Last week, we looked at the history of synthetic diamond manufacturing, and saw how it began in the late 1800s. While the machines for growing diamonds have improved tremendously over the past few decades, the methodology of today is not very different than it was in the 1950s, when GE pioneered the mass production of industrial-quality synthetic diamond. For the moment, there are two primary methods of manufacturing diamonds that most in the industry are familiar with. But there are, however, other processes that for the moment are cost prohibitive and do not possess the ability to mass-produce diamonds of any quality. But as technology improves, and as other technologies built for other industries become applicable to the manufacturing of diamonds, we may still see further developments. Let’s take a closer look at how diamonds are being synthesized in laboratories today.
The high-pressure, high-temperature method (HPHT) is the most common technique today for making diamonds. In fact, this is the technique with which early pioneers were experimenting. Just like its name implies, HPHT attempts to replicate the pressure and temperature within the Earth’s crust, where diamonds are formed. This typically requires pressure in excess of 5 GPa (Gigapascals), and temperatures in excess of 1500°C. HPHT requires a diamond ‘seed’ crystal, upon which successive layers of carbon grow, until it reaches its desired size, mimicking the actual growth process within the Earth. A carbon source is infused with a metal solvent. When heated, the metal will melt and dissolve the carbon, which solidifies into a diamond under the intense pressure. The cooling process must be closely monitored and controlled to achieve the desired growth.
Currently, there are three primary press designs in use for HPHT manufacturing. The first is the belt press, which was the original design of GE engineer Tracy Hall in 1954. In a belt press, two anvils supply pressure from above and below onto a belt of pressed steel bands, while at the same time pushing electrical currents through the steel. More modern belt presses use opposing hydraulic pressure, rather than steel bands, to control the lateral pressure exertion. The belt presses of today are generally much larger than those first used by GE in the 1950s, but the concept is much the same. A second press design is the cubic press, in which six anvils simultaneously apply equal force to a cube-shaped volume containing the diamond seed and metal medium. The cubic press can produce a diamond relatively quickly compared to a belt press, but is less effective in producing large diamonds, owing to the exponential increase in pressure required when increasing the volume of the cubic medium.
Perhaps the most common press design is the BARS apparatus, or ‘split-sphere’. It has proven to be the most economical, compact, and efficient press design for use in growing diamond crystals, and is also effective for growing larger diamonds, sometimes in excess of 10 carats. Originally invented by Russian scientists in the late 1980s, it can achieve pressures near 10 GPa, and temperatures of 2500°C. At the center of the ‘sphere’ is a cylindrical capsule that can be as large as one cubic inch, made of pressure-transmitting materials. The capsule is surrounded by six inner anvils of tungsten carbide, which fit together perfectly to form a double-sided pyramid. This pyramid is itself placed inside another series of steel anvils, arranged into two half spheres (hence the term ‘split-sphere’). The whole apparatus is placed inside a donut shaped housing, about one meter in diameter. The housing is then filled with oil, which is heated to extreme temperatures. The expansion of the oil exerts the necessary force and pressure onto the apparatus to produce diamond growth.
The second most common process for synthetic diamond manufacturing is Chemical Vapor Deposition, or CVD. Generally speaking, CVD is not useful in producing diamonds for the jewelry industry, as CVD diamonds tend to grow in more of a flat ‘wafer’ pattern than in the spherical model needed to polish a gem. CVD has many potential applications for use in semi-conductors and optical instruments, as the diamonds it produces can be grown over larger areas and on top of other substrates. CVD requires dramatically less pressure than HPHT, often as little as 27 kPa, or 0.0005% of the pressure needed in HPHT. Inside a CVD chamber, carbon-based gases, commonly methane, are added to the environment, and heated to several hundred degrees celsius. This causes the gases to break apart, and release their carbon atoms, which essentially ‘rain’ down onto a diamond-seeded substrate. This release of carbon builds layer upon layer of diamond, a few micrometers per hour. Most CVD diamonds grow with a black or brown hue, and must be put through other color enhancement treatments, commonly HPHT, to achieve colorless samples. CVD has the advantage of producing a more chemically pure diamond. However, it is currently limited in its ability to achieve mass-production scale and is therefore not well suited for the jewelry industry.
Detonation synthesis is a method that seeks to replicate observations from meteorite crash sites around the world, which have shown that diamonds can be created spontaneously through a powerful explosive event. In fact, much of the research into detonation synthesis has been clouded in secrecy, as the production of nano-sized diamonds was often an unexpected consequence of military research into the effects of enclosed explosions. When a carbon based explosive is detonated within a confined space, the shock wave from the explosion produces the necessary pressure and heat to convert the carbon into diamond. This typically results in an extremely fine diamond powder, with each crystal confined to about 4nm in diameter. While certainly not applicable to the jewelry industry, these tiny diamond crystals have significant application for use as abrasives and coatings. Detonation diamonds have been mass-produced in China, Russia, and Belarus, and began to hit the market in large quantities around 15 years ago.
Another method for diamond synthesis is in its earliest stages of development, but has some unique advantages over other methods that require significant amounts of energy to produce. Ultrasonic cavitation uses the properties of sound waves propagated through a liquid at room temperatures and at normal atmospheric pressures. Ultrasound through liquid can cause some very extreme effects on the nano-scale. High intensity ultrasound causes alternating high and low- pressure cycles that result in small vacuum bubbles, or voids in the liquid. When these bubbles attain a volume where they can no longer absorb energy, they collapse (cavitate) violently and produce very high temperatures and pressures locally. When a graphite solution is added to the liquid, the necessary conditions can be synthesized to convert the graphite into nano-sized diamonds. Ultrasonic cavitation is still mostly at the research and development stage. However, scientists have shown progress in developing new compounds, which combine diamonds with other materials. This may lead to the development of new, unforeseen products for future applications beyond our imaginations.
There’s no doubt that diamond synthesis has come a long way in recent years. For the moment, mass production for jewelry at an inexpensive price point has eluded researchers. But new developments are sure to come along, perhaps from unlikely sources of research. With so many companies involved in the development of synthetic diamonds, it may only be a matter of time before we see these economies of scale.
The views expressed here are solely those of the author in his private capacity. No one should act upon any opinion or information in this website without consulting a professional qualified adviser.
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Diamond industrialist Ehud Arye Laniado is a man passionate about diamonds. From his early 20s in Africa and later in Belgium honing his expertise in forecasting the value of polished diamonds by examining rough diamonds by hand, till today four decades later, as chairman of his international diamond businesses spanning mining, exploration, rough and polished diamond valuation, trading, manufacturing, retail and consultancy services, Laniado has mastered both the miniscule details of evaluating and pricing individual rough diamonds and the entire structure of the diamond industry. Today, his global operations are at the forefront of the industry, recognised in diamond capitals from Mumbai to Tel Aviv and Hong Kong to New York.