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Glossary of Capacitor Terms

-E-

• electrochemical capacitor
  
• electrolyte
  
• electrolyte, driving
  
• electrolytic capacitor
  
• equivalent conductivity
• ESL - Equivalent Series Inductance
  
• ESR - Equivalent Series Resistance
  
• expected life
  

electrochemical capacitor

This glossary entry is under construction.

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electrolyte

A conducting medium in which the flow of electric current takes place by migration of ions. Commonly an electrolyte is defined as a substance that when dissolved in a specific solvent, usually polar, like water or ethylene glycol, produces an ionically conducting solution.

Typical electrolytes are solutions of acids, bases, salts, and molten salts. Molten salts are a special class of electrolytes in which the ions and the solvent are the same species.

In an electrolytic capacitor the electrolyte constitutes the second electrode, or plate, separated from the anode, or positive plate by the barrier layer of oxide formed on the anode surface.

The electrolyte impregnated in the element (typically the wound anode foil, paper separator, and cathode foil of an electrolytic capacitor) is referred to as driving or working electrolyte and performs the following two functions:

1. it impregnates and adheres to the surfaces of the anode and cathode foils to realize 100 % of their capacitance (essentially a cathode action).
2. repairs defects in the anode oxide film.

Important: The characteristics of the electrolyte have a great influence on the temperature characteristic, frequency characteristic, high temperature load life, reliability, etc. of the capacitor, and so an electrolyte with a composition different from that of the formation electrolyte and one that also satisfies economic, safety and other requirements is, of course, required.

Ideally, a driving electrolyte should:

• be chemically inert to all capacitor components exposed to it
• have the ability to anodize the anode material
• have good temperature stability
• have a suitable conductivity
• be free of contaminating or corrosive agents
• additionally, if there is a spacer material separating the anode and cathode structures, then the electrolyte must also impregnate or "wet" the spacer very well without unduly decreasing its effective conductivity.

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electrolyte, driving

An electrolyte is broadly defined as any substance that passes electric current by the transport of ions. Electrolytes may be liquid solutions, fused salts or ionically conducting solids. An electrochemical process is a reaction that occurs across an interface, such as that between an electrolyte and an electrode. No electrochemical process can occur in isolation, that is a complete circuit must exist. Two electrodes separated by an electrolyte and connected by an external circuit (e.g., a wire) form a complete electrochemical cell. Thus, an electrolytic capacitor in circuit is an electrochemical cell.

Some compounds possess the ability to dissociate or ionize into positively and negatively charged species in solution. A common example of this is ordinary table salt, sodium chloride. Salts, however, are a subclass of all ionizing compounds because they exist as discrete ions in the solid state. Many molecular compounds, such as adipic acid dissociate in solution.

The anode oxide is formed on etched aluminum foil in an electrochemical process involving a so called formation electrolyte. Generally, these are aqueous solutions. Formation electrolytes are optimized to form aluminum oxide at maximum efficiency and highest oxide quality. Being aqueous, their use in finished capacitors is usually incompatible with the requirements of modern capacitors. The most obvious being the wide temperature range over which they are specified, e.g., -40°C to greater than 105°C. At lower temperatures aqueous systems solidify, resulting in loss of capacitance and excessive dissipation factors and ESR's. An aqueous electrolyte vaporizes in the upper temperature regime, accelerating the occurrence of the typical open circuit end-of-lifetime failure mode.

Thus modern capacitor electrolytes are based on such non-aqueous solvents as:

• ethylene glycol
• gamma-butyrolactone (GBL)
• dimethylformamide (DMF)
• N-methylpyrrolidinone (NMP)
• other solvents
• mixtures of the above

Usually, the concentration of water is maintained between 2 to 10 weight percent in order to balance physical and electrical properties. As operating temperatures of up to 150°C are specified, however, the trend towards completely "anhydrous" electrolytes has intensified. Nevertheless, the importance of water for proper oxide maintenance has been demonstrated. It is significant that paper contains approximately 8% w/w moisture at 73°F and 48% relative humidity because it is hygroscopic.

Electrolytes are characterized by their specific resistivity. For the sake of brevity, resistivity will be taken to mean specific resistivity. A more physically meaningful term is the equivalent conductivity, which considers the concentration number of ions generated by each compound in solution.

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electrolytic capacitor

A capacitor is typically made up of two (2) parallel plates, the electrodes, with a dielectric between them. The amount of capacitance is directly proportional to the surface area of the electrode and indirectly proportional to the dielectric thickness.

An electrolytic capacitor consists of a series combination of two capacitors (e.g. foil electrodes or plates) at least one of which is a valve metal, separated by an electrolyte, and between which a dielectric film is formed adjacent to the surface of one or both of the electrodes.

The high volumetric efficiency of an electrolytic capacitor is due to its enhanced plate surface area and a very thin dielectric layer. The dielectric is an oxide and has a high dielectric strength which is electrochemically deposited in very thin layers, usually on the order of hundreds to tens of thousands of Ångstrom units.

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equivalent conductivity

The concentration number of ions generated by each compound in solution is,

LAMBDA = KAPPA / C

where KAPPA, the specific conductance, or conductivity, equals d/(AxR) = 1/RHO, and C is the ionic concentration in equivalents per cm³. The resulting dimensions for equivalent conductivity, LAMBDA, are cm²ohm^-1equiv.^-1

Current is passed by the transport of oppositely charged ions. Since each ion, regardless of charge, contributes a fraction of the total current, the equivalent conductivity can be considered as the sum of two ionic conductances:

LAMBDA = LAMBDA₊ + LAMBDA_-

Ideally, the movement of ions depends only on the nature of the ion and such solvent properties as temperature and viscosity, but not on the opposite ion. This is formally known as Kohlrausch's law of Independent Migration of Ions and applies at infinite dilution.

The fraction of current carried by each ion is thus

t₊ = LAMBDA₊ / LAMBDA

t_- = LAMBDA_- / LAMBDA

and t₊ + t_- = 1

where t₊ and t_- are the transport numbers of their respective ions. Transport numbers vary somewhat with temperature and concentration.

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Updated: 24 March 2000

Copyright © 1996-2000 Tyra T. Buczkowski. All rights reserved.