Browsing by Subject "Electron Microscopy"

Now showing 1 - 4 of 4
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    Analytical and Experimental Nanomechanical Approaches to Understanding the Ductile-to-Brittle Transition
    (2015-10) Hintsala, Eric
    This dissertation presents progress towards understanding the ductile-to-brittle transition (DBT) using a mixture of nanomechanical experiments and an analytical model. The key concept is dislocation shielding of crack tips, which is occurs due to a dislocation back stress. In order to properly evaluate the role of these interactions, in-situ experiments are ideal by reducing the number of interacting dislocations and allowing direct observation of cracking behavior and the dislocations themselves. First, in-situ transmission electron microscope (TEM) compression experiments of plasma-synthesized silicon nanocubes (NCs) are presented which shows plastic strains greater than 50% in a semi-brittle material. The mechanical properties are discussed and plasticity mechanisms are identified using post-mortem imaging with a combination of dark field and high-resolution imaging. This observations help to develop a back stress model which is used to fit the hardening regime. This represents the first study of its kind where back stresses are used in a discrete manner to match hardening rates. However, the important measurable quantities for evaluating the DBT include fracture toughness values and energetic activation parameters for cracking and plasticity. In order to do this, a new method for doing in-situ fracture experiments is explored. This method is pre-notched three point bending experiments, which were fabricated by focused ion beam (FIB) milling. Two different materials are evaluated: a model ductile material, Nitronic 50, an austenitic steel alloy, and a model brittle material, silicon. These experiments are performed in-situ scanning electron microscope (SEM) and TEM and explore different aspects including electron backscatter diffraction (EBSD) to track deformation in SEM scale experiments, pre-notching using a converged TEM beam to produce sharper notches better replicating natural cracks, etching procedures to reduce residual FIB damage and elevated temperature experiments. Lastly, an analytical method to predict DBTs is presented which can account for effects of strain rate, temperature and impurity presence. The model is tested by pre-existing data on macroscopic compact tension specimens of single crystal Fe-3%Si. Next, application of the model to nano/micro scale fracture toughness experiments is explored and the large number of confounding variables is discussed in detail. A first attempt at fitting is also presented.
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    The Investigation Of New Magnetic Materials And Their Phenomena Using Ultrafast Fresnel Transmission Electron Microscopy
    (2017-02) Schliep, Karl
    State-of-the-art technology drives scientific progress, pushing the boundaries of our current understanding of fundamental processes and mechanisms. Our continual scientific advancement is hindered only by what we can observe and experimentally verify; thus, it is reasonable to assert that instrument development and improvement is the cornerstone for technological and intellectual growth. For example, the invention of transmission electron microscopy (TEM) allowed us to observe nanoscale phenomena for the first time in the 1930s and even now it is invaluable in the development of smaller, faster electronics. As we uncover more about the fundamentals of nanoscale phenomena, we have realized that images alone reveal only a snapshot of the story; to continue progressing we need a way to observe the entire scene unfold (e.g. how defects affect the flow of current across a transistor or how thermal energy propagates in nanoscale systems like graphene). Recently, by combining the spatial resolution of a TEM with the temporal resolution of ultrafast lasers, ultrafast electron microscopy  or microscope  (UEM) has allowed us to simultaneously observe transient nanoscale phenomena at ultrafast timescales. Ultrafast characterization techniques allow for the investigation of a new realm of previously unseen phenomenon inherent to the transient electronic, magnetic, and structural properties of materials. However, despite the progress made in ultrafast techniques, capturing the nanoscale spatial sub-ns temporal mechanisms and phenomenon at play in magnetic materials (especially during the operation of magnetic devices) has only recently become possible using UEM. With only a handful of instruments available, magnetic characterization using UEM is far from commonplace and any advances made are sparsely reported, and further, specific to the individual instrument. In this dissertation, I outline the development of novel magnetic materials and the establishment of a UEM lab at the University of Minnesota and how I explored the application of it toward the investigation of magnetic materials. In my discussion of UEM, I have made a concerted effort to highlight the unique challenges faced when getting a UEM lab running so that new researchers may circumvent these challenges. Of note in my graduate studies, I assisted in the development of three different magnetic material systems, strained Fe nanoparticles for permanent magnetic applications, FePd for applications in spintronic devices, and a rare-earth transition-metal (RE-TM) alloy that exhibits new magneto-optic phenomena. In studying the morphological and magnetic effects of lasers on these RE-TM alloys using the in situ laser irradiation capabilities of UEM along with standard TEM techniques and computational modeling, I uncovered a possible limitation in their utility for memory applications. Furthermore, with the aid of particle tracing software, I was able to optimize our UEM system for magnetic imaging and demonstrate the resolution of ultrafast demagnetization using UEM.
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    Strategies to Create Electrically Conductive Polymer/Graphene Composites
    (2021-08) Kou, Yangming
    Conductive polymer composites, typically constructed by melt compounding conductive fillers into a polymer matrix, enjoy specialized applications such as electrostatic discharge protection. Graphene nanoplatelets (GNPs) exhibit high inherent electrical conductivity and geometric anisotropy, thus require much lower loading (< 1 wt%) in a polymer matrix to achieve electric percolation while preserving good melt processability. However, due to their relative high cost, it is desirable to further reduce GNP loading while enhancing the polymer/GNP composite electrical conductivity. In this thesis, I demonstrate two formulation strategies to attain conductive polymer composites by controlling GNP localization in cocontinuous polymer blends using both miscible and immiscible systems. For the miscible system, poly(methyl methacrylate) (PMMA) and poly(styrene-co-acrylonitrile) (SAN) blends are selected. By first compounding PMMA, SAN, and GNP together at lower temperature and then inducing PMMA/SAN spinodal decomposition by heating, I create spatially regular, cocontinuous domains where GNPs preferentially localize within the thermodynamically preferred SAN-rich phase and form conductive networks. I develop a quantitative transmission electron microscopy (TEM) image analysis method to quantify both the polymer domain size and GNP localization. Dielectric measurements show that quiescent annealing improves particle connectivity of the GNP network, leading to further enhancement in electrical conductivity to ~ 10^[-8] S/cm at 1 wt% GNP concentration. For the immiscible system, polylactide/poly(ethylene-co-vinyl acetate) (PLA/EVA) blends are selected. PLA/GNP masterbatches are melt compounded with the EVA homopolymer. Since GNPs preferentially wet the EVA phase, they transfer from PLA to EVA but become kinetically trapped at the interface, as confirmed by electron microscopy. I achieve an ultra-low percolation threshold of 0.048 wt% GNPs and obtain blends with electrical conductivities of ~ 10^[-5] S/cm at 0.5 wt% GNP concentration. Rheology, in-situ dielectric measurements, and TEM imaging after nonlinear shearing and extensional deformations all show that interfacial GNP network remains at the PLA/EVA interface. Moreover, high electrical conductivity is maintained during a wide range of melt compounding times, between 2–10 minutes. In addition to cocontinuous blends, this thesis also addresses practical challenges related to homopolymer-based conductive composites. The effect of electric field-induced conductivity enhancement and dielectric breakdown due to electrical treeing formation within EVA/GNP composites is studied through in-situ measurement of the electrical conductivity. Furthermore, the relationship between shear rheology, filler dispersion, and electrical conductivity of industrially produced conductive polymer composites is studied. These analytical techniques allow for understanding of composite characteristics, enabling industrial partners to quickly determine which conductive fillers are best suited for the construction of conductive polymer composites.
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    The Use of Pulsed Electron Beams to Extend TEM Capabilities
    (2021-07) VandenBussche, Elisah
    Due to the high resolutions and high scattering cross-sections accessible using fast electrons, characterization of electronic materials using transmission electron microscopy (TEM) is indispensable in applied materials science and engineering. Modern TEMs are highly versatile, allowing high resolution characterization of structure and morphology, chemical analysis, beam-induced current mapping or even ultrafast structural dynamics at high spatial resolution. This dissertation discusses three ways in which pulsed electron beams can be used to extend these capabilities even further. First, the push to shrink dimensionality, and the subsequent impact of thermal effects, has led to the development of methods capable of being used to quantify nanoscale thermal transport. In order to use pulsed electron beams to determine transient temperatures at atomic length scales, it is possible to rely upon the Debye-Waller (DW) effect, in which the attenuation of Bragg scattering is related to atomic thermal energies. However, other factors, in addition to mean atomic displacements, can affect (and even dominate) the intensity of Bragg reflections, distorting the measurement. In this work, the degree to which structural specimen effects impact thermal measurements are quantitatively studied in order to better facilitate the use of pulsed electron beams to determine transient specimen temperatures. Second, the process by which photoexcited semiconductors return to the ground state consists of a series of strongly-correlated, many-body interactions which overlap in space and time. Such behaviors have both fundamental and practical implications, which include insights into the quantum nature of matter and control of device and materials behaviors in electronic and optoelectronic applications. Because the behavior of collective lattice oscillations depends on the structural and electronic properties of the material through which they propagate, coherent acoustic phonons (CAPs) resulting from such properties can be used as an intrinsic, multi-faceted characterization tool. Indeed, optical CAP spectroscopy has been used to access depth-dependent structural properties of materials and buried interfaces, but is limited in its in-plane spatial resolution. In this work, we demonstrate an analogous CAP spectroscopy using UEM, in which CAP phase velocity measured in real space is shown to be sensitive to changes in atomic structure. This facilitates future work in using UEM to access spatially-resolved information about buried structures and optoelectronic properties via high-resolution, real-space measurements. Third, the energy with which electrons propagate in the TEM column leads inevitably to damage, which particularly limits the study of radiation-soft materials such as organic materials, biological specimens, and any other samples containing low-Z atoms. In this work, we show that pulsed electron beams can be used to mitigate the damage caused to organic specimens compared to a stochastically-emitted electron beam. We demonstrate this effect both in the organic crystal hexatriacontane and in the hybrid metal halide perovskite, methylammonium lead iodide. Further, the temporal manipulation of pulsed beams can be leveraged to gain kinetic insights into the processes of beam damage. We show preliminary results in which the disparate damage rates in perovskites potentially elucidate a novel step in the damage mechanism.