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CARBON AT HIGH TEMPERATURES AND PRESSURES
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211
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It
appears that the thermal conductivity of the carbon or graphite at or
near the temperature of vaporisation is very greatly in excess of that
anticipated, or that the rapid transfer of heat is caused by carbon
vapour, which appears to have a great power of penetration through
carbon at high temperatures. The melting of the poles and the
destruction caused by short circuits which reached 80,000 amperes in
the mould were not only costly to remedy, but caused contamination of
the carbon from the metal of the poles and the insulating material.
In several experiments a nucleus of very soft graphite about 2-1/2 inches
in diameter was found in the centre. And in several experiments small
masses of iron, highly charged with graphite, were found in varying
positions among the carbon or graphite.
This
method, however, would probably be more successful if carried out on a
much larger scale, as for a given central temperature the transfer of
heat to the poles and mould would be less, and water jackets would then
prove more effective. It is, however, difficult to construct water
jackets to withstand more than 30 tons per square inch, and unless made
of hard steel they crush in. The maximum power of the press is 2500
tons, and with the apparatus at hand if the size of the mould was much
increased the pressure in the mould would have to be decreased.
Another
plan was then adopted of interposing an insulating barrier of some
refraetory material with a hole in it between the poles, the charge in
the first instance being graphite. It was hoped that by means of
electrical currents of higher potential and large volume the energy
would be so concentrated on the small volume in the neck as to melt it
before it had time to form carbides with the material of the barrier.
This was to some extent achieved in that the graphite in the centre was converted to a softer and more flaky nature.
In
one of these experiments the barrier was formed out of a block of fused
magnesium oxide, specific gravity 3-65, and the pressure in the mould,
which was 4 inches in internal diameter, was in this case raised to 100
tons per square inch. The strongest steel poles were required for this
pressure, also the mould of gun steel became permanently strained and
required reboring after each experiment.
A current at about 12 volts at the terminals in the mould, developing about 100 kilowatts, was turned on for 7 seconds.
The
initial diameter of the hole in the barrier was 5/8 inch and the
thickness about 3/4 inch.* This barrier was converted to magnesium
carbide of a green colour to a radial depth of about 3/8 inch. Thus
this magnesium oxide when heated under pressure with graphite readily
forms a carbide. The
*
The heat units delivered on to the neck being about four times that
required to raise the graphite column through 5000° C, taking the
specific heat at 0-5.
14-2
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