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

对无线高温传感器的需求很高，以便在高达900°C的高温下监控工艺和设备部件(例如涡轮叶片)，特别是在恶劣的大气和难以接近的位置。这种微型传感器设备需要高负载的衬底和金属化，因为高温强烈地促进了氧化和蠕变等过程。一个非常有前途的系统是RuAl金属化与高温稳定的压电性Ca3TaGa3Si2O14衬底相结合。由这些材料制成的传感器工作在声表面波(SAW)的原理上，因此金属化不仅暴露在高温下，而且还受到SAW产生的高频机械载荷的影响。为了了解导致器件失效的退化机制，并相应地改进电极系统，需要进行局部调查以揭示退化的起点。在本项目中，首次进行了在热和高频机械载荷作用下所需的纳米尺度的原位表征，并在扫描电子显微镜(STEM)中以高横向分辨率研究了微结构。现场退火和施加高频机械载荷将结合特殊的样品保持器和适当的样品几何形状进行。一个特点是通过SAW基板的压电效应实现了金属电极的高频负载。此外，声表面波传感器电极内的实际热致扩散条件将在薄的瞬变电磁薄层中重建，以实现与实际工作条件的良好相似性。因此，开发了一种新的工艺来实现电子透明样品的自由表面的钝化，以抑制在原位退火过程中沿TEM片层表面的扩散。为了评估现场实验结果在实际体器件上的可转换性，进行了额外的有限元模拟。结合这些现场技术，开发了一种尚未用于研究电极内发生的微结构变化的程序，并应用该程序来访问阻碍器件应用的这些过程的局部原因。所获得的关于这些机理的知识对RUL基电极的未来工业应用具有重要意义。对SAW器件退化过程的基本认识和模型的建立是提高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.