Browsing by Author "Gautam, Devendraprakash"

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    Characterization of the conduction properties of alkali metal ion conducting solid electrolytes using thermoelectric measurements
    (2006) Gautam, Devendraprakash; Aldinger, Fritz (Prof. Dr. rer. nat.)
    Under certain circumstances the electronic conductivity of the solid electrolyte may play a pivotal role for the behaviour of a solid state galvanic cell. Quantitatively, the extent of the electronic conductivity is expressed by the electronic conduction parameters, a⊕ and a⊖, that denote the alkali metal activities at which the n and p-type electronic conductivities, respectively, of the electrolyte are equal to its ionic conductivity. Previous findings demonstrated the existence of a finite, non-negligible electronic conductivity in various alkali metal ion conductors like Na-beta-alumina (NBA), K-beta-alumina (KBA) and NASICON when these materials are employed as solid electrolytes in potentiometric CO2 sensors or for certain thermodynamic investigations. Particularly, a number of previous studies brought into the light the tendency of the p-type electronic conduction parameter a⊕ to adapt to the chemical potential of those species that represent the neutral counterpart of the mobile ion of the electrolyte. These findings are in contradiction with the basic assumptions of the conventional defect chemical considerations underlying the Schmalzried-Wagner theory of mixed ionic-electronic conduction. Because of the fundamental relevance, they were supposed to be substantiated further, at best by an experimental approach independent of the previously applied ones. Thermoelectric power measurements, under certain prerequisites, allow to determine the proportion of the electronic charge carriers transport in the total conductivity of a mixed ionic-electronically conducting material. Moreover, this technique has an inherent advantage over the isothermal potentiometric technique insofar as the two surfaces of the solid electrolyte need not be exposed to different electrode media as the voltage is primarily generated due to a temperature gradient. This makes the approach an essential and so far disregarded tool to clarify the above mentioned unconventional phenomenon regarding a⊕. The objective of the present work is the quantitative determination of the p-electronic conduction parameter a⊕ of several sodium and potassium ion-conducting solid electrolytes, namely NBA, KBA, NASICON, Na2CO3 and K2CO3, by thermoelectric power measurements under a possibly wide spectrum of the chemical potential of the potential determining species and temperature. The measurements aimed at finding further evidence for the dependence of the p-electronic conduction parameter on the surroundings as was first described by Näfe. In addition, impedance spectroscopy studies were designed to characterize the conduction properties of some exemplary materials by another measuring technique and to independently check the results of the thermoelectric power measurements, at least qualitatively. In view of the paucity of the knowledge regarding the stability of the constituent phases of NASICON and because of the relevance of this topic for the characterization of the electronic conduction parameter of this material, the thermodynamic stability of NASICON was determined as another task to be fulfilled within the present work.