Browsing by Author "Murayama, Mitsuhiro"

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    Abundance and Speciation of Surface Oxygen on Nanosized Platinum Catalysts and Effect on Catalytic Activity
    Serra-Maia, Rui; Winkler, Christopher; Murayama, Mitsuhiro; Tranhuu, Kevin; Michel, F. Marc (2018-06-18)
    Oxygen at the surface of nanosized platinum has a direct effect on catalytic activity of oxidation−reduction chemical reactions. However, the abundance and speciation of oxygen remain uncertain for platinum with different particle size and shape characteristics, which has hindered the development of fundamental property−activity relationships. We have characterized two commercially available platinum nanocatalysts known as Pt black and Pt nanopowder to evaluate the effects of synthesis and heating conditions on the physical and surface chemical properties, as well as on catalytic activity. Characterization using complementary electron microscopy, X-ray scattering, and spectroscopic methods showed that the larger average crystallite size of Pt nanopowder (23 nm) compared to Pt black (11 nm) corresponds with a 70% greater surface oxygen concentration. Heating the samples in air resulted in an increase in surface oxygen concentration for both nanocatalysts. Surface oxygen associated with platinum is in the form of chemisorbed oxygen, and no significant amounts of chemically bonded platinum oxide were found for any of the samples. The increase in surface oxygen abundance during heating depends on the initial size and surface oxygen content. Hydrogen peroxide decomposition rate measurements showed that larger particle size and higher surface chemisorbed oxygen correlate with enhanced catalytic activity. These results are particularly important for future studies that aim to relate the properties of platinum, or other metal nanocatalysts, with surface reactivity.
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    Application of Steepest-Entropy-Ascent Quantum Thermodynamics to Solid-State Phenomena
    Yamada, Ryo (Virginia Tech, 2018-11-16)
    Steepest-entropy-ascent quantum thermodynamics (SEAQT) is a mathematical and theoretical framework for intrinsic quantum thermodynamics (IQT), a unified theory of quantum mechanics and thermodynamics. In the theoretical framework, entropy is viewed as a measure of energy load sharing among available energy eigenlevels, and a unique relaxation path of a system from an initial non-equilibrium state to a stable equilibrium is determined from the greatest entropy generation viewpoint. The SEAQT modeling has seen a great development recently. However, the applications have mainly focused on gas phases, where a simple energy eigenstructure (a set of energy eigenlevels) can be constructed from appropriate quantum models by assuming that gas-particles behave independently. The focus of this research is to extend the applicability to solid phases, where interactions between constituent particles play a definitive role in their properties so that an energy eigenstructure becomes quite complicated and intractable from quantum models. To cope with the problem, a highly simplified energy eigenstructure (so-called ``pseudo-eigenstructure") of a condensed matter is constructed using a reduced-order method, where quantum models are replaced by typical solid-state models. The details of the approach are given and the method is applied to make kinetic predictions in various solid-state phenomena: the thermal expansion of silver, the magnetization of iron, and the continuous/discontinuous phase separation and ordering in binary alloys where a pseudo-eigenstructure is constructed using atomic/spin coupled oscillators or a mean-field approximation. In each application, the reliability of the approach is confirmed and the time-evolution processes are tracked from different initial states under varying conditions (including interactions with a heat reservoir and external magnetic field) using the SEAQT equation of motion derived for each specific application. Specifically, the SEAQT framework with a pseudo-eigenstructure successfully predicts: (i) lattice relaxations in any temperature range while accounting explicitly for anharmonic effects, (ii) low-temperature spin relaxations with fundamental descriptions of non-equilibrium temperature and magnetic field strength, and (iii) continuous and discontinuous mechanisms as well as concurrent ordering and phase separation mechanisms during the decomposition of solid-solutions.
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    Characterizing the Effects of Mechanical Alloying on Solid State Amorphization of Nanoscaled Multilayered Ni-Ti
    Monsegue, Niven (Virginia Tech, 2010-07-09)
    Equiatomic composition of Ni and Ti was cryomilled with varying milling times to create Ni-Ti lamella structures with average spacings of 50 nm, 470 nm, and 583 nm in powder particles to vary the interfacial surface area per volume. These surfaces form interfaces for diffusion that are essential for solid state amorphization during low temperature annealing. To compare solid state amorphization in a relatively defect free multilayer system, elemental Ni and Ti were deposited by electron beam physical vapor deposition on titanium plates with comparable spacing as above. Both milled and deposited multilayers were annealed between 225 and 350°C for up to 50 hours. X-ray diffraction characterization and in situ annealing was conducted on cryomilled and deposited multilayers of Ni-Ti. Based on this characterization, an amorphization model based on the Johnson-Mehl-Avrami nucleation and growth equation has been established to predict the amorphization of both cryomilled and deposited multilayers. Cryomilled powders experienced much larger amorphization rates during annealing than that of deposited multilayer structures, for all layer spacings. This superior amorphization is seen despite the formation of amorphous phase during the milling process; the amount of which increases with increasing milling time. The difference in amorphization rates between cryomilled and deposited multilayers is attributed to excess driving force due to the extensive preexisting defects in the powders caused by cryomilling. Serial 3D reconstruction of cryomilled Ni-Ti powders was done by scanning electron microscopy and focused ion beam. Through 3D reconstruction it was observed that a random and non-linear lamella structure has been formed in cryomilled powders. Furthermore, lamellar spacing was seen to become smaller with increased milling time while at the same time becoming more homogeneous through the material's volume. 3D reconstruction of cryomilled Ni-Ti offers a unique insight into the microstructures and surface areas of cryomilled powder particles that has never been accomplished.
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    A correlation between grain boundary character and deformation twin nucleation mechanism in coarse-grained high-Mn austenitic steel
    Hung, Chang-Yu; Bai, Yu; Shimokawa, Tomotsugu; Tsuji, Nobuhiro; Murayama, Mitsuhiro (Nature Research, 2021-04-19)
    In polycrystalline materials, grain boundaries are known to be a critical microstructural component controlling material’s mechanical properties, and their characters such as misorientation and crystallographic boundary planes would also influence the dislocation dynamics. Nevertheless, many of generally used mechanistic models for deformation twin nucleation in fcc metal do not take considerable care of the role of grain boundary characters. Here, we experimentally reveal that deformation twin nucleation occurs at an annealing twin (Σ3{111}) boundary in a high-Mn austenitic steel when dislocation pile-up at Σ3{111} boundary produced a local stress exceeding the twining stress, while no obvious local stress concentration was required at relatively high-energy grain boundaries such as Σ21 or Σ31. A periodic contrast reversal associated with a sequential stacking faults emission from Σ3{111} boundary was observed by in-situ transmission electron microscopy (TEM) deformation experiments, proving the successive layer-by-layer stacking fault emission was the deformation twin nucleation mechanism, different from the previously reported observations in the high-Mn steels. Since this is also true for the observed high Σ-value boundaries in this study, our observation demonstrates the practical importance of taking grain boundary characters into account to understand the deformation twin nucleation mechanism besides well-known factors such as stacking fault energy and grain size.
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    Deep learning-based noise filtering toward millisecond order imaging by using scanning transmission electron microscopy
    Ihara, Shiro; Saito, Hikaru; Yoshinaga, Mizumo; Avala, Lavakumar; Murayama, Mitsuhiro (Nature Portfolio, 2022-08-05)
    Application of scanning transmission electron microscopy (STEM) to in situ observation will be essential in the current and emerging data-driven materials science by taking STEM's high affinity with various analytical options into account. As is well known, STEM's image acquisition time needs to be further shortened to capture a targeted phenomenon in real-time as STEM's current temporal resolution is far below the conventional TEM's. However, rapid image acquisition in the millisecond per frame or faster generally causes image distortion, poor electron signals, and unidirectional blurring, which are obstacles for realizing video-rate STEM observation. Here we show an image correction framework integrating deep learning (DL)-based denoising and image distortion correction schemes optimized for STEM rapid image acquisition. By comparing a series of distortion corrected rapid scan images with corresponding regular scan speed images, the trained DL network is shown to remove not only the statistical noise but also the unidirectional blurring. This result demonstrates that rapid as well as high-quality image acquisition by STEM without hardware modification can be established by the DL. The DL-based noise filter could be applied to in-situ observation, such as dislocation activities under external stimuli, with high spatio-temporal resolution.
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    E-SEM Characterization of Escherichia coli Biofilms Grown on Copper- and Silver-Alloyed Stainless Steels over a 48 -
    McMullen, Amelia Marie (Virginia Tech, 2018-06-01)
    The formation of bacterial biofilms on surfaces and their subsequent biofouling pose extensive safe and healthy concerns to a variety of industries. Biofilms are ubiquitous, and the biofilm state is considered the default mode of growth for the majority of the world's bacteria population. Once mature, biofilms are difficult to remove completely and have improved resistance against antibacterial agents. Given this, there has been significant interest to mitigate or at least manage biofilm formation on surfaces. One such method has been through the material design of surfaces, and to the interest of this study, through the development of antimicrobial stainless steels. Stainless steel is not an inherently antimicrobial material. Stainless steels alloyed with small amounts of either copper (Cu) or silver (Ag), both well-known natural antimicrobial agents, have been investigated since their initial development in the late 1990's onward. This class of materials have been proven to show significant antimicrobial effect over their traditional counterparts without compromising the characteristic mechanical properties of the stainless steels. However, most of the antimicrobial assessments for these materials documented within literature are conducted over a 24-hour timeframe and do not adequately account for the biofilm mode of growth. As so, this study aimed to assess how biofilms grow on this class of antimicrobial steels over a longer duration of growth and under growth conditions which more adequately modeled the biofilm mode of life. The same strain of Escherichia coli commonly used in antimicrobial surface testing, ATCC 8739, was grown on submicron-polished coupons of a ferritic Cu-alloyed stainless steel (1.50 wt. % Cu), an austenitic Ag-alloyed stainless steel (0.042wt. % Ag), and a standard 304 series stainless steel, used as a baseline. Following ASTM-E2647-13, the E. coli/SS coupons were grown using a drip flow bioreactor under low shear conditions at either ambient temperature or 37 ± 3 degrees C with a batch phase of 6 hours and a continuous phase of 48 hours up to 96 hours. Directly after harvesting, the coupons were analyzed with an Environmental Scanning Electron Microscope (E-SEM) under low vacuum with a water vapor environment. The effect of surface chemistry and alloy microstructure, surface roughness, rinsing the surfaces prior to inoculation and after harvesting, temperature, and growth duration on the resulting E. coli biofilms were all investigated in some capacity. Growth on the submicron finished surfaces indicated there were no significant differences between the biofilms grown on the three different steel compositions. Bacterial attachment appeared non-preferential to surface chemistry or alloy microstructure, suggesting that E. coli interacted with the surfaces effectively the same under the given growth conditions. To account for apparent randomness in bacterial attachment, it is hypothesized that the surface features of interest were on a size scale irrelevant to the size of single bacterial cells. To account for the lack of an observed biocidal effect from the Cu- and Ag-alloyed stainless steels, it is hypothesized that an organic conditioning film which developed on the surfaces from the fluid environment may have effectively inhibited the release of Cu and Ag ions from the steel surfaces.
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    The Effect of Austenitization Temperature on the Microstructure, Bend Properties, and Hardness of a Chrome-Bearing White Cast Iron
    Adelmann, Joshua Thomas (Virginia Tech, 2024-04-25)
    The goal of this line of research is to add to the existing body of literature on the effect of heat treatments on the microstructure of chrome-bearing white irons and add bend test data to the literature concerning chrome-bearing white irons. This project was intended to support an existing line of research into cast metal-ceramic lattices using chrome white iron as a substitute for sintered ceramic tiles used to defeat projectiles. Chrome-bearing white irons have a substantial quantity of chromium carbides, giving them high hardness and abrasion resistance. Additionally, tiles cast from white iron proved to be more durable than sintered ceramic tiles, breaking into large chunks rather than a powder following an impact. These properties make tiles cast from chrome-bearing white iron a low-cost alternative to sintered carbides. The alloy investigated contained nominally 14 chromium and 3.2 carbon by weight percent. Three-point bend test specimens were cut from cast plates. These specimens were austenitized, air cooled, then tempered prior to three-point bend and hardness testing. The microstructure of the specimens was evaluated using optical microscopy, SEM, and XRD. This line of research revealed that lower austenitization temperatures resulted in a martensitic matrix with fewer, smaller secondary carbides, resulting in higher strength and hardness. In contrast, higher austenitization temperatures produced an austenitic matrix with coarser carbides, resulting in lower strength and hardness. This research did not reveal an appreciable change in ductility over heat treatment temperature.
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    Electron tomography imaging methods with diffraction contrast for materials research
    Hata, Satoshi; Furukawa, Hiromitsu; Gondo, Takashi; Hirakami, Daisuke; Horii, Noritaka; Ikeda, Ken-Ichi; Kawamoto, Katsumi; Kimura, Kosuke; Matsumura, Syo; Mitsuhara, Masatoshi; Miyazaki, Hiroya; Miyazaki, Shinsuke; Murayama, Mitsuhiro; Nakashima, Hideharu; Saito, Hikaru; Sakamoto, Masashi; Yamasaki, Shigeto (Oxford University Press, 2020-06-01)
    Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) enable the visualization of three-dimensional (3D) microstructures ranging from atomic to micrometer scales using 3D reconstruction techniques based on computed tomography algorithms. This 3D microscopy method is called electron tomography (ET) and has been utilized in the fields of materials science and engineering for more than two decades. Although atomic resolution is one of the current topics in ET research, the development and deployment of intermediate-resolution (non-atomic-resolution) ET imaging methods have garnered considerable attention from researchers. This research trend is probably not irrelevant due to the fact that the spatial resolution and functionality of 3D imaging methods of scanning electron microscopy (SEM) and X-ray microscopy have come to overlap with those of ET. In other words, there may be multiple ways to carry out 3D visualization using different microscopy methods for nanometer-scale objects in materials. From the above standpoint, this review paper aims to (i) describe the current status and issues of intermediate-resolution ET with regard to enhancing the effectiveness of TEM/STEM imaging and (ii) discuss promising applications of state-of-the-art intermediate-resolution ET for materials research with a particular focus on diffraction contrast ET for crystalline microstructures (superlattice domains and dislocations) including a demonstration of in situ dislocation tomography.
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    Energetic Considerations and Structural Characterization of Twinning in Nanowires
    Wu, Chun-Hsien (Virginia Tech, 2013-05-08)
    Twins are a pair of adjoining crystal grains related to each other by a special symmetry. They are frequently observed in bulk materials and nanomaterials. The formation of twins is an important topic in materials science and engineering because it affects material behaviors such as plastic deformation of metals, yield strength, and band gap energy in nanoscale semiconductors. Because of these unique phenomena and properties that the twinning can bring to the materials, it is of interest to investigate the formation of twins. Our primary objective in this dissertation is to study twinning in nanowires. Both gold and platinum <111> oriented nanowires were fabricated by similar solution-phase chemical synthesis methods. High-resolution transmission electron microscopy and electron diffraction patterns were carried out to analyze the structures of the nanowires. Nanodiffraction was used to demonstrate twinning is a general structural feature of the growth of gold nanowires growing in a <111> direction.  A model was proposed to explain the conditions under which twinning is energetically favored during nanowire growth. The model, which is based on a maximum rate hypothesis, considers the nanowire geometry and the relative surface and stacking fault energies and predicts twins should appear in gold nanowires but not in platinum nanowires, in agreement with experimental observations. During the structural characterization of gold nanowires, our interest is to resolve 3D structure of twinning. However, the structure of twinning in gold nanowires is very fine and the average spacing between twin boundaries is only 0.57nm (+/- 0.38 nm); therefore, regular 3D electron microscopy technique is unable to reconstruct these defected structures. Here we present a stereo vision technique to reconstruct 3D atomic non-periodic structures containing defects. The technique employs intrinsic atomic planes as epipolar planes to achieve the alignment accuracy needed to reconstruct a crystal with atomic resolution. We apply it to determine the 3D geometry and atomic arrangements of twinning in gold nanowire. In addition, an iterated cross-correlation algorithm was developed to analyze electron diffraction fully automatically to facilitate structural analysis of nanowires. A time-temperature-transformation diagram of platinum nanowires in chemical synthesis was determined to help optimize the fabrication process of the nanowires.
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    Five-second STEM dislocation tomography for 300 nm thick specimen assisted by deep-learning-based noise filtering
    Zhao, Yifang; Koike, Suguru; Nakama, Rikuto; Ihara, Shiro; Mitsuhara, Masatoshi; Murayama, Mitsuhiro; Hata, Satoshi; Saito, Hikaru (Nature Portfolio, 2021-10-26)
    Scanning transmission electron microscopy (STEM) is suitable for visualizing the inside of a relatively thick specimen than the conventional transmission electron microscopy, whose resolution is limited by the chromatic aberration of image forming lenses, and thus, the STEM mode has been employed frequently for computed electron tomography based three-dimensional (3D) structural characterization and combined with analytical methods such as annular dark field imaging or spectroscopies. However, the image quality of STEM is severely suffered by noise or artifacts especially when rapid imaging, in the order of millisecond per frame or faster, is pursued. Here we demonstrate a deep-learning-assisted rapid STEM tomography, which visualizes 3D dislocation arrangement only within five-second acquisition of all the tilt-series images even in a 300 nm thick steel specimen. The developed method offers a new platform for various in situ or operando 3D microanalyses in which dealing with relatively thick specimens or covering media like liquid cells are required.
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    Gaining new insights into nanoporous gold by mining and analysis of published images
    McCue, Ian; Stuckner, Joshua; Murayama, Mitsuhiro; Demkowicz, Michael J. (Springer Nature, 2018-04-30)
    One way of expediting materials development is to decrease the need for new experiments by making greater use of published literature. Here, we use data mining and automated image analysis to gather new insights on nanoporous gold (NPG) without conducting additional experiments or simulations. NPG is a three-dimensional porous network that has found applications in catalysis, sensing, and actuation. We assemble and analyze published images from among thousands of publications on NPG. These images allow us to infer a quantitative description of NPG coarsening as a function of time and temperature, including the coarsening exponent and activation energy. They also demonstrate that relative density and ligament size in NPG are not correlated, indicating that these microstructure features are independently tunable. Our investigation leads us to propose improved reporting guidelines that will enhance the utility of future publications in the field of dealloyed materials.
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    Half-Heusler Thermoelectric Materials and Modules
    Kang, Han-Byul (Virginia Tech, 2019-08-29)
    High temperature waste heat recovery has been gaining attention in recent years as it forms one of the largest sources of available energy. A rapid development of thermoelectric (TE) materials that can directly convert heat into electricity through the Seebeck effect, opens promising pathway for harvesting the thermal energy from the surroundings. In order to harvest the high-quality waste heat at elevated temperature, excellent thermal and mechanical stability of the TE materials is critical for a sustainable energy harvesting. In this respect, half-Heusler (hH) alloys are one of the promising high-temperature TE materials due to their high dimensionless thermoelectric figure of merit (zT) along with excellent mechanical and thermal stability. This dissertation demonstrates novel hH compositions and microstructures for the waste heat recovery systems. Focus in the thesis is on development of high performance hH TE materials with excellent in-air thermal stability at high temperatures (>700K). This will allow manufacturing of high efficiency and durable high temperature thermoelectric generators (TEGs). In chapter 3 and 4, a comprehensive optimization of n-type MNiSn and p-type MCoSb (M = Hf, Zr, and Ti) compounds is investigated through systematic control of processing parameters during melting and sintering. The synthesis conditions were controlled to achieve the phase purity, desired microstructure and the enhanced charge-carrier transport. Optimized n-type and p-type compositions are found to exhibit zTmax ~ 1 at 773 K. Chapter 5 describes breakthrough in decoupling of TE parameters in n-type half-Heusler (hH) alloys through multi-scale nanocomposite architecture with tungsten nanoinclusions. The tungsten nanoparticles not only assist electron injection, thereby improving electrical conductivity, but also enhance the Seebeck coefficient through energy filtering effect. The microstructure comprises of disordered phases with feature sizes at multiple length scales, which assists in effective scattering of heat-carrying phonons over diverse mean-free-path ranges. Cumulatively, these effects are shown to result in outstanding thermoelectric performance of zTmax ~ 1.4 at 773 K and zTavg ~ 0.93 between 300 and 973 K. In order to deploy TE materials into a thermal energy conversion device, it is essential to understand the transformation behavior under thermal cycling at high temperatures. In-air thermal stability of the hH compositions is demonstrated in chapter 6. All the optimized compositions are found to be stable below 673 K in-air condition. The n-type MNiSn and p-type NbFeSb compounds were found to show good thermal stability even at higher temperatures (>773K), whereas MCoSb compounds did not exhibit similar level of stability. Building upon the improved material performance and thermal stability, uni-coupled TE generators are demonstrated that exhibit high power density of 13.81 W⸱cm-2 and conversion efficiency of 10.9 % under a temperature difference of 674 K. The uni-couple TEG device shows stable performance for more than 150 hours at 873 K in air. These results are very promising for deployment of TE materials in waste heat recovery systems.
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    The Impact of Aluminum on the Initiation and Development of MnOx(s) Coatings for Manganese Removal
    Hinds, Gary Stephen (Virginia Tech, 2015-06-23)
    Many treatment facilities remove soluble Mn by an autocatalytic adsorption-oxidation process involving manganese oxide (MnOx(s))-coated filter media and a free chlorine residual known as the natural greensand effect (NGE). In recent years, significant amounts of aluminum (Al) have been found integrated into MnOx(s) coatings on media from drinking water treatment facilities worldwide. The primary objective of this study was to characterize MnOx(s) coatings developed in the presence and absence of Al, and to further define the role played by Al in the coatings' initiation and development. A secondary objective of the study was to examine the potential for pre-filter oxidation of Mn and formation of nano-size MnOx(s) particles, which would be destabilized by Al(OH)3(s) and captured in the filter. This material could act as a seed for coating formation and help explain the integration of Al into MnOx(s) coatings. Bench-scale column tests were conducted to examine Mn removal and backwash composition, while centrifugation and ultrafiltration were utilized to examine the potential for rapid Mn oxidation. Results indicate that the presence of Al augments the initiation and development of MnOx(s) coatings. The backwash solids of columns loaded with Al were composed of a mixture of Mn and Al, suggesting that active adsorption-oxidation sites were present in the Al(OH)3(s) floc captured by the filter. These results suggest at least a small amount of pre-filter MnOx(s) formation by contact with free chlorine; further, that Al(OH)3(s) solids present may destabilize these negatively charged solids into a form that is important to MnOx(s) coating formation.
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    In situ electron tomography for the thermally activated solid reaction of anaerobic nanoparticles
    Ihara, Shiro; Yoshinaga, Mizumo; Miyazaki, Hiroya; Wada, Kota; Hata, Satoshi; Saito, Hikaru; Murayama, Mitsuhiro (Royal Society Chemistry, 2023-06)
    The nanoscale characterization of thermally activated solid reactions plays a pivotal role in products manufactured by nanotechnology. Recently, in situ observation in transmission electron microscopy combined with electron tomography, namely four-dimensional observation for heat treatment of nanomaterials, has attracted great interest. However, because most nanomaterials are highly reactive, i.e., oxidation during transfer and electron beam irradiation would likely cause fatal artefacts; it is challenging to perform the artifact-free four-dimensional observation. Herein, we demonstrate our development of a novel in situ three-dimensional electron microscopy technique for thermally activated solid-state reaction processes in nanoparticles (NPs). The sintering behaviour of Cu NPs was successfully visualized and analyzed in four-dimensional space-time. An advanced image processing protocol and a newly designed state-of-the-art MEMS-based heating holder enable the implementation of considerably low electron dose imaging and prevent air exposure, which is of central importance in this type of observation. The total amount of electron dose for a single set of tilt-series images was reduced to 250 e(-) nm(-2), which is the lowest level for inorganic materials electron tomography experiments. This study evaluated the sintering behaviour of Cu NPs in terms of variations in neck growth and particle distance. A negative correlation between the two parameters is shown, except for the particle pair bound by neighbouring NPs. The nanoscale characteristic sintering behavior of neck growth was also captured in this study.
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    Influence of nucleobase stoichiometry on the self-assembly of ABC triblock copolymers
    Zhang, Keren; Talley, Samantha J.; Yu, Ya Peng; Moore, Robert Bowen; Murayama, Mitsuhiro; Long, Timothy E. (Royal Society of Chemistry, 2016-05-11)
    ABC triblock copolymers bearing adenine- and thymine-functionalized external blocks self-assembled into long-range, ordered lamellar microphase-separated morphologies on non-patterned substrates. Intermolecular hydrogen bonding formed thymine–adenine triplets and promoted self-assembly into well-defined lamellae consisting of poly(n-butyl acrylate) soft domains and complementary nucleobase hard domains, while thymine–adenine duplets contributed to superior mechanical properties.
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    Investigating the dislocation reactions on sigma 3{111} twin boundary during deformation twin nucleation process in an ultrafine-grained high-manganese steel
    Hung, Chang-Yu; Shimokawa, Tomotsugu; Bai, Yu; Tsuji, Nobuhiro; Murayama, Mitsuhiro (Nature Portfolio, 2021-09-29)
    Some of ultrafine-grained (UFG) metals including UFG twinning induced plasticity (TWIP) steels have been found to overcome the paradox of strength and ductility in metals benefiting from their unique deformation modes. Here, this study provides insights into the atomistic process of deformation twin nucleation at Σ3{111} twin boundaries, the dominant type of grain boundary in this UFG high manganese TWIP steel. In response to the applied tensile stresses, grain boundary sliding takes place which changes the structure of coherent Σ3{111} twin boundary from atomistically smooth to partly defective. High resolution transmission electron microscopy demonstrates that the formation of disconnection on Σ3{111} twin boundaries is associated with the motion of Shockley partial dislocations on the boundaries. The twin boundary disconnections act as preferential nucleation sites for deformation twin that is a characteristic difference from the coarse-grained counterpart, and is likely correlated with the lethargy of grain interior dislocation activities, frequently seen in UFG metals. The deformation twin nucleation behavior will be discussed based on in-situ TEM deformation experiments and nanoscale strain distribution analyses results.
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    Investigating the Effect of Austenite Grain Size and Grain Boundary Character on Deformation Twinning Behavior in A High-Manganese TWIP Steel: A TEM In-Situ Deformation Study
    Hung, Chang-Yu (Virginia Tech, 2021-06-16)
    Nanocrystalline metals exhibit a high strength/hardness but generally poor ductility during deformation regardless of their crystal structure which is often called the strength-ductility trade-off relationship and generally appears in most ultrafine-grained metals. The ultrafine-grained (UFG) high manganese austenitic twinning-induced plasticity (TWIP) steels have been found to overcome the strength-ductility trade-off but their underlying mechanism of discontinuous yielding behavior has not been well understood. In this study, our systematic TEM characterization suggests that the plastic deformation mechanisms in the early stage of deformation, around the macroscopic yield point, show an obvious association with grain size and nucleation of deformation twin was promoted rather than suppressed in UFG. More specifically, the main mechanism shifts from the conventional slip in grain interior to twinning nucleated from grain boundaries with decreasing the grain size down to less than 1 m. We also provide insights into the atomistic process of deformation twin nucleation at 3{111} twin boundaries, the dominant type of grain boundary in the UFG-TWIP steel of interest. In response to the external tensile stresses, the structure of coherent 3{111} twin boundary changes from atomistically smooth to partly defective by the grain boundary migration mechanism thus the "kink-like" defective step can act as a nucleation site for deformation twin, which deformation process is different from the one induced by dislocation pile-ups in coarse-grained counterparts and explain why UFG TWIP steel can retain the moderate ductility. In addition to the effect of grain size on deformation twin nucleation, grain boundary character was also taken into account. In coarse-grained TWIP steel, we experimentally reveal that deformation twin nucleation occurs at an annealing twin () boundary in a high-Mn austenitic steel when dislocation pile-up at boundary produced a local stress exceeding the twining stress, while no obvious local stress concentration was required at relatively high-energy grain boundaries such as or  A periodic contrast reversal associated with a sequential stacking faults emission from boundary was observed by in-situ transmission electron microscopy (TEM) deformation experiments, proving the successive layer-by-layer stacking fault emission was the deformation twin nucleation mechanism. The correlation between grain boundary character and deformation behavior was discussed both in low- and high-sigma value grain boundaries. On the other hand, localized strain concentration causes the nucleation of deformation twins at grain boundaries regardless of the grain boundary misorientation character in UFG TWIP steel. The invisibility of stacking fault (zero contrast) was also observed to be emitted at 3{111} boundaries in the coarse-grained TWIP steel, which deformation twin nucleation mechanism is found to be identical to UFG Fe-31Mn-3Si-3Al TWIP steel.
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    Investigating the origin of localized plastic deformation in nanoporous gold by in situ electron microscopy and automatic structure quantification
    Stuckner, Joshua Andrew (Virginia Tech, 2019-05-06)
    Gold gains many useful properties when it is formed into a nanoporous structure, but it also becomes macroscopically brittle due to flow localization and may therefore be unreliable for many applications. The goal of this work was to establish processing/structure/property relationships of nanoporous gold, discover controllable structure features, and understand the role of structure on flow localization. The nanoporous gold structure, consisting of a 3D network of nanoscale gold ligaments, was quantified with an automatic software developed for this work called AQUAMI, which uses computer vision techniques to make statistically reliable numbers of repeatable and unbiased measurements per image. AQUAMI increased the efficiency and accuracy of characterization in this work, allowed for the conduction of more experiments, and provided better confidence in morphology and size distribution of the complex NPG microstructural features. Nanoporous gold was synthesized while varying numerous processing factors such as dealloying time, annealing time, and mechanical agitation. Through the expanded scope of synthesis experiments and detailed analysis, it was discovered that the curvature of the ligaments and the distribution width of ligament diameters could be controlled through processing. In situ tensile experiments in SEM and TEM revealed that large ligaments arrested crack propagation while curved ligaments increase ductility by straightening in the tensile direction and forming geometrically required defects, which inhibit dislocation activity. Through synthesis and microstructure characterization, two new controllable structure features were discovered experimentally. In situ mechanical testing revealed the role these structures play on the deformation behavior and flow localization of nanoporous gold.
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    Investigation of the Magnetic Properties of Non-Thiolated Au Nano-Structures Grown by Laser Ablation
    Zhao, Chenlin (Virginia Tech, 2014-09-09)
    Although it is known that gold (Au) is diamagnetic in bulk form, it has been reported that Au displays magnetic properties when reduced to the nano-scale. Researchers found magnetism in Au nanoparticles (NPs) in a size range from 2 to 10 nanometers. Moreover, the Au nanoparticles are usually coated by thiol-containing organic caps, which are believed to be responsible for the magnetism. However, others suggest that organic capping is not necessary to observe magnetism in Au NPs, and magnetism may be an intrinsic property for nano-structured gold. For this investigation, we used pulsed laser deposition to prepare nano-structured gold of different sizes and concentrations to investigate the magnetic properties. Our experiment results confirmed that for the samples in which Au is in the metallic state as nanoparticles with ~5 nm diameter, as well as inthe alloy form, bonded with indium, the samples show ferromagnetism when embedded in an Al2O3 matrix without any thiol-containing organic capping. Our results suggest that ferromagnetism is an intrinsic property of Au nano-structures, which means that it is not necessary to incorporate Au-S bonds with organic coatings in order to observe this phenomenon. We believe due to the significant broken symmetry at the surface of the nanoparticles, holes are generated in d bands of the surface Au atoms. These holes are most possibly responsible for ferromagnetism in Au nanoparticles. The realization of magnetism in Au coupled with the lack of clear understanding of its origin makes the investigation of magnetism of diamagnetic metals ripe for further inquiry.
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    Magnetoelectric Composites for On-Chip Near-Resonance Applications
    Zhou, Yuan (Virginia Tech, 2014-09-08)
    Magnetoelectric (ME) effect is defined as the change in dielectric polarization (P) of a material under an applied magnetic field (H) or an induced magnetization (M) under an external electric field (E). ME materials have attracted number of investigators due to their potential for improving applications such as magnetic field sensors, filters, transformers, memory devices and energy harvesters. It has been shown both experimentally and theoretically that the composite structures consisting of piezoelectric and magnetostrictive phases possess stronger ME coupling in comparison to that of single phase materials. Giant magnetoelectric effect has been reported in variety of composites consisting of bulk-sized ME composites and thin film ME nanostructures. In this dissertation, novel ME composite systems are proposed, synthesized and characterized in both bulk and thin films to address the existing challenges in meeting the needs of practical applications. Two applications were the focused upon in this study, tunable transformer and dual phase energy harvester, where requirements can be summarized as: high ME coefficient under both on-resonance and off-resonance conditions, broad bandwidth, and low applied DC bias. In the first chapter, three challenges related to the conventional ME behavior in bulk ME composites have been addressed (1) The optimized ME coefficient can be achieved without external DC magnetic field by using a self-biased ME composite with a homogenous magnetostrictive material. The mechanism of such effect and its tunability are studied; (2) A near-flat ME response regardless of external magnetic field is obtained in a self-biased ME composite with geometry gradient structure; (3) By optimizing interfacial coupling with co-firing techniques, the ME coefficient can be dramatically enhanced. Theses co-fired ME laminates not only exhibit high coupling coefficient due to direct bonding, but also illustrate a self-biased effect due to the built-in stress during co-sintering process. These results present significant advancement toward the development of multifunctional ME devices since it eliminates the need for DC bias, expands the working bandwidth and enhances the ME voltage coefficient. Next, magnetoelectric nanocomposites were developed for understanding the nature of the growth of anisotropic thin film structures. In this chapter following aspects were addressed: (1) Controlled growth of nanostructures with well-defined morphology was obtained. Microstructure and surface morphology evolution of the piezoelectric BaTiO3 films was systematically analyzed. A growth model was proposed by considering the anisotropy of surface energy and the formation of twin lamellae structure within the frame work of Structure Zone Model (SZM) and Dynamic Scaling Theory (DST). In parallel to BaTiO3 films, well-ordered nanocomposite arrays [Pb1.1(Zr0.6Ti0.4)O3/CoFe2O4] with controlled grain orientation were developed and investigated by a novel hybrid deposition method. The influence of the pre-deposited template film orientation on the growth of ME composite array was studied. (2) PZT/CFO/PZT thick composite film and BTO/CFO thin film were synthesized using sol-gel deposition (SGD) and pulsed laser deposition (PLD) techniques, respectively. The HRTEM analysis revealed local microstructure at the interface of consecutive constituents. The interfacial property variation of these films was found to affect the coupling coefficient of corresponding ME nanocomposites. Subsequently, a novel complex three-dimensional ME composite with highly anisotropic structure was developed using a hybrid synthesis method. The influence of growth condition on the microstructure and property of the grown complex composites was studied. The film with highly anisotropic structure was found to possess tailored ferroelectric response indicating the promise of this synthesis method and microstructure. Based on the laminated ME composites, three types of ME tunable transformer designs were designed and fabricated. The goal was to develop a novel ME transformer with tunable performance (voltage gain and/or working resonance frequency) under applied DC magnetic field. Conventional ME transformers need either winding coil or large external magnetic field to achieve the tunable feature. Considering the high ME coupling of ME laminate, two ME transformers were developed by epoxy bonding Metglas with transversely/longitudinally poled piezoelectric ceramic transformer. The influence of different operation modes toward magnetoelectric tunability was analyzed. In addressing the concern of the epoxy bonding interface, a co-fired ME transformer with unique piezoelectric transformer/magnetostrictive layer/piezoelectric transformer trilayer structure was designed. The design and development strategy of thin film ME transformer was discussed to illustrate the potential for ME transformer miniaturization and on-chip integration. Lastly, motivated by the increasing demand of energy harvesting (EH) systems to support self-powered sensor nodes in structural health monitoring system, a magnetoelectric composite based energy harvester was developed. The development and design concept of the magnetoelectric energy harvester was systematically discussed. In particular, the first dual-phase self-biased ME energy harvester was designed which can simultaneously harness both vibration and stray magnetic field (Hac) in the absence of DC magnetic field. Strain distribution of the EH was simulated using the finite element model (FEM) at the first three resonance frequencies. Additionally, the potential of transferring this simple EH structure into MEMS scalable components was mentioned. These results provide significant advancement toward high energy density multimode energy harvesting system.