Aluminas cationic conductivity

Figure 5.17 Schematic diagram of sodium cation conduction in sodium p-alumina...

Cation-conducting electrolytes (see Chapter 7) are also used in electrochemical sensors, one of the most widely used being 3 alumina [55, 56]. The 3 alumina structure consists of relatively densely packed spinel blocks that are separated by less densely packed planes through which ionic conduction occurs. The most common example is sodium 3 alumina, which is a Na+ ion conductor, although the sodium can be exchanged with other ions to create electrolytes that conduct other cations [57], as well as other species, such as O [58] (see also Chapter 8). 

Hence, a cation-conducting solid electrolyte can be used for a SO2 sensor. Figure 13.14 shows that the output of a sensor with an Ag P" alumina electrolyte [177, 178] and an Ag2SO4 electrode generates a response that is similar to those of sensors using Li2SO4-Ag2SO4 electrolytes [86, 87]. 

Auxiliary electrodes are also required if a cation-conducting electrolyte is used for measuring a metal which is different from the ion that is mobile in the electrolyte. For example, the use of Na [3 alumina for the measurement of antimony in zinc requires the use of a NaSbO3 auxiliary electrode [221]. Sodium [3 alumina has also been used for measuring magnesium and strontium concentrations in molten aluminum [222]. In this case, the auxiliary electrode forms in situ by an exchange reaction between... 

Beta Alumina and other Cation Conducting Oxides... 

Until 1995, it was accepted that trivalent cations were poorly migrating species in solids due to strong electrostatic interaction between the conducting trivalent cation and the surrotmding anions. Therefore, trivalent cations were commonly used as dopants in solid electrolytes for the adjustment of defect concentrations and lattice size. In order to realize trivalent cation conduction, such strong interactions between the mobile trivalent cations and the surrotmding anion framework must, at least, be reduced. Although some solids such as Ln -p/p -alumina [1-9], p-alumina related materials [10-13], and... 

In both of these materials the distribution of the ions in the conduction planes changes with temperature. At high temperatures the large cations tend to occupy all suitable sites in a random manner. Thus in (3-alumina the BR, aBR, and mO sites, and in (3"-alumina the BR-type and mO sites, are occupied statistically. 

There are two identical BR-type sites in the unit cell to accommodate the 1 + x Na+ ions, and there are always vacant BR-type sites in proximity to occupied sites. When cations of higher valence replace sodium, the number of vacant BR-type sites increase in proportion. Although there is no geometrical reason why large cations should occupy other sites, in many compounds, the large cations are located in both BR-type sites and mO sites. As in the case of (3-alumina, the defect structure of each compound is uniquely related to the chemical nature of the cations in the conduction layer. 

Another example of this type of intercalation compound is sodium beta alumina where the sodium ions are free to move between the spinel layers. The sodium ions can be replaced by almost any +1 cation such as Li. K, Rb+, Cs. NHJ, H 0 Tl+, Ga+, NO+, etc. The conductivity of these materials varies with the size of the ions moving between the fixed-distance (A)—0—Ai) layers. 

As long as the /1-alumina sensor remains homogeneous as far as Na+ is concerned (which is achieved by the high fraction of Na20), we see from Eqn. (15.6) that the electron potential varies inversely with the oxygen activity. We have already mentioned that /1-alumina is able to incorporate a number of different cations into the conducting plane. This non-specificity hampers the use of / -alumina as a universal sensor material under ordinary conditions. If more than one mobile component is... 

As the electrolytes, alkali metal sulfates(M=Li, Na, and K)(l-ll), 3-Alumina(12), and NASIC0N(13, 14) have been examined. Alkali metal sulfates are cation conductors at elevated temperature(>700 C). However, they have several disadvantages. One is the phase transformation of the sulfates(15-18). By this transformation, cracks occur in the electrolyte body and result in the permeation of the ambient gases. The other disadvantage is their low electrical conductivity. Mono, di, or tri-valent cations(19-24) have been doped so as to enhance their conductivity. Furthermore, they become ductile at a tem-... 

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