Abstract
Surfaces and interfaces in ionic ceramics play a pivotal role in defining the transport limitations in many of the existing and emerging applications in energy-related systems such as fuel cells, rechargeable batteries, as well as advanced electronics such as those found in semiconducting, ferroelectric, and piezotronic applications. Here, a variational framework has been developed to understand the effects of the intrinsic and extrinsic ionic species and point defects on the structural and electrochemical stability of grain boundaries in polycrystalline ceramics. The theory predicts the conditions for the interfacial electrochemical and structural stability and phase transitions of charged interfaces and quantifies the properties induced by the broad region of electrochemical influence in front of a grain boundary capable of spanning anywhere from a few angtroms to entire grains. We demonstrate the validity of this theory for YxZr1−xO2−x/2, cubic yttria stabilized zirconia. For small crystallographic misorientations, sharp Debye-type interfaces, D(1):{mathrm{C}}^{VY}{mathrm{S}}^{underline {VY} }, are favored and promote high ionic conductivity in materials in polycrystalline form. For large grain boundary misorientations and large amounts of [Y2O3] substitutions, three Mott–Schottky interfaces, MS(2):{mathrm{C}}^{VY,underline h e}{mathrm{S}}^{underline {VYh} e}, MS(2):{mathrm{C}}^{VYunderline h e}{mathrm{S}}^{eunderline {hVY} }, and MS(2):{mathrm{C}}^{VY}{mathrm{S}}^{underline h e} are responsible for controlling grain boundary segregation and the observed poor macroscopic ionic transport, in great agreement with the scientific literature.
Highlights
Over the last fifty years, a great deal of work describing the structural disorder,[1,2,3] solute segregation, and associated mechanical properties,[4,5] at the grain boundaries of polycrystalline materials has been made.[6]
These interfaces, defined as “complexions,” do not adhere to the rigorous Gibbsian definition of phase,[11] and its understanding has led to the development of an entire new field that has evolved into the rationalization and identification of discrete two-dimensional interfacial phases such as mono- and multilayered structures.[12,13]
The structural state of the interface is directly coupled to the electrochemical state, so that charged grain boundaries that display an opposite polarity with respect to the dominant charge species are more likely to be disordered
Summary
Over the last fifty years, a great deal of work describing the structural disorder,[1,2,3] solute segregation, and associated mechanical properties,[4,5] at the grain boundaries of polycrystalline materials has been made.[6] the thermodynamic formation of nanoscale intergranular thin films (IGFs), has been considered a class of interfacial adsorbates which exhibit structural properties that are spatially discernible and measurable at the boundary where two phases meet.[1,7] Tang, Carter, and Cannon pioneered its description by starting from a phase field formulation and acknowledged that these two-dimensional interfacial features could undergo transitions, bifurcations, and stabilization/destabilization events as a result of externally applied thermal, structural, and chemical stimuli.[8,9,10] These interfaces, defined as “complexions,” do not adhere to the rigorous Gibbsian definition of phase,[11] and its understanding has led to the development of an entire new field that has evolved into the rationalization and identification of discrete two-dimensional interfacial phases such as mono- and multilayered structures.[12,13] Based on these ideas, the community has built design and thinking tools to rationalize technologically relevant mechanisms, including solid-state activated sintering and abnormal grain growth.[10,14,15]. Analytical solutions found in the literature incorporate: (a) Guoy–Chapman-type descriptions to account for the effects of interfacial charge away from an infinitesimally thin grain boundary core;[18,19,20,21,22] or (b) artificial corrections to the effect of the grain boundary thickness to fit the experimentally observed behavior of the charge layer through Mott-Schottky-type solutions.[23,24,25,26,27] While existing descriptions correspond to piecewise solutions that are adapted by hand to meet specific applications in energy-related systems,[25] the charged region has been demonstrated to control fundamental properties, such as electrical conductivity and ionic diffusivity.[16,28,29] These properties can differ by several orders of magnitude with respect to the one displayed by a single-crystal
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