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Structure of Aluminum Electrolytic Capacitor

Contents
 1.Basic Model of Aluminum Electrolytic Capacitors
 2.Structure of Aluminum Electrolytic Capacitor
 3.Features of Capacitor Materials


1 Basic Model of Aluminum Electrolytic Capacitors
 Capacitors are passive components. Among the various kinds of capacitors, aluminum electrolytic capacitors offer larger CV product per case size and lower cost than the others. In principles of capacitor, its fundamental model is shown in Fig. 1 and its capacitance (C) is expressed by Equation (1) below: ε : Dielectric constant 
S : Surface area of dielectric(m2) 
d : Thickness of dielectric(m))
Equation (1) shows that the capacitance (C) increases as the dielectric constant (ε) and/or its surface area (S) increases and/or the dielectric thickness (d) decreases. An aluminum electrolytic capacitor comprises a dielectric layer of aluminum oxide (Al2O3), the dielectric constant (ε) of which is 8 to 10. This value is not significantly larger than those of other types of capacitors. However, by extending the surface area (S) of the aluminum foil electrode by means of etching, and by electrochemically forming a thinner but highly voltage-withstandable layer of oxide layer dielectric, the aluminum electrolytic capacitor can offer a larger CV product per case size than other types of capacitors. A basic model of aluminum electrolytic capacitor is shown in Fig. 2.
An aluminum electrolytic capacitor comprises:
Anode  …Aluminum foil 
Dielectric…E Electrochemically formed oxide layer (Al2O3) on the anode
Cathode …A true cathode is electrolytic solution (electrolyte).

Other component materials include a paper separator that holds electrolyte in place and another aluminum foil that functions as a draw-out electrode coming into contact with the true cathode (electrolyte). In general, an aluminum electrolytic capacitor is asymmetrical in structure and polarized. The other capacitor type known as a bi-polar (non-polar) comprises the anodic aluminum foils for both electrodes.



 
2 Structure of Aluminum Electrolytic Capacitor
The aluminum electrolytic capacitor has, as shown in Fig. 3, a roll of anode foil, paper separator, cathode foil and electrode terminals (internal and external terminals) with the electrolyte impregnated, which is sealed in an aluminum can case with a sealing material. The terminal draw-out structure, sealing material and structure differ depending on the type of the capacitor. Figure 4 shows typical examples.

3 Features of Capacitor Materials
Aluminum, which is main material in an aluminum electrolytic capacitor, forms an oxide layer (Al2O3) on its surface when the aluminum is set as anode and charged with electricity in electrolyte. The aluminum foil with an oxide layer formed thereon, as shown in Fig. 5, is capable of rectifying electriccurrent in electrolyte. Such a metal is called a valve metal. 



<Anode aluminum foil>
First, the foil material is electromechanically etched in a chloride solution to extend the surface area of the foil. Secondly, for the foil to form an aluminum oxide layer (Al2O3) as a dielectric, more than the rated voltage is applied to the foil in a solution such as ammonium borate. This dielectric layer is as dense and thin as 1.1 - 1.5 nm/volt and showing a high insulation resistance (108 - 109 Ω/m). The thickness of the oxide layer determines the withstand voltage according to their direct proportional relationship. For the etching pits to be shaped to the intended thickness of the oxide layer, the pit patterns have been designed to have efficient surface area extension depending on the intended withstand voltage (see Fig. 6)
<Cathode aluminum foil>
An etching process is performed to the cathode aluminum foil as well as the anode foil. However, the formation process for oxide layer is generally not performed. Therefore, the surface of the cathode foil only has an oxide layer (Al2O3) that has spontaneously formed, which gives a withstand voltage of about 0.5 volt



<Electrolyte>
The electrolyte, an ion-conductive liquid functions as a true cathode coming into contact with the dielectric layer on the surface of the anode foil. The cathode foil serves as a collector electrode to connect the true cathode with the external circuit. Electrolyte is an essential material that controls the performance of the capacitor (temperature characteristics, frequency characteristics, service life, etc.).
<Paper separator >
The separator maintains uniform distribution of the electrolyte and keeps the anode-to-cathode foil distance unchanged.
<Can case and sealing materials>
An aluminum can case and seal materials mainly consisting of rubber are used for the purpose of keeping airtightness.
 

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