High-accuracy determination of nonstoichiometry and chemical expansion of thin praseodymium-cerium oxide films at high temperatures

• Applicants: Professor Dr.-Ing. Holger Fritze; Professor Dr.-Ing. Christian Rembe
• Subject Area: Synthesis and Properties of Functional Materials ; Measurement Systems
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 404875250

In the first funding period, the chemical expansion of thin films was investigated in detail using the example of praseodymium cerium oxide solid solutions (PCO) in the temperature range from 500 to 950 °C. To this end, a novel differential laser Doppler vibrometer (D-LDV) was realized that exhibits significantly suppressed signal noise, particularly for periodic changes in oxygen activity of only a few millihertz, which are required to reach the oxygen equilibrium in the films. The solution to this metrological challenge allows non-contact measurement of thickness changes of the layers as small as 1 nm. Essential is the close guidance of the measuring and reference beams and their focusing on the sample and the sample holder, so that disturbances such as thermal turbulence are strongly suppressed. This beam path leads to an efficient compensation of disturbances at frequencies down to 0.1 Hz. Density cell sizes of the turbulent air, which are smaller than the remaining distance between the beams, cause disturbances above the mentioned frequency, which are still not fully compensated. The new measurement method has an international unique position and is meanwhile used in other projects for the investigation of very small and slow displacements at high temperatures and compensation of turbulence effects in the atmosphere in a broad frequency range. The chemical expansion leads to both a bending of the substrate and a change in layer thickness. The latter results in strains approximately twice that of bulk samples. It is essential that the associated Poission number is on the one hand reliable based on the D-LDV measurements and on the other hand at the upper limit of the spectrum of literature values. Consequently, highly accurate determination of the non-stoichiometry of PCO emerges as a further research goal to correlate non-stoichiometry, chemical expansion and defect formation at high mechanical stresses and high temperatures. Within the intended follow-up project, we aim to realize a new metrological approach, which allows the high-precision measurement of the non-stoichiometry of thin films with resonant high-temperature nanobalances in combination with a differential scanning confocal D-LDV. For the piezoelectric thickness-shear resonators that we want to explore, the distribution of the vibration amplitude and thus the mass sensitivity strongly depends on the temperature and the properties of the layers under investigation. Furthermore, bending of the resonators can occur, which leads to an unwanted frequency change. Therefore, in order to precisely determine the mass sensitivity and to detect possible disturbances, a differential scanning confocal D-LDV will be designed and investigated that allows lateral vibration amplitudes as well as (unwanted) transverse movements of the resonators with a resolution of 1 nm. The scanning system can measure 3D vibrational modes at high temperatures and thus has a very high scientific impact.

• DFG Programme: Research Grants
• Major Instrumentation: Kompakter Laser
• Instrumentation Group: 5700 Festkörper-Laser