Formation of the Pore Structure of Tantalum and Niobium Powders during Magnesiothermic Reduction of Lithium Tantalate an
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ation of the Pore Structure of Tantalum and Niobium Powders during Magnesiothermic Reduction of Lithium Tantalate and Lithium Niobate V. M. Orlova, *, M. V. Kryzhanova, and E. N. Kiseleva aTananaev Institute of Chemistry and Technology of Rare Elements and Minerals (separate subdivision), Kola Scientific Center
Federal Research Center, Russian Academy of Sciences, Apatity, Murmansk oblast, 184209 Russia *e-mail: [email protected] Received February 10, 2020; revised March 16, 2020; accepted March 25, 2020
Abstract—We have studied characteristic features of the reduction of single-crystal lithium tantalate and lithium niobate powders with magnesium vapor in the temperature range 1023–1123 K. The results demonstrate that the reduction of single-crystal particles is accompanied by their spontaneous mechanical disintegration. The presence of Li2O in the double oxides has not led to an increase in the specific surface area of the metallic powders obtained. Li2O has been completely reduced in the final stage of the process, without contributing to an increase in the number of pores in the structure of the powder particles. Keywords: tantalum, niobium, powder, pore structure, magnesiothermic reduction, lithium tantalate, lithium niobate DOI: 10.1134/S0020168520080117
INTRODUCTION Nanomaterials are playing an increasingly important role in the development of modern engineering. In particular, tantalum and niobium nanopowders prepared by reducing tantalum and niobium pentoxides with magnesium vapor have found application in the manufacture of high-capacity electrolytic capacitors [1–3]. The specific surface area of such tantalum and niobium powders reaches 15 and 30 m2/g, respectively [4]. The use of such powders enabled the advent of electrolytic capacitors with a specific capacity of 150000 μC/g and more [5]. Magnesiothermic tantalum and niobium powders are distinctive in that they have a mesoporous structure due to specific features of the growth of metal particles during the reduction process. As shown by Müller et al. [6] and Gille et al. [7], a reduced pentoxide particle consists of alternating layers of reaction products: tantalum metal and magnesium oxide. After leaching the magnesium oxide with acid solutions, the particle has a spongy pore structure, which ensures that the powder has a large specific surface area. In those studies, the following reduction mechanism was examined: magnesium penetrates into the bulk of a pentoxide particle through cracks and grain boundaries, so that the reduction process occurs not only on its surface but also in its bulk.
With this mechanism, it would be expected that the addition of extra magnesium oxide layers would increase the porosity of the metal powder. Indeed, the use of precursors for reduction in the form of double oxides of Group V and VI refractory metals containing a refractory oxide (CaO or MgO) allowed the specific surface area of the resultant metallic powders to be increased many times [8–11]. The large specific surface area of tantalum and niobium powders, reaching
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