Niobium capacitor

A niobium electrolytic 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 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 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 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 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 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 and the dielectric permittivity, and decreases with the dielectric thickness.
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:

Anode material	Dielectric	Relative permittivity	Oxide structure	Breakdown voltage	Dielectric layer thickness
Tantalum	Tantalum pentoxide Ta₂O₅	27	amorphous	625	1.6
Niobium or Niobium oxide	Niobium pentoxide Nb₂O₅	41	amorphous	400	2.5

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.

Electrolytic capacitor family	Electrolyte	Capacitance range	Max. rated voltage	Max. temperature
Tantalum electrolytic capacitor, sintered anode	Non-solid, sulfuric acid	0.1...18,000	630	125/200
Tantalum electrolytic capacitor, sintered anode	Solid, manganese dioxide	0.1...3,300	125	125/150
Tantalum electrolytic capacitor, sintered anode	Solid, polymer	10...1,500	25	105
Niobium oxide electrolytic capacitor, sintered anode	Solid, manganese dioxide	1...1,500	10	105
Niobium oxide electrolytic capacitor, sintered anode	Solid, polymer	4.7...470	16	105

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.

Electrolytic capacitor family	Type ¹	Dimension DxL, WxHxL	Max. ESR 100 kHz, 20 °C	Max. Ripple current 85/105 °C	Max. Leakage current after 2 Min. ²
Tantalum capacitors, MnO₂ electrolyte	Kemet T494 330/10	7.3x4.3x4.0	100	1285	10
Tantalum capacitors, Multianode, MnO₂ electrolyte	Kemet T510 330/10	7.3x4.3x4.0	35	2500	10
Tantalum capacitors, Polymer electrolyte	Kemet T543 330/10	7.3x4.3x4.0	10	4900	100
Tantalum capacitors, Multianode, polymer	Kemet T530 150/10	7.3x4.3x4.0	5	4970	100
	-	-	-	-	-
Niobium capacitors, MnO₂ electrolyte	AVX, NOS 220/6,3	7.3x4.3x4.1	80	1461	20
Niobium capacitors, Multianode, MnO₂ electrolyte	AVX, NBM 220/6.3	7.3x4.3x4.1	40	2561	20
Niobium-caps Polymer electrolyte	NEC, NMC 100/10	7.3x4.3x2.8	-	-	20
	-	-	-	-	-
Aluminum capacitors, Polymer electrolyte	Panasonic SP-UE 180/6.3	7.3x4.3x4.2	7	3700	100
Aluminum capacitors, Polymer electrolyte	Kemet A700 100/10	7.3x4.3x4.0	10	4700	40

100 μF/10 V, unless otherwise specified,
calculated for a capacitor 100 μF/10 V,

History

The phenomenon that can electrochemically form an oxide layer on aluminum and metals like tantalum or niobium, blocking an electric current in one direction but allowing it to flow in the other direction, was discovered in 1875 by the French researcher Eugène Ducretet. He coined the term "valve metal" for such metals. Charles Pollak used this phenomenon for an idea of a polarized "Electric liquid capacitor with aluminum electrodes". In 1896 Pollak obtained a patent for the first electrolytic capacitor.
The first tantalum electrolytic capacitors with wound tantalum foils and non-solid electrolyte were developed in 1930 by Tansitor Electronics Inc., USA, and used for military purposes.
Niobium is a sister metal to tantalum and serves as valve metal generating an oxide layer during anodic oxidation. Niobium as raw material is much more abundant in nature than tantalum and is less expensive. Niobium capacitors were developed in the United States and the former Soviet Union in the late 1960s. It received more use in the Soviet Union as niobium was much more available than tantalum.
Another price explosion for tantalum in 2000/2001 forced the commercial development of niobium electrolytic capacitors with manganese dioxide electrolyte, which became available since 2002. The materials and processes used to produce niobium-dielectric capacitors are essentially the same as for existing tantalum-dielectric capacitors. The characteristics of niobium electrolytic capacitors and tantalum electrolytic capacitors are roughly comparable.