Modifying Grain Boundary Ionic/Electronic Transport in Nano-Sr- and Mg- Doped LaGaO_3-δ by Sintering Variations

Abstract

is one of the fastest known oxide ion conductors, with reported enhanced p-type electronic transference numbers at grain boundaries, attributed to space charge effects. As this material is applied as a solid oxide fuel/electrolysis cell electrolyte, it is of interest to learn how its mixed conductivity may be tailored. Field assisted sintering technique/spark plasma sintering (FAST/SPS) and conventional sintering without field or pressure were employed to prepare pellets with various grain sizes, in order to systematically assess the influence of processing route on the mixed conductivity. AC-impedance spectroscopy and the brick layer model were applied to determine local conductivities as a function of temperature, oxygen partial pressure, and dc bias. With increasing sintering temperature and grain size, the following trends were observed: larger electrical grain boundary (GB) widths, higher GB potentials, lower specific GB conductivity, greater dc-bias dependence of GB conductivity, higher pO 2 -dependence of GB conductivity indicating higher electronic transference numbers, and lower pre-exponential factor for specific GB conductivity. These results suggest an increasing GB space charge effect with increasing sintering temperature/grain size, which coincided with increasing compositional uniformity by TEM and EDS. The results confirm that sintering route is an important variable for tailoring mixed conduction.

Electrochemistry and energy conversion features of protonic ceramic cells with mixed ionic-electronic electrolytes

“…The migration of oxygen vacancies in the perovskite structure results in oxygen-ionic transport, which dominates, for example, for the conventional oxygen-ionic electrolytes based on ZrO 2 , Bi 2 O 3 , CeO 2 , LaAlO 3 , LaGaO 3 . 19,45–47 However, proton transport can also dominate over other charge carriers due to features of crystal structures. In this case, protons are formed due to a dissociative water adsorption mechanism resulting in the oxygen vacancies being filled with steam and the subsequent formation of protons localized at the oxygen-sites: 48 Here K w is the equilibrium constant of protons formation:Since this reaction is exothermic in nature, protons exist in the structure in a meaningful quantity at relatively low temperatures.…”

Dislocation-Mediated Conductivity in Oxides: Progress, Challenges, and Opportunities

“…Composite and effective medium models 64 may be applied to model macroscopic transport behavior considering the dislocations as one phase and the bulk as the other phase, as has been applied with some success to the case of grain boundary transport in polycrystalline ceramics. 13,65,66 As with grain boundaries, the models may be tailored to account for locally anisotropic transport in the extended defects, for shape effects (perhaps considering bulk regions bounded by dislocation arrays as "grains"), and for volume fraction of the extended defect "phase". 65 Nonetheless, biphasic composite models may fail to account adequately for (a) interdefect variability and (b) intradefect inhomogeneity inherent in space charge-type concentration profiles or inhomogeneous dopant/impurity segregation and (c) irregular (nonperiodic) or hierarchical Significant computational and experimental research opportunities exist broadly in the area of understanding and classifying dislocation structure− transport property relationships across multiple length scales and compositions and in applying these insights in advanced manufacturing strategies toward transport-by-design.…”

“…Although there are ultimate limits on the extent to which charge carrier and point-defect populations can be tailored in the crystalline bulk by compositional means (e.g., solubility limits, accessible gas pressures, generation of compensating defects, defect association, Fermi level pinning), extended defects provide an extra degree of freedom for introducing novel transport and other functional behavior. Although there are ultimate limits on the extent to which charge carrier and point-defect populations can be tailored in the crystalline bulk by compositional means, extended defects provide an extra degree of freedom for introducing novel transport and other functional behavior. Exotic effects that are not achievable in pristine bulk crystals can be realized through the disruption of regular crystallographic mass and charge periodicity at these extended defects. For example, the discoveries of anomalous enhancements in ionic conductivity at heterointerfaces in thin-film multilayers and ceramic composites, heightened electronic conductivity , and ionic diffusivity , at grain boundaries, novel “mesoscopic” conductivity effects , due to the dominance of interacting interfaces, 2D electron/hole gases at interfaces in oxide heterostructures, , and tailorable ionic/electronic transference numbers via varied interface density , have not only established the field of “nano-ionics,” but also provided possibilities for novel materials design strategies, devices, and performance.…”

“…Further, in ionically bonded solids, extended defects can be charged at their core, giving rise to adjacent space-charge regions (length scale: nanometers to tens of nanometers) comprising accumulation of mobile carriers of the opposite charge or depletion of mobile carriers of the same charge . The extent and character of these space-charge profiles depend on the doping or impurity levels and thermal and electrical history, among other factors. In addition, inhomogeneous strain fields (length scale: nanometers) can surround dislocations, resulting in locally varying defect formation energies and mobilities, as well as a possibility for local defect association and dopant segregation. , At sufficient concentrations, extended defects may interact through overlapping space-charge regions or strain fields or intersecting cores, creating “mesoscopic” transport properties , or the possibility for percolation behavior (length scale: micrometers to millimeters).…”

“…Exotic effects that are not achievable in pristine bulk crystals can be realized through the disruption of regular crystallographic mass and charge periodicity at these extended defects. For example, the discoveries of anomalous enhancements in ionic conductivity at heterointerfaces in thinfilm multilayers 4 and ceramic composites, 5 heightened electronic conductivity 6,7 and ionic diffusivity 8,9 at grain boundaries, novel "mesoscopic" conductivity effects 4,10 due to the dominance of interacting interfaces, 2D electron/hole gases at interfaces in oxide heterostructures, 11,12 and tailorable ionic/electronic transference numbers via varied interface density 13,14 have not only established the field of "nanoionics," 15 but also provided possibilities for novel materials design strategies, devices, and performance.…”