Thin Pellets: Fast Electrochemical Preparation of Capacitor Tantalum Powders

Electrochemical reduction of solid Ta 2 O 5 to Ta metal in molten CaCl 2 at 1123 K has been studied by cyclic voltammetry and potentiostatic electrolysis, together with spectroscopic, electron microscopic, and elemental analyses. The initial step in the electro-reduction process is the electrochemically driven oxide−Ca 2+ interaction, leading to the formation of two or more stable calcium-containing compounds which are likely Ca 0.5 Ta 2 O 5 and CaTa 2 O 5 . These compounds are then reduced to TaO and/or δ-TaO before full metallization. At potentials more negative than −0.9 V vs Ag/AgCl, fine tantalum powder (aggregates of nodular ∼200 nm particles) can be prepared by direct electrolysis of thin and porous pellets of the Ta 2 O 5 powder (aggregates of nodular particles of ∼300 nm in size). The current efficiency and energy consumption are satisfactory, for example, achieving ∼78% and ∼2.4 kWh/kg-Ta at −1.0 V, respectively. The oxygen concentration in the produced Ta powder can be reduced to as low as 1200 ppm by electrolysis at a cell voltage of 3.1 V for 10 h. The specific capacitance of the electrolytic Ta powder reaches 40 kμFV/g.