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Σάββατο 31 Μαΐου 2014
Παρασκευή 30 Μαΐου 2014
Πέμπτη 29 Μαΐου 2014
Creating Man-Made Diamonds
Currently, there are two primary technologies that can create diamonds: HPHT and CVD.
HPHT - High Pressure High Temperature
The
high pressure, high temperature process recreates conditions found deep
inside the earth, in which diamonds naturally grow under. There are
multiple designs for HPHT growth units.
Belt Press
A
belt press was the first successful design, and some belt presses can
be many stories tall and weigh thousands of pounds. This design has two
anvils that press together and can grow many diamonds in one cycle,
though conditions cannot be controlled as easily, creating varying
quality within a single cycle. These presses are most commonly used for
industrial diamonds and powder, though are capable of producing
gem-quality stones.
Cubic Press
A
cubic press uses six different anvils pressing onto a cube. These
presses can vary greatly in size and are most commonly used to produce
industrial diamond powder.
BARS Press
A
BARS press was developed by Russian scientists and is around the size
of a large household appliance. It uses six inner anvils and eight
spherical outer anvils to apply hydraulic pressure to the growth cell.
One cycle of a BARS machine produces one diamond crystal. These BARS
presses are to-date, the most effective process for growing
comparatively large gem-quality diamonds.
Inside HPHT
At
the core of the HPHT machines is a growth cell. This cell contains all
the elements and materials necessary to grow the diamond. This includes a
tiny diamond seed, highly refined and purified graphite (carbon) and a
catalyst of mixed metals and powders that facilitates the diamond
growth. The growth cell is placed at the center of the machine and is
heated to over 1,300 degrees Celsius and over 50,000 atmospheres of
pressure is applied. As the temperature and pressure increase, the
catalyst turns into a molten metal solution.
Once
the ideal conditions are reached, the graphite dissolves into this
solution. Through a controlled cooling process over the course of many
days, the carbon atoms slowly build upon the crystal structure of the
diamond seed. The diamond normally grows in a truncated octahedron or a
hexa-cubic shape, depending on parameters of the growth process. Once
the machine runs its full cycle, the growth cell is removed. The new
diamond is cleaned and ready to be cut and polished, just like any other
rough diamond.
During
the growth, the temperature and pressure need to be maintained within a
very strict set of parameters. If there is any fluctuation, the diamond
can either stop growing, or become so heavily included, the usable
gem-quality size is quite limited. It is not possible to see the diamond
during growth, so in most cases the machine runs a complete planned
cycle, even though the diamond may have become heavily included or
stopped growing part way through the cycle.
The different colors grow at different rates, which is why the size availability
varies based on the color. It is exponentially more difficult to grow
larger sizes, as the longer the cycle runs, the more prone it is to
failure.
CVD - Chemical Vapor Deposition
Chemical
vapor deposition is a very different process from HPHT, though they
both ultimately grow diamond. The environment inside a CVD machine is a
small fraction of one atmosphere of pressure. Carbon-based gasses,
commonly methane, are added to the environment and are heated to many
hundreds of degrees Celsius which causes the gasses to break apart and
release their carbon atoms. The carbon then "rains" down onto a diamond
substrate, in which it slowly grows in layers (a few micrometers per
hour) as a crystal.
This
technology is best suited for applications utilizing thin layers of
diamonds, such as optics and semiconductors. CVD is capable of producing
gem-quality diamonds, though the size of the polished diamond is
limited by the thickness of the diamond wafer.
Most
CVD diamonds grow with brown or black hues and must be put through an
additional color treatment process to turn them near-colorless.
Source: d.neadiamonds.com
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