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 = \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 File:Niobium sintered pellet.png| The capacitor cell of a niobium electrolytic capacitor consists of sintered niobium or niobium monoxide powder File:Niobium sintered slug.png| Schematic representation of the structure of a sintered niobium electrolytic capacitor with solid electrolyte and the cathode contacting layers File:Niobium-SMD-Chip.png| Construction of a typical SMD niobium electrolytic chip capacitor with solid electrolyte 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.
Comparison of electrical parameters of niobium and tantalum capacitor types In order to compare the different characteristics of the different electrolytic chip capacitor types, specimens with the same dimensions and of comparable capacitance and voltage are compared in the following table. In such a comparison the values for ESR and ripple current load are the most important parameters for the use of electrolytic capacitors in modern electronic equipment. The lower the ESR the higher the ripple current per volume, thus the better the functionality of the capacitor in the circuit. (1) 100 μF/10 V, unless otherwise specified, (2) calculated for a capacitor 100 μF/10 V, ==History==