Modern Engineering Thermodynamics

3.7 Phase Diagrams 71
considerable sum of money to purchase a small diamond, then he burned it in a controlled oxygen environment. When he
analyzed the resulting combustion gas, he found it to be just carbon dioxide.
FIGURE 3.12
A real diamond.
From that time forward, many attempts have been made to make synthetic diamond from pure carbon (graphite). However,
diamond was not successfully synthesized until 1955, at the General Electric Corporation in Schenectady, New York,
when GE researchers compressed graphite to a pressure exceeding 1.5 × 10 6 psi (10. GPa) and 5000.°F (~3000.°C). Industrial
and gemstone quality synthetic diamonds have been commercially available since 1960.
The pressure-temperature phase diagram for carbon is shown in Figure 3.13. At low pressure and temperature, the solid carbon
phase is called graphite. At very high pressures and temperatures, a second solid carbon phase appears with a different
atomic structure. This phase is the valuable gemstone diamond with which we are all familiar. However, the phase diagram
clearly indicates that graphite, not diamond, is the equilibrium form of solid carbon at room temperature and pressure.
Since phase changes are rate processes that increase rapidly with increasing temperature, what is happening to all diamonds
that are exposed to room temperature and pressure? What would happen to a diamond ring if you put it into an oven at
1000 or 2000°F? Can you suggest a practical way of making a synthetic diamond? Do you think that exposing graphite to
explosive loading using dynamite under controlled conditions would work?
10,000,000
1,000,000
100,000
Pressure (psia)
10,000
1,000
100
Carbon II
(diamond)
Carbon I
(graphite)
Triple
point
Liquid
Vapor
Center
of Earth
240 miles
below Earth’s
surface
Breech of highest
power rifles
Deepest part
of ocean
Automobile
engine cyclinder
pressure
10
Atmospheric
pressure
1
FIGURE 3.13
Phase diagram of a diamond.
0.1
0 2,000 4,000 6,000 8,000 10,000
Temperature (°F)