| The thermomechanical field is the service condition for many structural and some functional materials,under which both the microstructure and mechanical behavior of the materials change significantly with time.For the high-temperature deformation mechanism of materials,on the one hand,research can characterize the microstructure after different service times by means of ex situ studies to infer the microscopic deformation mechanism of materials under thermomechanical conditions,but the deformation process will inevitably be missed.On the other hand,researchers have also developed a series of in situ study methods based on transmission electron microscopy,but there are still a series of technical difficulties to meet the practical research needs.These difficulties include.(1)Low heating temperatures,with the current maximum heating temperature of only 677? for thermomechanical devices,which is lower than the service temperature of structural materials,such as 1150? for nickel-based single crystal superalloy;and(2)atomic-scale resolution characterization in contradiction with high temperatures and stress application.So far,atomic-scale thermomechanical platforms with temperatures higher than 372? have not been reported.In response to the problems,this thesis develops a MEMS device for transmission electron microscopy with integrated high-temperature heating,temperature dissipation,and structural support functions,which,together with a micro-nano actuator and a double tilting mechanical structure developed by our group,can achieve atomic-scale in situ observation of material microstructure evolution under thermomechanical condition at temperatures higher than 1150?.The main research results of this paper are as follows.1.An atomic-scale in situ thermomechanical MEMS based on transmission electron microscopy is designed and developed.The innovative design of complex truss-like structure and"TC"shaped heating area solves the problem of temperature constraint of small size at extreme high temperature.The maximum operating temperature(at the sample)of the MEMS device is 1238?,and the heating response rate is 45.24?/ms,which can effectively simulate the service temperature of high-temperature materials such as nickel-based single-crystal superalloys;the four-electrodes temperature measurement method is designed to accurately measure the sample temperature with a temperature accuracy of better than 0.3? and a relative error of less than 0.15%.The lightweight and highly stable structure was designed to reduce the thermal mass and heating power consumption,and the atomic resolution was observed,with the measured spatial resolution of the material better than 1(?)(0.88(?)at 1150? using a superalloy for high temperature mechanical testing).The device adopts a split design to carry the MEMS device through the focused ion beam technology,which can achieve a variety of sample types,including:block films,nanowires,thin films,etc.The structure can be designed to combine with micro-nano actuators to achieve multi-mode mechanical loading such as tensile,compression,and bending.2.We prepared a polycrystalline molybdenum thin film heating resistive materials that can operate stably at 1200? for a long time(>190 min).The trend of process parameters was calculated by theoretical analysis,and the parameters of magnetron sputtering pressure,temperature,power,and holding time were adjusted in combination with the material properties to prepare a precursor film with low resistivity(209.3n?·m).The etching process of the heating material was optimized,and the ratio of tetramethylammonium hydroxide and hydrogen peroxide etching solution and etching conditions were improved to obtain a very high graphic transfer morphology.A high vacuum(<10-4 Pa)annealing process for 4-inch SOI wafers,including multilayer film annealing,was designed to prepare polycrystalline molybdenum MEMS heated films with a large grain size(?160 nm)and low resistivity of 101 n?·m.3.In response to the problems of over-etching and lateral drilling problems in deep silicon etching of complex feedthrough structures,the MEMS etching and integration process was optimized to improve processing accuracy and guarantee process stability and yield.For different structures of MEMS device layers,a reasonable layout pattern and size of top silicon sacrificial structure is designed.The design effectively reduces the longitudinal over-etching and lateral drilling of the device during ICP deep silicon etching.The silicon dry etching method with a complex penetration structure is improved to realize the effective conduction of crystal heat during etching and separation process after etching.The process is designed to use sputtered metal,silicone oil adhesion,wet separation,and plasma cleaning to ensure the uniformity of device structure on SOI wafers and improve the device yield.4.For the difficult problem of temperature calibration in the vacuum micro scale region,the temperature interpolation method and vacuum Raman spectroscopy was developed to calibrate the micro region temperature.The method improves the accuracy of temperature calibration and in situ temperature measurement in the electron microscope with an accuracy error of 2.3%at 1083?.To solve the technical challenges of oxidation,contamination,and internal stress of micron-sized samples that affect temperature measurement errors,a clean sample preparation technique was developed to de-stress and polish the calibrated metal elements and grow protective silicon oxide films on the surface,effectively reducing the temperature measurement errors of the melting point method.We designed a high vacuum Raman spectroscopy temperature calibration method for MEMS devices and designed a vacuum package with internal and external leads according to the device size and the working distance of the Raman objective.The device can be vacuum-encapsulated to achieve a high vacuum level(<10-4 Pa),which effectively avoids the influence of thermal convection and air conduction on temperature measurement,and combines with Raman spectroscopy to achieve accurate temperature measurement in the micro scale area. |
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