Aluminum electrolytic capacitor

Aluminium electrolytic capacitors are polarized electrolytic capacitors whose anode electrode is made of a pure aluminium foil with an etched surface. The aluminum forms a very thin insulating layer of aluminium oxide by anodization that acts as the dielectric of the capacitor. A non-solid electrolyte covers the rough surface of the oxide layer, serving in principle as the second electrode of the capacitor. A second aluminum foil called "cathode foil" contacts the electrolyte and serves as the electrical connection to the negative terminal of the capacitor.
Aluminium electrolytic capacitors are divided into three subfamilies by electrolyte type:

non-solid aluminium electrolytic capacitors,
solid manganese dioxide aluminium electrolytic capacitors, and
solid polymer aluminum electrolytic capacitors.

Aluminum electrolytic capacitors with non-solid electrolyte are the most inexpensive type and also those with widest range of sizes, capacitance and voltage values. They are made with capacitance values from 0.1 μF up to 2,700,000 μF, and voltage ratings ranging from 4 V up to 630 V. The liquid electrolyte provides oxygen for re-forming or "self-healing" of the dielectric oxide layer. However, it can evaporate through a temperature-dependent drying-out process, which causes electrical parameters to drift, limiting the service life time of the capacitors.
Due to their relatively high capacitance values aluminum electrolytic capacitors have low impedance values even at lower frequencies like mains frequency. They are typically used in power supplies, switched-mode power supplies and DC-DC converters for smoothing and buffering rectified DC voltages in many electronic devices as well as in industrial power supplies and frequency converters as DC link capacitors for drives, inverters for photovoltaic, and converters in wind power plants. Special types are used for energy storage, for example in photoflash or strobe applications or for signal coupling in audio applications.
Aluminium electrolytic capacitors are polarized capacitors because of their anodization principle. They can only be operated with DC voltage applied with the correct polarity. Operating the capacitor with the wrong polarity, or with AC voltage, leads to a short circuit which can destroy the component. The exception is the bipolar or non-polar aluminum electrolytic capacitor, which has a back-to-back configuration of two anodes in a single case, and which can be safely used in AC applications.

Basic information

Oxide layer

Electrolytic capacitors use a chemical feature of some special metals, earlier called "valve metals". Applying a positive voltage to the anode material in an electrolytic bath forms an insulating oxide layer with a thickness corresponding to the applied voltage. This oxide layer acts as the dielectric in an electrolytic capacitor. The properties of this aluminum oxide layer compared with tantalum pentoxide dielectric layer are given in the following table:

Anode- material	Dielectric	Oxide structure	Relative permittivity	Breakdown voltage	Electric layer thickness
Aluminum	Aluminium oxide Al₂O₃	amorphous	9.6	710	1.4
Aluminum	Aluminium oxide Al₂O₃	crystalline	11.6...14.2	800...1000	1.25...1.0
Tantalum	Tantalum pentoxide Ta₂O₅	amorphous	27	625	1.6

After forming a dielectric oxide on the rough anode structures, a counter-electrode has to match the rough insulating oxide surface. This is provided by the electrolyte, which acts as the cathode electrode of an electrolytic capacitor. Electrolytes may be "non-solid" or "solid". Non-solid electrolytes, as a liquid medium that has an ion conductivity caused by moving ions, are relatively insensitive to voltage spikes or current surges. Solid electrolytes have an electron conductivity, which makes solid electrolytic capacitors sensitive to voltages spikes or current surges.
The anodic generated insulating oxide layer is destroyed if the polarity of the applied voltage changes.
Image:Parallel plate capacitor.svg|thumb|right|A dielectric material is placed between two conducting plates, each of area A, and with a separation d.
Every electrolytic capacitor in principle forms a "plate capacitor" whose capacitance is greater the larger the electrode area A and the permittivity ε, and the thinner the thickness of the dielectric.
The capacitance is proportional to the product of the area of one plate multiplied with the permittivity, divided by the thickness of the dielectric.
Electrolytic capacitors obtain their large capacitance values by a large area and small dielectric thickness. The dielectric thickness of electrolytic capacitors is very thin, in the range of nanometers per volt, but the voltage strengths of these oxide layers are quite high. All etched or sintered anodes have a much higher surface compared to a smooth surface of the same area. This increases the capacitance value by a factor of up to 200 for aluminum electrolytic capacitors.

Construction of non-solid aluminum electrolytic capacitors

An aluminum electrolytic capacitor with a non-solid electrolyte always consists of two aluminum foils separated mechanically by a spacer, mostly paper, which is saturated with a liquid or gel-like electrolyte. One of the aluminum foils, the anode, is etched to increase the surface and oxidized. The second aluminum foil, called the "cathode foil", serves to make electrical contact with the electrolyte. A paper spacer mechanically separates the foils to avoid direct metallic contact. Both foils and the spacer are wound and the winding is impregnated with liquid electrolyte. The electrolyte, which serves as cathode of the capacitor, covers the etched rough structure of the oxide layer on the anode perfectly and makes the increased anode surface effectual. After impregnation the impregnated winding is mounted in an aluminum case and sealed.
By design, a non-solid aluminum electrolytic capacitor has a second aluminum foil, the so-called cathode foil, for contacting the electrolyte. This structure of an aluminum electrolytic capacitor results in a characteristic result because the second aluminum foil is also covered with an insulating oxide layer naturally formed by air. Therefore, the construction of the electrolytic capacitor consists of two single series-connected capacitors with capacitance C_A of the anode and capacitance C_K of the cathode. The total capacitance of the capacitor C_e-cap is thus obtained from the formula of the series connection of two capacitors:
It follows that the total capacitance of the capacitor C_e-cap is mainly determined by the anode capacitance C_A when the cathode capacitance C_K is very large compared with the anode capacitance C_A. This requirement is given when the cathode capacitance C_K is approximately 10 times higher than the anode capacitance C_A. This can be easily achieved because the natural oxide layer on a cathode surface has a voltage proof of approximately 1.5 V and is therefore very thin.

Comparison of non-solid and solid types

Although the present article only refers in essence to aluminum electrolytic capacitors with non-solid electrolyte, an overview of the different types of aluminum electrolytic capacitors is given here in order to highlight the differences. Aluminum electrolytic capacitors are divided into two sub-types depending on whether they make use of liquid or solid electrolyte systems. Because the different electrolyte systems can be constructed with a variety of different materials, they include further sub-types.

Aluminum electrolytic capacitors with non-solid electrolyte
* may use a liquid electrolyte based on ethylene glycol and boric acid, so-called "borax" electrolytes, or
* based on organic solvents, such as DMF, DMA, GBL, or
* based on high water containing solvents, for so-called "low impedance", "low ESR" or "high ripple current" capacitors
Aluminum electrolytic capacitors with solid electrolyte
* have a solid manganese dioxide electrolyte, see solid aluminum capacitor, or
* a solid polymer electrolyte, see polymer aluminum electrolytic capacitor, or
* hybrid electrolytes, with both a solid polymer and a liquid, see also polymer aluminum electrolytic capacitor

Description of the materials

1: Anode foil, 2: Anode oxide layer, 3: Cathode foil, 4: Cathode oxide layer, 5: Non-solid electrolyte, 6: Paper spacer soaked with electrolyte, either non-solid or polymer, 7: Conducting polymer, 8: Manganese oxide, 9: Graphite, 10: Silver

The following table shows an overview over the main characteristics of the different types of aluminum electrolytic capacitors.

Electrolyte	Capacitance range	Rated voltage range	Typical ESR ¹⁾ 100 kHz, 20 °C	Typical ripple current ¹⁾ 100 kHz, 105 °C	Leakage current ¹⁾ after 2 minutes at 10 V
Non-solid borax or organic	0.1 μF–2.7 F	4–630 V	800 mΩ	130 mA	< 10 μA
Non-solid water-based	1–18000 μF	4–100 V	360 mΩ	240 mA	10 μA
Solid manganese dioxide	0.1–1500 μF	6.3–40 V	400 mΩ	620 mA	12 μA
Solid conducting polymer	2.2–2700 μF	2–125 V	25 mΩ	2.5 A	240 μA
Solid and non-solid hybrid electrolyte	6.8–1000 μF	6.3–125 V	40 mΩ	1.5 A	100 μA

¹⁾ Values for a typical capacitor with 100 μF/10–16 V
Aluminum electrolytic capacitors with non-solid electrolyte are the best known and most widely used electrolytic capacitors. These components can be found on almost all boards of electronic equipment. They are characterized by particularly inexpensive and easy to process base materials.
Aluminum capacitors with liquid electrolytes based on borax or organic solvents have a large range of types and ratings. Capacitors with water-based electrolytes are often found in digital devices for mass production. Types with solid manganese dioxide electrolyte have served in the past as a "tantalum replacement". Polymer aluminum electrolytic capacitors with solid conductive polymer electrolytes are becoming increasingly important, especially in devices with a flat design, such as tablet PCs and flat panel displays. Electrolytic capacitors with hybrid electrolytes are relatively new on the market. With their hybrid electrolyte system, they combine the improved conductivity of the polymer with the advantage of liquid electrolytes for better self-healing properties of the oxide layer, so that the capacitors have the advantages of both low ESR and low leakage current.