  Citation: Lyakhov, N.; Grigoreva, T.; Talako, T.; Udalova, T.; Vosmerikov, S.; Devyatkina, E. A Carbon-Free Way for Obtaining Nanoscale Silicon. Powders 2022, 1, 18–32. https:// doi.org/10.3390/powders1010003 Academic Editor: Khaled Morsi Received: 30 December 2021 Accepted: 10 February 2022 Published: 17 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article A Carbon-Free Way for Obtaining Nanoscale Silicon Nikolay Lyakhov 1,2, * , Tatiana Grigoreva 1, *, Tatiana Talako 3 , Tatyana Udalova 1,4 , Sergey Vosmerikov 1 and Evgeniya Devyatkina 1 1 Institute of Solid State Chemistry and Mechanochemistry of the Siberian Branch of the Russian Academy of Sciences, 18 Kutateladze Street, 630128 Novosibirsk, Russia; udalova@solid.nsc.ru (T.U.); vosmerikov@solid.nsc.ru (S.V.); devyatkina@solid.nsc.ru (E.D.) 2 Department of Chemical Materials Science, Faculty of Natural Sciences, Novosibirsk State University, 1 Pirogov Street, 630090 Novosibirsk, Russia 3 Department of Physical and Technical Sciences of National Academy of Sciences of Belarus, 66 Nezavisimosti Avenue, 220072 Minsk, Belarus; talako@presidium.bas-net.by 4 Departmentof Chemistry and Chemical Technology, Faculty of Mechanics and Technology, Novosibirsk State Technical University, 20 K. Marks Avenue, 630073 Novosibirsk, Russia * Correspondence: lyakhov@solid.nsc.ru (N.L.); grig@solid.nsc.ru (T.G.); Tel.: +7-383-2332410 (ext. 1111) (N.L.); +7-383-2332410 (ext. 1546) (T.G.) Abstract: The nanosized silicon powder has been produced by reduction of silica with magnesium in an argon medium using both the mechanically activated self-propagating high-temperature synthesis and the direct mechanochemical synthesis and has been investigated by X-ray phase analysis, Infrared spectroscopy, electron scanning microscopy, and energy dispersive X-ray spectroscopy. The optimal Mg:SiO 2 ratio has been found to provide the minimum content of contaminant impurities of mag- nesium silicide and silicate in mechanically activated self-propagating high-temperature synthesis. For the first time, direct mechanochemical synthesis of Si via reduction of silica with magnesium has been implemented. Optimal component ratio and mechanical activation parameters have been determined, yielding Si/MgO composites without impurity phases (magnesium silicide and silicate). A purification procedure has been proposed for separating silicon obtained from magnesium oxide and other impurity phases. The ratio of initial components has been determined, at which purified silicon has the least amount of impurities. The particle size of silicon powder obtained was 50–80 nm for the mechanically activated self-propagating high-temperature synthesis, and 30–50 nm for the direct mechanochemical synthesis. Keywords: mechanical activation; self-propagating high-temperature synthesis; mechanochemical synthesis; silica; magnesium; silicon; reduction reactions 1. Introduction Nanostructured silicon is a promising material for lithium-ion batteries [1,2], pho- tovoltaics systems [3,4], photocatalysis [5], nanoenergetics materials [6], and thermo- electrics [7,8]. Metallurgical-grade silicon is industrially produced by carbothermal re- duction of silica [9]. However, this process cannot fabricate nanostructured material as the temperature of carbothermic reduction (over 1900 ◦ C) is higher than the melting point of silicon (1414 ◦ C). In addition, this process is multistage, energy consuming, and rather dangerous ecologically. Therefore, the development of alternative time- and energy-saving technologies that also reduce the cost of the final product is an extremely desirable task. One of the most common carbon-free methods for obtaining silicon for most highly exothermic systems is self-propagation high-temperature synthesis (SHS) [10–12]. However, in systems where the combustion temperatures are significantly higher than the melting temperatures of the reactants (first of all, magnesium- and aluminothermic processes), the preparation of powder nanosized products is a serious problem. Usually, a decrease in Powders 2022, 1, 18–32. https://doi.org/10.3390/powders1010003 https://www.mdpi.com/journal/powders