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Re: Better Electrode Material

Tesla List wrote:
> 
> Original Poster: "David Huffman" <huffman-at-fnal.gov>
> 
> Hello All,
> Just a thought here. Has anyone ever tested the effect of various metals in
> a spark gap? I remember hearing zinc is bad for several reasons. Brass is
> zinc and copper so maybe it would not be the best choice. It would be nice
> to know the performance of a SG with different types of metals.
> Dave Huffman

Hi Dave!

There's actually quite a bit of research that's been done in this area
by spark gap researchers in the high-energy/pulsed-power arena. For a
variety of reasons, the hands-down winner for repetitive spark gaps is
either pure tungsten or a composite alloy of copper and tungsten
(commercially known as Elkonite) where tungsten is imbedded in a matrix
of copper. Because of tungsten's high melting and vaporization
temperatures, it's highly resistant to erosion as long as you don't
overheat it (i.e., by causing the electrode tips to become incandescent
and oxidize). The key to long electrode life is to use materials with
high melting and evaporation temperatures. Other refractory materials
that have been used with some success include molybdenum and niobium
(which is much easier to machine).

However, because of its excellent thermal conductivity, copper is still
excellent for relatively low power static gaps as long as you keep the
electrodes cool. The copper oxide (the black or dark green material that
forms in the gaps) can tend to foul gaps with close spacings, however. A
build-up of oxide can increase breakdown voltage, and it can also
seriously degrade the gap's quenching capability. BUT... it's cheap, you
can easily machine and cut it, and readily available. Brass tends not to
last as long, particulalry for heavier current levels. On a high-power
triggered gap I've noticed that the zinc tends to evaporate from the
alloy forming a coating of white zinc oxide in the immediate vicinity,
and leaving electrode surfaces that resemble a cracked, dry lake-bed. 

Excellent technical sources for spark gap information are Glasoe's
"Pulse Generators", McGraw-Hill, 1949 (still referenced as the best
source of information for rotary spark gaps by modern-day texts!), and
"Gas Discharge Closing Switches", Plenum Press, 1990 (the "bible" from
which much of the electrode information above was extracted). While both
are out of print, they are available in many technical libraries.

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
http://www.teslamania-dot-com