Elasticity of Solids with a Large Concentration of Point Defects II. The Chemical Strain Effect in Ce_0.8Gd_0.2O_1.9

Abstract

The chemical strain effect describes a mechanism of stress relaxation in solids that can be attributed to the conversion of elastic energy into chemical energy of point defects. Experimental confirmation of this effect is presented here for the case of thin self‐supported films of the ionic conductor Ce0.8Gd0.2O1.9. If heated slowly, (< 5 °C min–1) these films remain flat within the temperature range of 25–180 °C. If heated more rapidly than ∼ 20 °C min–1, the films buckle above 53 °C, but after ∼ 3–30 min at elevated temperatures, they become flat again, demonstrating that stress‐relaxation has taken place. The degree of stress reduction observed is consistent with the value calculated for this system using Boltzmann statistics in the case of small strain. These findings confirm the concept of stress adaptability in solids that we introduced in Part I and suggest that a large class of such materials, which exhibit the chemical strain effect, is likely to be found.

Influence of Point‐Defect Reaction Kinetics on the Lattice Parameter of Ce0.8Gd0.2O1.9

“…5.3). This imposes an upper limit on the biaxial modulus [5] of the substrate-supported films of Ce 0.8 Gd 0.2 O 1.9 of <30 MPa/ 0.18% ¼ 17 GPa, which is more than an order of magnitude lower than the biaxial modulus of bulk Ce 0.8 Gd 0.2 O 1.9 (282 GPa) [12] and matches earlier observations. [5] We performed in situ monitoring of the temperature evolution of the d 220 spacing of the as-deposited substrate-supported Ce 0.8 Gd 0.2 O 1.9-x films (10 8C min À1 heating and 1 8C min À1 cooling rates) (Fig.…”

“…The scientific interest in ceria-based materials is inspired by the richness of their behavior: ionic conductivity due to the high mobility of oxygen vacancies, the existence of a series of different phases formed upon reduction, [1] the dependence of the lattice parameter and properties on grain size, [2] and nonlinear elastic effects, which have been named ''chemical stress'' [3] and ''chemical strain''. [4,5] The ''chemical strain'' effect, that is, the ability of a ceramic to exhibit two different elastic moduli depending on the time scale, is attributed to the volume change caused by the association/dissociation reaction between point defects. [4,5] In the case of Gd-doped ceria, these defects are presumably aliovalent ions and oxygen vacancies and, in a strongly simplified approximation, the association/dissociation reaction can be written as [7] Gd 0…”

“…[4,5] The ''chemical strain'' effect, that is, the ability of a ceramic to exhibit two different elastic moduli depending on the time scale, is attributed to the volume change caused by the association/dissociation reaction between point defects. [4,5] In the case of Gd-doped ceria, these defects are presumably aliovalent ions and oxygen vacancies and, in a strongly simplified approximation, the association/dissociation reaction can be written as [7] Gd 0…”

“…Reactions between point defects may strongly influence elastic properties even if the relative changes in defect concentration are only a few percent at most. [4,5,17] Tracking the changes in the defect concentration in bulk samples associated with such reactions is very difficult. On the other hand, tracking stress and strain in thin films is far more accessible experimentally and may provide information on the evolution of the state of association of the point defects.…”

Sub-nA spatially resolved conductivity profiling of surface and interface defects in ceria films

“…29,30 Ionic conduction is strongly enhanced in pulsed-laser deposition grown films, which often have high oxygen non-stoichiometry. We expect nonstoichiometric ceria to be an n-type semiconductor in which oxygen deficiency leads to oxygen vacancies in the cubic fluorite lattice with electrons from the removed oxygen atom interacting with neighboring cerium cations.…”