Establishment of controlled thermal conditions in in-situ TEM heating experiments: Calibration and Application

This thesis presents the development of a framework to establish controlled thermal conditions in in-situ heating experiments in scanning/ transmission electron microscopy (S/TEM), with the aim of advancing our understanding of structure-property correlations in materials under heating process. The integration of three methods has been employed to characterize the thermal conditions that can be established with microelectronic mechanical systems (MEMS)-based heating devices: finite element methods (FEM) for simulating temperature distribution, plasmon energy expansion thermometry (PEET) for in-situ temperature measurement and Raman spectroscopy for ex-situ temperature measurements. Tungsten (W) samples were used as a model material to investigate how accurately we can predict, calibrate, and verify the local temperature variation in a sample during in-situ S/TEM heating experiments. Finally, a phase transformation was investigated in the nickel titanium alloy (NiTi) sample under controlled thermal conditions as an application example to highlight the importance of in-situ S/TEM heating studies.

With investigations of precision and uncertainty of temperature measurement using PEET on W samples, it has been shown that the effects of sample thickness should be taken into account in the application of PEET. The common variations of sample thickness (30 to 70 nm) across a TEM sample seem to influence the plasmon energy of the material, leading to deviations in the determination of the temperature. However, these deviations, caused by thickness-dependent plasmon energy shifts, can be corrected by considering the plasmon peak broadening effect, which correlates with the sample thickness measured by electron energy loss spectroscopy (EELS). In the case of W, a temperature resolution of ± 30°C can be achieved in PEET with sample thickness larger than 60 nm.

When verifying whether controlled thermal gradients can be generated in TEM samples by MEMS-based heating devices, the results from FEM simulation and Raman spectroscopy show strong agreement. This agreement indicates that thermal gradients on the order of 10⁷ K/m can be achieved at set temperature (T_set) values between 800°C and 1100°C. However, the PEET measurements did not agree with the expected trends in the thermal gradients. This further highlights limitations in PEET, as morphological changes and strain effects in the sample during heating experiments could also influence variation in the plasmon energy of the material.

Finally, the reversible martensite phase transformation in NiTi alloys was investigated in in-situ TEM heating experiments, where controlled thermal conditions are important to understand the structure/temperature dependence of this transformation. Grain-by-grain phase transformations were observed under homogeneous temperature distributions, with local structural variations driving the transformation. The in-situ heating experiments revealed that phase transformation initiates near grain boundaries and at the intersections of the primary and secondary twin boundaries. These findings highlight that precisely controlled thermal conditions in in-situ heating experiments can enhance the study of complex materials during the heating process using TEM.