Thin-film chemical expansion of ceria based solid solutions: laser vibrometry study

Abstract

The chemical expansion of Pr0.1Ce0.9O2–δ (PCO) and CeO2–δ thin films is investigated in the temperature range between 600 °C and 800 °C by laser Doppler vibrometry (LDV). It enables non-contact determination of nanometer scale changes in film thickness at high temperatures. The present study is the first systematic and detailed investigation of chemical expansion of doped and undoped ceria thin films at temperatures above 650 °C. The thin films were deposited on yttria stabilized zirconia substrates (YSZ), operated as an electrochemical oxygen pump, to periodically adjust the oxygen activity in the films, leading to reversible expansion and contraction of the film. This further leads to stresses in the underlying YSZ substrates, accompanied by bending of the overall devices. Film thickness changes and sample bending are found to reach up to 10 and several hundred nanometers, respectively, at excitation frequencies from 0.1 to 10 Hz and applied voltages from 0–0.75 V for PCO and 0–1 V for ceria. At low frequencies, equilibrium conditions are approached. As a consequence maximum thin-film expansion of PCO is expected due to full reduction of the Pr ions. The lower detection limit for displacements is found to be in the subnanometer range. At 800 °C and an excitation frequency of 1 Hz, the LDV shows a remarkable resolution of 0.3 nm which allows, for example, the characterization of materials with small levels of expansion, such as undoped ceria at high oxygen partial pressure. As the correlation between film expansion and sample bending is obtained through this study, a dimensional change of a free body consisting of the same material can be calculated using the high resolution characteristics of this system. A minimum detectable dimensional change of 5 pm is estimated even under challenging high-temperature conditions at 800 °C opening up opportunities to investigate electro-chemo-mechanical phenomena heretofore impossible to investigate. The expansion data are correlated with previous results on the oxygen nonstoichiometry of PCO thin films, and a defect model for bulk ceria solid solutions is adopted to calculate the cation and anion radii changes in the constrained films during chemical expansion. The constrained films exhibit anisotropic volume expansion with displacements perpendicular to the substrate plane nearly double that of bulk samples. The PCO films used here generate high total displacements of several 100 nm’s with high reproducibility. Consequently, PCO films are identified to be a potential core component of high-temperature actuators. They benefit not only from high displacements at temperatures where most piezoelectric materials no longer operate while exhibiting, low voltage operation and low energy consumption.

Chemical lattice strain in nonstoichiometric oxides: an overview

“…2 shows the experimental data and model functions for chemical strain of CeO 2-d and Ce 0.9 Gd 0.1 O 2-d . 84,92,94 It is seen that Kim's 76 and Bishop's 84 models (eqn ( 13) and ( 22), respectively) agree much better with experimental data for CeO 2-d , while for Ce 0.9 Gd 0.1 O 2-d even eqn (22) where oxygen vacancy ordering is taken into account does not describe the experimental 3 chem within the whole nonstoichiometry range.…”

“…The latter is not an exaggeration: in the studies of chemical expansivity, detectable changes in linear dimensions of as small as 5 pm were reported. 22 However fascinating the ever-increasing magnication of modern analytical methods, more "conventional" ones, such as thermogravimetric analysis, dilatometry, and X-ray and neutron diffraction, despite having been with us for many decades, are still indispensable when it comes to the properties of bulk materials. 23 Imaging techniques such as scanning electron microscopy 24,25 or even X-ray tomography 7 are useful for assessment of the consequences of chemical expansion, e.g., the damage caused to electrochemical devices.…”

Chemical expansion of CeO2−δ and Ce0.8Zr0.2O2−δ thin films determined by laser Doppler vibrometry at high temperatures and different oxygen partial pressures

“…Alternatively, the oxygen activity in the thin-film samples can be adjusted by pumping of oxygen using the electrochemical approach mentioned above. Related periodic deformations of samples are observed [9][10][11]. Oxygen activity and therefore deficiency strives toward equilibrium with the surrounding atmosphere.…”

“…Especially in automotive exhaust catalysts, ceria-zirconia is a standard oxygen buffer material [3]. The insertion and release of oxygen leads to a change in stoichiometry, which can be associated with a change in volume [1,[4][5][6][7][8][9][10][11] called chemical expansion. Since a thin film adhering firmly to a negligibly expanding substrate cannot expand freely like the respective bulk material, the chemical expansion leads to the build-up of lateral mechanical stress in the film.…”

“…A particularly suited method is laser Doppler vibrometry (LDV), as it can be applied without mechanical contact at high temperatures if a specially adapted setup is used, as done here. Previous studies by the authors [9,11] using praseodymia-ceria (PCO) solid solutions have shown the unique ability to detect chemical expansion and related effects in thin-film samples without influencing the sample. PCO is known to show large chemical expansion.…”