Structure of new form of super-hard carbon identified

November 8, 2010 by Lin Edwards, report

Views along [100]/[001], and [010] directions of 2x2x2 supercell of bct-Carbon,the dotted-dashed in (b) indicate the perpendicular graphene-like structure of bct-Carbon. Image credit: Xiang-Feng Zhou,
( -- An experiment in 2003 formed what was believed to be a new form of carbon, but the findings were controversial. Now two teams of scientists have used different means to identify a three-dimensional network structure called "bct-carbon," which they say could have been the structure formed in 2003.

Pure carbon exists in a variety of structures, including and diamond. The new , body-centered tetragonal carbon or bct-carbon, is unexpectedly simple and consists of sheets of squares of four carbon atoms each, joined by “short” bonds perpendicular to the sheets. This form of carbon is created when graphite is exposed to high pressure at normal temperatures.

It has been known for nearly 50 years that graphite subjected to cold compression (high pressure at ambient temperatures) undergoes a transformation that is reversible, and in 2003 researchers at Stanford University compressed graphite in a diamond anvil press, while simultaneously obtaining the x-ray diffraction pattern to help them study the bonds within the structure. They found that when the pressure exceeded 17 gigapascals (GPa) (170,000 atmospheres) the carbon atoms in the normally soft graphite formed a material hard enough to crack diamond, but its structure remained unclear.

Now a team of scientists led by Hui-Tian Wang of Nankai University at Tianjin, China, have shown through computer simulations that the super-hard carbon may be at least partly composed of bct-carbon, since this takes the least energy to form. Bct-carbon has a structure part-way between diamond’s cubes of carbon atoms and graphite’s linked sheets of in a hexagonal lattice. Bct-carbon consists of sheets of four-atom carbon rings linked together by strong bonds.

The team studied 15 possible structures and found the transparent bct-carbon not only required lower energies to form but that its shear strength is 17 percent greater than diamond’s. If the results are confirmed, this means it may be possible to produce a material stronger than diamond at normal temperatures.

Another group of scientists, including Renata Wentzcovitch of the University of Minnesota and Takashi Miyake from the National Institute of Advanced Industrial Science and Technology in Japan, came to similar conclusions earlier this year, but by a different method. This group analyzed the proposed bct-carbon structure using quantum mechanical simulations. They found bct-carbon was more stable than graphite at 18.6 GPa, and that when mixed with M-carbon it would produce an x-ray diffraction pattern closely matched to that found in 2003. (M-carbon is a structure consisting of layers of in rings of five and seven members.)

The paper from Hui-Tian Wang’s team was published in the journal Physical Review B, while the US/Japan research was reported in Physical Review Letters in March this year.

Explore further: Meteorite yields carbon crystals harder than diamond

More information:
-- Ab initio study of the formation of transparent carbon under pressure, Phys. Rev. B 82, 134126 (2010) DOI:10.1103/PhysRevB.82.134126

-- Body-Centered Tetragonal C4: A Viable sp3 Carbon Allotrope, Phys. Rev. Lett. 104, 125504 (2010) DOI:10.1103/PhysRevLett.104.125504

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5 / 5 (1) Nov 08, 2010
hard to see applications when the structure only appears at 17 GPa's and reverses without constant pressure. But you never know where a "pure science" advance will lead. Even if all it does is help someone make a new hypothesis, its worth it.
2.3 / 5 (3) Nov 08, 2010
What's the pressure seen on the wings of a hypersonic plane travelling near 20-30 mach? I don't know, but it'd be a fun calculation which may near 17 GPa...
3 / 5 (2) Nov 08, 2010
sorry fastest plane SR-71 Blackbird -- speed mach 3.2 -- fastest vehicle space shuttle/ rocket / anything in orbit
5 / 5 (2) Nov 09, 2010
Nonsense, NASA's X-41A achieved Mach 10 for several seconds using scramjet technologies. Nasa is planning to further develop this engine to achieve Mach 30.
A re-entering space shuttle has speeds of about Mach 28. The SR-71 Blackbird is CERTAINLY NOT faster than the space shuttle or anything in orbit, you silly, silly person.
1 / 5 (14) Nov 09, 2010
The X-15 remains the fastest [acknowledged] manned aircraft ever. It once reached Mach 6.7 or 4,520 mph

Aurora- 7,000 mph (Mach 10.5, or 11,265 km/h)
not rated yet Nov 10, 2010
Obviously he was referring to theoretical hypersonic aircraft of the future. Who would bother applying this tech to an aging relic?

Not any military industrial complex I know. :)

not rated yet Nov 11, 2010
i say boo to the X-41'ers why count a plane that has no humans. I will submit to the X-14/15'ers as I did not know about those trials but give me a break these were not production aircraft doing missions.

I got a 1 rating for talking about planes that had to fly more than one mission, were used extensively and as recently as the Iraq war (1st one) Planes that engines could handle the pressures and could be reused... You all are talking about one time drones that I equate to silly cars - run em once, they go up in flames then rebuild the whole damn thing.

the X-41/43 was scuttled into the ocean or blown up - cause it costs to much to recover or scared of stolen technology
not rated yet Nov 13, 2010
Aurora- 7,000 mph (Mach 10.5, or 11,265 km/h)

The Aurora is real. I first heard of this back in 1998 directly from an retired marine.

When I asked about how they deal with the g-forces and such, he claimed that it, "Wasn't a problem. They've got that taken care of."

I don't know if he was implying some sort of top secret inertial dampening technology or what, but it's real. This guy worked in counter intelligence.

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