Nano-tubes harder than diamonds

Earth scientists discover the densest and harshest matter form

The random distribution of the diamond nano-tubes and the plurality of grain boundaries in the material (below) are responsible for the high rupture strength of the ADNR's compared to a diamond single crystal (above) which has a preferred cleavability after a facet. © University of Bayreuth
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Diamonds were previously considered the hardest and least compressible material on earth. Researchers at Bayreuth University's Bavarian Geoinstitute (BGI) have now discovered diamond nano-tubes that are denser than any previously known form of carbon and barely reduce their volume under extreme pressure.

Density and compressibility are essential material properties with which every child collects his or her first experiences, for example when playing with balloons or when forming sand pies. We also know from everyday life that gaseous substances are very easy to compress. The combustion engines of our cars are operated with a compression ratio of about nine to one, so the gasoline-air mixture is compressed ninefold.

We also recognize in our car that liquids are much more difficult to compress. Even with a powerful full braking, the volume of brake fluid in the hydraulic brake system usually does not decrease. Even less compressible are solids. They and liquids are considered almost uncompressible.

Diamant replaced as record holder

All the elastic properties of a material together determine how much a material compresses under certain external pressures. The measure of the relative volume change in response to the change in an externally applied pressure is called compressibility. The reciprocal of the compressibility is called the compression modulus.

So far, Diamond held the record as the hardest and least compressible material. There is an obvious correlation between the modulus of compression (K) and hardness (H): the larger H, the larger K. It has often been found that the positive properties of matter improve even more when matter is nano-crystalline rather than macrocrystalline Form is present. Until now, it was hard to imagine that there could be other, even less compressible substances with larger compression moduli than diamond. display

Nano-wires and nano-tubes with outstanding features

In theoretical studies, outstanding properties of nano-wires and nano-tubes (fracture toughness, toughness) were predicted, making these materials important and viable goals of synthetic experiments. The production of nano-tubes from different starting materials is very successful; So far, there are no findings from experiments with nano-tubes made of diamond.

Initial success in the synthesis of this novel material from aggregated diamond nano-tubes (ADNR's = aggregated diamond

nanorods) were recently presented by Natalia Dubrovinskaia, Leonid Dubrovinsky and Falko Langenhorst, scientists at the Bavarian Geoinstitute of the University of Bayreuth. Together with colleagues from Grenoble / France and the Technical University of Applied Sciences Wildau, they examined the elastic properties of the material.

Higher density than ordinary diamond

They found that this material has an extremely high compression modulus and a higher density than ordinary diamond. It has now been proven that ADNRs are the densest known form of carbon and possess the least compressibility of all known materials.

The ADNR's were synthetically produced in the special high-pressure presses at the Bavarian Geoinstitut at pressures such as those found in the interior of the Earth at the boundary between the upper and lower mantle (24 GPa - 240, 000 atm). Under such enormous pressure conditions, even the volume of water (ice) would be reduced by 35 percent, that of steel by eleven percent, and that of ADNR's by only about four percent.

The three scientists involved have patented their newly developed method for the synthesis of superhard, wear-resistant and thermally stable aggregated diamond nano-tubes (ADNRs) and their use. The results, which are also of great interest to users from material science research, have just been published in the renowned journal Applied Physics Letters.

(idw - University of Bayreuth, 24.08.2005 - DLO)