Saint Petersburg Mining University, Saint Petersburg, Russia

А. S. Mustafaev, Professor, Head of the Department of General and Technical Physics, Doctor of Physical and Mathematical Sciences, e-mail: alexmustafaev@yandex.ru

V. Yu. Bazhin, Professor, Head of the Department of Metallurgy, Doctor of Technical Science, e-mail: bazhin_vyu@pers.spmi.ru
N. A. Bukovetskiy, postgraduate student of the Department of General and Technical Physics, e-mail: bukovetn@gmail.com

Saint Petersburg State University, Saint Petersburg, Russia
V. S. Sukhomlinov, Professor of the Department of Optics, Doctor of Physical and Mathematical Sciences, e-mail: v_sukhomlinov@mail.ru

Institute of Electrophysics and Electric Power Engineering of the Russian Academy of Sciences, Saint Petersburg, Russia
А. V. Surov, Head of Plasmadynamic Systems Laboratory3, Candidate of Technical Sciences, e-mail: avsurov@ieeras.ru

The article presents the results of testing plasma technology for producing ultrapure corundum. This material is widely used as an abrasive to produce microcircuits in the electronics industry and impact-resistant glasses in the optical industry. The article describes a rationale for choosing the technology parameters, the type of a plasma torch used, plasma gas, the design of the installation, and the parameters of the thermal insulation of the reactor. The key requirement to the technology to be developed is purity of the melt by reducing a concentration of impurities, when selecting temperature modes of the process. To carry out experiments on the technology, the authors chose a high-voltage three-phase plasma torch with electrodes from a tungsten-copper pseudo-alloy. To upgrade the process, a four-layer thermal shield of the reactor was designed with the first layer coated with molybdenum sheets to minimize its interaction with plasma gas and prevent premature melt crystallization. The experiments were conducted at the Institute of Electrophysics and Electric Power Engineering of the Russian Academy of Sciences. The produced samples of corundum were studied at the laboratories of the Common Use Center of Saint Petersburg Mining University. The studies proved a high-temperature transition and provided a rationale for conditions of recrystallization of aluminum oxide and a polymorphic transition to α-Al₂O₃. As a result, the samples of corundum were produced with an aluminum oxide of 99.8% and hardness of up to 92 HRC. The content of impurities in the samples was reduced by 4.67 times compared with the feedstock, and for some individual elements, for example, silicon, by 14 times. This is the first time when such results have been produced using plasma technology.
The research was funded by the Russian Science Foundation, grant No. 21-19-00139; grant No. 22-1-1-61-1 of Basis Foundation of Advances in Theoretical Physics and Mathematics.
The authors are grateful to specialists of the Institute of Electrophysics and Electric Power Engineering of the Russian Academy of Sciences, the Institute  of Solid State Chemistry of the Ural Branch, the Russian Academy of Sciences, Yekaterinburg, and the Common Use Center of Saint Petersburg Mining University.

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