The systems Cs2O-CaO-Al2O3, K2O-CaO-Al2O3, Cs2O-MgO-Al2O3 and K2O-MgO-Al2O3 were investigated. Three new compounds were found and characterized (The stable compounds Cs2O·2CaO·4Al2O3 and 2K2O·3CaO·5Al203 and the metastable compound CaO·10Al2O3 = Ca-ß-alumina). From the phase relations a conclusion is drawn about the influence of CaO or MgO dopants (sintering aids) on the alkali (K, Cs) corrosion resistance of Al2O3. For example, we found that CaO will have a negative influence on the caesium
corrosion resistance because of the formation of a very stable quasi-ternary compound (Cs2O·2CaO·4Al2O3). This in contrast to the Cs2O-MgO-Al2O3 system, where no such compound was found and where the phase relations do not indicate a negative influence of MgO. A comparison is made of the four systems mentioned and of the analogous sodium systems.
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