Advanced Transmission Electron Microscopy Research Articles

The formation and accumulation of radiation-induced defects and the role of lamellar interfaces in radiation damage of titanium aluminum alloy irradiated with Kr-ions at room temperature

A transmission electron microscopy (TEM) specimen of a titanium aluminide (TiAl) alloy was irradiated in-situ, at the IVEM-TANDEM facility, with 1 MeV Kr ions to a maximum fluence of 1.25 × 1019 ions m−2 at room temperature. The irradiated microstructure was then investigated ex-situ using advanced analytical TEM in combination with TEM image simulations and molecular dynamics (MD) simulations. The TEM examination showed that dot-like defects first formed in both the α2-Ti3Al and γ-TiAl phases of the irradiated microstructure. With increasing irradiation dose planar defects formed and propagated within the γ phase, and most of the planar defects were accompanied by the dot-like defects. The TEM image simulations and MD irradiation simulations revealed that the origins of the dot-like and planar defects were interstitial clusters and stacking faults, respectively, and large interstitial clusters were surrounded by dislocation loops. The interstitial clusters formed and grew in the irradiated microstructure due to much faster diffusion of interstitials than that of vacancies at room temperature. Local stress concentrations increased near large interstitial clusters and lamellar interfaces, which resulted in the nucleation and propagation of stacking faults. Moreover, the configurations of the lamellar interfaces in the start microstructure were found to play an important role in the formation and accumulation of the radiation-induced interstitial clusters and stacking faults at room temperature.

First Atomic-Scale Insight into Degradation in Lithium Iron Phosphate Cathodes by Transmission Electron Microscopy.

The capacity-voltage fade phenomenon in lithium iron phosphate (LiFePO4) lithium ion battery cathodes is not understood. We provide its first atomic-scale description, employing advanced transmission electron microscopy combined with electroanalysis and first-principles simulations. Cycling causes near-surface (∼30 nm) amorphization of the Olivine crystal structure, with isolated amorphous regions also being present deeper in the bulk crystal. Within this amorphous shell, some of the Fe2+ is transformed into Fe3+. Simulations predict that amorphization significantly impedes ion diffusion in LiFePO4 and even more severely in FePO4. The most significant barrier for ion transfer will be in the partially delithiated state due to the presence of FePO4, resulting in the inability to extract the remaining Li+ and the observed capacity fade. The pyrrole coating suppresses the dissolution of Fe and allows for extended retention of the Olivine structure. It also reduces the level of crossover of iron to the metal anode and stabilizes its solid electrolyte interphase, thus also contributing to the half-cell cycling stability.

(Invited) Phase Evolution and Interfacial Strain in Electrode Materials for Batteries

Advanced (scanning) transmission electron microscopy ((S)TEM) techniques have been intensively applied to study the materials for energy storage. With/Combining different TEM techniques, including in-situ TEM and diffraction, HAADF-STEM, and STEM-electron energy-loss spectroscopy, researchers are able to probe local structural and chemical information of electrode materials, at a resolution of nanoscale. This talk will focus on my recent work on using in-situ and analytical TEM to characterize the conversion-reaction oxide material (Fe3O4) for electrode of lithium ion batteries [1-3]. The dynamical process of the redox reaction of Fe3O4 revealed by in-situ TEM may help us to understand how reaction pathways affect the batteries’ kinetic properties. Reference s : [1] He K. et al., Visualizing non-equilibrium lithiation of spinel oxide via in situ transmission electron eicroscopy, Nature Communications, 7,11441 (2016)[2] Hwang S. et al., Strain coupling of conversion-type Fe3O4 thin film for lithium ion battery, Angewandte Chemie, 56,7813, (2017)[3] Li J. et al. Phase evolution of conversion-type electrode for lithium ion batteries, Nature Communications, 10,2224 (2019)

Direct visualization of the magnetostructural phase transition in nanoscale FeRh thin films using differential phase contrast imaging

To advance the use of thermally activated magnetic materials in device applications it is necessary to examine their behavior on the localized scale operando conditions. Equiatomic FeRh undergoes a magnetostructural transition from an antiferromagnetic (AF) to a ferromagnetic (FM) phase above room temperature (\ensuremath{\sim}350--380 K), and hence is considered a very desirable material for the next generation of nanomagnetic or spintronic devices. For this to be realized, we must fully understand the intricate details of the AF to FM transition and associated FM domain growth on the scale of their operation. Here we combine in situ heating with a comprehensive suite of advanced transmission electron microscopy techniques to investigate directly the magnetostructural transition in nanoscale FeRh thin films. Differential phase contrast imaging visualizes the stages of FM domain growth in both cross-sectional and planar FeRh thin films as a function of temperature. Small surface FM signals are also detected due to interfacial strain with the MgO substrate and Fe deficiency after HF etching of the substrate, providing a directional bias for FM domain growth. Our work provides high resolution imaging and quantitative measurements throughout the transition, which were previously inaccessible, and offers fundamental insight into their potential use in magnetic devices.

Site occupancy of alloying elements in γ′ phase of nickel-base single crystal superalloys

L12 structured γ′ phase is the main strengthening phase of Ni-base superalloys, which contain multiple alloying elements. Locations of these elements in the lattice are crucial for understanding the mechanical/physical properties of γ′ phases. Here, we reveal the site occupancy of various elements in γ′ phases of two multi-component Ni-base single crystal superalloys by using advanced scanning transmission electron microscopy and energy disperse spectroscopy. Cr, Mo, Re, Ta, W and Ti atoms preferentially locate at Al sublattices, and Co and Ru atoms prefer Ni sublattices. Moreover, the addition of Ru does not affect the site occupancy of other alloying elements.

Study on Ca Segregation toward an Epitaxial Interface between Bismuth Ferrite and Strontium Titanate.

Segregation is a crucial phenomenon, which has to be considered in functional material design. Segregation processes in perovskite oxides have been the subject of ongoing scientific interest, since they can lead to a modification of properties and a loss of functionality. Many studies in oxide thin films have focused on segregation toward the surface using a variety of surface-sensitive analysis techniques. In contrast, here we report a Ca segregation toward an in-plane compressively strained heterostructure interface in a Ca- and Mn-codoped bismuth ferrite film. We are using advanced transmission electron microscopy techniques, X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. Ca segregation is found to trigger atomic and electronic structure changes at the interface. This includes the reduction of the interface strain according to the Ca concentration gradient, interplanar spacing variations, and oxygen vacancies at the interface. The experimental results are supported by DFT calculations, which explore two segregation scenarios, i.e., one without oxygen vacancies and Fe oxidation from 3+ to 4+ and one with vacancies for charge compensation. Comparison with electron energy loss spectroscopy (EELS) measurements confirms the second segregation scenario with vacancy formation. The findings contribute to the understanding of segregation and indicate promising effects of a Ca-rich buffer layer in this heterostructure system.

TEM investigation of the role of the polycrystalline-silicon film/substrate interface in high quality radio frequency silicon substrates

The microstructural characteristics of two polycrystalline silicon (poly-Si) films with different electrical properties produced by low-pressure chemical vapour deposition on top of high resistivity silicon substrates were investigated by advanced transmission electron microscopy (TEM), including high resolution aberration corrected TEM and automated crystallographic orientation mapping in TEM. The results reveal that the nature of the poly-Si film/Si substrate interface is the main factor controlling the electrical resistivity of the poly-Si films. The high resistivity and high electrical linearity of poly-Si films are strongly promoted by the ∑3 twin type character of the poly-Si/Si substrate interface, leading to the generation of a huge amount of extended defects including stacking faults, ∑3 twin boundaries as well as ∑9 grain boundaries at this interface. Furthermore, a high density of interfacial dislocations has been observed at numerous common and more exotic grain boundaries deviating from their standard crystallographic planes. In contrast, poly-Si film/Si substrate interfaces with random character do not favour the formation of such complex patterns of defects, leading to poor electrical resistivity of the poly-Si film. This finding opens windows for the development of high resistivity silicon substrates for Radio Frequency (RF) integrated circuits (ICs) applications.

Anisotropic Defect Distribution in He+-Irradiated 4h-Sic: Effect of Stress on Defect Distribution

Irradiation-induced anisotropic swelling in hexagonal α-SiC is known to degrade the mechanical properties of SiC; however, the associated physical mechanism and microstructural process remain insufficiently understood. In this study, an anisotropic swelling condition where the surface normal direction was allowed to freely expand with constraint in the lateral direction was introduced in 4H-SiC using selected-area He + irradiation, and the internal defect distribution was investigated using transmission electron microscopy (TEM) and advanced scanning TEM. The defect distribution was compared to that in non-selected-area He + -irradiated 4H-SiC and electron-irradiated TEM-foil 4H-SiC. An anisotropic defect distribution was observed in the selected-area He + -ion-irradiated 4H-SiC, with interstitial defects preferentially redistributed in the surface normal direction ([0004]) and negative volume defects (such as vacancies and/or carbon antisite defects) dominantly located in the lateral directions ([ ] and [ ]). This anisotropy of the defect distribution was substantially lower in the non-selected-area He + -irradiated and electron-irradiated samples. The stress condition in the three samples was also measured and analyzed. In the selected-area He + -irradiated 4H-SiC, compressive stress was introduced in the lateral directions (([ ] and [ ])), with little stress introduced in the surface normal direction ([0004]); this stress condition was introduced at the beginning of ion irradiation. The compressive stress likely inhibits the formation of interstitial defects in the lateral directions, enhancing the anisotropy of the defect distribution in SiC.

Detailed investigation of the shearing mechanism of β" precipitates in Al-Mg-Si alloys

The mechanism behind shearing of β11 precipitates in Al-Mg-Si alloys during deformation is investigated by applying advanced transmission electron microscopy (TEM) techniques and frozen phonon multislice TEM image simulations on a selection of shearing configurations. In particular, the results indicate that the needle-like precipitates are sheared several times in single matrix Burgers vector steps. The multislice image simulations suggest that shearing events are most likely achieved in single Burgers vector steps, and there are some experimental evidence that the shearing planes are the matrix glide planes.

Evidence of Copper Separation in Lithiated Cu6Sn5 Lithium-Ion Battery Anodes

Intermetallics such as Cu6Sn5, NiSi2, and CuGa2 etc., are promising candidate materials to replace carbon-based lithium-ion battery anodes. However, the lithiation reactions of these anodes often involve the separation of the inactive phases, a slow process that retards the lithiation kinetics and deactivates their role as a stress buffer. This research visualizes the separated Cu in a lithiated Cu6Sn5 anode by advanced transmission electron microscopy techniques. Cu nanospheres of 3–4 nm are found homogeneously distributed in both Li(13+y)Sn5 and Li13Cu6Sn5 phases, suggesting that Cu is transported by long-range diffusion from the reaction site at the Li(13+y)Sn5/Li13Cu6Sn5 phase boundaries.

Self-assembled gold nanostructures in complex oxide thin films

Recently, noble metal- complex oxide nanocomposites with a high interface-to-volume ratio have attracted considerable interest for their potential applications in advanced devices. In this work, Nb:SrTiO3 epitaxial thin films embedded with different ratios of Au nanophase were fabricated using pulsed laser deposition. Detailed heterostructures at an atomic scale have been investigated based on advanced transmission electron microscopy. The results reveal that the pattern tunability of Au nanostructures can be realized by controlling the volume ratios between Au and Nb:SrTiO3, resulting in the manipulation of absorption characteristics in the visible light region. It is suggested that elastic energy and surface energy anisotropy are important factors to control the growth of the present Au/Nb:SrTiO3 heterostructures. This work offers a desirable platform to design nanocomposites with novel optical properties.

Dynamic imaging of Ostwald ripening in copper oxide nanoparticles by atomic resolution transmission Electron microscope

Structural evolution of copper oxide nanoparticles is examined, especially with respect to Ostwald ripening under electron beam irradiation. Dissolution of the smaller particles into the larger one was clearly observed at the atomic scale using advanced transmission electron microscope.

Nano-scale characterisation of sheared \u03b2\u201d precipitates in a deformed Al-Mg-Si alloy

This paper compares the nano-scale structure of β” precipitates in a peak-aged Al-Mg-Si alloy before and after deformation. Three complementary advanced transmission electron microscopy techniques are used to reveal the structures and elucidate the interaction between dislocations and β” precipitates. We show that the needle-like and semi-coherent β” precipitates are sheared several times on different planes by dislocations during deformation, with no indications that they are bypassed or looped. Our results show that dislocations cut through precipitates and leave behind planar defects lying on planes inclined to 〈100〉 directions inside the precipitates. The results also indicate that precipitates are sheared in single steps, and the implication of this observation is discussed in terms of slip behaviour.

Elucidating the microstructural development of refractory metal high entropy superalloys via the Ti–Ta–Zr constituent system

The recently developed Refractory Metal High Entropy Superalloys have the potential to replace Ni-based alloys in very high temperature structural applications. However, the microstructures of these new alloys typically consist of refractory metal based solid solution precipitates within an ordered superlattice structured matrix, which is likely to compromise key properties such as toughness. As such, there is significant interest in inverting this arrangement, such that superlattice precipitates form within a disordered refractory metal matrix. Yet the mechanisms by which these microstructures form and how they might be modified with compositional variations are currently unclear. To elucidate these mechanisms, the microstructural evolution of a series of compositionally simpler alloys from the Ti–Ta–Zr system have been studied following long term exposures at 700, 900 and 1000 °C. Exposures of up to 1000 h were used as a proxy to equilibrium and the resulting microstructures were analysed using advanced scanning and transmission electron microscopy methods. The microstructures of these alloys were found to predominantly contain one or two bcc phases, the lengthscale and morphology of which changed with exposure temperature. From these results it is established that the fine-scale microstructure, which is very similar to that widely reported in the more compositionally complex refractory metal high entropy superalloys, forms via spinodal decomposition during cooling. It is also shown, for the first time, how compositional modification can lead to a refractory metal solid solution based matrix. It is believed that these results provide key insights that can guide further development in the more complex systems that will be required for commercial applications.

Uncompensated Polarization in Incommensurate Modulations of Perovskite Antiferroelectrics.

Complex polar structures of incommensurate modulations (ICMs) are revealed in chemically modified PbZrO_{3} perovskite antiferroelectrics using advanced transmission electron microscopy techniques. The Pb-cation displacements, previously assumed to arrange in a fully compensated antiparallel fashion, are found to be either antiparallel, but with different magnitudes, or in a nearly orthogonal arrangement in adjacent stripes in the ICMs. Abinitio calculations corroborate the low-energy state of these arrangements. Our discovery corrects the atomic understanding of ICMs in PbZrO_{3}-based perovskite antiferroelectrics.

Tailoring the surface structures of iron oxide nanorods to support Au nanoparticles for CO oxidation

AbstractIron oxide supported Au nanomaterials are one of the most studied catalysts for low-temperature CO oxidation. Catalytic performance not only critically depends on the size of the supported Au nanoparticles (NPs) but also strongly on the chemical nature of the iron oxide. In this study, Au NPs supported on iron oxide nanorods with different surface properties through β-FeOOH annealing, at varying temperatures, were synthesized, and applied in the CO oxidation. Detailed characterizations of the interactions between Au NPs and iron oxides were obtained by X-ray diffraction, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy. The results indicate that the surface hydroxyl group on the Au/FeOOH catalyst, before calcination (Au/FeOOH-fresh), could facilitate the oxygen adsorption and dissociation on positively charged Au, thereby contributing to the low-temperature CO oxidation reactivity. After calcination at 200 °C, under air exposure, the chemical state of the supported Au NP on varied iron oxides partly changed from metal cation to Au0 along with the disappearance of the surface OH species. Au/FeOOH with the highest Au0 content exhibits the highest activity in CO oxidation, among the as-synthesized catalysts. Furthermore, good durability in CO oxidation was achieved over the Au/FeOOH catalyst for 12 h without observable deactivation. In addition, the advanced identical-location TEM method was applied to the gas phase reaction to probe the structure evolution of the Au/iron oxide series of the catalysts and support structure. A Au NP size-dependent Ostwald ripening process mediated by the transport of Au(CO)x mobile species under certain reaction conditions is proposed, which offers a new insight into the validity of the structure-performance relationship.

The grain‐boundary resistance of CeO2 ceramics: A combined microscopy‐spectroscopy‐simulation study of a dilute solution

AbstractWeakly acceptor‐doped ceria ceramics were characterized structurally and compositionally with advanced transmission electron microscopy (TEM) techniques and electrically with electrochemical impedance spectroscopy (EIS). The grain boundaries studied with TEM were found to be free of second phases. The impedance spectra, acquired in the range 703 ≤ T/K ≤ 893 in air, showed several arcs that were analyzed in terms of bulk, grain‐boundary, and electrode responses. We ascribed the grain‐boundary resistance to the presence of space‐charge layers. Continuum‐level simulations were used to calculate charge‐carrier distributions (of acceptor cations, oxygen vacancies, and electrons) in these space‐charge layers. The acceptor cations were assumed to be mobile at high (sintering) temperatures but immobile at the temperatures of the EIS measurements. Space‐charge formation was assumed to be driven by the segregation of oxygen vacancies to the grain‐boundary core. Comparisons of data from the simulations and from the EIS measurements yielded space‐charge potentials and the segregation energy of vacancies to the grain‐boundary core. The space‐charge potentials from the simulations are compared with values obtained by applying the standard, analytical (Mott–Schottky and Gouy–Chapman) expressions. The importance of modelling space‐charge layers from the thermodynamic level is demonstrated.

Oxidation-Related Microstructural Changes at a Crack Tip in Waspaloy After Elevated-Temperature Dwell-Fatigue Testing

Using advanced aberration-corrected scanning transmission electron microscopy coupled with high-resolution energy-dispersive spectroscopy techniques, transitions to microstructure and composition occurring at a grain boundary (GB) crack-tip in Waspaloy after dwell-fatigue at 704 °C are characterized at the nano to atomic scale. The simple microstructure of this γ/γ′ superalloy, which enables elucidation of oxygen effect ahead of the oxide intrusion—dissolution of the precipitates fostering selective segregation of Al to the GB to form oxides—has been experimentally observed.

Identification of interstratified mica and pyrophyllite monolayers within chlorite using advanced scanning/transmission electron microscopy

Abstract Interstratified clay minerals reflect the weathering degree and record climatic conditions and the pedogenic processes in the soil. It is hard to distinguish a few layers of interstratified clay minerals from the chlorite matrix, due to their similar two-dimensional tetrahedral-octahedral-tetrahedral (TOT) structure and electron-beam sensitive nature during transmission electron microscopy (TEM) imaging. Here, we used multiple advanced TEM techniques including low-dose high-resolution TEM (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging combined with energy-dispersive spectroscopic (EDS) mapping to study interstratified layers in a chlo-rite sample from Changping, Beijing, China. We demonstrated an interstratified mica or pyrophyllite monolayer could be well distinguished from the chlorite matrix by projected atomic structures, lattice spacings, and chemical compositions with advanced TEM techniques. Further investigation showed two different transformation mechanisms from mica or pyrophyllite to chlorite: either a 4 Å increase or decrease in the lattice spacing. This characterization approach can be extended to the studies of other electron-beam sensitive minerals.

Unraveling Structural Information of Turkevich Synthesized Plasmonic Gold-Silver Bimetallic Nanoparticles.

For the synthesis of gold-silver bimetallic nanoparticles, the Turkevich method has been the state-of-the-art method for several decades. It is presumed that this procedure results in a homogeneous alloy, although this has been debatable for many years. In this work, it is shown that neither a full alloy, nor a perfect core-shell particle is formed but rather a core-shell-like particle with altering metal composition along the radial direction. In-depth wet-chemical experiments are performed in combination with advanced transmission electron microscopy, including energy-dispersive X-ray tomography, and finite element method modeling to support the observations. From the electron tomography results, the core-shell structure can be clearly visualized and the spatial distribution of gold and silver atoms can be quantified. Theoretical simulations are performed to demonstrate that even though UV-vis spectra show only one plasmon band, this still originates from core-shell type structures. The simulations also indicate that the core-shell morphology does not so much affect the location of the plasmon band, but mainly results in significant band broadening. Wet-chemistry experiments provide the evidence that the synthesis pathway starts with gold enriched alloy cores, and later on in the synthesis mainly silver is incorporated to end up with a silver enriched alloy shell.