The Role of Point Defects in the Mechanical Behavior of Doped Ceria Probed by Nanoindentation

Abstract

The influence of dopant size and oxygen vacancy concentration on the room temperature elastic modulus and creep rate of ceria doped with Pr4+, Pr3+, Lu3+, and Gd3+, is investigated using a nanoindentation technique. Measurements are conducted with both fast (15 mN s−1) and slow (0.15 mN s−1) loading modes, including a load‐hold stage at 150 mN of 8 s and 30 s, respectively. Based on the data obtained using the fast loading mode, it is found that: 1) the dopant size is a primary determinant of the elastic modulus—the larger dopants (Pr3+ and Gd3+) produce lower unrelaxed moduli which are independent of the oxygen vacancy concentration. 2) The rearrangement of point defects is the major source of room temperature creep observed during load‐hold. Pr3+‐ and Gd3+‐doped ceria display the higher creep rates: due to their large size, they repel oxygen vacancies (VO), thereby promoting the formation of O7–CeCe–VO complexes that are capable of low temperature rearrangement. Lower creep rates are observed for Pr4+‐ and Lu3+‐doped ceria: the former has no vacancies and the latter, immobile vacancies. 3) Nanoindentation is a practical technique for identifying materials with labile point defects, which may indicate useful functionality such as high ionic conductivity, large electrostriction, and inelasticity.

Suitability of Raman Spectroscopy for Assessing Anisotropic Strain in Thin Films of Doped Ceria

“…Ceria, especially doped ceria, displays anelastic mechanical behavior, due, at least in part, to the dipolar strain fields developed in the vicinity of oxygen vacancies . As the dopant concentration increases, the elastic moduli of doped ceria decrease; and in the presence of applied anisotropic stress, the mechanical response may be time dependent . When stress is applied at sufficiently high frequency and/or isotropically, the dipolar strain fields do not respond and the material behaves elastically .…”

“…In such devices, films can experience large tensile or compressive strain (≈0.5%), either retained from deposition processes or developed during heterogeneous layer stacking. Doped ceria has been reported to be anelastic, that is, it exhibits time‐dependent elastic moduli. On a time scale shorter than a few seconds, the moduli are unrelaxed and Young's modulus exceeds 200 GPa, while on a time scale of minutes to months, the biaxial Young's modulus of Gd‐doped ceria is <30 GPa .…”

Electro‐chemomechanical Contribution to Mechanical Actuation in Gd‐Doped Ceria Membranes

“…This complexity has two sources. i) CGO is strongly anelastic . The frequency range of the anelasticity (<0.5 Hz) clearly overlaps the frequency range within which nonclassical electrostriction is observed, making precise identification of the nature of the response difficult.…”

TiO2 doped UO2 fuels sintered by spark plasma sintering

“…For doped UO 2 fuels, another feature of the Raman spectra is the peaks between 500 and 700 cm -1 . The intensity of the peaks in this region appeared to increase with accumulation of structure defects introduced to UO 2 crystal lattice by oxide doping [48]. Another study underlined the appearance of peaks at this range for U 4 O 9 ; whereas no peaks were found in this region for UO 2 [49].…”

“…Another peak, located at ~ 630 cm -1 , displayed noticeable intensity changes which can attributed to cuboctahedra constituting U 4 O 9 phase [47,49]. However, this assignment has been questioned recently since this peak can also be observed at stoichiometric SIMFUEL UO 2 (simulated high burn-up fuel) [48]. Similarly, this peak appears on the spectrum obtained from stoichiometric 1500-undoped UO 2 pellet.…”