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

Abstract

The chemical expansion of ceria (CeO2−δ) and ceria-zirconia (Ce0.8Zr0.2O2−δ, CZO80) thin films is investigated by high-temperature laser Doppler vibrometry (LDV) at temperatures from 600 to 950 °C. The films are deposited on single-crystalline 8 mol-% yttria-stabilized zirconia substrates, which act as pumping cells to adjust oxygen non-stoichiometry in the thin films. Oxygen deficiency causes film expansion, leading to mechanical strain that bends the sample. The total displacement, i.e., the sum of bending and film-thickness change, is determined contact-less by LDV. A differential laser Doppler vibrometer (D-LDV) is realized to enable measurements on a very long time scale, which is necessary due to the long equilibrium times of the ceramic films. These displacements are compared to those acquired with a commercial single-point laser Doppler vibrometer (SP-LDV) for motions above 1 Hz. Here, both devices yield similar results. CZO80 films are found to bend a substrate much more than ceria films under similar experimental conditions. A model describing the displacement of the sample is derived from the Stoney model and applied to calculate deflections using literature data. The displacements at the center of the CZO80 sample measured with the SP-LDV increase from 0.18 nm at 10 Hz and 600 °C to 32.7 nm at 0.1 Hz and 800 °C. For ceria, the displacements range from 1.6 nm (10 Hz, 800 °C) to 79.4 nm (0.1 Hz, 900 °C). The D-LDV enables the detection of quasi-static displacements at very low frequencies. The ceria sample exhibits 218 nm at 0.001 Hz and 800 °C.