In-situ TEM analysis of the degradation processes in Ru-Al electrodes for high-temperature SAW devices on CTGS substrates under impact of temperature and varying mechanical stress using passivated TEM lamellas

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There is a high demand for wireless high-temperature sensors to monitor processes and equipment parts (e.g. turbine blades) at high temperatures up to 900°C, especially in aggressive atmospheres and at locations which are difficult to access. Such miniature sensor devices require substrates and metallizations which can be highly loaded, since the high temperatures strongly promote processes like oxidation and creep. A very promising system is the RuAl metallization in combination with the high-temperature stable piezoelectric Ca3TaGa3Si2O14 substrate. The sensors made of these materials work on the principle of surface acoustic waves (SAW), so that the metallization is not only exposed to the high temperature, but in addition also to a high-frequency mechanical load caused by the SAW. To understand the resulting mechanisms of degradation which lead to a failure of the devices and to improve the electrode systems accordingly, local investigations are required to reveal the starting points of the degradations. In this project, the required in-situ characterizations on the nano-scale during thermal and high-frequency mechanical load are carried out for the first time.The microstructure is investigated with high lateral resolution in a scanning transmission electron microscope (STEM). The in-situ annealing and application of a high-frequency mechanical load will be performed in combination with a special sample holder and an appropriate sample geometry. A special feature is the realization of the high-frequency load of the metal electrode via the piezoelectric effect of the SAW substrate. Furthermore, the actual heat-induced diffusion conditions within the electrodes of a SAW based sensor will be reconstructed in the thin TEM lamellas to realize a good comparability with the real operating conditions. Therefore, in addition, a new procedure is developed to realize a passivation of the free surfaces of the electron transparent samples to suppress a diffusion along the surfaces of the TEM lamellas during the in-situ annealing. To estimate the transferability of the results achieved with the in-situ experiments to the real bulk devices additional FEM simulations are carried out.With these combined in-situ techniques, a procedure which has not yet been applied for the investigations of the changes of the microstructure which take place within the electrodes is developed and applied to access the local causes of these processes which impede the application of the devices. The knowledge gained about these mechanisms is relevant for future industrial applications of the RuAl-based electrodes. The increased basic understanding as well as the models of the degradation processes which are developed are an important basis for the improvement of the high-temperature stability of the SAW devices.