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The state of fundamental knowledge on the subject of graphite and
the graphitisation process is reviewed. The principle methods of
manufacture may be considered to include (1) conventional graphitisation
of a coke filler-binder mix, (2) the compaction at high pressure and
temperatures of natural or artificial graphite particles without a binder,
(3) pyrolytic graphites derived from gaseous deposition, and (4)
conventional graphites impregnated by liquid or gas and re-graphitised.
The present state of development of these processes is examined. The
erosion of graphite by high velocity gases at high temperatures is due
primarily to oxidation effects which occur preferentially at crystallite
boundaries. Coatings of carbides and nitrides improve the resistance
at temperatures below about 1700 degrees C, but above this, pyrolytic coatings
are more successful. The addition of vapourising compounds, iodides and
fluorides, or the addition of carbides and nitrides to the graphite
mix, are both beneficial, but of little value at very high temperatures.
The development of new graphites, either the impregnated type, or those produced by pressure baking, may offer a margin of improvement, as the
best surface structure at temperatures of 3000 degrees C and above appears to
be simply graphite. Additions may do little to improve the mechanism
of erosion, but they may usefully lower the surface temperature.
Considerations relating to thermal shock, creep and fabrication are
surveyed. Some of the conclusions are: that graphite is of singular
importance to high temperature technology; that commercial issues
cannot be allowed to impede vigorous development towards more resistant
forms; that much is to be gained by viewing graphite from a metals
standpoint; that the fundamental theory of the basic crystal mechanics
is undeveloped; that the present wide variability in properties should
not be regarded overseriously; that non-destructive assessment by
damping measurements needs development, that coatings and impregnants
are of high priority, and that, of all factors, oxidation is the most
serious limitation to use at the present time. |
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