Thin-Film Chemical Expansion of Ceria Based Solid Solutions Determined by Laser Vibrometry up to 800 °C
Chemical expansion in oxides and oxide solid solutions is a known phenomenon. In the past, most studies focused on its minimization to reduce stress in thin-film systems. In this study, it is used to explore its applicability for a new type of high-temperature actuators. State-of-the-art actuators are mostly based on polycrystalline piezoelectric ceramics, like PZT (Pb-Zr-Ti-O) or doped Bi perovskites. Their main disadvantages are low Curie temperatures and, therefore, low application temperatures. Further, excitation voltages of several 10 V result in displacements in pm range only. High-temperature stable oxide solid solutions may overcome these deficits. Key factors are thermal and mechanical stability and a reversible displacement. Low energy consumption is beneficial, especially for energy self-sufficient systems.
By applying an electrical potential, oxygen nonstoichiometry in the films is generated, leading to chemical expansion. At 800 °C Pr(0.1)Ce(0.9)O(2-δ) (PCO) films with a thickness of 1.35 µm showed expansions of up to 4 nm which is equivalent to 0.3 %. For this comparably high expansion, a quite low potential of only 0.75 V is already sufficient.
The PCO thin films are deposited on yttria stabilized zirconia (YSZ) substrates that serve simultaneously as oxygen pump to adjust the oxygen activity in the films. The chemical expansion in the films generates stress and leads to a deflection of the substrates that is several times higher than the film thickness change caused directly by chemical expansion. The latter and the resulting displacement are determined by high-temperature laser Doppler vibrometry in dependence of temperature, excitation voltage and excitation frequency.
For the films and conditions mentioned above the deflection is about 19 times higher than the film thickness change when deposited on a YSZ substrate of 520 µm thickness. The sum of chemical expansion and deflection results in displacements of several hundred nm. For example, reversible displacements of up to 244 nm are achieved for PCO films (1.35 µm) on YSZ (326 µm) at 700 °C. Calculations based on the Stony equation are performed to compare these measurements to the bulk limits. The influence of the Pr dopant is investigated by comparing these measurements with undoped ceria films. The reversibility is proven by repeated measurements at 800 °C over five consecutive days. Here, only the very first measurements show a slight decrease in displacement.