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High-Pressure Synthesis (Chemistry) 375

conditions. Selection is not done by theory because of the
complexities of the grinding process.
The need for industrial diamond in sizes over 1 mm
has largely been met by a sintered diamond material
made of smaller grains. The sintering process uses pres-
sures and temperatures similar to those for initial syn-
thesis with a few percent by volume of a sintering aid,
usually cobalt. The cobalt helps to form direct diamond-
to-diamond bonds, which give the mass high hardness
and thermal conductivity. The randomly oriented poly-
crystalline structure gives good strength and shock resis-
tance. Such pieces of sintered diamond are widely used
for cutting tools for hard materials, including rock (but
not iron, nickel, or cobalt-based alloys), for dies for draw-
ing wire, and for dressing abrasive wheels. The residual
cobalt weakens them at temperatures above about 800 C,
◦
FIGURE 9 Freshly grown diamonds bearing a thin film of nickel.
but those from which the cobalt has been leached, though
The arrow indicates a bare octahedral face about 0.1 mm in size.
not as strong, are durable to 1200 C. Various shapes are
◦
available in sizes up to about 2 cm. Some have been used
crystals for particular uses (e.g., dendritic, friable crys- for special very high pressure apparatus reaching 50 GPa.
tals cut hard metals more efficiently than blocky crys- Single diamond crystals of gem quality can be grown at
tals, while strong cubo–octahedra are preferred for cutting high pressure using the reaction cell arrangement shown
rock). Commercial diamond grains are now available in in Fig. 10. The carbon source is a mass of small diamond
◦
sizes up to about 1 mm. Progress in synthesis and applica- crystals maintained at 1450 C at the top of a bath of molten
tion has dramatically reduced the costs of using diamond catalyst metal alloy (e.g., iron). Diamond grows on a seed
abrasives since the introduction of synthesized diamond in crystal held at about 1425 C at the bottom of the bath.
◦
1957. The price as of 1986 ranges from about $5 per gram Stray diamond nuclei tend to float up out of the growing
for powders to about $15 per gram for 0.7 mm rock-sawing zone. The bath is held in a sodium chloride vessel whose
◦
crystals. meltingtemperatureisabove1450 Cattheoperatingpres-
The abrasive grains are usually held in the rim of a sure, about 5.5 GPa. About a week is needed to grow an
wheel by a matrix of resin or sintered or electroplated acceptable single crystal about 1.3 carat and about 6 mm
metal. Usually the diamond-bearing part contains 25% in maximum dimension; recently a 3.5 carat crystal was
or less diamond by volume. The grains themselves may grown in about 200 hours. The process is not econom-
be bare, or they may be precoated with a thin layer of ically practical, but special crystals of scientific interest
copper or nickel applied by electroless plating techniques are grown. Many of these are more pure and more inter-
(controlled reduction of a solution of metal salt). Diamond nally perfect than any natural diamond. A few parts per
surfaces are generally difficult to wet or adhere to, but the million of nitrogen yield yellow crystals in which nitro-
metal provides a mechanical grip on the grain while the gen atoms replace carbon atoms. A few parts per million
metal itself is easier to bond to a matrix. As a wheel is of boron yield blue, p-type semiconducting crystals.
used, the diamond grains break up or wear down. In many Figure 11 shows several crystals of various types.
uses the metal coating helps hold fragments in place and
dissipate heat.
The grinding process is more rubbing than cutting, so
that local pressures and temperatures are high at the con-
tact areas. Indeed, the processes of abrasion could be
considered a branch of high-pressure, high-temperature
chemistry, although the conditions are complex and tran-
sient. For example, diamond rubbing on clean, hot iron is
rapidly attacked, but diamond lasts a long time rubbing or
cutting glass. The selection of abrasive grain, type, size,
matrix, and so on, depends on the particular application FIGURE 10 High-pressure cell for growing single diamond
and is usually done by extensive testing under various crystals.