 
lighters), but lanthia is now more popular. 1-1/2% lanthia works 
about as well as 2% thoria (it's less dense, so the volume ratio is 
probably about the same), but is more expensive.


I suspect that the change from thoria in lantern mantles is the 
same.  If it is ytrria, it's a LOT more expensive than 
thoria.  Thorium is found in uranium ore, which is everywhere, and cheap.


From the web: Nevertheless, NUREG-1717 assumed that a typical mantle 
contained 250 mg (0.027 uCi) for the purpose of its dose 
calculations. They noted that some mantles contained up to 400 
mg.  Luetzelschwab and Googins measured a total of 0.065 uCi in two mantles.

In 1952, approximately 65 % of the thorium produced in the U.S. was 
being used in lantern mantles. It has been estimated that the annual 
production/sales of mantles in the U.S. was 20 million in 1974, 25 
million in 1981 and 50 million in 2000.  Sometime around 1990, the 
Coleman Company replaced the thorium in their mantles with 
yttrium.  The yttrium mantle produces about 20% less light, but it 
lasts longer. According to NUREG-1717, about half of the 50 million 
mantles sold in the U.S. in 2000 (or thereabouts), contained thorium.

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25 million mantles at 0.25 g each.. 6000 kg of thorium a year.. I 
suspect it's a byproduct of the nuclear fuel industry.