Ionic conduction mechanisms in nano-composite electrolyte and their relationship to micro-structural features
Shalima, Shawuti (2014) Ionic conduction mechanisms in nano-composite electrolyte and their relationship to micro-structural features. [Thesis]
Official URL: http://192.168.1.20/record=b1558965 (Table of Contents)
This study is based on a nano-composite electrolyte that is made up of samarium doped cerium oxide (SDC) skeleton within a sodium carbonate matrix. These nanocomposites have high ionic conductivities, even at temperatures as low as 400 °C. Our work revealed that the high interfacial interaction enhanced the ionic conduction behavior of the nano-composites by forming an interlayer between the oxide phase particles and the carbonate matrix which also provided structural connectivity. Consequently, the objective in this work is to address how the inter-phase affects ionic transport mechanisms, and under which circumstances the ionic conduction properties can be improved, especially in a two-phase nano-composite. The strength of influence of interfaces in ionic transport was also controlled by varying the amount of specific interface areas of the samarium doped ceria (SDC) particles in the nano-composite. In addition, the measurements conducted with composites with different amounts of the specific interface area of the SDC oxide particles in the electrolyte revealed insights into ionic conductivity of the composite. To control the value of specific surface area (SSA), the inverse relationship between the average particle size and SSA for SDC particles was employed. SDC particles with micron and nano-meter size distributions were mixed in order to obtain differing amounts of SSA in the composites between 47 m2.g-1 and 203 m2.g-1. The micro-structural investigations with SEM and TEM revealed that the Na2CO3 phase served as the glue in the composite. The glass-transition-like behavior was apparent in the thermal response of the nano-composite at 350 °C. Furthermore, the experimental results demonstrated that the overall ionic conductivity below 400 °C was controlled by the SSA. The activation energies for ionic conductivity were determined in temperature range of 25-600 °C using the Arrhenius conductivity (σT) versus inverse temperature plots in order to identify the ionic conductivity mechanisms. The activation energies are consistent with the calculated dissociation energy of the carbonate phase. The spectral elemental mapping by TEM-EELS mode showed that carbonate phase constituted the majority of the matrix. A rim around the SDC oxide particles with a high concentration of carbon was imaged. The strong dependence of the conductivity on the SSA, the differences in the activation energies, and spectral elemental mapping results suggested that the oxide surface acted as a dissociation agent for the carbonate phase. As a conclusion, the high ionic conductivities in the nano-composite electrolyte were the consequence of the oxide surface "liberating" ions, which can move more easily in the interaction region surrounding the oxide particles. The dependence of the ionic conductivity on the oxide particle amount was consistent with percolation type behavior of this interaction region, termed the "interphase" in this work.
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