In-situ non-equilibrium nanomechanics in a proton-conducting ceramic at low temperatures

TL;DR

This study reveals that proton-conducting ceramics undergo non-equilibrium structural changes, including defect formation and grain cracking, at low temperatures, challenging the assumption of their stability during operation.

Contribution

The paper demonstrates in-situ 3D imaging of structural dynamics in PCC grains at 200°C, uncovering non-equilibrium defect processes and structural degradation mechanisms.

Findings

Non-equilibrium defect generation observed at 200°C

Grain cracking and facet formation within hours

Correlation between structural changes and H+ transport heterogeneity

Abstract

Nanostructured proton-conducting ceramics (PCCs) have attracted considerable interest as moderate-temperature proton conductors. Structure dynamics during proton conduction, particularly at grain boundaries, are crucial for stability and proton transport in nanostructured PCCs. A common assumption is that PCCs are structurally stable at low operating temperatures; however, material polycrystallinity, absorption, and reactive operating conditions have so far prevented verifying this assumption by nano resolved in-situ structure measurements. Here, in an archetypal PCC BaZr0.8Y0.2O3-d the premise of structural stability is demonstrated to be inaccurate at temperatures as low as 200 {\deg}C. Coherent X-ray diffraction on a nanostructured BaZr0.8Y0.2O3-d sintered pellet is adapted to image in-situ three-dimensional structural processes inside the constituent submicron grains in a humid nitrogen atmosphere at 200 {\deg}C. Direct observation reveals non-equilibrium defect generation and subsequent grain cracking on a timescale of hours, forming new, otherwise energetically unfavorable facets in BaZr0.8Y0.2O3-d. Furthermore, the structural rearrangements correlate with dynamic inhomogeneities of the lattice constant within the grains, showing potential heterogeneous H+ transport. Our results elucidate the mechanisms behind PCCs structural degradation, overturn existing assumptions about the structure dynamics in PCCs, and fill a method gap for further in-depth in-situ studies of the PCC nanostructure.