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374 High-Pressure Synthesis (Chemistry)

mercury, and tantalum, generally show a fall in their super-
conducting critical temperatures as pressure increases. In-
teresting exceptions are lanthanum, silver, Mo 6 Se 8 , and a
few other ternary compounds. However, some substances
require pressure for superconductivity—for example, an-
timony, arsenic, barium, yttrium, germanium, or cesium
in their high-pressure forms. The critical temperatures of
the latter at first rise with increasing pressure but then
usually fall at still higher pressures. Phase changes may
complicate this simple picture. So far it appears that
the superconductivity in nearly all materials can be ex-
plained by the BCS type of electron pairing. Increased
understanding will follow as X-ray diffraction studies re-
veal the crystal structures of the various superconducting
substances.

V. PRACTICAL USES OF VERY
HIGH PRESSURES FIGURE 8 Carbon phase diagram showing diamond synthesis
regions.
Some modern metalworking processes use very high pres-
sures in extrusion or cold-forming operations simply be- sures of this level are easily reached and tiny diamonds
cause the metals being worked are relatively strong and can be made for lapping and polishing, as mentioned
the tools are made of even stronger cemented tungsten earlier.
carbides. High hydrostatic pressures, 1–2 MPa, have been The bulk of industrial diamond production is done at
used to form special pieces that could otherwise be formed pressures and temperatures in the range 4.5–6.0 GPa and
only by more expensive methods. Usually the high ini- 1400–1800 K, a range indicated in Fig. 8 by a cross-
tial and continuing costs of very high pressure equipment hatched area. The transformation of graphite to diamond is
make it a last resort. madepossiblebyusingcatalystsolvents,whicharemolten
(carbon-saturated) metals such as alloys picked from man-
ganese, iron, cobalt, and nickel. Platinum and palladium
A. Diamond Synthesis
are also effective but cost more and require higher tem-
Very high pressures probably find their widest use in the peratures and pressures. (Some carbon solvents, such as
commercial synthesis of diamond from graphite. The high AgCl or CdO, do not form diamond from graphite at
valueoftheproductsmakestheefforteconomicallyviable, 5.5 GPa. Diamond-forming catalysts usually carry pos-
and several tons of industrial diamonds are synthesized itively charged carbon in solution.)
each year in dozens of plants throughout the world. Apparatus of the “belt” type is often used. Pieces
Figure 8, the carbon phase diagram, forms a basis for of graphite and metal occupy the heated zone of the
discussing the processes involved. Ideal graphite has a high-pressure chamber. When the chamber pressure has
density of 2.2 and diamond, 3.52, so 1 ml of graphite become suitably high, the hot zone temperature is in-
becomes 0.63 ml of diamond, a relatively large change. creased until the metal melts and becomes saturated with
Diamond is favored to form at pressures and temper- carbon. At this point, diamond begins to deposit from the
atures where it is stable, but the carbon atoms must molten metal and graphite dissolves. Only a thin layer of
be in the proper environment, particularly at the milder metal is involved, and the diamond replaces the graphite.
conditions. Figure 9 is a photograph of a thin layer of nickel catalyst
At temperatures above about 2500 K, thermal agitation on the surface of a mass of freshly grown diamonds. The
alone is usually sufficient to make the stable phase form diamonds are recovered after acid treatment.
in a few seconds or less. Diamond can form from molten The higher the pressure over equilibrium, the higher
carbon (4000 K) in a few milliseconds. The pressures re- the diamond nucleation and growth rate and the smaller
quired for diamond stability are then upwards of 10 GPa, and less perfect the crystal. Lower synthesis temperatures
which are not economic for static apparatus, and the di- favor cubes and higher ones, octahedra. Suitable control
amond crystals are very small. However, dynamic pres- of these variables permits the growth of selected types of