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  • Dec 21, 2014
  • Updated: 1:41am
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SCIENCE

Chinese scientists develop what they say are tougher synthetic diamonds

Chinese scientists develop microscopic industrial gems they say are harder and more stable, but critics question the testing methods

PUBLISHED : Thursday, 12 June, 2014, 9:13pm
UPDATED : Friday, 13 June, 2014, 5:56am
 

Diamonds may be a girl's best friend, but they are also prized by oil drillers, road crews and experimental physicists for their unparalleled ability to bore, grind and cut.

Now scientists in China have synthesised microscopic diamonds that are harder, tougher and more stable than any made in nature or in a lab.

These new nanodiamonds boast a special pattern in their crystal structure called nanotwinning, in which adjacent crystals share an interlacing boundary and grow into mirror images of one another. That twinning gives the diamonds their extraordinary hardness, the researchers say in a study published in yesterday's edition of the journal Nature.

Most diamonds are used not in engagement rings but for industrial purposes. Diamonds make sharp drill bits that penetrate the earth's crust in search of fossil fuels and minerals.

They form blades that grind down uneven road surfaces and immaculate crystals for conducting precise scientific experiments. You can even purchase fine diamond powders for polishing glass from Amazon.com.

But these functional diamonds need more than hardness to withstand the rigors of rough work.

Paradoxically, harder diamonds tend to fracture more easily, so scientists have long sought ways to make tougher materials that won't break under stress.

Unfortunately, synthetic diamonds often involved trade-offs - when one quality (such as hardness) got better, another (toughness) got worse.

Until now. Scientists at Yanshan University in Hebei province say that diamonds made with their new method surpass existing diamonds in every category.

The new nanodiamonds measure between 20 and 50 nanometres across, slightly larger than other nanodiamonds. A nanometre is equal to one billionth of a metre. But unlike their predecessors, the new stones contain twinning structures as small as five nanometres. That is roughly twice the thickness of a single strand of DNA.

"At these twinning boundaries, the crystals on each side are bonding together much better," said Bo Xu, a materials scientist who helped lead the study.

The nanotwinning accounts for the diamonds' hardness while the small crystal size lends stability and makes them more resistant to fractures.

Each pinhead-size sample contains a multitude of nanodiamonds whose many twinning boundaries help absorb a force that would otherwise break it, Xu said.

All these advantages can be traced to the properties of a nanoparticle called onion carbon, the raw ingredient in the scientists' diamond recipe that they say deserves credit for their success.

With a dozen or so nested spheres of carbon atoms, the molecule bears an obvious resemblance to its namesake. The scientists cooked these molecules at extremely high temperatures and pressures-akin to the conditions deep in earth's molten mantle-until tiny diamond crystals began to grow.

The scientists then put their nanotwinned diamonds to the test by crushing them with a natural diamond crystal and looking for an imprint. They found that the new diamonds could withstand pressures about two to three times greater than natural diamonds.

They also weathered strong forces without cracking and remained stable at temperatures up to 1,056 degrees Celsius - more than 200 degrees hotter than natural diamonds tend to.

These nanodiamonds are "extraordinary", Jim Boland, a diamond expert at the Commonwealth Scientific and Industrial Research Organisation in Australia, wrote in a commentary that accompanies the study.

But others are not so sure. Dr Natalia Dubrovinskaia, a crystallographer at the University of Bayreuth in Germany, questioned the logic of testing something that is supposedly harder than a diamond by comparing it to diamond.

It would be like trying to measure the hardness of a stainless steel knife by pressing it against an aluminium fork, she said: "The result of such an attempt will be broken teeth of the fork."

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