Mechanochemical synthesis of intermetallic compounds in the system gallium – ruthenium Chimica Techno Acta ARTICLE published by Ural Federal University 2021, vol. 8(1), № 20218104 journal homepage: chimicatechnoacta.ru DOI: 10.15826/chimtech.2021.8.1.04 1 of 4 Mechanochemical synthesis of intermetallic compounds in the system gallium – ruthenium T.F. Grigoreva a* , E.A. Pavlov b , P.A. Vitiaz c , N.Z. Lyakhov a a: Institute of Solid State Chemistry and Mechanochemistry SB RAS, Kutateladze St. 18, Novosibirsk, 630090, Russia b: JSC “Zelenyj Gorod”, 60 let Oktyabrya St. 126, Krasnoyarsk, 660079, Russia c: Joint Institute of Mechanical Engineering of the National Academy of Sciences of Belarus, Akademicheskaya St. 12, Minsk, 220072, Belarus * Corresponding author: corresponding_grig@solid.nsc.ru This article belongs to the PCEE-2020 Special Issue. © 2021, The Authors. This article is published in open access form under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Abstract The interaction of a solid inert metal Ru with liquid active metal Ga during mechanical activation in a high-energy planetary ball mill was studied using the X-ray diffraction and the high resolution scan- ning electron microscopy with energy dispersive X-ray microa- nalyses. This paper considers mechanical activation effects on for- mation of intermetallic compounds GaxRuy and their solubility in concentrated acids. Gallium is a surface-active substance with re- spect to Ruthenium. Under intensive mechanical treatment, liquid Gallium penetrates into grain boundaries of polycrystalline Rutheni- um particles and sharply reduces their strength. Because of severe mechanical deformation, an intensive increase of contact surface be- tween solid and liquid metals observed, which a place of rapid for- mation of intermetallic compounds. This processing leads to high re- active products of mechanical activation of Ga + Ru. Their interac- tion with a mixed concentrated hydrochloric and nitric acid allows Ruthenium (~37%) to pass into an acidic solution, forming complex compounds of the HxRuCly type (H2RuCl6). Keywords mechanochemical synthesis intermetallics gallium ruthenium complex compounds of ruthenium Received: 28.12.2020 Revised: 05.02.2021 Accepted: 05.02.2021 Available online: 08.02.2021 1. Introduction Ruthenium is the most chemically and heat-resistant metal of the platinum group metals, so Ru is applied as alloys additive for manufacturing turbine blades of jet engines, high temperature parts of missiles, equipment for aircraft devices [1]. A promising area for using Ruthenium is solar energy. The ability of Ruthenium to bind catalytically atmospheric nitrogen at room temperature is also unique. The alloys of Ruthenium with Platinum and Iridium are used for manu- facturing dies in the production of glass fibers and viscose. An efficient approach to obtain ultra-fine high purity metals can be their dissolution in acids with formation of ammonium salts and subsequent reduction. For Rutheni- um, there is a problem with the dissolution, because this metal does not interact even with highly concentrated ac- ids. It is known that intermetallic compounds (IMC) and alloys dissolve faster than their constituent inert metals, especially in case of well-developed interfacial or inter- granular surface [2,3]. The conversion of an inert metal into a soluble form becomes possible when one of the components of the IMC dissolves easily. This effect is much more pronounced for ultra-dispersed IMC, and mechanochemical synthesis is one of the most promising techniques for their production. In systems with interacting solid and liquid metals, the adsorption–active properties of the liquid metal with re- spect to most hard solid metals were verified [4,5]. Pene- tration of liquid metal along the grain boundaries sharply reduces the strength characteristics of the latter [6–9]. The work of destruction of polycrystalline metal in contact with the melt reduces by hundreds of times, which pro- vides a high rate of formation of a "fresh" (native) surface of a solid metal. Due to the high wettability of the solid surface with liquid metal, the contact surface of the solid– liquid metal can reproduce itself permanently, and IMC synthesis continues [8,10–12]. From this point of view, it is very likely that the inert Ruthenium should chemically interact with the active liquid Gallium, since it is known http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2021.8.1.04 http://creativecommons.org/licenses/by/4.0/ http://orcid.org/0000-0002-7264-0862 Chimica Techno Acta 2021, vol. 8(1), № 20218104 ARTICLE 2 of 4 that Ga3Ru, Ga2Ru, and GaRu are able to form in the Ga– Ru system at high temperature and pressure. The full equilibrium state diagram of the Ga–Ru system has not been evaluated yet [13]. The presence of several interme- tallic compounds in the Ga–Ru system is probably due to the negative enthalpy of mixing. In a pair of solid Ru – liquid Ga (with melting temperature 29.8 °C), the mecha- nochemical synthesis is possible thanks to formation of large contact surface Ru/Ga. Gallium is highly soluble in acidic solutions [14]. This makes it possible to leach Ga from intermetallic compounds GaxRuy obtained mechano- chemically, thus increasing the ability for Ruthenium to dissolve in concentrated acids. Analysis of scientific literature data shows that there are no studies of mechanical interaction of platinum satel- lite metals (Ir, Ru, Rh) with liquid Gallium, so this study is relevant. The aim of this is work to study the process of mecha- nochemical synthesis of intermetallic compounds in the Ga–Ru system, as well as the acid separation of Gallium from synthesized IMC in order to convert Ruthenium into a soluble form. 2. Experimental Liquid Gallium and Ruthenium powder were used to pre- pare the intermetallics GaxRuy. Mechanochemical synthesis of intermetallic com- pounds in the inert solid metal (Ruthenium) – active liquid metal (Gallium) system was carried out in a high-energy planetary ball mill AGO-2. The volume of milling drums was 250 cm 3 . Steel balls of 5 mm diameter with total mass 200 g were used to achieve the ball-to-powder ratio 20:1. The rotation speed of the drums around the common axis was 1000 rpm. During milling, drums and balls were cooled with water [15]. To determine the degree of dissolution of the initial Gallium and Ruthenium, mechanochemically synthesized intermetallic compounds GaxRuy, they were treated with a mixture of concentrated acids (60 ml HCl + 20 ml HNO3), for three hours, at a temperature of 75 °C. Samples of 1 g each were weighed directly in 400 ml glasses with analyt- ical Mettler Toledo RT 503 balance (weighing accuracy of 0.001 g). Separation of insoluble sediments from solutions was carried out in several stages using paper filters with a pore size of 2–3 μm. Glass measuring cylinders were used to determine the volume of filtrate and washing water. X-ray phase analysis of the powders was carried out using an XPERT-PRO diffractometer (Cu Kα1 radiation, wavelength = 1.54051 Å, the 2θ angle range from 3.0131° to 90.91°, step Δ2θ = 0.001°). Morphological characteristics and elemental composi- tion of the initial Ruthenium powder, as well as mechano- chemically synthesized GaxRuy intermetallides and GaxRuy/Ru composites, before and after treatment with a mixture of concentrated acids, were obtained on a scan- ning electron microscope JEOL 6601 LV (accelerating volt- age 20 kV, magnification up to ×10000) with an energy- dispersive X-ray microanalyzer (EDXA). The error of the elemental content analysis on the EDXA was 0.2-0.3 mass. %. 3. Results and Discussion According to scanning electron microscopy, the initial Ruthenium powder consists of primary particles with size within 1–3 μm; round shaped particles form porous ag- glomerates with the size in between 5 to 20 μm (Fig. 1). According to the results of EDXA, the Ru content is 100%. According to [16,17], for interacting metals an interme- tallic compound with the maximum content of fusible metal is first formed on the contact surface. Accordingly, in the interacting Ga-Ru system, some phases with the highest Gallium content must be formed first, and when Gallium is totally spent the mechanochemical interaction of the IMC with Ru will start forming intermetallic phases with a lower Gallium content (GaRu) [18]. It can be expected that at the initial stage of the me- chanical activation of the Ga+Ru mixture the bulk Gallium will cover the Ruthenium particles with an extremely thin layer, forming Ga3Ru or Ga2Ru on the contact surface of the compound, since, according to Miedema calculations, the mixing enthalpy is about –8 kJ/mol [19–21]. These IMCs will further interact with Ruthenium to form GaRu. By means of X-ray phase analysis, the products of the chemical interaction of ruthenium with gallium (with mo- lar ratio Ga:Ru = 1:1) are detected only after 2 minutes of mechanical activation. (Fig. 2, curve 1). A noticeable amount of this phase is formed in the mix- ture when activated for 14 min (Fig. 2, curve 2), at the same time a weak diffraction peak appears on the X-ray pattern, which may be due to the formation of GaRu as a result of the interaction of Ga2Ru with Ruthenium. As the activation time increases up to 20, 34, and 62 min, the newly formed Ga2Ru is consumed to form GaRu, resulting in the increasing diffraction peak intensities of secondary phase (Fig. 2, curves 3–5). Electron microscopic studies showed the small parti- cles of ~0.2–0.5 μm after 2 min of activation (Fig. 3, photo a). After activation for 14 min, the sample contains the Fig. 1 SEM image and particle size of the as received Ru powder (×4000) Chimica Techno Acta 2021, vol. 8(1), № 20218104 ARTICLE 3 of 4 small particles of ~0.2–0.5 μm and agglomerates of differ- ent types: dense ~0.5-5 μm and porous (loose) - up to 10 μm (Fig. 3, photo b). In the sample activated 34 min, there are significantly fewer small particles of ~0.2–0.5 μm, and much more of agglomerates with a wide size distribution from ~0.5 to 5 μm (Fig. 3, photo c). According to the EDXA, the iron content in the samples increases from 0.5–0.7 to 6–8 wt. % due to the wear con- taminations from balls and drums, depending on time of mechanical activation. Preliminary experiments showed the starting Gallium totally dissolved in a mixture of concentrated acids within 40 min. The highly dispersed Ruthenium powder does not dissolve under the above conditions, remaining the same morphological features as the original Ruthenium (Figs. 1 and 4). It was found that the amount of Ruthenium in a mix- ture of concentrated acids increases with the increase in the content of intermetallic compounds in mechanocom- posites obtained. So, after 20 min of mechanical activation it was 27.09 wt.%, after 34 min the value increased to 36.78 wt. %. The insoluble precipitate after MA for 34 min is an ultrafine powder with an average particle size of less than 0.5–0.6 μm (Fig. 5). We can expect that Ruthenium is oxidized to form a complex acid HxRuCly in the course of transition from a mechanocomposite to an acidic solution. 30 40 50 60 70 80 90    - Ru [6-663]  - Ga 2 Ru: [23-261]  - GaRu [17-440]                  In te n s it y , a rb . u n . 2 theta, degree 1 2 3 4 5                  Fig. 2 X–ray patterns of the products of mechanochemical interac- tion of Ru with Ga in depending on the duration of mechanical activation (min): 2 (1), 14 (2), 20 (3), 34 (4), 62 (5). a b c Fig. 3 SEM images of the products of mechanochemical interac- tion of Ru with Ga depending on the duration of mechanical acti- vation (min): 2 (a), 14 (b), 34 (c). Fig. 4 SEM image of the insoluble precipitate particles of the orig- inal Ru powder after treatment with acid mixture (×6000) Fig. 5 SEM image of the insoluble precipitate obtained from Ru– Ga intermetallic (MA for 34 min) after treatment with concen- trated acid mixture (×7000) Chimica Techno Acta 2021, vol. 8(1), № 20218104 ARTICLE 4 of 4 4. Conclusions For metallic systems with Platinum satellite metals, the example of the Ru-Ga system shows that the problem of converting chemically inert Ruthenium into a soluble form can be solved by mechanochemical synthesis of interme- tallic compounds Ru with liquid Ga and subsequent extrac- tion of Gallium into acidic solutions. Being a surface-active substance in relation to Ruthenium, liquid Gallium pene- trates into grain boundaries of polycrystalline Ruthenium particles, which strongly reduces their strength. 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