Device construction & distinguishing traits
Niobium oxide capacitors are similar in construction to tantalum and manganese dioxide (Ta/MnO2) devices, using sintered niobium oxide (NbO) in lieu of tantalum metal as the anode material. Produced chiefly by AVX as an alternative to Ta/MnO2 capacitors that doesn’t have the nasty inclination to deflagrate upon failure and also having the potential for improved raw material supply logistics, niobium oxide capacitors are in competition with tantalum polymer devices for a variety of applications.
Construction of niobium oxide capacitors is similar to that of Ta/MnO2 devices; the anode material consists of a highly porous, sponge-like mass of niobium (mon)oxide (NbO) on which a dielectric layer of niobium (pent)oxide (Nb2O5) is established and around which a counter electrode of manganese dioxide is built up, in a fashion similar to the common Ta/MnO2 devices. Capacitors based on niobium metal (rather than the oxide NbO) and polymer electrolyte technologies have also been developed, but are not being produced in any significant quantity at the time of writing.
Why niobium?
A shortfall in tantalum supply amid high demand around the turn of the millennium resulted in tantalum capacitors being a rare and costly item for a season, causing production headaches that motivated the development of devices based on niobium. Relative to tantalum, which finds its primary use in the electronics industry, niobium is estimated to be some 20 times more abundant in nature, and is also used widely as an alloying element in steel production in quantities much larger than would conceivably be required for electronics purposes. Between there being more of the stuff to start with and the electronics industry not being the primary buyer for it, the long-term supply prospects for raw materials were thought to favor niobium over tantalum.
Application strengths & weaknesses
Niobium oxide/manganese dioxide capacitors have a significant advantage over their tantalum kin, in that they generally don’t ignite when they fail catastrophically. This is attributed to a much larger amount of energy required to ignite niobium oxide compared to tantalum, as well as a secondary self-healing effect wherein the niobium oxide anode material exposed at a fault site is further oxidized to a less conductive state. Between the two effects, the behavior of niobium oxide capacitors experiencing catastrophic failures is said to be a high-impedance short circuit in the Kohm range; a value high enough to prevent the resulting fault currents from delivering enough energy to ignite the device at rated voltages.
Relative to Ta/MnO2 devices, NbO/MnO2 capacitors currently are a bit behind in terms of performance, being limited to voltage ratings of 10V or less, having leakage currents roughly double those of tantalum devices, slightly lower capacitance per volume, and higher temperature de-rating over 85°C. On the other hand, “doesn’t burst into flame” is a very nice feature to have, and the issue of better raw materials availability offers promise of lower costs. Though the tantalum polymer approach to the pyrotechnic capacitor problem seems to be gaining greater popularity, the niobium oxide technology is said to retain advantages in terms of long-term service life and environmental tolerance, particularly in high-humidity applications. If for no other reason, it’s an interesting technology simply because bringing up the subject with the sales & marketing folks representing the different factions seems to elicit decidedly different perspectives and opinions…
Application considerations
The ignition-resistant character of niobium oxide capacitors permits a more aggressive application of NbO-based devices relative to their tantalum-based counterparts. Whereas the rule of thumb for designing with Ta/MnO2 capacitors is to de-rate voltage by 50% (or more if series resistance is very low) the leading manufacturer of NbO-based devices (AVX) has suggested that de-rating voltage by only 20% is sufficient for safe operation. Additional de-rating beyond these levels can improve long-term reliability of both device types significantly however. Also, since the device’s internal structure and thermo-mechanical properties of the solid MnO2 electrolyte remain, users of niobium oxide capacitors are advised to be mindful of the potential for failures induced by the assembly process.