For the self-taught or formally-trained experts on this forum who
have vacuum electron tube material knowledge, ignore this note.
For those interested, there is an out of print book which pretty much
summarizes everything about the stuff tubes are made of, and how they
interact and operate in the vacuum. Its Walter Kohl's Handbook of
Materials and Techniques for Vacuum Devices. I have in front of me
the 1967 edition published by Reinhold Publishing Corp. The earlier
editions went by the names Materials and Techniques for Electron
Tubes (1960) and Materials Technology for Electron Tubes (1959). Kohl
was with GTE at that time, I think. In 1967 he was with NASA.
Chapter titles:
Glass
Ceramic
Mica
Carbon and graphite
Iron and Steel
Copper and Copper Alloys
Nickel and Nickel Alloys
Precious Metals and Their Alloys
Tungsten and Tungsten Allows
Molybdenum and Molybdenum Alloys
Tantalum and Columbium (Niobium) and their Alloys
Titanium, Zirconium and Hafnium and their Alloys
Soldering and Brazing
Glass to Metal Sealing
Ceramic to Metal Sealing
Cathodes and Heaters
Grid Structures and Coatings
Getter Materials
Secondary Emission
Voltage Breakdown
Quotes from the chapter on Carbon and Graphite:
"Graphite Anodes
Graphite offers many advantages as an anode material in power tubes,
high voltage rectifiers and ignitrons where large overload capacity
is required.... At equal power input, the high radiancy of graphite
results in a substantially lower temperature of operation than that
attained by other anode materials. Although the vapor pressure of
graphite is somewhat higher than that of tungsten and tantalum at a
given reference temperature, the much lower operating temperature of
graphite at a given power input gives it a marked advantage...."
"..Another difficulty frequently encounted with graphite parts in the
past was the danger of having loose carbon particles released during
the operation of the tube by abrasion of the graphite surface where
it is in contact with metal parts, or by the action of high-voltage
gradients. Such loose carbon particles might find their way to the
cathode and cause poisoning of the emission or produce noise in the
tube, These difficulties have now largely been overcome. Not only is
graphite now being produced from a finer and more uniform grade of
carbon so that it takes on a hard surface and can be polished, but it
is also possible to electroplate the surface with chromium, or apply
a coating of zirconium, or metallize the surface with molybdenum or a
combination of molybdenum and vanadium. "
"...Since the density of graphite is quite low, a substantial
increase of wall thickness can be tolerated without increasing the
weight of the part. The wall thickness of anodes is usually held to
the range from 60 to 80 mils...."
"....Although dense, vacuum tight, graphite is now available,
material chosen for anode structures may be of a a porous nature and
consequently absorb large quantities of gas. A special outgassing
treatment therefore becomes necessary before such anodes or other
graphite parts are mounted in the tube..."
"....The carburizing of thoriated tungsten filaments by flashing in a
hydrocarbon atmosphere must be done before the graphite anodes are
mounted because the removal of absorbed hydrocarbons from the porous
graphite body would require a much longer pump cycle. "
In the voltage breakdown chapter:
Sorry to only summarize what it says in a short series of quotes, but
i am working (employed) right this minute.
"...The many parameters that enter into voltage breakdown under
various conditions have made it difficult to consistently explain all
the observed effects.... no theory as yet describes quantitatively,
or even qualitatively, all the experimental results....."
"Field emission of electrons from protrusions at the cathode surface
has long been recognized as an important mechanism in the initiation
of pre-discharge currents......FE sites on the cathode surface are
not so much microscopic protrusions but rather submicroscopic
whiskers that appear in a time of milliseconds in the presence of
high fields.... "
"A more recent investigation... further elucidated the cause for
whisker growth. The experiments were conducted with a 0.25 mm gap at
a pressure of 10^-8 Torr, using aluminum, copper, and 304 Stainless
steel electrodes. It could be shown that whiskers appear at the
splash edge of craters on the cathode which are caused by the impact
of particles released by the anode, and that they grow from molten
metal in the presence of high fields, as previously stated. The
mechanism for the ejection of the anode "clumps" is not yet
established."
"In the presence of thermionic cathodes from which active materials
evaporate, the condensation of these evaporants on whisker sites
causes breakdowns at fields well below those to which the surfaces
where previously conditioned."
Want to know more, or put some scientifically proven work behind your
theories? Get the book. The references in each chapter are quite
extensive.
73
John
K5PRO
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