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MEI Online: General Minerals Engineering: Latest News: October 10th 2002

 
    
:: Big Diamonds Grown Faster  

Large, perfect crystals should broaden diamond's technological applications
PHILIP BALL

Researchers in the United States are making small diamonds bigger. In 12 hours they can expand a 3.5-mm square slab of synthetic diamond to one 4.2 mm across. Large, perfect crystals like this should broaden diamond's technological applications.

There is nothing new about making and growing diamond artificially. But Russell Hemley of the Geophysical Laboratory of the Carnegie Institution of Washington and colleagues have developed a method for depositing fresh diamond rapidly and without flaws onto existing gems1.

Bigger diamonds will enable researchers, like those at the Geophysical Lab, to test more material in diamond-toothed anvils. These are used to study the composition and behaviour of rocks under the kinds of immense pressures that they experience deep inside the Earth.

Hard graft

Diamond is currently synthesized in two main ways. One involves squeezing graphite, or some other form of carbon, at very high temperatures and pressures (HPHT) to transform it into transparent diamond. The other technique, chemical vapour deposition (CVD), uses heat, and radio or microwaves to break a carbon-rich gas into fragments that reassemble into diamond on a surface.

Both methods have drawbacks. HPHT diamonds, like natural ones, are pure and flawless, but generally only a few millimetres across. It is tricky to squeeze that much material to pressures of around 60,000 atmospheres.

CVD diamonds, on the other hand, are thin, flawed films that grow very slowly - typically by about one thousandth of a millimetre per hour. This limits their economic viability as diamond windows for high-power lasers or space-based infrared sensors.

Hemley and colleagues put together two tricks to improve CVD films. They grow them on pure diamond to reduce flaws: the carbon atoms line up with those on the existing surface. And they hasten the growth of the film by increasing the pressure of the carbon-rich gas - usually methane mixed with hydrogen - and adding to it a small amount of nitrogen gas.

This combination increases the growth rate of diamond films about a hundred-fold. They deposit new diamond onto seeds of HPHT diamond using microwave-assisted CVD at 150 micrometres per hour. Better still, the new diamond grows as a single crystal, making it virtually indistinguishable from the seed.

 

   

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See:
Yan, C.-S., Vohra, Y. K., Mao, H.-K. & Hemley, R. J. Very high growth rate chemical vapor deposition of single-crystal diamond. Proceedings of the National Academy of Sciences USA, September 9, (2002)

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