Sintered Tantalum and Niobium Sub-micron Powders of Solid Electrolite Capacitors
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Sintered Tantalum and Niobium Sub-micron Powders of Solid Electrolite Capacitors Y. Pozdeev Freeman,1 A. Gladkikh,2 and Yu. Rosenberg2 1 Vishay Sprague Inc., 1754 Main St. Sanford, ME 04073, USA 2 Wolfson Applied Materials Research Center, Tel Aviv University, Tel Aviv 69978, Israel. ABSTRACT The results of x-ray study and electrical measurements are brought together in order to understand a difference in performance and failures of solid electrolyte capacitors made of sintered tantalum and niobium sub-micron powders. It is shown that bulk Ta and Nb exhibit different abilities for the formation and decomposition of oxygen solid solution during manufacturing steps which may be critical for stability of metal/oxide interface of capacitor.
INTRODUCTION Sintered tantalum (Ta) and niobium (Nb) micro-particles are widely used in modern electronics as porous anodes of Ta and Nb capacitors. Amorphous Ta2O5 and Nb2O5 film, formed by anodizing of the particle surface, is employed as the dielectric in these capacitors. Due to high surface area of the anode, Ta and Nb capacitors demonstrate excellent volumetric and mass efficiency. Evolution of these capacitors involves gradual reducing of the average particle size from tens of microns initially to less than one micron currently. This resulted in the specific charge increase from about 10,000 µQ/g initially to more than 100,000 µQ/g currently. At the same time, the DC and AC characteristics of some of these capacitors degrade during life test or in the field. In the Ta capacitors, the degradation usually displays as an abrupt increase of the DC leakage, causing catastrophic failure of the Ta capacitor. In the Nb capacitors, the DC leakage typically increases gradually, causing parametric failure of the Nb capacitor. The earlier works, performed on Ta and Nb foil models and coarse powders, showed that the nature of the degradation relates to thermodynamic non-equilibrium of the amorphous Ta2O5 (Nb2O5) film and its interface with Ta (Nb) anode [1,2]. Relaxation into a thermodynamically stable state proceeds through crystallization of the Ta2O5 (Nb2O5) film and oxygen migration from the film into the Ta (Nb) anode through the Ta2O5/Ta (Nb2O5/Nb) interface. Both processes, crystallization and depletion of oxygen due to the oxygen migration, degrade the dielectric, causing DC leakage to increase. It was also demonstrated, that oxygen in sintered particles has strong influence on the degradation processes in the anodic oxide films on Ta and Nb [3,4]. When bulk oxygen content approaches solubility limit, oxide crystals precipitate from solid solution of oxygen in Ta (Nb). These crystals act as nuclear initiating crystallization of the amorphous Ta2O5 (Nb2O5) film. On the other hand, when the oxygen content in the Ta (Nb) is low, it stimulates oxygen migration out of the anodic oxide film. In other words, oxygen content in Ta (Nb) particles defines which of the degradation mechanisms is dominating. The current work is dedicated to sub-micron Ta and Nb powders and capacitors m
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