A niobium electrolytic capacitor (historically also Columbium capacitor) is an electrolytic capacitor whose anode (+) is made of passivated niobium metal or niobium monoxide, on which an insulating niobium pentoxide layer acts as a dielectric. A solid electrolyte on the surface of the oxide layer serves as the capacitor's cathode (−). Niobium capacitors are available in SMD packaging and compete with tantalum chip capacitors in certain voltage and capacitance ratings. They are available with a solid manganese dioxide electrolyte. Like most electrolytic capacitors, niobium capacitors are polarized components. Reverse voltages or ripple currents higher than specified tolerances can destroy the dielectric and thus the capacitor; the resulting short circuit can cause a fire or explosion in larger units. Niobium capacitors were developed in the United States and the Soviet Union in the 1960s. Since 2002 they have been commercially available in the West, taking advantage of the lower cost and better availability of niobium relative to tantalum.
Basic information Niobium is a sister metal to tantalum. Niobium has a similar melting point (2744 °C) to tantalum and exhibits similar chemical properties. The materials and processes used to produce niobium-dielectric capacitors are essentially the same as for existing tantalum-dielectric capacitors. However, niobium as a raw material is much more abundant in nature than tantalum and is less expensive. The characteristics of niobium electrolytic capacitors and tantalum electrolytic capacitors are roughly comparable. Niobium electrolytic capacitors can be made with high purity niobium as the anode but the diffusion of oxygen from the dielectric (Nb2O5) into the niobium anode metal is very high, resulting in leakage current instability or even capacitor failures. There are two possible ways to reduce oxygen diffusion and improve leakage current stability – either by doping metallic niobium powders with nitride into passivated niobium nitride or using niobium oxide (NbO) as anode material. Niobium oxide is a hard ceramic material characterized by high metallic conductivity. Niobium oxide powder can be prepared in a similar structure to that of tantalum powder and can be processed in a similar way to produce capacitors. It also can be oxidized by anodic oxidation (anodizing, forming) to generate the insulating dielectric layer. Thus two types of niobium electrolytic capacitors are marketed, those using a passivated niobium anode and those using a niobium oxide anode. Both types use niobium pentoxide (Nb2O5) as the dielectric layer.
Anodic oxidation
Niobium, similarly to tantalum and aluminum, is a so-called valve metal. Placing such a metal in contact with an electrolytic bath and applying a positive voltage to it forms a layer of electrically insulating oxide whose thickness corresponds to the applied voltage. This oxide layer acts as the dielectric in an electrolytic capacitor. This property of niobium was known since the beginning of the 20th century. Although niobium is more abundant in nature and less expensive than tantalum, its high melting point of 2744 °C hindered the development of niobium electrolytic capacitors. In the 1960s, the higher availability of niobium ore compared with tantalum ore prompted research into niobium electrolytic capacitors in the Soviet Union. Here they served the same purpose as tantalum capacitors in the West. With the collapse of the Iron Curtain, the technology became better-known in the West, with major capacitor manufacturers taking interest in the late 1990s. The materials and processes used to produce niobium capacitors are essentially the same as for tantalum capacitors. Rising tantalum prices in 2000 and 2001 encouraged the development of niobium electrolytic capacitors with manganese dioxide and polymer electrolytes, which have been available since 2002.
Every electrolytic capacitor can be thought of as a "plate capacitor" whose capacitance increases with the electrode area (A) and the dielectric permittivity (ε), and decreases with the dielectric thickness (d).
C = ε ⋅ A d {\displaystyle C=\varepsilon \cdot {\frac {A}{d}}}
The dielectric thickness of niobium electrolytic capacitors is very thin, in the range of nanometers per volt. This very thin dielectric layer, combined with a sufficiently high dielectric strength, allows niobium electrolytic capacitors to achieve a high volumetric capacitance comparable to tantalum capacitors. The niobium anode material is manufactured from a powder sintered into a pellet with a rough surface structure intended to increase the electrode surface area A compared to a smooth surface with the same footprint. This increase in surface area can increase the capacitance by a factor of up to 200 for solid niobium electrolytic capacitors, depending on the rated voltage. The properties of the niobium pentoxide dielectric layer, compared with a tantalum pentoxide layer, are given in the following table:
The higher permittivity and lower breakdown voltage of niobium pentoxide relative to tantalum pentoxide results in niobium capacitors and tantalum capacitors having similar sizes for a given capacitance and a rated voltage.
Basic construction of solid niobium electrolytic capacitors
A typical niobium capacitor is a chip capacitor and consists of niobium or niobium oxide powder pressed and sintered into a pellet as the anode of the capacitor, with the oxide layer of niobium pentoxide as dielectric, and a solid manganese dioxide electrolyte as the cathode.
Comparison of niobium and tantalum electrolytic capacitor types The combination of anode materials for niobium and tantalum electrolytic capacitors and the electrolytes used has formed a wide variety of capacitor types with different properties. An outline of the main characteristics of the different types is shown in the table below.
Tantalum and niobium electrolytic capacitors with solid electrolyte as surface-mountable chip capacitors are mainly used in electronic devices in which little space is available or a low profile is required. They operate reliably over a wide temperature range without large parameter deviations.
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