Influence of microstructure on the electrical conductivity of Tysonite-type fluorine-ion conductors

In the present study, Tysonite-type fluorine ionic conductors La0.93Ba0.07F2.93 dense sintered polycrystalline briquettes (relative density: >95%) with different grain sizes were prepared by discharge plasma sintering and their electrical conductivities were investigated. An evaluation was carried out to understand the effect of microstructure on ionic conductivity. The electrical conductivity of the prepared briquettes increased when the sintering temperature was raised to 1273 K and exhibited the highest value, e.g. 7.1 × 10-5Scm-1 at 300 K, which is much higher than the reported value for sintered bodies of the same material. This is due to the fact that the application of discharge plasma sintering technology enables us to achieve high densification of the sintered body and the increase in sintering temperature promotes grain growth, leading to a decrease in grain boundary resistance. When the density of the polycrystalline composite is high enough, the inhibitory effect of grain boundaries on ionic conduction is not significant. On the other hand, when the sintering temperature is higher than 1273 K, small pores are formed along the grain boundaries, and the electrical conductivity decreases as the sintering temperature increases. This decrease in conductivity can be explained as an increase in grain boundary resistance due to hole formation resulting from high temperature sintering. This work shows that for sintered polycrystalline (La,Ba)F3-based fluoride ion conductors, there is an optimal preparation condition to achieve the best microstructure, i.e., high density, large grain size, and clean grain boundaries for high fluoride ion conduction.

In this thesis, observations were made using a high-frequency analyzer, 4990EDMS-120K LN-Z2-HF, Toyo Co., Japan, on LBF93 blocks sintered at 1073, 1273, 1373, and 1473 K, respectively, at room temperature.
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Influence of microstructures on conductivity in Tysonite-type fluoride ion conductors
Author: K. Motohashi,T. Nakamura,Y. Kimura,Y. Uchimoto,K. Amezawa
Publication: Solid State Ionics
Publisher: Elsevier
Date: 1 October 2019

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