| The development of a protonic solid electrolyte that is structurally stable and ionically conductive in the temperature range 200-400(DEGREES)C would revolutionize fuel cell and water electrolysis technology. While many protonic solids exhibit high ionic conductivities at room tem- perature, they generally decompose to less conductive forms at ele- vated temperatures. The protonic (beta)- and (beta)'' aluminas do not follow this trend; they are stable and conductive above 100(DEGREES)C. This work was an investigation of the most stable and conductive member of this group, ammonium/hydronium (beta)'' alumina: (NH(,4))(,y)(H(,3)O)(,1.67-y)- Mg(,0.67)Al(,10.33)O(,17) where 1.0 (LESSTHEQ) y < 1.67.; Samples of ammonium/hydronium (beta)'' alumina were prepared by ion-exchange of sodium (beta)'' alumina in molten ammonium nitrate at 200(DEGREES)C. Chemical analysis showed that the ratio of ammonium/ hydronium is variable, and that it can be controlled by varying the duration of exchange. After very long exposures to the exchange bath, the composition reaches a steady-state (y = 1.0).; Thermal analysis of fine powders of ammonium/hydronium (beta)'' alumina showed that water and ammonia are liberated from the sample in discrete steps below 330(DEGREES)C. These losses are reversible, and the sample can be re-hydrated or re-ammoniated by thermal cycling in either water vapor or ammonia. Above 400(DEGREES)C, the decomposition is irreversible.; The ionic conductivity of single crystals of ammonium/hydronium (beta)'' alumina were measured using AC methods. For samples with the composition y = 1.0, the conductivity follows an Arrhenius relation- ship in the temperature range -40 to 200(DEGREES)C (10('-3)(ohm-cm)('-1) at 25(DEGREES)C; Ea = 0.26 eV). Samples with higher concentrations of ammonium ions (y = 1.4) are slightly more conductive.; Above 200(DEGREES)C, samples become less conductive as ammonia and water are evolved. After extensive thermal treatment in air, the con- ductivity falls into the range of hydronium (beta)'' alumina (10('-5) (ohm-cm)('-1) at 25(DEGREES)C; Ea = .5 eV). Thus, thermal treatment of ammonium/ hydronium (beta)'' alumina might provide a new fabrication route to hydronium (beta)'' alumina.; This work shows that there is a subtle balance of ammonium and hydronium ions in ammonium/hydronium (beta)'' alumina. The combined results of FTIR, thermal analysis, and conductivity studies provide a basis for describing the roles of ammonium and hydronium in the conduction process in ammonium/hydronium (beta)'' alumina. |
