Institute of High-Temperature Electrochemistry of the Ural Branch of the Russian Academy of Sciences, Ekaterinburg, Russia:

A. A. Chernyshev, Engineer, e-mail: info@ihte.uran.ru
A. P. Apisarov, Researcher, e-mail: aap@ihte.uran.ru
Yu. P. Zaykov, Research Manager of Instutute

We studied the production of Ta powders by the method of secondary reduction in the NaCl – KCl and KCl – CaCl₂ melts. The study was carried out without the addition of potential-determining ions in the melt composition in the temperature range of 700–900 ^oC. The X-ray diffraction analysis and electron microscopy coupled with micro-X-ray analysis showed the formation of tantalum powders with a developed core-shell surface (Ta – TaxOy) during the reduction process. The main stoichiometric oxide phase in the particles shell was Ta₂O₅. Regularities of the precipitation formation were studied in order to obtain powders of the required size. The use of a calcium chloride containing melt expands the range of the particles up to 1000 μm with a maximum in the region of 20–40 μm. Applying a laser size analysis and BET method, we found that the use of different ratios of the cathode and anode current densities effects the size and specific area of the tantalum powder. The powders with the high specific surface area up to 22.9 m²/g were obtained. The number of the largest particles increases with rising temperature. This is confirmed by the tendency to reduce the specific surface area. It is shown that the dependence of the particle size on the volume fraction of particles depends on the process conditions. For powders obtained in the melts NaCl – KCl and CaCl₂ – KCl with the ratio of current densities i_c /i_a = 1 и i_c /i_a = 2, the indicative for the particle size distribution was the range of 0.05–10 and 0.5–20 μm, respectively. The increase in the ratio of the current densities to i_c /i_a = 5 in the melt of CaCl₂ – KCl makes it possible to obtain particles with a maximum of 30 μm and a high volume fraction of such particles. We found the optimal electrolysis conditions for obtaining particles suitable for the process of spherization and subsequent use in additive technologies.
This work was carried out with the financial support of the Program of Ural Branch of RAS 2015–2017 “Materials and technologies for nuclear, alternative and renewable power generation” (Project No. 15-20-3-20).

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