Crystalline electrolytes 15-aluminas

Previously reported thermal analysis data (Brown et al., 2006 Kirchner et al., 2007a, 2008) shows that further heating causes a sharp exothermic event due to formation of crystalline transition alumina phase(s). This event occurs at 850 °C for H3PO4 electrolyte, 900 °C for H2C2O4 electrolyte and 970 °C for H2SO4 eledrolyte, although the precise crystallisation... 

Besides the amorphous alumina films formed in the majority of acidic electrolytes (except those formed in chromic acid and exhibiting traces of y-Al203194), there are possibilities of forming oxides with a more or less pronounced crystallinity. These oxides are formed in alkaline solutions195 and especially in sodium carbonate baths.196 According to the data provided by Hiroshi and Yoshimura,197 these oxides contain a y- A1203 phase easily hydrated and converted into a bayerite-like substance. 

Crystalline solid electrolytes such as a-Agl, ji-alumina, NASICON, and LISICON, LLN, oxide ion conductors such as yttria-stabilized zirconia, etc ... 

There are several important hydrated forms of alumina corresponding to the stoichiometries A1(0)0H and Al(OH)3. Addition of ammonia to a boiling solution of an aluminum salt produces a form of A1(0)0H known as boehmite, which may be prepared in other ways also. A second form of A1(0)0H occurs in Nature as the mineral diaspore. The true hydroxide Al(OH)3 is obtained as a crystalline white precipitate when carbon dioxide is passed into alkaline aluminate solutions. It occurs in Nature as the mineral gibbsite. Materials sometimes referred to as /1-aluminas have other ions such as Na+ and Mg2+ present. They possess the idealized composition Na2011Al203. They can act as ion exchangers, have high electrical conductivity, and are potential solid state electrolytes for batteries. 

Sodium ion conduction in /S"-alumina is anisotropic and ceramic electrolytes produced from this material not only can be sensitive to exposure to moisture but also are somewhat difficult to densify (e.g., because of problems related to NaaO evaporation, conversion to single-phase / -alumina, and control of exaggerated grain growth). For these reasons. Serious attempts have been made to find alternative materials that have Na" ion conductivities comparable to or higher than / "-alumina but are easier and/or potentially more economical to fabricate. Here we review attempts to develop alternatives. They fall into two classes of materials the crystalline NASICON family of compounds and conducting glasses. 

Crystalline solid electrolytes have been subdivided into soft ionic crystals such as P-Pbp2 and hard covalent crystals such as P-alumina. The conduction mechanism can be pictured as involving a liquid-like charge carrier array moving in the vibrating potential energy profile set up by the immobile counterions. 

A second crystalline modification of similar composition, called sodium /3"-alumina, permits exchange forNa+ by metal ions and supports the rapid migration of most cations in the periodic table, including multivalent ones. Because it can serve as a single crystal or powder host for multivalent cations, sodium /3"-alumina has a variety of potential uses as a solid electrolyte and in optical and electrical devices and has excited much current interest. 

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