Harder than diamond

Brussels, 31 Aug 2005

Physicists in Germany have created a material that is harder than diamond. Natalia Dubrovinskaia and colleagues at the University of Bayreuth made the new material by subjecting carbon-60 molecules to immense pressure. The new form of carbon, which is known as 'aggregated diamond nanorods' (ADNR), is expected to have many industrial applications.

Due to combination of unique physical and chemical properties such as hardness, high thermal conductivity, wide band gap, high electron and hole mobility and chemical inertness, diamond has been used for a wide range of applications in modern science and technology. There is growing demand for diamond-like materials in electronic applications.

Although there is little prospect of diamond-based microelectronics ousting silicon totally, diamond devices could function in situations when silicon electronics fail: diamond chips potentially could still work at temperatures of several hundred degrees, whereas silicon devices generally fail above 450 degrees Kelvin. Electrochemical applications of diamond-based films have been developed significantly in the last few years and are considered a promising research area. Pure diamond is a perfect insulator and conducts electricity very poorly. But, like silicon, it can be turned into a semiconductor by traces of boron or nitrogen impurities.

Diamond derives its hardness from the fact that each carbon atom is connected to four other atoms by strong covalent bonds. The new material is different in that it is made of tiny interlocking diamond rods. Each rod is a crystal that has a diameter of between 5 and 20 nanometres and a length of about 1 micron. The hardness of a material is defined by its resistance to another material penetrating its surface and it is measured by its isothermal bulk modulus. Bulk modulus gives the change in volume of a solid substance as the pressure on it is changed. Aggregated diamond nanorods have a modulus of 491 gigapascals (GPa), compared with 442 GPa for conventional diamond.

The team created the ADNRs by compressing the carbon-60 molecules to 20 GPa, which is nearly 200 times atmospheric pressure, while simultaneously heating to 2500 degrees Kelvin. According to Dr Dubrovinskaia, 'the synthesis was possible due to a unique 5000-tonne multianvil press at the Bayerisches Geoinstitut in Bayreuth that is capable of reaching pressures of 25 GPa and temperatures of 00 Kelvin at the same time'. The Bayerisches Geoinstitut is receiving funding from the EU's 'Research Infrastructures' Programme for four years.

The properties of the samples were measured with a diamond anvil cell - a device capable of generating pressures almost as great as those found at the centre of the Earth - at the European Synchrotron Radiation Facility at Grenoble, France. These measurements indicated that ADNRs are about 0.3 per cent denser than diamond, and that the new material has the lowest compressibility of any known material.

In addition to working out why the new material is so hard, the Bayreuth team also hopes to exploit its industrial potential. Dr Dubrovinskaia and two of her colleagues have patented the process used to make the new material. 'We have developed a concept for innovative technology to produce the novel material in industrial-scale quantities and now we are looking for partners in order to realise our ideas,' she said. To download the abstract of the Applied Physics Letters paper, please: click here Remarks: Reference document: Natalia Dubrovinskaia, Leonid Dubrovinsky, Wilson Crichton, Falko Langenhorst, Asta Richter. Aggregated diamond nanorods, the densest and least compressible form of carbon. Applied Physics Letters 87, 22 August 2005.

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