Chemical expansion of praseodymium-cerium mixed oxide films at high temperatures

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The primary objective of the research project is to investigate the chemical expansion of thin films at high temperatures. The material to be studied is praseodymium-cerium mixed oxide (PCO) in the temperature range from 400 °C to 700 °C. The choice is motivated by considerably higher mechanical expansion upon insertion or removal of oxygen in comparison to e.g. high-temperature piezoelectric crystals. Compared to bulk material, PCO films offer the advantage of low time constants as short transport distances for oxygen occur. In addition to the chemical expansion, the defect structure in the layers and the time-determining atomic transport mechanisms have to be determined. The development, realization and test of a special interferometer or more precisely vibrometer, which is able to measure changes of the layer thickness in the sub-nanometer range even at frequencies of a few millihertz, represents an essential technical task. Through the development of a differential vibrometer and the use of signal diversity, the noise of the signal should be drastically reduced. The results and the new measuring method can be applied to other materials than the PCO model system, so that further insights into chemical expansion and small displacements in other mechanical systems can be obtained.The topic is highly relevant. However, hardly any data on the chemical expansion of thin films is available. They have been determined largely indirectly and show some contradictions. In contrast to structural materials and electronic functional materials, high-temperature actuator materials require extensive research. Such high temperature "artificial muscles" are e.g. demanded for energy conversion and space exploration.The applicants have complementary competences in the field of high-temperature materials and the development of vibrometers, which are well suited for solving the scientific challenge and which are unique in this combination. Furthermore, the work will be carried out in close cooperation with an author of the apparently only available publication on mechanical determination of PCO film expansion, so that the planned sample exchange provides comparable results.The preliminary work has shown that vibrometers are generally able to detect changes in the thickness of PCO films which are caused by electrochemical pumping of oxygen. A significant restriction, however, is the signal noise of standard vibrometers at low frequencies. Oxygen-equilibrium in the PCO films requires excitation frequencies of a few millihertz, where no meaningful measurements are possible so far. On the instrumentation side, for example, it has been shown that signal diversity significantly reduces noise.