Researchers from the Technological Institute for Superhard and Novel Carbon Materials in Troitsk, the Moscow Institute of Physics and Technology (MIPT), National University of Science and Technology (MISiS), and Moscow State Univ. (MSU) have developed a new method for the synthesis of an ultrahard material that exceeds diamond in hardness. An article recently published in the journal Carbon describes in detail a method that allows for the synthesis of ultrahard fullerite, a polymer composed of fullerenes, or spherical molecules made of carbon atoms.
In their work, the scientists note that diamond hasn’t been the hardest material for some time now. Natural diamonds have a hardness of nearly 150 GPa, but ultrahard fullerite has surpassed diamond to become first on the list of hardest materials with values that range from 150 to 300 GPa.
Diamond anvils malformed during synthesis of ultrahard fullerite. Note the dent in the center.
Credit: Image courtesy of Moscow Institute of Physics and Technology
All materials that are harder than diamond are called ultra hard materials. Materials softer than diamond but harder than boron nitride are termed superhard. Boron nitride, with its cubic lattice, is almost three times harder than the well-known corundum.
Fullerites are materials that consist of fullerenes. In their turn, fullerenes are carbon molecules in the form of spheres consisting of 60 atoms. Fullerene was first synthesized more than 20 years ago, and a Nobel Prize was awarded for that work. The carbon spheres within fullerite can be arranged in different ways, and the material’s hardness largely depends on just how interconnected they are. In the ultrahard fullerite discovered by the workers at the Technological Institute for Superhard and Novel Carbon Materials (FSBI TISNCM), C60 molecules are interconnected by covalent bonds in all directions, a material scientists call a three-dimensional polymer.
However, the methods providing production of this promising material on an industrial scale are not available yet. Practically, the superhard carbon form is of primary interest for specialists in the field of metals and other materials processing: the harder a tool is, the longer it works, and the more qualitatively the details can be processed.
What makes synthesizing fullerite in large quantities so difficult is the high pressure required for the reaction to begin. Formation of the three-dimensional polymer begins at a pressure of 13 GPa, or 130,000 atm. But modern equipment cannot provide such pressure on a large scale.
The scientists in the current study have shown that adding carbon disulfide (CS2) to the initial mixture of reagents can accelerate fullerite synthesis. This substance is synthesized on an industrial scale, is actively used in various enterprises, and the technologies for working with it are well-developed. According to experiments, carbon disulfide is an end product, but here it acts as an accelerator. Using CS2, the formation of the valuable superhard material becomes possible even if the pressure is lower and amounts to 8 GPa. In addition, while previous efforts to synthesize fullerite at a pressure of 13 GPa required heating up to 1100 K (more than 820 C),in the present case it occurs at room temperature.
Abstract – Synthesis of ultrahard fullerite with a catalytic 3D polymerization reaction of C60
3D polymerization of C60 realizes under conditions of large plastic deformation at pressure 6–7 GPa and room temperature in the presence of CS2. The phase of 3D-polymerized C60 is identical to ultrahard fullerite synthesized from pure C60 at 18 GPa pressure: in both cases, the samples plough diamond during the rotation of the sample in a shear diamond anvil cell, bulk module is 585 GPa, and a sequence of phase transitions preceding to ultrahard phase is also the same in both cases (in the presence of CS2, the phase transitions take place at lower pressures than in pure C60). Raman and transmission microscope studies confirm the structure equivalence of samples of both types. The absence of sulfur in the structure of ultrahard fullerite synthesized in the presence of CS2 proves the catalysis role of CS2 in the 3D polymerization of C60.
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