Growing diamonds in the rough
( 2003-10-16 08:47) (China Daily)
A Chinese research team has proclaimed success in synthesizing diamond crystals as large as half a millimetre across using new, unconventional methods.
The team from the University of Science and Technology of China (USTC) based in Hefei, capital of East China's Anhui Province, produced the crystals through chemical reduction of magnesium carbonate, a compound widely used in the manufacture of inks and glass, with metallic sodium.
They put the magnesium carbonate and the metallic sodium in an autoclave and heated the mixture up to 500 C, when large grains of diamonds were produced alongside graphite.
"We found well-crystallized diamonds with an average diameter of 0.12 millimetres through electron microscopy, with the largest being 0.51 millimetre in diameter," said team leader Professor Chen Qianwang, a research fellow at the Structure Research Laboratory of the USTC.
Traditional methods for producing synthesized diamonds normally result in crystals in the range of scores of micrometres (a millimetre equals 1,000 micrometres), and under more stringent conditions.
For example, the temperature required in the traditional methods normally may be up to 1,400 C. That makes generation of large-size, gem-quality diamonds even more costly than mining the natural ones.
Chen's methods may open the door to low-cost synthesis of gem-quality diamonds and prove a "profitable alternative" to previous methods, according to a report from the Angewandte Chemie, a Germany-based scientific journal which published the Chinese team's research paper two weeks ago.
The diamond yield depends strongly on the exact reaction conditions. When the temperature in the sealed autoclave is less than 500 C, only graphite is produced. When it goes up, more diamond crystals are generated, Chen said. Under optimal conditions, a diamond yield of 6.6 per cent can be obtained, according to their research.
More significant is that Chen's method is such that the diamond crystals in the autoclave can grow with continuous addition of the reactants and maintaining of the temperature and pressure. That is impossible in conventional high-temperature diamond synthesis.
Chen said he would not be surprised if the crystalline diamond grows to over two millimetres in size. "That would be large enough for jewelry making," he added.
Synthetic diamonds, ever since they appeared on the market in the 1950s, have been primarily used for industrial purposes - to coat saws, drill bits, and grinding wheels. "They are too small to make jewels," Chen said.
Chen's unconventional methods come as no surprise.
Three months earlier his team published a paper in the Journal of the American Chemical Society, in which Chen and his team discussed synthesizing diamond crystals by using carbon dioxide.
The diamonds are made by reacting carbon dioxide with metallic sodium in an oven heated to 440 C under a pressure of 800 atmospheres. They later improved the process to be able to produce gemstones as large as 1.2 millimetres in size, Chen claims.
He noted that the newly developed method using magnesium is partly similar to using carbon dioxide in that they both employ chemical reduction with sodium and graphite as by-products.
But the magnesium-based method may be more practical for industrial use, Chen said, because preparation of solid carbon dioxide in the other method may prove too costly and inconvenient in large-scale manufacture.
Both methods may allow the diamond crystals to grow, he added.
Low temperature synthesis of diamonds has been studied for some time and a variety of methods, such as the chemical vapor deposition have been developed. However, Chen's method is revolutionary not only because it requires less stringent conditions and produces larger gemstones, but also because the ideas that inspired the research may lead to the rewriting of our understanding of the origin of diamonds.
Indeed, making diamonds out of cheap minerals is not confined to the fantasies of ancient alchemists. It has been the dream of serious scientists, too, for more than 200 years.
In 1792 French chemist Antoine Lavoiser burned diamonds in oxygen and obtained carbon dioxide as the only combustion product. He concluded that diamonds are comprised of only the element carbon.
From that point, some chemists were trying to convert the common mineral graphite, which was also known to be of carbon, into diamonds.
Success was not forthcoming until a relatively plausible theory on natural diamond formation was adopted to produce artificial gems.
The theory stated that diamonds are the product of fire deep beneath the planet's surface.
It was thought that 3 billion years ago, when Earth was in its infancy, diamonds were formed by the high temperature and high pressure in the Earth's mantle, some 200 kilometres below the surface.
When volcanoes erupted, lava pushed some of these precious stones to the surface, where they cooled as octahedral crystals hidden in rocks. These formed diamond lodes.
That theory inspired people to try to make artificial diamonds by creating similar conditions. After 150 years of failed trials, the General Electric Co managed to build a high-temperature, high-pressure sealed container to synthesize diamonds with graphite for the first time.
Chen began to challenge the theory in 1998 when he was leading his team doing research on the origin and evolution of life following the Big Bang.
One aspect of the research focused on the variation of chemical compounds in the Earth's mantle, where carbonates and carbon dioxide are plentiful. At the same time, a parallel research on the chemical characteristics of diamonds found that most of them contain similar elements.
"It occurred to us that the mantle may probably be a natural reactor in which diamonds were formed," Chen said. "And carbon dioxide or certain carbonates may be the materials."
Lavoiser's experiment 200 years ago also gave Chen some inspiration. "If diamonds can burn into carbon dioxide in oxygen, why cannot the reverse process be possible?"
Rather than merely be formed by the high temperature and pressure in the depth of the mantle, diamonds may also have originally formed through chemical reduction of the carbon dioxide that requires lower temperature and pressure, Chen hypothesized.
They began experiments in September last year and succeeded in making colourless diamond crystals by the chemical reduction of solid carbon dioxide with metallic sodium.
Over the next six months they fine-tuned the technology to grow larger stones and experiment with other carbonates.
"All this originated from a flicker of inspiration," Chen said.
Having applied patents for their findings and waiting for approval, he said his team is now concentrating on the growth of the diamond crystals and continuing to improve related techniques to make the process more stable and cost-efficient in terms of industrial manufacturing.
"We have seen the beautiful glitter of the precious stones," he said. "But more work needs to be done before we can see them on the market."
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