Solid ionic conductors, high temperature

Nemst [1,2] studied the behavior of solid ionic conductors— high temperamre ceramics—for use as filaments in light bulbs. Nernst made a breakthrough discovery based on the observation of different types of conductivity in stabilized zirconia, or zirconium oxide doped by a few mole per cent of calcia, magnesia, yttria, etc. At that time, the use of other materials for this purpose was problematic due to their unfavorable resistance characteristics—a rise in temperature caused an increase in resistance, and thus the metal wires were not able to obtain a... 

Because of the high values of conductivity which in individual cases are found at room temperature, such compounds are often called superionic conductors or ionic superconductors but these designations are unfounded, and a more correct designation is solid ionic conductors. Strictly unipolar conduction is typical for all solid ionic conductors in the silver double salts, conduction is due to silver ion migration, whereas in the sodium polyaluminates, conduction is due to sodium ion migration. 

Transport of alkali metal ions through the tunnels in Nasicons can be extremely rapid, particularly at elevated temperatures, although the electronic conductivities are low. For these reasons, these materials were originally proposed for use as solid ionic conductors (e.g., to replace 3" alumina in high temperature Na/S batteries). In spite of their low electronic conductivities, researchers recognized that Nasicon structures with redox-active transition metals and related three-dimensional framework compounds could function as electrode materials as early as the late 1980s [229-231] and numerous materials were investigated [232, 233]. In many cases, the electrochemical... 

Solid ionic conductors that can be used in electrochemical cells as an electrolyte are called solid electrolytes. In such compotmds only one ion is mobile (see entry. Solid State Electrochemistry, Electrochemistry Using Solid Electrolytes). Generally, any conductor with a high ionic transference number can serve as an electrolyte. Often, the definition after Patterson is used who described solids with a transference number > 0.99 as solid electrolytes [1]. The transference number is not a fixed value. It depends on the temperature and the partial pressure of the gas involved in the chemical reaction with the mobile ion. Therefore, all solids are more or less conductors with a mixed ionic and electronic conductivity, so-called mixed conductors. For the application in sensors and fuel cells, only a window concerning temperature and partial pressure is suitable. This is also called as electrolytic domain. The phenomenon that solids exhibit a high ionic conductivity is also designed as fast ion transport. 

Another very similar method can also be used - particularly to measure the activity of an element in a metal alloy at high temperature. We create a cell with two electrodes, one of which is the pure metal and the other is the solid solution, using a solid ionic conductor as an electrolyte. By measuring the electromotive force of the cell, at the chosen temperature, we will be able to calculate the activity of an element of the alloy at the desired temperature. 

Very recently, new solid ionic conductors, so-called polymer in-salt materials, have been reported, in which lithimn salts are mixed with small quantities of the polymers PEO and PPO, while conventional polymer electrolytes ( salt-in-polymer ) contain only one Li per about ten repeat units of ether. The reported conductivity in the AlCl3-LiBr-LiC104-PP0 system is as high as 0.02 S cm at room temperature. 

The yttria addition to the zirconia-yttria solid solution has two functions to stabilize the cubic structure type fluorite and to form oxygen vacancies in concentrations proportional to the yttria content. These vacancies are responsible for high ionic conductivity. Yttria stabilized zirconia is a suitable ionic conductor at temperatures above 800 °C, since thin dense membranes (less than 20 pm) can be manufactured. These membranes should be free of impurities. The stabilized zirconia is chemically inert to most reactive gases and electrode materials. 

SOE cells utilize solid ceramic electrolytes (e.g. yttria stabilized zirconia) that are good oxygen ion (0 ) conductors at very high temperatures in the range of 1000°C [8]. The operating temperature is decided by the ionic conductivity of the electrolyte. The feed gas, steam mixed with hydrogen, is passed through the cathode compartment. At the cathode side, the reaction is... 

Some ionic solids have been discovered that have a much higher conductivity than is typical for such compounds and these are known as fast-ion conductors. One of the earliest to be noticed, in 1913 by Tubandt and Lorenz, was a high temperature phase of silver iodide. 

A large number of oxides which are commonly solid insulators at room temperature (e.g., A1203) yield highly conducting (specific conductance > 10 ohm-1 cm-1) melts on fusion. Some of these melts are undoubtedly ionic conductors whereas others are electronic conductors. It is, however, not always possible to differentiate between these two general types since both mechanisms may be operative to comparable extents at a particular temperature. Relatively few experiments have been reported in which reliable estimates of the transport numbers of either ions or electrons could be made. The subdivision in this section into the two general types of conductors is thus not to be interpreted as rigid. 

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