Ionic conductivity (solid state)
Ionic conductivity (denoted by λ) is a measure of a substance's tendency towards ionic conduction. This involves the movement of an ion from one site to another through defects in the crystal lattice of a solid or aqueous solution.
Ionic conduction is one mechanism of current. In solids, ions typically occupy fixed positions in the crystal lattice and do not move. However, ionic conduction can occur, especially as the temperature increases. Materials exhibiting this property are used in batteries. A well-known ion conductive solid is β''-alumina ("BASE"), a form of aluminium oxide. When this ceramic is complexed with a mobile ion, such as Na+, it behaves as so-called fast ion conductor. BASE is used as a membrane in several types of molten salt electrochemical cell.
Ionic conduction in solids has been a subject of interest since the beginning of the 19th century. Michael Faraday established in 1839 that the laws of electrolysis are also obeyed in ionic solids like lead(II) fluoride (PbF2) and silver sulfide (Ag2S). In 1921, solid silver iodide (AgI) was found to have had extraordinary high ionic conductivity at temperatures above 147 °C, AgI changes into a phase that has an ionic conductivity of ~ 1 –1 cm−1. This high temperature phase of AgI is an example of a superionic conductor. The disordered structure of this solid allows the Ag+ ions to move easily. The present record holder for ionic conductivity is the related material Ag2HgI4. β''-Alumina was developed at the Ford Motor Company in the search for a storage device for electric vehicles while developing the sodium-sulfur battery.
Ion conduction in disordered solids like glasses, polymers, nanocomposites, defective crystals and other disordered solids plays an important role in technology.
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