There are currently two methods of synthesizing diamonds.
High pressure - high temperature (HPHT) methods subject graphite to conditions similar to those
under which natural diamonds are formed in the Earth’s mantle. Most large, gem quality synthetic
diamonds are grown using variants of this method. The most successful involves growing the diamonds
in a ‘flux’ of molten metal alloy.
There are also vapour deposition methods which operate at lower temperatures and pressures, and are
usually used to produce thin films for industrial applications.
Researchers at the Carnegie Institution's Geophysical Laboratory have learned to produce 10-carat,
half-inch thick single-crystal diamonds at rapid growth rates (100 micrometers per hour) using a chemical
vapor deposition (CVD) process. This size is approximately five times that
of commercially available diamonds produced by the standard high-pressure/high-temperature
(HPHT) method and other CVD techniques.
In addition, the team has made colorless single-crystal diamonds, transparent from the ultraviolet to
infrared wavelengths with their CVD process.
"High-quality crystals over 3 carats are very difficult to produce using the conventional approach,"
commented Dr. Russell Hemley who leads the diamond effort at Carnegie. "Several groups have begun to
grow diamond single crystals by CVD, but large, colorless, and flawless ones remain a challenge. Our
fabrication of 10-carat, half-inch, CVD diamonds is a major breakthrough." The results were reported
at the 10th International Conference on New Diamond Science and Technology, Tsukuba, Japan, on May 12,
and will be reported at the Applied Diamond Congress in Argonne, Illinois, May 18.
Most HPHT synthetic diamond is yellow and most CVD diamond is brown,
limiting their optical applications.
Colorless diamonds are costly to produce and so far those reported are small. This situation limits general
applications of these diamonds as gems, in optics, and in scientific research. Last year, the Carnegie
researchers found that HPHT annealing enhances not only the optical properties of some CVD diamond, but
also the hardness [1]. Using new techniques, the Carnegie scientists have now produced transparent diamond
using a CVD method without HPHT annealing.
To further increase the size of the crystals, the Carnegie researchers grew gem-quality diamonds
sequentially on the 6 faces of a substrate diamond plate with the CVD process. By this method,
three-dimensional growth of colorless single-crystal diamond in the inch-range (~300 carat) is
achievable.
The standard growth rate is 100 micrometers per hour for the Carnegie process, but growth rates in
excess of 300 micrometers per hour have been reached, and 1 millimeter per hour may be possible. With
the colorless diamond produced at ever higher growth rate and low cost, large blocks of diamond should
be available for a variety of applications. "The diamond age is upon us," concluded Hemley.
From Carnegie Institution
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