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Capacitance is defined as the system’s ability to store electric charge. The component that can store energy electrostatically is called a capacitor.
A capacitor consists of two parallel conducting plates separated by an insulating material called the dielectric. Capacitance value of above capacitor is inversely proportional to the separation or distances between the plates and directly proportional to its area.
Where C = capacitance,
E = the permittivity of the dielectric material,
A = the area that the plates overlap, and
D = the distance between them.
Where C = capacitance,
E = the permittivity of the dielectric material,
A = the area that the plates overlap, and
D = the distance between them.
Cross section of a capacitor is shown in the above figure. When voltage is applied across the capacitor, the plate connected to the negative terminal of the battery accepts the electrons and the plate connected to positive terminal loses the electron. Due to this, a potential difference is developed across the capacitor. The flow of electrons is continued until this potential difference across the capacitor is equal to the voltage of the battery. When battery is removed, the charge is stored across the capacitor and the plate that accepted the electrons will have negative charge and that which lost electrons will have a positive charge. Now it behaves like a battery with positive and negative terminals. Stored charge is discharged when a closed path is formed between this positive and negative terminal.
In ideal condition, the charge stored across the capacitor plates discharges only when the circuit is closed. But in practical conditions due to leakage, capacitor is gradually discharged even if the circuit is open. The most important advantage of using a capacitor is that, it can provide its entire charge in a tiny fraction of time, while, a battery requires few seconds to do the same. This property is used in the camera flash. Capacitor can also be used to reduce spikes and ripples in the DC circuits. Functionally, a capacitor restricts the change of voltage across its terminals by charging and discharging with increase and decrease of terminal voltage.
Ceramic or Disc Capacitor is a non-polarized capacitor made by coating two sides of a ceramic disc with conductive material. Many such capacitors are arranged together to get desired capacitance value. Dielectric constant is very high for ceramic material so it has relatively high capacitance value in a small size.
Film capacitors use insulating plastic film as the dielectric. Because of that film, capacitor is also called plastic capacitor. Metallized aluminum or zinc is applied on plastic film, may be on one or both sides of it and is used as electrodes.
Electrolytic capacitors are constructed with thin metallic film as one electrode and an electrolytic semi-liquid solution as the other. Dielectric is a very thin layer of oxide. The flexibility of these materials allows them to be rolled up and provide a large surface area, so an electrolytic capacitor has high capacitance per unit volume than other types. Most of the electrolytic capacitors are polarized, so if incorrectly connected, polarization will break oxide layer. For applications in AC circuits, bipolar electrolytic capacitors are also available. Aluminum electrolytic capacitors and tantalum capacitors are the two most commonly seen electrolytic capacitors.
Aluminum electrolyte capacitors are made by depositing oxide film on aluminum foils. It has high capacitance to volume ratio and it is cheaper and easily available.
Tantalum electrolyte capacitor looks like ceramic capacitors. Tantalum capacitors are more resilient for reversed polarities when compared to aluminum electrolyte capacitor. Smaller size and lower leakage current is its main advantage. But tantalum capacitors are costlier than aluminum electrolytes and have lower maximum voltage up to 50V.
Super capacitors are also called electrolyte double layer capacitor. It doesn’t have a conventional solid dielectric. Instead, super capacitor cells consist of two collectors, two electrodes, an electrolyte and a separator. Electrolyte contains dissolved ions that migrate to the electrodes during charging and away from the electrode during discharge. As shown in the figure, two layer formed by ion movement electrolyte-electrode interface will cause a double layer capacitance effect.
They are again divided into three families
Supercapacitor is a high capacity electrochemical capacitor that can store 10 to 100 times more energy per unit volume than ordinary capacitors. They have very short charging and discharging times and can withstand frequent charging and discharging activities for a very long time. They are thus preferred over electrochemical cells in many applications like regenerative braking, short-term energy storage or burst-mode power delivery in lifts, elevators, trains and cars.
Early evidence for supercapacitance was discovered by the engineers of General Electric in 1950s while working on the design of fuel cells and rechargeable batteries. "Low voltage electrolytic capacitor with porous carbon electrodes" was developed in 1957 by H. Becker but General Electric did not pursue this work. Standard Oil of Ohio (SOHIO) developed a similar product and sold the license to NEC who commercialized it as supercapacitor.Even though supercapacitors were developed and used the reason for their exhibition of unique properties was unknown due to lack of knowledge about the double-layer phenomenon. Later, in the 1990s Brian Evans Conway could describe the reason behind supercapacitance – Helmholtz double-layer and redox reactions.
As explained by Conway, supercapacitance is the result of formation of Helmholtz double-layer and occurrence of redox reactions which give rise to a phenomenon called psuedocapacitance. Either of the two phenomena may dominate the other or both may occur with same vigor based on which a supercapacitor may be called Electrostatic Double-layer Capacitors(EDLCs), electrochemical psuedocapacitors, or hybrid capacitors.
EDLCs are basically electrochemical capacitors. When voltage is applied on the electrodes electric double layers are formed. These layers are oppositely charged – one being an electronic layer on the surface of the electrode and the other one being a layer of dissolved ions. A monolayer of the dielectric solvent separates the two charged layers. This is similar to the two oppositely charged plates separated by a dielectric in a parallel plate capacitor. Capacitance of EDLC is given by the same equation that governs the parallel plate capacitor.
C = € x (A/D)
Here, € is the dielectric constant of the solvent, A is the area of plate surface and D the width of solvent monolayer.
Fig : A basic design of EDLC
In EDLCs the solvent is normally water with a high value of dielectric constant. It forms a monolayer that is only about one molecule thick. Also, supercapacitors are generally large in size and the surface area of electrodes must be on the higher side too. Taking these facts and the equation for capacitance in EDLC into consideration we can understand why EDLCs have very high values of capacitance.
In psuedocapacitors electrical energy is stored as a result of faradaic charge transfer originated by a fast sequence of redox reactions and other processes. This is always accompanied by an electron charge transfer between electrolyte and electrode. This charge comes from the ions adsorbed on the electrode surface. Here, only charge transfer takes place and no chemical reaction occurs.
It may be noted that while double-layer capacitance is an electrostatic phenomenon psuedocapacitance is an electrochemical phenomenon. Psuedocapacitance does not occur independently but only when there is presence of a double layer. Psuedocapacitors are called so because the magnitude of psuedocapacitance is multiple times greater than that of double-layer capacitance.
Hybrid capacitors exploit both the phenomena to generate capacitance. Unlike most capacitors they have electrodes made from different materials. One electrode exhibits more electrostatic behavior while the other one exhibits more electrochemical behavior. This induces asymmetry in internal resistance and consequently in the potential distribution. In the case of double-layer capacitors and pseuedocapacitors they are symmetric.
Electrodes are crucial in supercapacitors. The different types of supercapacitors are made of electrodes that exhibit the property suitable for their working. EDLCs are generally made using carbon-based electrodes. This is very obviously due to the fact that these electrodes do exhibit significant double-layer capacitance. The earliest EDLCs had electrodes made of activated carbon. This extremely porous material gives very high surface area and thus greater value of capacitance. Later activated carbon fibers(AFCs) were introduced which have very low electrical resistance. Further decease in resistance is achieved by the use of carbon aerogels instead of activated carbon. Other electrodes used in EDLCs include carbon-derived carbon, graphene and carbon nanotubes.
Carbon-based electrodes also exhibit psuedocapacitance In addition to the dominant double-layer capacitance but the magnitude is too small for them to be used in psuedocapacitors. Thus, psuedocapacitors use different electrodes that are capable of triggering fast redox reactions though even they exhibit a little double-layer capacitance. Oxides and sulfides of transition metals and their combinations are good examples of this. Conductive polymers like polyaniline are another alternative which provide high conductivity.
Hybrid supercapacitors need electrodes that can provide both double-layer capacitance and psuedocapacitance. Composite electrodes are thus used in them for they are made from a combination of psuedocapacitive and carbon-based materials. Electrodes of rechargeable batteries along with electrodes of EDLCs are also used that gives asymmetric properties to the hybrid capacitors. Hybrid capacitors with a positive metal oxide and a negative carbon electrode have also been developed.
The electrolyte of supercapacitors are generally chemicals dissolved in aqueous or organic solvents. Capacitors using water as a solvent provide high power density and low energy density. Those using organic solvents provide the opposite. Porous, chemically inert materials like glass fibers or woven ceramic fibers are used as separators to avoid short circuit between electrodes but facilitate ion flow. Current collectors that connect electrodes to terminal and housing are generally made of Aluminum.
Supercapacitors are used in a variety of applications ranging from codless screwdrivers to powering light rail vehicles in areas where overhead lines are not desirable. They also provide power to low power consuming devices in computing devices during emergency shutdown. Supercapacitors are voltage stabilizers and are also used for energy storage in energy harvesting systems.
Surface mount capacitors uses surface mount technology in which devices are directly placed on PCB, no holes and connecting leads are required in this method. Instead, metallization at the end is used to fix the device on the PCB, so noise inductance can be reduced. SMD capacitors are very small in size, so high component density can be achieved.
Capacitance value depends on the distance between the plates and area of cross section. It can be given by the relation,
Where C = capacitance,
E = the permittivity of the dielectric material,
A = the area that the plates overlap, and
D = the distance between them.
Therefore, capacitance can be varied by one of the possible variables and keeping other two constant. Variation can be done either mechanically or electrically.
In mechanically controlled variable capacitor, either distance between the plates or cross sectional area can be changed to modify the capacitance value. Cross sectional area is varied in most common devices. A group of metal plates are arranged on a stator and a rotor as shown in the figure. By rotating the axis of the rotor, amount of plate surface area which overlaps will change, there by changing the capacitance value.
(b). Electronically Controlled
In reverse-biased semiconductor diode, thickness of the depletion layer can be varied by the DC voltage applied across it. Since depletion region is sandwiched between conductive P and N region, it behaves like a capacitor. Capacitance value can be changed by varying DC voltage across the diode. The device is called Varactor diode or Varicap (variable capacitor). Depletion width of diode for a low reverse bias voltage is shown below.
When the reverse voltage is increased, depletion width is also increased. This is analogous to the distance between conductive parallel plates in a capacitor. Thus, capacitance of the device can be electrically controlled.
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