Experiments on the conversion of diamond to graphite
A
clear octahedral diamond was placed in a small carbon crucible and
packed loosely with Acheson graphite and heated for 10 minutes to about
1400° G. The diamond was coated with a firm layer of graphite.
After
two prolonged treatments with fuming nitric acid and potassium
chlorate, alternating with boiling sulphuric acid and nitre, the opaque
coating was removed and there remained a blackish translucent skin.
When fractured the interior was unaltered and perfectly transparent.
A
piece of bort somewhat laminated, after the same treatment, showed the
laminations separated by cracks starting from the outside. Upon
breaking, the interior surface of the fissures showed an incipient
change to graphite, but less rapid than on the outside surface. There
was a sinuous pitting, deepest near the outside and diminishing
inwards. The substance of the bort between the fissures was unaltered.
The
change of diamond to graphite under the conditions described is
gradual, the surrounding gases, carbon monoxide, carbon dioxide,
nitrogen., hydrogen, and also vapour of iron (as an impurity in the
graphite) singly, or collectively, probably play a part, and further
investigation as to this seems to be desirable.
Sir
James Dewar, in 1880, heated a diamond in a carbon tube to a
temperature of 2000° C, while a flow of pure hydrogen was maintained
through the tube. The diamond soon became covered with a coating of
graphite (Proceedings of the Royal Institution).
A
clear diamond plunged into molten iron saturated with carbon at about
1400° C. for 5 minutes was deeply pitted. When removed from the iron
small globules of iron adhered to the surface and the pits appeared to
occur at these spots.
A
clear diamond was disintegrated by cathode rays, the temperature by
pyrometer being 1890° C, the splinters were quite black and opaque, but
after several prolonged treatments with fuming nitric acid and
potassium chlorate, alternating with boiling sulphuric acid and nitre,
the coating that remained was a dusky grey, but semi-transparent, the
gas present being chiefly hydrogen. (Paper by Parsons and Swinton,
January 16th, 1908, Roy. Soc. Proc. A, Vol. lxxx.)
In
this latter experiment the surface action appeared to be much less in
proportion to the incipient change of the under layer to graphite, and
the impression is that at 1890° C. the temperature of bulk
transformation is being approached, also that carbon monoxide, carbon
dioxide, nitrogen, hydrogen, and iron, one or more, act as catalysts in
the change of diamond to graphite.
