No high pressure required, lower cost Scientists discover new superhard materials

No high pressure required, lower cost Scientists discover new superhard materials

2020-05-15 13:48:11 Henry Wang

When looking for new super-hard compounds, the researchers predicted stable molybdenum boride and its crystal structure. They found that the highest boride contains 4 to 5 boron atoms per molybdenum atom. MoB5 has an estimated Vickers hardness of 37 to 39 GPa, which makes it a potentially superhard material. The research was published in "The Journal of Physical Chemistry Letters".
       Previously, a group of researchers led by Professor Artem Oganov of the Moscow Institute of Physics and Technology (MIPT) published a study in the Journal of Applied Physics, which comprehensively compiled a list of superhard materials that may have industrial application potential. The list is made using a new model of the evolutionary algorithm USPEX, Vickers hardness (the pressure required to leave pyramidal indentations in the material), and fracture toughness (the ability of the material to resist fracture transfer), known as the "treasure map "For the author's use.
       In this new paper, scientists from Moscow Skoltech University, Moscow Institute of Physics and Technology, AM Prokhorov RAS Institute of General Physics, Pirogov Russian State Medical University, and Northwestern Polytechnical University in China studied boronization on the map Molybdenum area. The transition metal boride is a potential substitute for traditional cemented carbide and superhard materials in technical applications. Unlike widely used diamond and cubic boron nitride, the synthesis of transition metal boride does not require high pressure, so the production cost is lower.

Atlas of superhard materials
Atlas of superhard materials

       Since electrons repel each other, the high-valence electron density of metal atoms can resist compression, while covalent boron-boron and boron-metal bonds can resist elastic and plastic deformation.
       "Usually the simulated X-ray diffraction (XRD) pattern is compared with the experimental figure to determine whether the predicted structure is compatible with the experimental structure. However, considering the transition metal boride, such as molybdenum boride, the XRD pattern can only show The signal of heavier atoms, while the position of lighter boron atoms is basically invisible, which is why crystal structure models based on experimental data alone are often unrealistic and unstable, therefore, a comprehensive method to determine the crystal structure requires The most advanced theoretical calculation method. "Said Alexander Kvashnin, a senior researcher at Skoltech and MIPT.
       It was found that the molybdenum pentaboride MoB5 is the highest stable boride, but the simulated X-ray diffraction pattern is close to but not completely the same as the experimental data. The predicted pentaboride has some weak peaks that were not observed in the experiment. This implies a higher symmetry in the experimental samples. The key structural element of this new compound is the triangular arrangement of boron atoms, molybdenum layers and boron atoms in the graphene-like layer. Boron and molybdenum atoms are arranged alternately, and some Mo atoms are replaced by B3 triangles evenly distributed throughout the crystal volume.
       "We made a hypothesis that the structure of the highest boride is disordered, and the boron triangle statistically replaces the molybdenum atom. To prove this, we developed a lattice model that allows us to define the control boron The rules of how the unit should be positioned on it. "Said Dmitry Rybkovskiy, the first author of the paper and a researcher at Skoltech and AM Prokhorov Institute of General Physics.

Atom arrangement in MoB5-x crystal
Atom arrangement in MoB5-x crystal

       Through a brute force search of the positions of molybdenum atoms and boron triangles, sampling different variants, we found patterns related to stability. Each metal atom in the stable phase contains 4 to 5 boron atoms. MoB4.7 is the most stable among these compounds and has the best matching with the experimental X-ray diffraction pattern.
       "This study is an interesting example of the interaction between theory and experiment. Theory predicts a compound that exhibits peculiar properties and a new structure, but experiments show that the actual material is more complex and its structure is partially disordered. We based on The theory developed by these findings allows us to reproduce all the experimental observations, understand the exact composition and structure of this material, and its detailed characteristics. In particular, the calculated hardness is close to the range of superhard materials. " Artem Oganov, professor of Skoltech and MIPT and head of the research team, said.
       Superhard materials have a wide range of industrial applications, such as machine tool manufacturing, jewelry or mining. They are used for cutting, polishing, grinding, and drilling, so finding new superhard compounds is an important task.
       The title of the paper is "Structure, Stability, and Mechanical Properties of Boron-Rich Mo--B Phases: A Computational Study"


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