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.Tantalum-Niobium International Study Center, Tantalum and Niobium – Early History and Applications for Niobium 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.Ch. Schnitter, A. Michaelis, U. Merker, H. C. Starck, Bayer, New Niobium Based Materials for Solid Electrolyte Capacitors, Carts 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).
The dielectric thickness of niobium electrolytic capacitors is very thin, in the range of nanometers per volt.J. Moore, Kemet, Nb capacitors compared to Ta capacitors a less costly alternative PDF 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.Niobium Powder for Electrolytic Capacitor, JFE Technical Report No. 6 (October 2005) PDF
The properties of the niobium pentoxide dielectric layer, compared with a tantalum pentoxide layer, are given in the following table:T. Kárník, AVX, Niobium oxide for capacitor manufacturing, Metal 2008, 2008-05-13 – 2008-05-15, PDF
+ Characteristics of the different tantalum and niobium oxide layersAnode material Dielectric Relative permittivity Oxide structure Breakdown voltage (V/μm) Dielectric layer thickness (nm/V)Tantalum Tantalum pentoxide Ta2O5 27 amorphous 625 1.6Niobium 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. + Overview of the key features of niobium and tantalum electrolytic capacitorsElectrolytic capacitor familyElectrolyteCapacitance range (μF)Max. rated voltage (V)Max. temperature (°C)Tantalum electrolytic capacitor, sintered anode Non-solid, sulfuric acid 0.1...18,000 630 125/200 Solid, manganese dioxide 0.1...3,300 125 125/150Solid, polymer 10...1,500 25 105Niobium oxide electrolytic capacitor, sintered anode Solid, manganese dioxide 1...1,500 10 105Solid, 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.T. Zednicek, S. Sita, C. McCracken, W. A. Millman, J. Gill, AVX, Niobium Oxide Technology Roadmap, CARTS 2002 PDF Y. Pozdeev-Freeman, P. Maden, Vishay, Solid-Electrolyte Niobium Capacitors Exhibit Similar Performance to Tantalum, 2002-02-01, Rutronik, Tantalum & Niobium Capacitors, Technical Standards and Benefits PDF
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. + Comparison of the most important characteristics of different types of electrolytic chip capacitors Electrolytic capacitor family Type 1 Dimension DxL, WxHxL (mm) Max. ESR 100 kHz, 20 °C (mΩ) Max. Ripple current 85/105 °C (mA) Max. Leakage current after 2 Min. 2 (μA) Tantalum capacitors, MnO2 electrolyte Kemet T494 330/10 7.3x4.3x4.0 100 1285 10 (0.01CV) Tantalum capacitors, Multianode, MnO2 electrolyte Kemet T510 330/10 7.3x4.3x4.0 35 2500 10 (0.01CV) Tantalum capacitors, Polymer electrolyte Kemet T543 330/10 7.3x4.3x4.0 10 4900 100 (0.1CV) Tantalum capacitors, Multianode, polymer Kemet T530 150/10 7.3x4.3x4.0 5 4970 100 (0.1CV) Niobium capacitors, MnO2 electrolyte AVX, NOS 220/6,3 7.3x4.3x4.1 80 1461 20 (0.02CV) Niobium capacitors, Multianode, MnO2 electrolyte AVX, NBM 220/6.3 7.3x4.3x4.1 40 2561 20 (0.02CV) Niobium-caps Polymer electrolyte NEC, NMC 100/10 7.3x4.3x2.8 - - 20 (0.02CV) Aluminum capacitors, Polymer electrolyte Panasonic SP-UE 180/6.3 7.3x4.3x4.2 7 3700 100 (0.1CV) Aluminum capacitors, Polymer electrolyte Kemet A700 100/10 7.3x4.3x4.0 10 4700 40 (0.04CV)
(1) 100 μF/10 V, unless otherwise specified,
(2) 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 (born Karol 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.Charles Pollack: D.R.P. 92564, filed 1896-01-14, granted 1897-05-19 D.R.P. 92564 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.D. F. Tailor, Tantalum and Tantalum Compounds, Fansteel Inc., Encyclopedia of Chemical Technology, Vol. 19, 2nd ed. 1969 John Wiley & sons, Inc.
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.Y. Pozdeev-Freeman, P. Maden, Vishay, Solid-Electrolyte Niobium Capacitors Exhibit Similar Performance to Tantalum, Feb 1, 2002,
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.Ch. Schnitter, A. Michaelis, U. Merker, H.C. Starck, Bayer, New Niobium Based Materials for Solid Electrolyte Capacitors, Carts 2002T. Zednicek, S. Sita, C. McCracken, W. A. Millman, J. Gill, AVX, Niobium Oxide Technology Roadmap, CARTS 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.
Electrical characteristics Series-equivalent circuit