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

无线高温传感器在高达900°C的高温下监控过程和设备部件（例如涡轮机叶片）的需求很高，特别是在腐蚀性气氛中和难以接近的位置。这样的微型传感器装置需要能够高负载的衬底和金属化，因为高温强烈地促进了诸如氧化和蠕变的过程。一个非常有前途的系统是RuAl金属化与高温稳定的压电Ca3TaGa3Si2O14衬底相结合。由这些材料制成的传感器根据表面声波（SAW）的原理工作，使得金属化不仅暴露于高温，而且还暴露于由SAW引起的高频机械负载。为了了解导致器械失效的退化机制，并相应地改进电极系统，需要进行当地调查，以揭示退化的起点。在本项目中，首次实现了在热载荷和高频机械载荷作用下纳米尺度的原位表征，并在扫描透射电子显微镜（STEM）上以高横向分辨率研究了微观结构。将结合特殊样品保持器和适当的样品几何形状进行原位退火和高频机械载荷的应用。其特点是通过SAW基片的压电效应实现金属电极的高频负载。此外，基于SAW的传感器的电极内的实际热致扩散条件将在薄TEM薄片中重建，以实现与真实的操作条件的良好可比性。因此，另外，开发了一种新的过程来实现电子透明样品的自由表面的钝化，以抑制在原位退火期间沿着TEM薄片的表面的扩散。为了估计与现场实验所取得的结果的可转移性的真实的散装设备进行额外的FEM simulations.With这些组合的现场技术，尚未被应用于电极内发生的微观结构的变化的调查的程序开发和应用访问这些过程的本地原因，阻碍了设备的应用。关于这些机制所获得的知识与RuAl基电极的未来工业应用相关。增加的基本理解以及开发的退化过程的模型是提高SAW器件高温稳定性的重要基础。

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.