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Anelastic and Electromechanical Properties of Doped And& PROGRESS REPORT Ceria www.advmat.de Anelastic and Electromechanical Properties of Doped and Reduced Ceria Ellen Wachtel, Anatoly I. Frenkel, and Igor Lubomirsky* activity of ceria: the temporal, thermal, Room-temperature mechanical properties of thin films and ceramics of doped and compositional influence on the mate- and undoped ceria are reviewed with an emphasis on the anelastic behavior rial elasticity was little examined. How- of the material. Notably, the unrelaxed Young’s modulus of Gd-doped ceria ever, during the last decade, considerable ceramics measured by ultrasonic pulse-echo techniques is >200 GPa, while interest has developed in doped ceria as an essential component in micrometer-sized the relaxed biaxial modulus, calculated from the stress/strain ratio of thin fuel cells,[6] sensors, and even electrome- films, is≈ 10 times smaller. Oxygen-deficient ceria exhibits a number of chanical actuators.[7] These scaled-down anelastic effects, such as hysteresis of the lattice parameter, strain-dependent devices demand particularly precise Poisson’s ratio, room-temperature creep, and nonclassical electrostriction. mechanical engineering, e.g., because Methods of measuring these properties are discussed, as well as the thermal cycling of ceria films during device operation can be associated with applicability of Raman spectroscopy for evaluating strain in thin films of the generation of unpredictable mechan- Gd-doped ceria. Special attention is paid to detection of the time dependence ical strain. To give an example, the linear of anelastic effects. Both the practical advantages and disadvantages of thermal expansion coefficient of undoped anelasticity on the design and stability of microscopic devices dependent ceria is ≈11 × 10−6 K−1, which predicts that on ceria thin films are discussed, and methods of mitigating the latter are heating from 300 to 500 K will generate suggested, with the aim of providing a cautionary note for materials scientists strain 0.22%. The fractional linear expan- sion of silicon within the same tempera- and engineers designing devices containing thin films or bulk ceria, as well ture range is only 0.063%; and therefore, as providing data-based constraints for theoreticians who are involved in heating a ceria film deposited on a silicon modeling of the unusual electrical and electromechanical properties of substrate would produce in-plane com- undoped and doped ceria. pressive strain in the film of 0.16%. Taking into account that the literature value for the elastic (Young’s) modulus of undoped ceria is ≈200–300 GPa[8–11] and that Pois- 1. Introduction: Point Defects and the Elasticity son’s ratio is 0.28, a thin film of ceria should develop in-plane of Ceria compressive stress ≈430 MPa. A ceria film with thickness on the order of a few hundred nanometers cannot sustain such Thin films and ceramics of undoped and doped ceria have been stress: the elastic energy stored exceeds typical adhesion energy. the subject of many studies and review articles in the materials Therefore, the film would be expected to delaminate from the science literature. This persistent interest is due, at least in substrate, i.e., blister. However, delamination is not observed; part, to the broad versatility of ceria in industrial applications: and as discussed below, there is evidence from measurements devices that require high levels of oxygen ion conductivity, e.g., of the substrate curvature that, following thermal cycling, the fuel cells and sensors[1–3] as well as catalytic converters bene- anelastic properties of oxygen-deficient ceria appear to reduce fiting from the redox properties of the Ce ion.[4,5] In the past, the relaxed biaxial Young’s modulus of the substrate-supported reviews have generally focused on the conductivity or catalytic films to much lower values. Point defects in solids can give rise to anelastic behavior— i.e., the material completely recovers from deformation due Dr. E. Wachtel, Prof. I. Lubomirsky to anisotropic application of stress, but with variable rates of Department of Materials and Interfaces Weizmann Institute of Science strain release—provided that the defects behave as elastic Rehovot, Israel dipoles, producing an anisotropic, local distortion of the crystal E-mail: [email protected] lattice. When thin films of ceria experience substrate clamping, Prof. A. I. Frenkel only in-plane stress components are present. The in-plane Department of Materials Science and Chemical Engineering strain is fully recoverable if and when the substrate is removed. Stony Brook University Similarly, if ceramics are subjected to arbitrary mechanical NY, USA loading (not necessarily isostatic), lattice deformation is fully The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201707455. recoverable, unless loading is performed at very high tempera- tures (above 1000 °C) when dislocation movement and grain DOI: 10.1002/adma.201707455 boundary creep induce plastic (irrecoverable) deformation. Adv. Mater. 2018, 1707455 1707455 (1 of 17) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advmat.de Introduction of a dopant ion produces local distortion of the lattice, which is present even in the absence of external stress. Ellen Wachtel received her Since the lattice constant determined by XRD is actually an Ph.D. degree in applied average of the total irradiated volume of the sample, the pres- physics from Yale University ence of dopants imposes an overall change in the lattice param- in 1970, and since 1976 eter, which may be viewed as isostatic strain. This isostatic has been a member of the strain is not accompanied by mechanical stress, and therefore Faculty of Chemistry of does not modify mechanical response. On the other hand, local the Weizmann Institute of distortions have been viewed as the structural origin of the Science in Rehovot, Israel. elastic dipoles, leading to the observed anelastic response to Until her retirement in 2009, anisotropic stress. Consequently, no competition exists between she was the head of the these two factors: isostatic strain and substrate-induced com- X-Ray Diffraction Laboratory, pressive in-plane strain. studying the structure of The anisotropic application of elastic stress, as, for example, powders, fibers, and dispersions of both organic and in mechanical oscillations, leads to thermally activated reorien- inorganic materials. Currently, she is a consultant in the tation of the elastic dipoles.[12] However, when the applied stress Department of Materials and Interfaces. is isotropic or hydrostatic, then no change in the elastic dipolar strain fields would be expected and a time-dependent relaxation Anatoly I. Frenkel is a process should not occur. In industrially useful forms of ceria, professor in the Department point defects include oxygen vacancies and dopant atoms sub- of Materials Science and stituting for the host at cation positions and they can occupy Chemical Engineering, Stony a significant fraction of the lattice sites. The fluorite crystal Brook University, USA. lattice (Fm-3m) is tolerant toward ion substitution; therefore, He received his M.Sc. in ceria is able to accommodate a relatively large concentration of physics from St. Petersburg dopants. For instance, in a frequently studied doped ceria com- University, Russia, and his position, Ce Gd O , 20% of Ce ions are replaced by the alio- 0.8 0.2 1.9 Ph.D. degree in physics valent dopant (Gd3+ ) and 5% of the oxygen sites are vacant. We from Tel Aviv University. He note that a number of acronyms for the material composition studies the physicochemical may be found in the literature, e.g., Gd-doped ceria is referred properties of nanomaterials to as xCGO, CGOx, or xGDC (used here), where x is the molar and the mechanisms of work of nanocatalysts, and fraction of Gd. In undoped ceria, facile loss of oxygen and Ce develops operando methods of characterization using redox reactions (Ce4+ ↔ Ce3+ ) can produce additional com- synchrotron X-ray techniques. He is a founding Principal plicating factors related to interaction between point defects. Investigator and the Spokesperson of the Synchrotron Nonstoichiometric, reduced ceria CeO presents a rich phase 2−x Catalysis Consortium at Brookhaven National Laboratory. diagram at temperatures between ambient and 700 K and as a function of oxygen partial pressure.[13,14] In a number of these crystal phases, neutron diffraction and reverse Monte Carlo Igor Lubomirsky is a modeling have shown that the oxygen vacancies preferentially professor in the Department align as pairs along the 〈111〉 direction rather than being ran- of Materials and Interfaces, domly distributed among the anion sites.[14] In Gd-, Sm-, or Nd- Weizmann Institute of doped ceria, onset of oxygen vacancy ordering occurs when the Science (WIS), Israel. aliovalent dopant concentration requires (for charge compensa- He obtained his B.Sc. tion) that ≥6–10% of the oxygen sites are vacant,[15–17] resulting in chemical engineering in a change from Fm-3m to the double fluorite lattice (Ia-3). at Kharkov Polytechnic Thus, point defects on the anion and cation sublattices of Institute (Ukraine) and his ceria can result in thermal and mechanical properties that are Ph.D. degree in solid-state both difficult to characterize experimentally and also to model. chemistry at WIS. He held Although the most striking of these properties may be attributed postdoctoral positions to the anelasticity of the material, quantitatively describing the in electrical engineering at UCLA and the Max Planck particular sources of the anelastic behavior of ceria is beyond Institute for Solid State Research (Stuttgart, Germany). the scope of this review. The early, insightful book-length trea- His studies involve local symmetry reduction leading to tise on this topic by Nowick and Berry,[12] “Anelastic Relaxation anelastic and electrostrictive effects in solids with a large in Crystalline Solids,” has provided the initial foundation for our concentration of point defects, in general, and in ion approach.
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