Study Of In-Situ Transmission Electron Microscopy In Functional Materials Under Thermal Field

The microstructure of materials determines their properties,and their development is also a hot topic of current research in condensed matter physics,materials science,and biomedicine.Over the past decades,a deeper and more comprehensive understanding of the relationship between structure and properties of materials has emerged as a result of the continuous accumulation of results in the field of materials science.However,the understanding of the dynamical evolution processes and mechanisms of materials at the atomic level,such as size,shape,structure and chemical composition,is not clear sufficiently.In order to establish a more efficient synthesis/manufacturing-structure-property relationship,in-situ electron microscopy has been developed.In-situ transmission electron microscope（in-situ TEM）provides the key information of structural dynamics of materials in the process of transformation,and can correlate the structure and properties of materials,which provides strong support in the process of exploring the relationship between the structure and properties of functional materials.In this paper,the advanced spherical aberration-corrected（Cs-corrected）transmission electron microscope combined with in-situ heating technology,its related analysis functions and image processing methods are used to thoroughly analyze the functional materials of different systems at atomic scale,and the regulation mechanism of structure on material properties is obtained by studying the fine structure of materials.The contents of this dissertation are divided into three parts.The first part focuses on the multi-stage growth mechanism of Kirkendall voids（KVs）at the electrode interfaces of Bi₂Te₃-based thermoelectric devices.In the second section,the evolution of the microstructure of crystalline amorphous oxides is studied by in-situ electron microscopy.The third part mainly studies the growth mechanism of single crystalline metal oxide nanowires induced by metal catalytic particles through in-situ thermal analysis.The details are as follows:1.In the in-situ study of electrode interface of Bi₂Te₃-based thermoelectric devices,the thermal stability of thermoelectric generator（TEG）device interface based on Ni/Bi₂Te_2.7Se_0.3 and Ni/Bi_0.4Sb_1.6Te₃ was systematically studied by using high-resolution transmission electron microscope and in-situ heating technology.KVs were directly observed in the electrode interfaces of both Ni/Bi₂Te_2.7Se_0.3 and Ni/Bi_0.4Sb_1.6Te₃,providing thus the microscopic reason for the naked-eye cracks causing thermal failure.The growth of KVs of the as-investigated interfaces shows multi-stage behavior.This effect is attributed to the superimposition of vacancy coalesce due to the inter diffusion and interface stress mechanisms owing to the plastic difference and volume shrinkage relative to the interface reaction.Among the various interface reactions,the reaction of 3Ni+2Bi₂Te₃=3Ni Te₂+4Bi has the largest volume shrinkage,and hence decisively affects the growth of KVs.An outlook relative to the design of the thermal stability is also provided from the point of view of reducing the local stress to suppress the formation of KVs,which is regarded as a valuable guideline for the electrode interface design of TEGs.2.In the in-situ electron microscopy study of two-step crystallization of amorphous oxides,taking Y₃Al₅O₁₂（YAG）as an example,the crystallization process of amorphous YAG oxide is investigated from an atomic point of view through the combination of Cs-corrected transmission electron microscopy with an in situ heating method,and the morphology and structural evolution of the surface and interface are explored.It is observed for the first time that the crystallization proceeds on the surface and interior of the amorphous YAG separately by means of multistage layer-by-layer growth.Through atomic-scale imaging,it is determined that semi-ordered structures are formed in a few atomic layers at the amorphous/crystalline interface and that the structural transformation process from the amorphous to the semi-ordered state and then to the crystal structure may be the main reason for the multiple stages of the crystallization of amorphous oxides.This study demonstrates the mechanism of the amorphous-to-crystal transformation on the atomic scale and contributes to gaining a better understanding of the crystallization process of bulk oxides,especially complex metal oxides.These findings will promote the application of metal oxides in optics,electronic devices,and catalysis,and provide a reference for studies on material structure regulation.3.In-situ electron microscopy study of metal particle-induced metal oxide nanowire growth has successfully prepared single crystal metal oxide nanowires,which is different from the traditional growth of metal oxide nanowires in gas phase or liquid phase,through the combination of high-resolution transmission electron microscopy and in-situ heating technology,a method of directly inducing the growth of metal oxide nanowires on solid phase by using solid metal catalytic particles is proposed,and its growth mechanism is deeply explored.Under the driving force of the thermal motion of metal particles and surface atomic traction force between nanoscale particles,metal particles diffuse into the amorphous phase in a liquid-like form,continuously absorbing surrounding amorphous atoms at the front end,and the concentration gradient between the front and rear ends promotes the diffusion of atoms to the rear end.After saturation,the crystal continuously precipitates from the rear end to form a single crystal nanowire.The precipitated metal oxide single crystal nanowires are mainly grown along the<211>direction under the thermodynamic driving force provided by the surface free energy,and their growth direction and diameter depend on the shape and structure of the metal particles.The catalytic metal particles are not only affected by the surface energy of metal particles and a-YAG,metal particles and nanowires,and nanowires and surrounding a-YAG,but also by the dynamic difference of diffusion rates between the front and rear interfaces（metal particle/a-YAG,metal particle/nanowire）.The prepared metal oxide single-crystal nanowires have good crystal quality,few defects,and uniform shape and size,which means that the nanowires produced in this way will have the same physical properties along their entire length.