Allotropes of Carbon
The impetus for this project came from a patent
I read a while back. U.S. Patent 5,437,243
"Process for Fabricating Diamond by Supercritical Electrical Current"
claims "A method of producing allotropic transformations of graphite to
diamond, said method comprising the steps of : providing a specimen of
selected shape and of the substance to be transformed; physically confining
the specimen with electrically insulating high strength material so that
the specimen will hold its shape under pressure; applying pressure to the
specimen between electrodes of a refractory material; applying to the specimen
through the electrodes a pulse of electrical current of super critical
density and extremely short duration; and causing said current to flow
through the entire specimen to produce an internal electrical field inside
the entire specimen for a duration of only a few microseconds during which
the allotropic transformation of the entire specimen takes place without
changing the original shape of the specimen." Sounds simple enough,
zap some pencil shavings and, presto-chango, diamond. My first reaction
was: "bullshit!" Reading the patent in greater depth convinced me
that, if it worked at all, I could do it my basement.
This picture shows my diamond rig assembled,
loaded with graphite and ready to fire. The electrical bits are identical
to those described for my improved can crusher. The press frame is
made from four lengths of 1/2-13 threaded steel rods seated in a 12" X
12" X 1/2" mild steel base. The top plate is an 8" X 8" X 1/2" thick
piece of 316 stainless steel. (This was scavenged from the scrap
pile at work, originally it held a blow down pot for a high pressure reactor.)
The middle platform is an 8" X 8" X 1/2" piece of polycarbonate that floats
on the threaded rods and has two 4" X 4" X 1/2" aluminum plates epoxied
on either side. A 1/2" square copper bus bar rests on the upper aluminum
plate and is connected to the output of the triggered spark gap.
Another 4" X 4" X 1/2" piece of aluminum is glued to the underside of the
SS plate with silver filled conductive epoxy. The entire frame is
connected to the low potential side of the capacitor bank with 1/2" square
copper bus bar. A 2-ton hydraulic jack forces the floating plate,
and bus bar against the sample cell, which is restrained by the upper plate.
The cell consists of two 1/2" copper cubes, drilled 1/4" deep with an F
size drill (0.257", nominally) and a two inch length of 1/4" OD X 1/8"
ID high alumina ceramic (a.k.a. sapphire). The ceramic tube was tightly
packed with graphite, using a hardened steel rod and ball peen hammer.
The sample cell is surrounded by a 3" length of 4" OD X 3 3/4" ID polycarbonate
tubing, to slow down any shrapnel. The aluminum plates, polycarbonate
tubing, copper bus bar, sapphire tube and letter size F, titanium nitride
coated, drill bit were all purchased from McMaster-Carr.
A quick resistance check of the sample showed
1.7 ohms. With the spark gap set for 25,000 volts, I charged the
caps (1.46 micro Farads) and triggered the relay. This resulted in
the normal flash and snap from the gap. After shorting the capacitor
to make sure it was discharged, I tested the sample again and found a resistance
of 1.9 ohms. A second shot at the same voltage raised the resistance
to 2.1 ohms. Next, I cranked the gap out to 40,000 volts and tried
again. The results are shown below.
Apparently, the earlier shots had loosened the graphite (explaining
the resistance drop), allowing some air to enter the cell. This run
heated the air enough to spring open the cell and expel part of the sample
(with an 1/8" diameter cross section under 2 tons of force, this means
that the cell pressure must have exceeded 300,000 psi). Amazingly,
the sapphire tube was unaffected. I repacked it with graphite and
added a pair of 1/4" OD X 1/32" thick graphite sheet gaskets at each end
to help keep air out. The measured resistance was 1.6 ohms.
Another shot at 40,000 volts had no effect on the resistance and the cell
Closer scrutiny of the patent suggests that my failure can be traced
to too large of a sample cross section. The patent specifies that
a super critical current must be applied for transformation to occur.
What this current is was not apparent at first reading. The author suggested
that a one micro Farad cap and a 20,000 volt supply were sufficient to
demonstrate the patent. However, he also claimed that the transformation
requires ~ 4 microseconds and a critical charge density of 1,000,000 coulombs/square
centimeter. This translates to a critical current of 250 giga-amps/square
centimeter. Either I need a bigger capacitor (2 FARADS! at 40,000
volts) or a smaller sample (1/10,000" in diameter).
Nuts. Well, at least this didn't cost me much. Most of the
bits and pieces were scrap from other projects. The jack was $12
at K-Mart, the sapphire tube $6 for a 12 inch length, the graphite powder
$6 for a pound and $5 for the drill bit. Time to find another project
(but, I'll probably keep an eye out for giant caps and extra fine graphite