Enhanced ionic conductivity in electroceramics by nanoscale enrichment of grain boundaries with high solute concentration

Abstract

The enhancement of oxygen ionic conductivity by over two orders of magnitude in an electroceramic oxide is explicitly shown to result from nanoscale enrichment of a grain boundary layer or complexion with high solute concentration. A series of CaCeO polycrystalline oxides with fluorite structure and varying nominal Ca solute concentration elucidates how local grain boundary composition, rather than structural grain boundary character, primarily regulates ionic conductivity. A correlation between high grain boundary solute concentration above ∼40 mol%, and four orders of magnitude increase in grain boundary conductivity is explicitly shown. A correlated experimental approach provides unique insights into fundamental grain boundary science, and highlights how novel aspects of nanoscale grain boundary design may be employed to control ion transport properties in electroceramics.

Correlated Electron Microscopy across Length Scales to Elucidate Structural, Electrical and Chemical Properties of Oxide Grain Boundaries

“…Electron diffraction orientation imaging microscopy is employed to assess microscopic grain boundary character, and links macro-and nanoscopic techniques. These correlated experimental approaches provide unique insights into fundamental GB science, and highlights how novel aspects of nanoscale GB engineering may be manipulated to control ion transport properties in electroceramics.In the model system CaxCe1-xO2-x, the enhancement of oxygen ionic conductivity by over two orders of magnitude is explicitly shown to result from modulation of the local composition of a GB layer or complexion a few nanometers in width [3]. EBSD from a series of polycrystalline samples with varying nominal Ca concentration (see Fig 1c,d for example) showed that all samples possessed random grain boundary character.…”

“…In the model system CaxCe1-xO2-x, the enhancement of oxygen ionic conductivity by over two orders of magnitude is explicitly shown to result from modulation of the local composition of a GB layer or complexion a few nanometers in width [3]. EBSD from a series of polycrystalline samples with varying nominal Ca concentration (see Fig 1c,d for example) showed that all samples possessed random grain boundary character.…”

“…Charge transport in these materials is influenced strongly by grain boundaries, which exhibit fluctuations in composition, chemistry and atomic structure within Ångstroms or nanometers [1][2][3]. Here, studies are presented that elucidate the interplay between macroscopic electrical conductivity, microscopic character, and local composition and electronic structure of grain boundaries in polycrystalline CeO2-based solid solutions.…”

Probing Functionality for Energy Related Materials; Opportunities for Advanced Electron Microscopy

“…Electron energy-loss spectroscopy (EELS) has also undergone a revolution with 10 meV energy resolution now possible. We have been able to use this approach to investigate electronic, optical and vibrational properties of nanoscale systems [2][3][4][5][6][7][8][9]. STEM bright and dark-field images along with techniques like negative Cs imaging now allow atomic columns to be observed with atomic number spanning the entire periodic table in favorable cases.…”

“…For example, in polycrystalline solid electrolytes of relevance to fuels cells and batteries, grain boundaries often play an important role in regulating ionic transport. By combining electron backscatter diffraction in a scanning electron microscope with STEM precession electron diffraction and EELS, it becomes possible to infer the nanoscopic characteristic of 100,000 grain boundaries and relate this to macroscopic grain boundary conductivity [5,10]. The continued development of diverse techniques and more sophisticated data sampling will allow a stronger statistical link between atomic-level structure, composition and bonding with macroscopic functionality.…”