Abstract
The technologically important frequency range for the application of electrostrictors and piezoelectrics is tens of Hz to tens of kHz. Sm3+- and Gd3+-doped ceria ceramics, excellent intermediate-temperature ion conductors, have been shown to exhibit very large electrostriction below 1 Hz. Why this is so is still not understood. While optimal design of ceria-based devices requires an in-depth understanding of their mechanical and electromechanical properties, systematic investigation of the influence of dopant size on frequency response is lacking. In this report, the mechanical and electromechanical properties of dense ceria ceramics doped with trivalent lanthanides (RE0.1Ce0.9O1.95, RE = Lu, Yb, Er, Gd, Sm, and Nd) were investigated. Young’s, shear, and bulk moduli were obtained from ultrasound pulse echo measurements. Nanoindentation measurements revealed room-temperature creep in all samples as well as the dependence of Young’s modulus on the unloading rate. Both are evidence for viscoelastic behavior, in this case anelasticity. For all samples, within the frequency range f = 0.15–150 Hz and electric field E ≤ 0.7 MV/m, the longitudinal electrostriction strain coefficient (|M33|) was 102 to 104-fold larger than expected for classical (Newnham) electrostrictors. However, electrostrictive strain in Er-, Gd-, Sm-, and Nd-doped ceramics exhibited marked frequency relaxation, with the Debye-type characteristic relaxation time τ ≤ 1 s, while for the smallest dopants—Lu and Yb—little change in electrostrictive strain was detected over the complete frequency range studied. We find that only the small, less-studied dopants continue to produce useable electrostrictive strain at the higher frequencies. We suggest that this striking difference in frequency response may be explained by postulating that introduction of a dopant induces two types of polarizable elastic dipoles and that the dopant size determines which of the two will be dominant.
Highlights
Undoped and aliovalent cation-doped ceria has a wide range of applications as intermediate-temperature (IT) oxygen ion conductors[1,2] and in the field of catalysis.[3]
The doping level was limited to 20 mol % to avoid the double fluorite phase. Such point defect-derived local distortion is viewed as giving rise to the formation of elastic dipoles with a broad distribution of dipole strengths and relaxation times.[8,12−14,17,26] On the basis of density functional theory (DFT) modeling of reduced ceria,[31] Qi and co-workers have suggested that charge disproportionation in the vicinity of oxygen vacancies induces strongly anisotropic local strain, forming a polarizable elastic dipole which contributes to anelastic behavior
The dependence of the averaged low-frequency electrostriction coefficients, M33100Hz. Such strikingly different behavior implies that lattice defects oxygen vacancies and aliovalent dopants may produce more than one type of polarizable elastic dipoles and that the crystal radius of the dopant may determine which dipole controls the ceramic response as a function of electric field frequency
Summary
Undoped and aliovalent cation-doped ceria has a wide range of applications as intermediate-temperature (IT) oxygen ion conductors[1,2] and in the field of catalysis.[3]. The doping level was limited to 20 mol % to avoid the double fluorite phase Such point defect-derived local distortion is viewed as giving rise to the formation of elastic dipoles (see Supporting Information, Section S2) with a broad distribution of dipole strengths and relaxation times.[8,12−14,17,26] On the basis of density functional theory (DFT) modeling of reduced ceria,[31] Qi and co-workers have suggested that charge disproportionation in the vicinity of oxygen vacancies induces strongly anisotropic local strain, forming a polarizable elastic dipole which contributes to anelastic behavior. Local lattice distortion in aliovalent-doped ceria appears to correlate with the crystal radius of the dopant.[36] we expect that measuring electrostrictive strain as a function of dopant size should provide information on the nature of the elastic dipole’s dominating response at high and low electric field frequencies. The choice of the 10 mol % dopant was determined by the need to remain well within the fluorite phase[38−40] while generating a sufficient number of point defects, including 2.5% charge-compensating oxygen vacancies
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