Niobium and Niobium Oxide Capacitors

C 3.7. SOLID NIOBIUM and NIOBIUM OXIDE CAPACITORS

Niobium is a sister metal to tantalum, and shares many chemical characteristics with it, in addition to a few disadvantages and advantages of its own when used as a capacitor dielectric.

Niobium capacitor technology has existed for decades, but its limitation in maximum rated voltage, lower volumetric efficiency, incompatibility with low ESR polymer electrode, limited range and number of vendors keeps this technology as a niche line of tantalum capacitor industry today. On the other hand, there are still some advantaged (such as lower cost, safety and reliability) that worth a closer look and consideration.

Brief History

Capacitor manufacturers have been evaluating niobium material for years every time the tantalum supply situation becomes unstable. Its lower cost base was also evaluated in order to replace low-voltage aluminium electrolytics.

Development of the first niobium capacitors started in former USSR already in 1960’s motivated by shortage of tantalum there. However, at that time, niobium powders were not available in appropriate low impurity, high quality level and the capacitor reliability was inferior to the tantalum capacitors.

Niobium-based material as easier to get and more abundant in nature captured attention by most of the leading tantalum manufacturers especially after the tantalum shortage and supply chain issues around year 2000. Beside niobium metal powder, NbO material has been introduced with metal conductivity level by J.Fife (U.S. Patent 6,391,275 B2) resulting in some advantage for capacitor application properties. Upon the new development and availability of low impurity Niobium/NbO powders within years 2001-2003 companies AVX, EPCOS (tantalum division acquired by Kemet in 2007), Hitachi (bought by Holy Stone in 2009, acquired by Vishay in 2014), Kemet, NEC and Vishay announced some early sampling and pre-production of niobium-based capacitors in 2002.

Construction of SMD chip niobium capacitors is identical to tantalum including manufacturing process route. Niobium or Niobium oxide powder are used instead of tantalum and the dielectric is Nb2O5 instead of Ta2O5 see Fig. below. Dielectric constant of Nb2O5 is 41 ~ twice higher compare to tantalum pentoxide, hower its density and electrical breakdown strength is about a half. This does not bring CV advantage of Nb/NbO capacitors over tantalum, however as the dielectric has to be ~ twice thicker compare to tantalum it brings benefits to stability and reliability.

NMC series of niobium based capacitor structure; source Vishay

Nevertheless, despite the very optimistic first results, niobium posed some early technical challenges that capacitor makers were struggling to overcome. The obstacles included high dc-leakage current (two to five times worse than for tantalum capacitors), increase of DCL with lifetime and susceptibility to damage by thermal shock during reflow manufacturing process. Low-CV capabilities have also been a problem because of the lack of high grade, high purity capacitor-grade powders at that time.

Tantalum supply chain has consolidated in further years and the urgency for niobium-based capacitor development dropped versus requirements for lower ESR tantalum capacitors with conductive polymer electrodes. In co-incidence, conductive polymer junction to Nb₂O₅ dielectric is resulting in even higher DCL that practically disqualify its use with niobium-based capacitors. This next technical challenge would require additional development resources to address some basic physics issues related to potential development of polymer niobium-based capacitors. Hence, most of the manufacturers terminated its further development effort, focusing its development power to the other priorities and obsoleted their niobium-based capacitor lines.

Niobium-based capacitors with MnO₂ solid electrolyte are still on the market with high reliability versus cost unique value in certain applications. Vishay obsoleted its SMD chip niobium capacitors in 2017 and the major supplier of SMD NbO capacitors today is AVX Corporation that dedicate its product to mainly automotive, aircraft, defence and high safety applications. Axial niobium capacitors are also still made in Russia by company OJSC ELECOND (figure on right) for industrial, defence and high reliable applications.

Niobium-based Capacitor Technology and Features Background

Volumetric efficiency

Niobium, as mentioned, is in many ways behaving very similar to tantalum capacitors. Niobium or NbO material can be processed into a form of powder with a stable oxide Nb₂O₅ (vs Ta₂O₅ in case of tantalum) as a high dielectric constant insulation. Niobium pentoxide has a dielectric constant about twice as high as that of tantalum oxide, however the density of niobium is only half that of tantalum, resulting in similar or slightly lower volumetric efficiency compare to tantalum capacitors.

Abundance

an important advantage of niobium is the abundance of niobium ore in nature relative to tantalum ore; this relative abundance equates to lower cost and better availability within the capacitor marketplace.

Flame Retardancy

Niobium metal is behaving like tantalum capacitors. However, as one of key advantages niobium oxide has a far higher ignition energy than tantalum which results in a significant reduction (up to 95%) of the ignition failure mode of niobium oxide capacitors when compared to conventional tantalum devices.

Sub-oxides stability

One of the “small” differences between niobium and tantalum that makes a big impact to capacitor features is that niobium sub-oxides are stable unlike on tantalum. There are no long term stable suboxides between tantalum and the dielectric tantalum pentoxide (Ta₂O₅). There are at least two suboxides on niobium-based materials that are stable: NbO and NbO₂.

NbO is a material with metallic type of conductivity that can be processed and used as the main anode material same way as Nb or Ta metals (used by NbO capacitors as anode material)

NbO₂ is an oxide with semiconductive behaviour that exists together with the “main” dielectric Nb₂O₅ on the junction with Nb/NbO anode. The NbO₂ oxide may grow with temperature (such as during the hot spot in dielectric failure site) and its responsible for the unique “self-arresting” mechanism.

Self-arresting mechanism:

In addition to the self-healing known on conventional tantalum MnO₂ capacitors, the NbO capacitors are featuring one more feature when it is exposed to local breakdown, the NbO₂ layer on dielectric will grow as a “second” insulation protecting the part to go to short circuit mode. Such part then continues in normal operation even in occasion of the main Nb₂O₅ dielectric breakdown. Under a specified operating conditions, NbO capacitors are thus one of the safest capacitor technologies on the market.

Operating temperature:

Another difference to tantalum capacitors is that Nb₂O₅ dielectric is more sensitive to higher temperature operation above 85°C. Reliability up to 85°C may be considered as better, about equal at 105°C and inferior at 125°C requiring higher derating compare to tantalum capacitors.

The natural high reliability performance of niobium-based capacitors up to 85°C (including safety margin) makes them suitable for high performance industrial, automotive, defence, medical support, aerospace applications even at safety critical applications.

Rated voltage:

As mentioned, the density of niobium is only half that of tantalum, so twice as much material is needed per unit volume to provide the same charge. The Nb₂O₅ dielectric constant is higher, but you must form a thicker niobium oxide dielectric for the same voltage. That is good to reduce further the electrical field stress to the dielectric (and achieve better reliability) on one side, but there is a certain limitation in maximum dielectric thickness / maximum rated voltage, driven by powder impurities and formation techniques on the other site. In result, niobium-based capacitors are featuring significantly lower maximum rated voltage compare to tantalum capacitors.

Limitations

One limitation of niobium-based technologies, is a lower maximum rated voltage (~10-16V) and lower CV in comparison to conventional tantalum capacitors with MnO2 (still higher compare to other capacitor technologies).

As tantalum supply chain consolidated, niobium-based technology has been ceased by number of manufacturers, thus limited sources and product range are the main current issues of the technology apart of the technical borders discussed above. Case sizes, mounting guidelines etc of the niobium-based chip capacitors are however identical to tantalum and tantalum polymer capacitors, that can be considered as drop-in alternative on the same footprint to avoid single source issues. Thus, the niobium-based capacitors can be considered as additional option where tantalum capacitor designs are considered with enhanced safety feature.

Capacitance versus temperature

In comparison with solid tantalums the temperature dependence is slightly higher.

Capacitance versus frequency

The higher the porosity and the finer the powder, the sooner the capacitor starts loosing capacitance due to the increasing time constant of the farthest in localized capacitor elements in the pellet. Because niobium powders have a slightly higher porosity than tantalum its frequency dependence is correspondingly higher. Figure above.

ESR versus temperature

Because ESR is dominated by the MnO2 counter electrode we may expect approximately the same behavior as in solid tantalums.

ESR versus frequency

The ESR in niobium electrolytics is slightly higher than that of solid tantalums at frequencies around 100 Hz. The difference decreases successively and is at 1 to 10 MHz negligible.

Leakage current versus ambient temperature

Applications

Where the niobium capacitors bring its benefits:

• High reliability, long term operation, safety circuits, with a “standard ESR requirements” up to 8V application voltage operating in environment up to 85°C safety (105°C qualification) with a little more space on board needed compare to tantalum capacitors (still smaller then aluminium electrolytics). The parts showing high mechanical robustness against shocks and vibrations and stable electrical performance (over MLCC class II capacitors).

There is a wide range of applications were niobium-based capacitors can bring its benefits in low cost for high safety and reliability standards.

applications with benefits from downsizing of aluminium electrolytics capacitor:

• Consumer such as home theatres, game controllers, white goods controllers.

high value in lower cost versus safety and reliability

• Industrial with enhanced safety feature such as smoke detector, security electronics etc
• Automotive such as cabin electronics and telematics
• Aircraft – on board entertainment, telematics, defence (COTS+ version available)

Summary

Niobium-based capacitors present a niche capacitor market share within the tantalum capacitor industry. Nevertheless, some of its unique features, especially high safety, no short circuit failure mode under standard operating conditions and high reliability bring its value to the end users to remain as a viable capacitor solution in longer term future.

The NbO capacitor within its rated voltage is not likely to fail as a short circuit in applications that makes it one of the safest capacitor technologies on the market at the class of up to 8V operation voltage – counting with 20% derating of its maximum 10V range. The main applications are then 3.3 and 5V lines where safety is the prime focus such as aircraft or automotive electronic hardware.

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ABC of CLR: Chapter C Capacitors

Niobium and Niobium Oxide Capacitors

EPCI licenced content by:
[1] EPCI European Passive Components Institute experts original articles
[2] CLR Passive Components Handbook by P-O.Fagerholt*
*used under EPCI copyright from CTI Corporation, USA

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