| dc.description.abstract | Solid State Ionic (SSI) materials are key materials for devices where high ionic diffusivity and conductivity are crucial for their proper operation. The most common SSI materials for these applications are doped zirconia or ceria based materials, with yttria-stabilized zirconia (YSZ) being the benchmark one. The mechanical properties are extremely important for their use as electrochemical devices which frequently operate under high temperature, mechanical stresses, electric fields and reducing or oxidizing environments. In this study, Resonant Ultrasound Spectroscopy (RUS) is used to evaluate the mechanical properties of these materials at high temperature and under electric fields.
A high temperature RUS system was developed for measuring moduli of solids from room temperature up to 1300 ⁰C in controlled environments. Elastic moduli of different materials were obtained using high temperature RUS and were statistically analyzed and systematically compared to the values obtained using other high temperature techniques. This system was used for the remainder of the study.
The elastic properties of polycrystalline Yttria-stabilized Zirconia (YSZ), Scandia-Ceria-Stabilized Zirconia (SCZ) and Gadolinia Stabilized Zirconia (GSZ) were analyzed using Resonant Ultrasound Spectroscopy (RUS) in air, from room temperature up to 1000 ⁰C. In all samples, the both Young’s and shear moduli were found to decrease significantly, i.e. for up to ~50%, in the 250 ⁰C to 600 ⁰C. In the same temperature range, two major frequency dependent attenuation (Q^-1) peaks are observed that can be attributed to the anelastic relaxation of oxygen vacancy – cation complexes. Assuming single Debye relaxation model, activation energies for the anelastic relaxation were calculated and a linear trend of increasing activation energy with increasing ionic radii mismatch was observed. The effect of dopant cation (Gd^3+, La^3+, Sm^3+, and Y^3+) on elastic properties and anelastic relaxation of doped ceria. For comparison, elastic properties of pure stoichiometric ceria (CeO2) and reduced ceria (CeO2-δ) were also examined. It was found that although the elastic moduli decrease monotonically with temperature, Q^-1 shows a frequency dependent maximum at different temperatures ranging from 100 ⁰C to 300 ⁰C for differently doped ceria. Unlike the doped zirconias, only a slight trend was observed in the activation energy for anelastic relaxation due to the more complex defect clusters that can form in doped ceria.
The mechanical damping peaks produced by RUS of 8 mol% yttria stabilized zirconia (8YSZ) were further studied to understand the number of relaxation mechanisms occurring within the material. The previous assumption of two Debye peaks did not match the data when the relaxation curves were reconstructed. It was found that a six peak model minimized the RMS error between the reconstructed curve and the experimental data. In addition, 8YSZ was studied under electric fields of 0 V/mm, 50 V/mm, 100 V/mm and 150 V/mm. It was observed that the elastic moduli become stiffer as a result of the applied electric field. The mechanical damping curves change slightly as a result of some defect clusters becoming frozen in place while others are more likely to move. | en |
