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High-Pressure Synthesis (Chemistry) 367

by pressure, according to whether the intermediate state is light elements, with diamond at the top. Usually, hard
less or more voluminous. The intermediate states in vis- materials are brittle because the strongly directed bonds
cous flow or diffusion are more voluminous, and these that favor hardness do not favor plasticity, which involves
processes are strongly hindered by pressure. the intersite motions of atoms during which the attractive
If the density change on melting is large, pressure will forcesontheatomsremainrelativelyconstant.However,at
have a large effect on the melting point. The melting tem- sufficiently high ambient pressures many normally brittle
◦
perature of NaCl, for example, rises from 801 Cat1atm materials become plastic as the overall compressive stress
◦
to about 1900 C at 10 GPa. The melting point of iron makes repulsive forces predominate between atoms. Thus,
◦
rises from 1535 C at 1 atm to about 1700 Cat5MPa. cracks become energetically unfavorable, although the re-
◦
For materials such as bismuth, water, silicon, and proba- sistance to deformation may increase. This phenomenon
bly diamond, the liquid is more dense than the solid and has some applications in industrial processes and in
the melting point decreases with pressure. The variation geology.
of melting temperature with pressure is given by: The long chains of atoms present in oils, greases, and
polymersbecometightlyentangledathighpressures;most
dt/dp = V/ S (1) atomic displacements then involve breaking of chemical
bonds, and the viscosity or shear strength rises markedly.
where S and V are the entropy and volume changes Such “hardening” of oil is probably important in the lu-
associated with melting. brication of highly stressed areas on cams, gear teeth, etc.
Generally speaking, it is easy to find a substance that
will exhibit some kind of a phase change as the result of
compression, but it is more difficult to find a substance III. METHODS FOR GENERATING
that will retain its high-pressure form after the pressure VERY HIGH PRESSURES
on it is reduced to 1 atm. In most substances the internal
bonding of the high-pressure phase is too weak to pre- Two general methods are available. In the “static” method,
serve the structure against decompression or thermal agi- the substance is confined by the strength of materials and
tation. Hence, most high-pressure forms must be studied the exposure times are long—seconds to months. In the
at high pressure, and many ingenious devices have been dynamic method, the substance is confined by inertia and
made for such studies. The few high-pressure forms that the exposure times are short, of the order of microseconds,
can be “brought back alive” are typically hard, refractory due to the difficulty of maintaining large accelerations for
materials such as carbon, silicon, and silicates. These are long time periods. Nevertheless, the highest pressures are
usually formed at high temperatures and pressures and achieved by dynamic methods.
then quenched for leisurely study at low pressure.
The problem of recovery leads to the question of hard-
A. Static Apparatus
ness. Hard substances have a high number of strongly
directed, covalent chemical bonds per unit volume. Soft The simplest apparatus is the piston and cylinder, shown
substances generally have fewer bonds per unit volume or in Fig. 1. The pressure is the force on the piston di-
bonds that are weak or weakly directed, such as ionic or vided by its area, after allowing for friction and distortion.
dipole attractive forces. Bond energy per unit volume has The strongest practical piston material is cobalt-cemented
the same dimensions as pressure (force per unit area), and tungsten carbide. In certain compositions, around 3–
a plot of hardness measured by the Knoop indenter versus 6 wt%, cobalt can have a compressive strength of 4–5GPa
the bond energy per molar volume for various substances along with sufficient ductility to absorb inevitable local
is essentially linear, provided that one chooses substances high stresses without failure. The strongest cylinders are
for which the bonding is predominantly of one type (i.e., made with a stiff cemented tungsten carbide inner shell
not mixed, as in graphite or talc). that is supported against bursting (and partly against axial
Covalent (electron pair) bond strengths vary between delamination) by prestressed steel rings.
approximately 60 and 90 kcal/mol for most elements Let us examine the stresses and distortions that accom-
present in hard materials, but the cube of covalent bond pany the generation of pressure in this apparatus. In Fig. 1
˚ 3
length varies even more: approximately 3.65 A for C C, the original (zero pressure) shapes of piston and cylinder
˚ 3
˚ 3
6.1 A for Si O, and 14.3 A for Ni As. The heavier are shown by dotted lines; the distortions, shown by the
elements generally offer more bonds per atom, but this solid lines, due to pressure are exaggerated. The bulging
usually does not compensate for the larger molar volumes of the piston is most pronounced above the cylinder; inside
except in certain interstitial compounds such as WC and the cylinder the piston is supported by, and rubs on, the
TiN. Thus, the hardest materials are generally made of wall of the cylinder. The sharp change in radial bursting