Model of fullerite surrounded by diamond shell. (Courtesy of Alexander Kvashnin.)
Model of fullerite surrounded by diamond shell. (Courtesy of Alexander Kvashnin.)

Russian researchers believe that they have solved the mystery of why fullerite nanocomposites are so ultrahard [Kvashnina et al., Carbon 115 (2017) 546].

Nearly 20 years ago, a team of scientists at the Technological Institute for Superhard and Novel Carbon Materials led by Vladimir Blank synthesized a material based on polymerized fullerite with outstanding stiffness and hardness called ‘tisnumit’. Fullerite is a molecular crystal lattice made up of fullerene molecules – hollow spheres of carbon atoms. But the atomic structure of fullerite and the origin of its exceptional mechanical properties remained a mystery.

Now a team of researchers from the same institute, along with colleagues from Moscow Institute of Physics and Technology, Skolkovo Institute of Science and Technology, Emanuel Institute of Biochemical Physics, and the National University of Science and Technology, has come up with a new model of fullerite, which closely matches experimental data.

The researchers suggest that when the fullerite is compressed at high temperature, some of the fullerenes transform into polycrystalline diamond while the rest remains in a compressed state (SH-phase).

“The amorphous structure of ultrahard fullerite led us to assume that the compressed polymerized fullerite is surrounded by an amorphous shell made of carbon atoms with diamond-like sp3 bonds, which does not allow the structure to expand,” explains researcher Alexander G. Kvashnin.

In other words, fullerite could be considered as a grain of nanocomposite with a shell of diamond. The fullerite grains are arranged in a period pattern in single crystal diamond like raisins in a cake, says Kvashnin.

“It is known from the experiments and theory, that a material in a compressed state will display greater mechanical properties compared to relaxed state,” he explains. “In this nanocomposite with nanoparticles in the SH-phase clamped in a diamond-like amorphous matrix, the improved mechanical properties remain preserved.”

Those mechanical properties include ultrahigh mechanical stiffness, higher even than that of diamond. If such outstanding properties could be realized in materials that could be readily synthesized, it could lead to mechanical parts with reduced wear and longer lifetimes in many industries. But such ultrahigh hard materials, which are likely to require high pressures to produce, could be difficult to handle.

Kvashnin believes the next step forward is to try to synthesize the new material under different high pressure and temperature conditions and investigate its properties. Researchers around the world are looking anew at ultrahard carbon and Kvashnin hopes their new model will help understand these exceptional materials.

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