In situ measurements of degraded mixed ionic conductors reveal temperature’s role

Due to their ability to transduce chemical signals, permeate chemical components, and separate charger carriers, mixed ionic conductors have found a wide variety of industrial uses including in sensors, capacitors and actuators. In the presence of stressors like heat, however, these conductors degrade and lose their dielectric properties. Tests to predict their operational lifetime lack the ability to fully describe impedance changes in these materials over time. A new technique described in the Journal of Applied Physics looks to better investigate the dynamics of how mixed ionic conductors degrade.

Researchers demonstrated a procedure for analyzing the temperature-dependent properties of degraded strontium titanate (SrTiO₃) crystals in situ by conducting impedance studies with applied direct current (DC) voltage. The results reveal new and more direct insight on how activation energy in capacitor cathodes change and how degraded states evolve under external field stress.

A leading reason that mixed ionic conductors experience changes in resistance is the movement of oxygen vacancies. Previous studies measured degraded conductors in equilibrium states, but failed to analyze how these changes occur over time and could not include effects driven by electric fields.

The in situ impedance approach used DC voltage to degrade the crystals while alternating current (AC) frequency sweeps determined how the conductivity distributions changed during the process. Comparing the behavior of Fe-doped SrTiO₃ crystals to undoped crystals revealed crucial differences in behavior. Fe-doped crystals experienced a significantly different activation energy for the cathode region, while the anode region demonstrated expected degradation behavior.

With these findings, the authors proposed a new model for electron conductivity in the cathode region that is strongly temperature-dependent, and highlight the important distinction between doped and undoped charge transfer behavior. They hope these findings will provide a framework to study more mixed ionic materials.

Source: “Determination of electrical properties of degraded mixed ionic conductors: Impedance studies with applied dc voltage,” by T. J. M. Bayer, J. J. Carter, Jian-Jun Wang, Andreas Klein, Long-Qing Chen, and C. A. Randall, Journal of Applied Physics (2017). The article can be accessed at https://doi.org/10.1063/1.5006062 .