Relation of capacitance to geometry and dielectric constant of a capacitor
The capacitance of a capacitor is proportional to the area of the contact plates and the dielectric constant of the medium between the plates, and it is inversely proportional to the separation between the plates (see the Appendix). In relation to electrochemical capacitors, to be discussed below, the capacitance of small dielectric capacitors is very small being on the order of microfarads or nanofarads (millionth or billionth of a farad, respectively) for small devices on the order of mm or cm in dimensions. By having very thin insulating films, on the order of 10 to 100 nanometers, formed anodically on the plate of a two-electrode capacitor, substantially larger specific capacitances (that is per cm2) can be attained. Such devices are called ?a href="art-c04-electr-cap.htm">electrolytic capacitors?because the thin dielectric oxide films are formed on the plates by an anodic electrolysis procedure applied at metals such as aluminum, tantalum, titanium, niobium, etc. Such capacitors are still of the dielectric type (the dielectric medium being here the thin, insulating oxide film, usually having a relatively high dielectric constant) and should not be confused with the "electrochemical" capacitor type of device which is the topic of this article.
Electrochemical capacitors are a special kind of capacitor based on charging and discharging the interfaces of high specific-area materials such as porous carbon materials or porous oxides of some metals. They can store electric charge and corresponding energy at high densities in an highly reversible way, as does a regular capacitor, and hence can be operated at specific power densities (in watts/kg) substantially higher than can most batteries. Their capacitance for a given size of the device is thus much higher, by a factor of 10,000 or so, than those achievable with regular capacitors. For this reason proprietary names such as "Supercapacitors" or "Ultracapacitors" have been coined to describe their performance.
While they function formally like rechargeable batteries in storing or delivering electric charge, their mechanisms of charge storage are quite different, in most cases, from those operating in batteries. Thus, electrochemical capacitors are not substitutes for batteries but rather are to be regarded as complementary to them for charge storage or delivery. They can offer advantageously fast charging or discharging rates over most batteries of comparable volume but their energy density is usually less, by a factor of 3 to 4, than that of batteries. Their high power or power densities, however, enables them to be employed in interesting complementary ways in hybrid systems with batteries.
An important difference between charging a capacitor and charging a battery is that there is always an intrinsic increase of voltage on charge (or decrease on discharge) of a capacitor as the charge per cm2 is increased or decreased. In contrast, an ideal battery has a constant voltage during discharge or recharge except as the state of charge approaches 0 or 100%. Although practical batteries exhibit some dependence of cell voltage on state of charge, especially lithium-intercalation batteries, the latter for fundamental reasons arising from intercalation. (See the Appendix for further details.)
Suntan offer all kinds of Ceramic Capacitor .
