The effect of precursor composition and reduction conditions on the characteristics of niobium powders obtained by magnesium reduction were studied. The hypothesis of the possible increase in the specific surface area of the powders by a large factor by using complex oxides as precursors was confirmed. With magnesium niobate Mg4Nb2O9 as the precursor, mesoporous powders, unprecedented in the world, with a specific surface area of up to150 m2 g–1 and an average pore size of 4.6 nm were produced.

Magnesium-vapor reduction of niobium oxide compounds

The metallic tantalum powder was successfully synthesized via reduction of tantalum pentoxide (Ta2O5) with magnesium gas at 1073~1223 K for 10 h inside the chamber held under an argon atmosphere. The powder obtained after reduction shows the Ta–MgO mixed structure and that the MgO component was dissolved and removed fully via stirring in a water-based HCl solution. The particle size in the tantalum powder obtained after acid leaching was shown to be in a range of 50~300 nm, and the mean internal crystallite sizes measured by the Scherrer equation varied from 11.5 to 24.7 nm according to the increase in reduction temperatures. The temperature satisfactory for a maximal reduction effect was found to be 1173 K because the oxygen content was minimally saturated to about 1.3 wt %.

Extraction of Tantalum Powder via the Magnesium Reduction of Tantalum Pentoxide

In this work we investigate influence of the composition and specific surface of a precursor on the porous structure of tantalum powders obtained via magnesium vapor reduction of the Та2О5, MgТа2O6 and Mg4Та2O9. Sorption and nitrogen capillary condensation have allowed an increase in the specific powder surface to be established due to the increasing amount and specific surface of mesopores with a size below 10 nm.

Tantalum powders with a mesoporous structure

The influence of the composition and the conditions of the reduction of niobium oxide compounds with magnesium vapor on the specific surface area of the produced metallic powder is studied. When magnesium niobate Mg4Nb2O9 is used as a precursor, the specific surface of the powder increases by several times compared to that of an Nb2O5 precursor. Niobium powders with the specific surface up to 150 m2 g–1 (calculated particle size 4.7 nm) and a bulk density of 0.8 g cm–3 are formed by the reduction of Mg4Nb2O9. The powders are characterized by a mesoporous structure, and the most part of the specific surface consists of pores with a diameter less than 4 nm.

Influence of the precursor composition and the reduction conditions on the characteristics of magnesium-thermic niobium powders

The reduction of tungsten and molybdenum oxide compounds (WO3, MoO3, MgWO4, MgMoO4, and CaMoO4) with calcium vapor at 800–860°С and a residual argon pressure of 5–10 kPa has been studied. As previously during the reduction with magnesium vapor, the spatial separation of the reaction products was observed, namely, the major portion of the calcium oxide formed in the reaction was deposited outside the reaction zone. A specific feature of the reduction of MgWO4 and MgMoO4 is that the magnesium is first replaced by calcium. The resulting magnesium metal acts as a reducing agent, and the magnesium oxide, along with calcium oxide, forms a crust on the surface of the reaction mass. Analysis of the experimental results shows that the reduction of oxide compounds with magnesium and calcium vapors at a residual argon pressure of more than 5 kPa proceeds via the electronically mediated reaction mechanism without direct physical contact between the reactants.

M. V. Kryzhanov

Papers

Magnesium reduction of tantalum oxide compounds

Magnesium-vapor reduction of niobium oxide compounds