Page 330 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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376 High-Pressure Synthesis (Chemistry)

B. Cubic Boron Nitride
Boron nitride, BN, exists in three forms: (1) a hexagonal
form, such as graphite; (2) a dense cubic form (zincblende
structure, such as diamond); and (3) a dense hexagonal
form (wurtzite, such as lonsdaleite). The two dense forms
are thermodynamically stable only at higher pressures,
but, like diamond, can be formed at high pressures and
high temperatures and then quenched and recovered for
use or study at atmospheric pressure. The equilibrium
pressures for cubic BN are about 10% lower than those for
diamond. The dense hexagonal form is slightly less sta-
ble than the cubic form and is usually prepared by shock
compression or catalyst-free pressure and heat (10 GPa,
1000 C) from crystalline graphitic BN.
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FIGURE 11 Synthesized high-quality diamond crystals, showing Cubic BN is usually manufactured at about 5 GPa and
◦
their typical growth faces. 1500 C from a mixture of graphitic hexagonal BN and
a catalyst solvent such as lithium or magnesium nitride.
Many other catalyst solvent systems have been found and
most of them involve a nitride-forming element. As pres-
Specialsinglediamondcrystalscontainingabout99.9%
sure and temperature increase, the catalyst requirements
of the carbon-12 isotope have been grown to about 5 mm
relax as with carbon.
in size using the method described above. The carbon-12
The cubic form is widely used as an abrasive or as sin-
source diamond crystals were made by the low-pressure
tered cutting tools for grinding or shaping hard ferrous-,
(10 torr) decomposition of carbon-12 methane at about
nickel-, or cobalt-based alloys. It is not quite as hard as
950 Cinthepresenceofhydrogenatomsgeneratednearby
◦
diamond, but it is more resistant to oxidation and alloy-
by a hot tungsten wire. (The H atoms keep the solid car-
ing with the workpiece metal. Its low wear rate and cool
bon surface atoms in tetrahedral bonding states.) These
cutting action make it a favorite for high-precision work
diamonds are noteworthy for their excellent thermal con-
on cutting tools, cylinders, and rotors. Its price is similar
◦
ductivity at 20 C, about 8 times that of copper and 5 times
to that of synthesized diamond. So far, high-quality single
that of most diamonds.
crystals up to about 4 mm in size have been grown at high
When graphite of good crystalline perfection is com-
pressures using the temperature-difference technique used
◦
pressed to 10–14 GPa and heated to about 1000 C, it
with diamond. The bath was an alkaline earth nitride-BN
mostly collapses into diamond. Much of this diamond is
complex contained in a molybdenum can. It was possible
not cubic but hexagonal, like the wurtzite structure. This
to grow n-type (S-doped) BN on a p-type (Be-doped) BN
happens because melting did not occur, and the diamond
seed crystal to form a p–n junction diode a few millime-
form was forced by the form of the graphite. The graphite
ters in size which emitted blue light when carrying current
collapsed in a direction parallel with the hexagonal sheets
in the forward direction.
of atoms, like squeezing a deck of cards on the edges,
not the faces. Traces of hexagonal diamond, called lons-
daleite, also appear in shock-formed diamond, natural or
C. Synthesis of Other Inorganic Materials
synthetic. Lonsdaleite is slightly less stable than regular
cubic diamond and changes to cubic if heated hot enough Although at least hundreds of new high-pressure phases
or exposed to solvent catalysts at high pressures. have been made in the search for other materials with
The tendency for diamond formed under nonfluid con- useful applications, the primary benefit has been greater
ditions to be influenced by the structure of the precursor understanding in solid-state chemistry and physics. The
carbon can be noted when hydrocarbons are decomposed closely related effort to understand the properties of the
at 12 GPa. Aliphatic hydrocarbons, which already posses deeper materials of Earth and the other planets will con-
tetrahedral carbon bonding, seem to slowly lose hydrogen tinue to be one of the driving forces for high-pressure
and approach cubic diamond. Purely aromatic molecules studies. Metallic ammonia and metallic hydrogen are of
such as anthracene change to graphite, then finally to di- direct interest in the structure of the larger planets, and
amond at higher temperatures. Adding aliphatic carbon it is hoped that the conditions for synthesis of metallic
atoms to the molecules or the mixture favors diamond for- hydrogen might be attained in the diamond anvil. It is es-
mation at lower temperatures. timated that above about 300 GPa would be required. This