Niobium Capacitors: From Raw Materials to Finished Parts

摘要

The major feature of high CV Nb capacitors that distinguishes them from low CV Nb capacitors and Ta capacitors is saturation of Nb anode with oxygen during the capacitor manufacturing process. This is a result of a high diffusion coefficient for oxygen in Nb at typical manufacturing temperatures and the low effective radius of the powder particles. The combination of these two parameters enables oxygen diffusion into the core of the powder particles and their gradual saturation with oxygen. When the Nb anode becomes saturated with oxygen during the capacitor manufacturing process, further oxygen redistribution during life testing or in the field is suppressed. In this case the probable mechanism of Nb capacitor degradation is crystallization of the anodic Nb_2O_5 film. The crystal growth in the amorphous matrix of the film results in its disruption and, thereby, in the DCL increase. NbO and Nb_2N phases precipitate in the Nb anode during sintering of the high CV Nb powder. This is caused by dissolving of surface oxide and redistribution of oxygen and nitrogen into the bulk of the powder particles. Precipitates of NbO and Nb_2N phases may work as nuclei for Nb_2O_5 crystals, provoking the crystallization process. No precipitates were detected when sintering of the non-doped Nb powder was combined with deoxidizing. The higher the thickness of the amorphous oxide film, the more susceptible it is to crystallization. That is why the usage of Nb anodes with high chemical purity and homogenous phase composition is especially important for the manufacture of capacitors with 10 V and above rated voltages. Obviously, other parameters of the powder, like morphology and pore size distribution, are also important for the higher voltage applications.