Editing Electrolytic capacitor (section)

==General information==

===Electrolytic capacitors family tree===
As to the basic construction principles of electrolytic capacitors, there are three different types: aluminium, tantalum, and niobium capacitors. Each of these three capacitor families uses non-solid and solid manganese dioxide or solid polymer electrolytes, so a great spread of different combinations of anode material and solid or non-solid electrolytes is available.

[[File:E-cap-family-14-02-02.jpg|thumb|400px|center| Depending on the nature of the anode metal used and the electrolyte used, there is a wide variety of electrolytic capacitors]]

===Charge principle===
Like other conventional capacitors, electrolytic capacitors store the [[electric energy]] [[Static electricity|statically]] by [[Electric charge|charge]] separation in an [[electric field]] in the dielectric oxide layer between two [[electrode]]s. The non-solid or solid [[electrolyte]] in principle is the cathode, which thus forms the second electrode of the capacitor. This and the storage principle distinguish them from electrochemical capacitors or [[supercapacitor]]s, in which the electrolyte generally is the ionic conductive connection between two electrodes and the storage occurs with statically [[double-layer capacitance]] and electrochemical [[pseudocapacitance]].

===Basic materials and construction===
[[File:Al-Elko-Formier-Prinzip-Wiki-07-02-18-en.svg|thumb|right|Basic principle of anodic oxidation (forming), in which, by applying a voltage with a current source, an oxide layer is formed on a metallic anode]]

Electrolytic capacitors use a chemical feature of some special metals, previously called "valve metals", which on contact with a particular electrolyte form a very thin insulating oxide layer on their surface by [[anodizing|anodic]] [[oxidation]] which can function as a dielectric.  There are three different anode metals in use for electrolytic capacitors:
# [[Aluminum electrolytic capacitor]]s use a high-purity etched [[aluminium]] foil with [[aluminium oxide]] as dielectric
# [[Tantalum capacitor|Tantalum electrolytic capacitors]] use a [[sintered]] pellet (“slug”) of high-purity [[tantalum]] powder with [[tantalum pentoxide]] as dielectric
# [[Niobium capacitor|Niobium electrolytic capacitors]] use a sintered "slug" of high-purity [[niobium]] or [[niobium oxide]] powder with [[niobium pentoxide]] as dielectric.

To increase their capacitance per unit volume, all anode materials are either etched or sintered and have a rough surface structure with a much higher surface area compared to a smooth surface of the same area or the same volume. By applying a positive voltage to the above-mentioned anode material in an electrolytic bath an oxide barrier layer with a thickness corresponding to the applied voltage will be formed (formation). This oxide layer acts as the dielectric in an electrolytic capacitor. The properties of these oxide layers are given in the following table:

<div class="center">
{| class="wikitable"
|+ Characteristics of the different oxide layers in aluminium, tantalum and niobium electrolytic capacitors<ref name=Nano>J.L. Stevens, A.C. Geiculescu, T.F. Strange, Dielectric Aluminum Oxides: Nano-Structural Features and Composites [http://ecadigitallibrary.com/pdf/CARTSUSA07/2_1063.pdf PDF] {{Webarchive|url=https://web.archive.org/web/20141229123213/http://ecadigitallibrary.com/pdf/CARTSUSA07/2_1063.pdf |date=2014-12-29 }}</ref><ref name=Karnik>T. Kárník, AVX, NIOBIUM OXIDE FOR CAPACITOR MANUFACTURING, METAL 2008, 13. –15. 5. 2008, [http://www.metal2014.com/files/proceedings/metal_08/Lists/Papers/087.pdf PDF]</ref>
!Anode-<br />material
! [[Dielectric]]
! Oxide<br />structure
! [[Relative permittivity|Relative<br />permittivity]]
! Breakdown<br/>voltage<br/>(V/μm)
! Electric<br />layer<br />thickness<br />(nm/V)
|-
| rowspan="2" |aluminium|| rowspan="2" |Aluminium oxide Al<sub>2</sub>O<sub>3</sub>|| amorphous||  9.6 || 710 ||1.4
|-
| crystalline ||11.6…14.2<ref>Jeng-Kuei Chang, Chia-Mei Lin, Chi-Min Liao, Chih-Hsiung Chen, Wen-Ta Tsai, Journal of the Electrochemical Society, 2004. Effect of Heat-Treatment on Characteristics of Anodized Aluminum Oxide Formed in Ammonium Adipate Solution  [http://ir.lib.ncku.edu.tw/bitstream/987654321/74554/2/3010500301037.pdf] {{Webarchive|url=https://web.archive.org/web/20210225132022/http://ir.lib.ncku.edu.tw/bitstream/987654321/74554/2/3010500301037.pdf|date=2021-02-25}} DOI: 10.1149/1.1646140</ref>||800...1000<ref>Th. F. Strange, T. R. Marshall, Very high volt oxide formation of aluminum for electrolytic capacitors, US Patent 6299752 B1, 9. Okt. 2001, [http://www.google.com.gt/patents/US6299752]</ref>|| 1.25...1.0
|-
|Tantalum|| Tantalum pentoxide Ta<sub>2</sub>O<sub>5</sub> || amorphous|| 27  || 625 || 1.6
|-
|Niobium or<br/>Niobium oxide || Niobium pentoxide Nb<sub>2</sub>O<sub>5</sub>|| amorphous ||41 || 400 || 2.5
|}
</div>

After forming a dielectric oxide on the rough anode structure, a counter electrode has to match the rough insulating oxide surface. This is accomplished by the electrolyte, which acts as the cathode electrode of an electrolytic capacitor. There are many different electrolytes in use. Generally they are distinguished into two species, “non-solid” and “solid” electrolytes. As a liquid medium which has [[ion]] [[Electrical resistivity and conductivity|conductivity]] caused by moving ions, non-solid electrolytes can easily fit the rough structures. Solid electrolytes which have electron conductivity can fit the rough structures with the help of special chemical processes like [[pyrolysis]] for [[manganese dioxide]] or [[polymerization]] for conducting [[polymer]]s.

Comparing the permittivities of the different oxide materials it is seen that tantalum pentoxide has a permittivity approximately three times higher than aluminium oxide. Tantalum electrolytic capacitors of a given CV value theoretically are therefore smaller than aluminium electrolytic capacitors. In practice different safety margins to reach reliable components makes a comparison difficult.

The anodically generated insulating oxide layer is destroyed if the polarity of the applied voltage changes.

===Capacitance and volumetric efficiency===
[[Image:Parallel plate capacitor.svg|thumb|right|A dielectric material is placed between two conducting plates (electrodes), each of area '''''A''''' and with separation '''''d'''''.]]
Electrolytic capacitors are based on the principle of a "plate capacitor" whose capacitance increases with larger electrode area A, higher dielectric [[permittivity]] ε, and thinness of dielectric (d).

:<math>C = \varepsilon \cdot \frac{A}{d}</math>

The dielectric thickness of electrolytic capacitors is very small, in the range of [[Meter|nanometers]] per volt. On the other hand, the voltage strengths of these oxide layers are quite high. With this very thin dielectric oxide layer combined with a sufficiently high dielectric strength the electrolytic capacitors can achieve a high volumetric capacitance. This is one reason for the high capacitance values of electrolytic capacitors compared to conventional capacitors.

All etched or sintered anodes have a much higher surface area compared to a smooth surface of the same area or the same volume. That increases the capacitance value, depending on the rated voltage, by a factor of up to 200 for non-solid aluminium electrolytic capacitors as well as for solid tantalum electrolytic capacitors.<ref name=Voltage>A. Albertsen, Jianghai Europe, Keep your distance – Voltage Proof of Electrolytic Capacitors, [http://jianghai-europe.com/wp-content/uploads/3-Jianghai-Europe-E-Cap-Voltage-Proof-AAL-2012-10-30.pdf PDF] {{Webarchive|url=https://web.archive.org/web/20130108014604/http://jianghai-europe.com/wp-content/uploads/3-Jianghai-Europe-E-Cap-Voltage-Proof-AAL-2012-10-30.pdf |date=2013-01-08 }}</ref><ref name="KDK">{{Cite web|url=http://www.kdk.com/e/productinfo/specifications/c-03-02-03-e.pdf|title=KDK, Specifications for Etched Foil for Anode, Low Voltage}}</ref><ref name="Horacek">I.Horacek, T.Zednicek, S.Zednicek, T.Karnik, J.Petrzilek, P.Jacisko, P.Gregorova, AVX, High CV Tantalum Capacitors - Challenges and Limitations [http://www.avx.com/docs/techinfo/highcvtant.pdf] {{Webarchive|url=https://web.archive.org/web/20140309021406/http://www.avx.com/docs/techinfo/highcvtant.pdf|date=2014-03-09}}</ref> The large surface compared to a smooth one is the second reason for the relatively high capacitance values of electrolytic capacitors compared with other capacitor families.

Because the forming voltage defines the oxide layer thickness, the desired voltage rating can be produced very simply. Electrolytic capacitors have high [[volumetric efficiency]], the so-called "CV product", defined as the product of capacitance and voltage divided by volume.

===Basic construction of non-solid aluminium electrolytic capacitors===
<gallery perrow="3" class="center" mode="packed" heights="130px" caption="Basic construction of a non-solid aluminium electrolytic capacitor">
File:Al-e-cap-winding-multi-tabs.jpg|Opened winding of an electrolytic capacitor with multiple connected foils
File: Elko-Prinzipschnittbild-english.png|Closeup cross-section of an aluminium electrolytic capacitor design, showing capacitor anode foil with oxide layer, paper spacer soaked with electrolyte, and cathode foil
File:Al-e-cap-construction.jpg| Construction of a typical single-ended aluminium electrolytic capacitor with non-solid electrolyte
</gallery>

===Basic construction of solid tantalum electrolytic capacitors===
<gallery perrow="3" caption="Construction of a solid tantalum chip capacitor with manganese dioxide electrolyte" class="center" mode="packed" heights="150px">
File:Tantalum sintered pellet.jpg| The capacitor cell of a tantalum electrolytic capacitor consists of sintered tantalum powder
File:Tantalum-Sintered-MnO2-slug.jpg| Schematic representation of the structure of a sintered tantalum electrolytic capacitor with solid electrolyte and the cathode contacting layers
File:Tantalum-SMD-Chip-Molded.jpg| Construction of a typical SMD tantalum electrolytic chip capacitor with solid electrolyte
</gallery>