Niobium metal has similar chemical properties to tantalum, and niobium ore is more abundant and less expensive than tantalum. So, why don't capacitor manufacturers use niobium instead of tantalum? In the past, the opportunity to evaluate niobium as a potential alternative to tantalum metal wasn't a possibility, due to direct current leakage (DCL) instability and the lack of high purity niobium metal powders necessary for capacitor manufacturing. However, in today's capacitor market, these barriers are being overcome with some interesting new developments.
Although the diffusion rate of oxygen in niobium systems is higher when compared to tantalum (resulting in DCL instability), we can alleviate this by doping metallic niobium powders with nitrogen or by using niobium oxide powder — which is a hard ceramic material characterized by high conductivity, a property usually associated with metals.
Niobium oxide powder has a similar morphology to that of tantalum, and niobium metals and can be processed the same way.
It seems the second barrier, a lack of high purity niobium metal powders necessary for capacitor manufacturing, shouldn't be a problem, being that niobium and niobium oxide are more abundant in nature than tantalum. These materials are common alloy elements widely used in the production of steel. Use of niobium for the production of capacitors is dwarfed by this major industry and this ensures a long-term stable supply. However, conversion of metallic niobium to capacitor-grade niobium (Nb) powder requires the same specialized processing as capacitor-grade tantalum powder, and shares the same supply chain.
Additionally, production of capacitor-grade niobium metal powder has not yet been scaled up to high volume. By contrast, niobium oxide (NbO) technologies have a much wider material supply base and higher volume availability (table).
Feature Overview
New generation niobium and niobium oxide capacitors share the same robust casing design and industry standard sizes as current tantalum chip capacitors. Yet, niobium oxide has two orders higher ignition energy and two times the specific heat compared to tantalum and niobium metals (Fig. 1).
This higher ignition energy results in a significant reduction (95%) of the ignition failure mode of niobium oxide capacitors compared with conventional tantalum and niobium metal capacitors. Coupled with the lower electrical stress within the dielectric (Nb2O5 dielectric grows thicker per applied volt than Ta2O5 and so operates at lower field strength for a given voltage rating), this also enables a higher ripple current load and reduced voltage derating requirements in low impedance circuits.
Unlike the tantalum and niobium metal capacitors that derate at 50% minimum, the niobium oxide capacitor can derate at 20% minimum. (i.e., 10V tantalum and niobium metal capacitors for 5V power rail, and 6.3V niobium oxide capacitors for 5V power rail).
Niobium oxide electrolytic capacitors have a high resistivity to short-circuit failure mechanism. Their oxide base significantly improves resistance against thermal runaway after dielectric breakdown and provides a genuine “non-burn” technology, when compared to metallic tantalum and niobium types either with or without a polymer electrolyte system.
More features of niobium oxide capacitors offer advantages in specific applications, including the following.
- Lead-free assembly
Lead-free assembly systems call for higher reflow temperatures. Not all capacitor technologies are ready to withstand these rigid conditions. Aluminum and foil capacitors are most sensitive to thermo-mechanical loads, especially reflow temperature/time soldering profiles that can result in catastrophic electrical failures. Ceramic capacitors have the most resilience to electrical overstress and are thermo-mechanically compatible with lead-free assembly. However, large parts can be sensitive to board flexure, so it's important to always follow manufacturers' handling guidelines. The general ceramic failure mechanism is low insulation resistance or a short circuit.
The new niobium oxide capacitors are of special interest, as they also show very good stability under thermo-mechanical stress and higher temperature, lead-free peak reflow conditions — but without any sensitivity to mechanical weakness.
- No piezo effect
High CV formulations of barium titanate used in ceramic capacitors exhibit a microphonic effect. For example, if you take a Y5V dielectric capacitor and subject it to a dc bias with a superimposed signal, the capacitor will vibrate. This mechanism is also reversible, which means mechanical vibration will generate noise on the electrical signal. Niobium oxide capacitors do not exhibit such a microphonic effect, which is particularly important in audio/video applications.
- Low weight.
Niobium oxide powder is half the density of tantalum powder. A typical “E” case size is about 25% lighter than the same capacitor made from tantalum powder. The practical usage is in weight sensitive applications such as mobile devices. Lower mass on the same component footprint will also improve p. c. board drop test strength.
- Lower ESR.
Temperature dependence behavior of NbO capacitors is identical to tantalum capacitors. ESR drops with temperature due to improvement of MnO2 (second electrode) conductivity. Thus, filtering at higher temperature is better than the room temperature specification.
Future Developments
Future developments can be subdivided into four main areas:
- Extension of the maximum temperature rating from 105°C to 125°C
- Increased product range, i.e. range extension and voltage extension to both 2.5V and 10V rated lower DCL (equivalent to tantalum capacitors)
- AVX is also currently working on niobium oxide capacitors utilizing our latest generation production technologies to manufacture low profile and microchip capacitors.
- One of the key requirements of the power supply sector is low ESR. We are working on several directions towards lower ESR. All of these developments stem from the underlying stability and reliability of the current niobium oxide capacitor.
Niobium and niobium oxide capacitors are now entering the high CV capacitor market place. They have a similar capacitance/voltage (CV) range to current tantalum chip and demonstrate ESR characteristics comparable to conventional tantalum ratings. Their parametric stability and less expensive material cost (especially in the case of niobium oxide capacitors), make these technologies promising alternatives to low voltage tantalum and ceramic capacitors, and allow downsizing of aluminum foil capacitors. The reduced burning of NbO also makes this a technology of interest.
New generation niobium and niobium oxide capacitors share the same robust casing design and industry standard sizes as current tantalum chip capacitors and are suited in low ESR applications. However, unlike niobium oxide, niobium metal capacitors have the disadvantage of higher cost and relatively higher failure rate.
Key benefits of NbO are long term stable electrical parameters, wide availability of materials, reduced burning and lower cost, which should form the basis for fast design-in cycles in this high growth application area.
References
This article adapted from a paper written by T. Zednicek, S. Zednicek, Z. Sita “Niobium Oxide Technology Roadmap” of AVX presented in CARTS EUROPE 2002.
For more information on this article, CIRCLE 335 on Reader Service Card