Advanced Electron Microscopy Techniques Research Articles

Lead-induced stress corrosion cracking (PbSCC) initiation in alloy 690TT in caustic environment

Pb-Caustic Stress Corrosion Cracking initiation was studied on Alloy 690TT at 315 °C. Advanced electron microscopy techniques revealed that cracking mainly initiated intergranularly following the oxidation of Cr-rich grain boundary carbides. This finding explains the higher resistance to crack nucleation of the as-received surfaces compared to polished ones, because the grain boundaries of the ultrafine-grained near-surface layer were not decorated with carbides. It is also proposed that dislocation entanglements, typical of a cold-worked surface, attenuated the emission of dislocations at the surface. Once initiated, cracking continued due to Pb incorporation in the oxide, which weakened the passive layer and enhanced slip dissolution.

On the bending of MS1-P20 hybrid steels additively manufactured using laser powder bed fusion

Maraging steel (MS1)-tool steel (P20) bimetals additively manufactured using the laser powder bed fusion technique were studied under different heat treatment cycles and loading conditions. The hardening of P20 and aging of MS1 were performed sequentially on the hybrid samples. The interfacial characteristics and microstructural evolution of the bulk materials were studied using various advanced electron microscopy techniques. The post-processing procedures successfully produced a uniform martensitic structure throughout the MS1-P20 hybrid steels, leading to a less detectable interface under electron backscatter diffraction (EBSD) imaging. The mechanical performance of heat-treated hybrid steels was evaluated using complex loading conditions. 3-point and 4-point bending tests were performed to assess the impact of heat treatments on the mechanical performance of the hybrid steels. The heat-treated samples exhibited higher strength with relatively homogeneous hardness variations and deformed more uniformly in bending conditions.

Effect of magnesium doping on discontinuous precipitation in age-hardenable copper–titanium alloys

One of the most important goals in the development of Cu-based alloys for electrical engineering applications is the realization of materials with both high mechanical strength and electrical conductivity. Cu-Ti alloys are promising alternatives to Cu-Be alloys, which are expensive and can be hazardous to health. Herein, an age-hardenable binary Cu96Ti4 alloy and its Mg-doped analog were compared by advanced electron microscopy techniques. The mechanical properties of the Mg-doped alloy are superior to those of the binary alloy. The effects of Mg-doping on the microstructure are investigated by atomic-scale scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy. The Cu96Ti4 (at.%) solid-solution alloy developed lamellar aggregates when subjected to prolonged aging treatment at 450 °C for 100 h owing to discontinuous precipitation (DP) along the grain boundaries. The DP consisted of a face-centered cubic (FCC) Cu solid-solution phase (denoted as α-Cuss) and a stable Cu4Ti intermetallic phase (β-Cu4Ti). Conversely, while the Cu94Ti4Mg2 counterpart had similar lamellar aggregates, the amount of these aggregates was reduced, and no β-Cu4Ti phase was observed. The lamellar in the Cu94Ti4Mg2 counterpart consisted of two distinct FCC Cuss phases with different Ti-solute contents. Accordingly, the possible effects of Mg doping on the local structures of the DP in the Cu-Ti alloy are explained, and the structural characteristics of the DP that may result in improved mechanical properties is discussed.

Co-localizing Kelvin Probe Force Microscopy with Other Microscopies and Spectroscopies: Selected Applications in Corrosion Characterization of Alloys.

Kelvin probe force microscopy (KPFM), sometimes referred to as surface potential microscopy, is the nanoscale version of the venerable scanning Kelvin probe, both of which measure the Volta potential difference (VPD) between an oscillating probe tip and a sample surface by applying a nulling voltage equal in magnitude but opposite in sign to the tip-sample potential difference. By scanning a conductive KPFM probe over a sample surface, nanoscale variations in surface topography and potential can be mapped, identifying likely anodic and cathodic regions, as well as quantifying the inherent material driving force for galvanic corrosion. Subsequent co-localization of KPFM Volta potential maps with advanced scanning electron microscopy (SEM) techniques, including back scattered electron (BSE) images, energy dispersive spectroscopy (EDS) elemental composition maps, and electron backscattered diffraction (EBSD) inverse pole figures can provide further insight into structure-property-performance relationships. Here, the results of several studies co-localizing KPFM with SEM on a wide variety of alloys of technological interest are presented, demonstrating the utility of combining these techniques at the nanoscale to elucidate corrosion initiation and propagation. Important points to consider and potential pitfalls to avoid in such investigations are also highlighted: in particular, probe calibration and the potential confounding effects on the measured VPDs of the testing environment and sample surface, including ambient humidity (i.e., adsorbed water), surface reactions/oxidation, and polishing debris or other contaminants. Additionally, an example is provided of co-localizing a third technique, scanning confocal Raman microscopy, to demonstrate the general applicability and utility of the co-localization method to provide further structural insight beyond that afforded by electron microscopy-based techniques.

Understanding the structural dynamics of electrocatalysts via liquid cell transmission electron microscopy

High-performance catalysts are essential for many electrochemical technologies, such as water splitting, fuel cells and electrosynthesis. Understanding the structure–performance correlation of the catalysts demands a real-time characterization of their structural evolutions under work conditions. Herein, we review recent advances in structural dynamics of electrocatalysts during reactions by highlighting the utilization of advanced liquid cell transmission electron microscopy techniques. The processes of corrosion, Ostwald ripening, and particle agglomeration are discussed. By incorporating the synchronous electrochemical measurements in the liquid cell, unique information may be retrieved toward improving the electrocatalytic performance.

Off-axis electron holography for the direct visualization of perpendicular shape anisotropy in nanoscale 3D magnetic random-access-memory devices

Perpendicular shape anisotropy (PSA) and double magnetic tunnel junctions (DMTJs) offer practical solutions to downscale spin-transfer-torque Magnetic Random-Access Memory (STT-MRAM) beyond 20 nm technology nodes, while retaining their thermal stability and reducing critical currents applied. However, as these modern devices become smaller and three-dimensionally (3D) complex, our understanding of their functional magnetic behavior is often indirect, relying on magnetoresistance measurements and micromagnetic modeling. In this paper, we review recent work that was performed on these structures using a range of advanced electron microscopy techniques, focusing on aspects specific to the 3D and nanoscale nature of such elements. We present the methodology for the systematic transfer of individual SST-MRAM nano-pillars from large-scale arrays to image their magnetic configurations directly using off-axis electron holography. We show that improved phase sensitivity through stacking of electron holograms can be used to image subtle variations in DMTJs and the thermal stability of <20 nm PSA-STT-MRAM nano-pillars during in situ heating. The experimental practicalities, benefits, and limits of using electron holography for the analysis of MRAM devices are discussed, unlocking practical pathways for direct imaging of the functional magnetic performance of these systems with high spatial resolution and sensitivity.

The in situ generated emerging phase inside dual phase oxygen transport membranes

The in situ generated emerging phase inside the dual-phase oxygen transport membranes (DP-OTMs) plays a crucial role in boosting the overall performance of DP-OTMs. However, its detailed structure and properties are still not fully understood. Utilizing advanced transmission electron microscopy (TEM) techniques, the emerging phase GdxCe1-xFeyCo1-yO3-δ (GCFCO) inside the CexGd1-xO2-δ-FeCo2O4 (CGO-FC2O) OTMs was successfully characterized at the atomic scale. The newly formed GCFCO is primarily surrounded by the CGO, and contributes to a significant reduction of non-solute segregation at the CGO grain boundaries. Electronic characteristics of the GCFCO shows a sensitive dependence on its chemical composition, including the valence state of Ce and Fe as well as the oxygen vacancies. Additional CGO-GCFCO interfaces were introduced, where almost intact crystal structures were observed with slight Gd and Co segregation ∼1 nm at the edges. Approaching the interface, on the CGO side, only a minimum drop of the Ce valence was determined. On the GCFCO side, mixed Ce3+ and Ce4+ are partially occupying the Gd sites, while Fe and Co valence stay constant until the edge. Our study provides novel insight into the phase information within CGO-FC2O composites, which paves the path towards superior performance of various DP-OTMs.

Crystallization of Ge-Rich GeSbTe Alloys: The Riddle Is Solved

Among the phase-change materials, Ge-rich GeSbTe (GST) alloys are of considerable interest as they offer a much higher thermal stability than their congruent contenders, a desirable characteristic for embedded digital memories and neuromorphic devices. Up to now, the mechanisms by which such alloys crystallize and progressively switch from one resistivity state to the other remain unclear and very controversial. Using in situ synchrotron X-ray diffraction during isothermal annealing and advanced transmission electron microscopy techniques, we solve this riddle and unveil the mechanisms leading to the overall crystallization of such alloys. During annealing at 310 °C, the initially homogeneous and amorphous material undergoes a progressive phase separation, leading to the formation of Ge-rich regions of different compositions. During this decomposition, the first formed GeTe embryos crystallize and trigger the heterogeneous crystallization of the Ge cubic phase. As the phase separation proceeds, these embryos dissolve and the Ge phase gradually builds up through the nucleation of small grains. Only when this Ge cubic phase is largely formed, the remaining amorphous matrix may locally reach the Ge2Sb2Te5 composition at which it can crystallize as large grains. Our density functional theory calculations confirm that the quite exotic Pnma GeTe structure we have experimentally identified is more stable than the regular R3m structure at nanometric sizes.

Ca Solubility in a BiFeO3-Based System with a Secondary Bi2O3 Phase on a Nanoscale.

In BiFeO3 (BFO), Bi2O3 (BO) is a known secondary phase, which can appear under certain growth conditions. However, BO is not just an unwanted parasitic phase but can be used to create the super-tetragonal BFO phase in films on substrates, which would otherwise grow in the regular rhombohedral phase (R-phase). The super-tetragonal BFO phase has the advantage of a much larger ferroelectric polarization of 130–150 μC/cm2, which is around 1.5 times the value of the rhombohedral phase with 80–100 μC/cm2. Here, we report that the solubility of Ca, which is a common dopant of bismuth ferrite materials to tune their properties, is significantly lower in the secondary BO phase than in the observed R-phase BFO. Starting from the film growth, this leads to completely different Ca concentrations in the two phases. We show this with advanced analytical transmission electron microscopy techniques and confirm the experimental results with density functional theory (DFT) calculations. At the film’s fabrication temperature, caused by different solubilities, about 50 times higher Ca concentration is expected in the BFO phase than in the secondary one. Depending on the cooling rate after fabrication, this can further increase since a larger Ca concentration difference is expected at lower temperatures. When fabricating functional devices using Ca doping and the secondary BO phase, the difference in solubility must be considered because, depending on the ratio of the BO phase, the Ca concentration in the BFO phase can become much higher than intended. This can be critical for the intended device functionality because the Ca concentration strongly influences and modifies the BFO properties.

Structure of the active pharmaceutical ingredient bismuth subsalicylate

Structure determination of pharmaceutical compounds is invaluable for drug development but remains challenging for those that form as small crystals with defects. Bismuth subsalicylate, among the most commercially significant bismuth compounds, is an active ingredient in over-the-counter medications such as Pepto-Bismol, used to treat dyspepsia and H. pylori infections. Despite its century-long history, the structure of bismuth subsalicylate is still under debate. Here we show that advanced electron microscopy techniques, namely three-dimensional electron diffraction and scanning transmission electron microscopy, can give insight into the structure of active pharmaceutical ingredients that are difficult to characterize using conventional methods due to their small size or intricate structural features. Hierarchical clustering analysis of three-dimensional electron diffraction data from ordered crystals of bismuth subsalicylate revealed a layered structure. A detailed investigation using high-resolution scanning transmission electron microscopy showed variations in the stacking of layers, the presence of which has likely hindered structure solution by other means. Together, these modern electron crystallography techniques provide a toolbox for structure determination of active pharmaceutical ingredients and drug discovery, demonstrated by this study of bismuth subsalicylate.

Precious metals can be green

In this chapter, a general view of metallic alloys nanoparticles from the atomic-scale view using advanced transmission electron microscopy techniques such as aberration-corrected electron microscopy (Cs-corrected) and high-resolution electron microscopy, is covered. It is well established that catalytic activity of metal nanoparticles is increased by roughness at the atomic scale. Indeed a flat smooth surface will be less active than its stepped counterpart. Metal nanoparticles have in most cases facets with different Miller indexes. However, the control of the shape and crystal structure is not easy to achieve. The possibility of passivizing the nanoparticle surface with organic molecules is a way to control the nanoparticle shape. Using separation techniques such as electrophoresis, it is possible to obtain a batch of particles with the same mass in average and hence the same number of atoms. Passivized particles still have enough free sites to allow catalytic activity. Then, it is possible by using aberration-corrected electron microscopy to obtain the structure and surface structure of nanoparticles. In this work, we will report those methods and demonstrate its application for mono- and multimetallic clusters. The surface of the metallic nanoparticles plays a crucial role in the catalytic activity and interaction between elements in the surface and in the core of the core-shell structures. In this way, the analysis of chemical composition and chemical bonding needs to be obtained using analytic techniques such as X-ray energy-dispersive spectroscopy and electron energy-loss spectroscopy. Herein, we also present quantitative analysis of particles with few atoms by atomic counting using a statistical method applied in Cs-corrected high-angle annular dark-field scanning transmission electron microscopy images.

Post-irradiation characterization of a high burnup mixed oxide fuel rod with minor actinides

The Advanced Fuels Campaign performed a series of irradiation tests of minor actinide-bearing mixed oxide fuel (MA-MOX), the so-called AFC-2C&D experiments, to investigate the transmutation of long-lived transuranic actinide isotopes contained in spent nuclear fuel via fast reactor technology at burnups exceeding 10 % fission of initial metallic atoms. This manuscript reports the test results derived from one of the five MA-MOX rodlets taken to higher burnup in the AFC-2D irradiation. This includes both non-destructive investigations, such as gamma and neutron spectrometry, and destructive investigations, such as fission gas release, ceramography, and chemical burnup analysis. In addition, the microstructure of the fuel was investigated using advanced electron microscopy techniques including electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). It was observed with EBSD that the pellet had subdivision of the grains and the TEM observed migration of cladding material into the 5 metal precipitates in the fuel which could have been from the higher than desired oxygen/metal ratio. The TEM also showed an enrichment of Cr in fuel clad chemical interaction (FCCI) layer.

Tuning the ceria interfaces inside the dual phase oxygen transport membranes

Dual-phase oxygen transport membranes (DP-OTMs) offer a good alternative for the separation of oxygen from gas mixtures. As a promising candidate, the overall performance of the fluorite-spinel composite CexGd1-xO2-δ-FeyCo3-yO4 (CGO-FCO) is often dominated by the behavior of the CGO-CGO grain boundaries (GBs), such as solute and non-solute segregations and structural disorders within the boundary. Due to the largely unknown atomistic GB environment, any attempt to control the GBs for an optimized membrane performance is still severely limited. In order to bridge this essential gap, utilizing advanced transmission electron microscopy (TEM) techniques, we quantified both atomistic and chemical structures at the CGO GBs inside the CGO-FCO DP-OTMs down to sub-nm scale. The atomic-site specific lattice distortions, elemental distributions, and valence state variations were found to be well confined within ∼2 nm around the GB, where a clear dependence on the structural coherence of individual GBs can be established. Our results further unraveled the complicated CGO GB environment inside real DP-OTMs, and paved the path towards optimized processing of various types of functional membranes.

Challenges of Overcoming Defects in Wide Bandgap Semiconductor Power Electronics

The role of crystal defects in wide bandgap semiconductors and dielectrics under extreme environments (high temperature, high electric and magnetic fields, intense radiation, and mechanical stresses) found in power electronics is reviewed. Understanding defects requires real-time in situ material characterization during material synthesis and when the material is subjected to extreme environmental stress. Wide bandgap semiconductor devices are reviewed from the point of view of the role of defects and their impact on performance. It is shown that the reduction of defects represents a fundamental breakthrough that will enable wide bandgap (WBG) semiconductors to reach full potential. The main emphasis of the present review is to understand defect dynamics in WBG semiconductor bulk and at interfaces during the material synthesis and when subjected to extreme environments. High-brightness X-rays from synchrotron sources and advanced electron microscopy techniques are used for atomic-level material probing to understand and optimize the genesis and movement of crystal defects during material synthesis and extreme environmental stress. Strongly linked multi-scale modeling provides a deeper understanding of defect formation and defect dynamics in extreme environments.

Polyallylamine assisted synthesis of 3D branched AuNPs with plasmon tunability in the vis-NIR region as refractive index sensitivity probes

This paper describes the synthesis of highly branched gold nanoparticles (AuNPs) through a facile seeded growth approach using poly(allylamine hydrochloride) (PAH) as shape inducing agent. The obtained branched AuNPs present highly tunable optical properties in the Vis-NIR region from ca. 560 nm to 1260 nm. We controlled the morphology, and therefore the optical response, of the NPs by either changing the gold salt to seeds ratio or by fine-tuning the solution pH. We proposed that the formation of size-dependent PAH-AuCl4- aggregates as demonstrated by dynamic light scattering measurements, together with pH-dependent gold salt speciation might be responsible for the branched morphology. Advanced electron microscopy techniques demonstrated the polycrystalline nature of the AuNPs and facilitated a better understanding of branched morphology. Additionally, the refractive index sensitivity estimated by the inflection point of the Localized Surface Plasmon Resonance (LSPR) band can be controlled by tuning the nanoparticle branching. Furthermore, the versatility of the PAH chemistry allowed the easy functionalization of the synthesized NPs.

3D Binary Mesocrystals from Anisotropic Nanoparticles.

Binary mesocrystals offer the combination of nanocrystal properties in an ordered superstructure. Here, we demonstrate the simultaneous self‐assembly of platinum and iron oxide nanocubes into micrometer‐sized 3D mesocrystals using the gas‐phase diffusion technique. By the addition of minor amounts of a secondary particle type tailored to nearly identical size, shape and surface chemistry, we were able to promote a random incorporation of foreign particles into a self‐assembling host lattice. The random distribution of the binary particle types on the surface and within its bulk has been visualized using advanced transmission and scanning electron microscopy techniques. The 20–40 μm sized binary mesocrystals have been further characterized through wide and small angle scattering techniques to reveal a long‐range ordering on the atomic scale throughout the crystal while showing clear evidence that the material consists of individual building blocks. Through careful adjustments of the crystallization parameters, we could further obtain a reverse superstructure, where incorporated particles and host lattice switch roles.

Quantifying inter-organelle membrane contact sites using proximity ligation assay in fixed optic nerve sections

Membrane contact sites (MCS) play crucial roles in cell physiology with dysfunction in several MCS proteins being linked with neurological and optic nerve diseases. Although there have been significant advances in imaging these interactions over the past two decades with advanced electron microscopy techniques, super-resolution imaging and proximity-dependent fluorescent reporters, a technique to observe and quantify MCS in mammalian optic nerve tissues has not yet been reported. We demonstrate for the first time that proximity ligation assay (PLA), a technique already used in mammalian cell lines, can be used as an efficient method of quantifying inter-organelle contact sites, namely mitochondria-endoplasmic reticulum (ER) and mitochondria-late-endosomes, in mammalian optic nerve tissues treated with adeno-associated virus (AAV) gene therapy with wild-type or phosphomimetic (active) protrudin. PLA utilises complementary single-stranded DNA oligomers bound to secondary antibodies that hybridise and complete a circular piece of DNA when the primary antibodies of interest interact. These interactions can be detected by amplifying the circular DNA and adding fluorescent probes. We show that PLA is a useful method that can be used to quantify MCS in optic nerve tissues. We have found that upregulation of protrudin with gene therapy significantly increases the number of mitochondria-ER and mitochondria-Rab7-late endosomes contact sites in optic nerves.

Microstructural characterization of 316L stainless steel fabricated by selective laser melting by advanced electron microscopy techniques

Microstructural characterization of 316L stainless steel fabricated by selective laser melting by advanced electron microscopy techniques

A Review of Grain Boundary and Heterointerface Characterization in Polycrystalline Oxides by (Scanning) Transmission Electron Microscopy

Interfaces such as grain boundaries (GBs) and heterointerfaces (HIs) are known to play a crucial role in structure-property relationships of polycrystalline materials. While several methods have been used to characterize such interfaces, advanced transmission electron microscopy (TEM) and scanning TEM (STEM) techniques have proven to be uniquely powerful tools, enabling quantification of atomic structure, electronic structure, chemistry, order/disorder, and point defect distributions below the atomic scale. This review focuses on recent progress in characterization of polycrystalline oxide interfaces using S/TEM techniques including imaging, analytical spectroscopies such as energy dispersive X-ray spectroscopy (EDXS) and electron energy-loss spectroscopy (EELS) and scanning diffraction methods such as precession electron nano diffraction (PEND) and 4D-STEM. First, a brief introduction to interfaces, GBs, HIs, and relevant techniques is given. Then, experimental studies which directly correlate GB/HI S/TEM characterization with measured properties of polycrystalline oxides are presented to both strengthen our understanding of these interfaces, and to demonstrate the instrumental capabilities available in the S/TEM. Finally, existing challenges and future development opportunities are discussed. In summary, this article is prepared as a guide for scientists and engineers interested in learning about, and/or using advanced S/TEM techniques to characterize interfaces in polycrystalline materials, particularly ceramic oxides.

The Morphology and Microstructure of Oxide Scale Grown on Austenitic Steel during Steam Oxidation at 700 °C for 500 h.

The modern power generation industry needs materials able to withstand severe conditions, such as high temperatures, steam pressure, and an aggressive environment, to create more electric power out of a decreasing amount of fuel. Therefore, new metallic materials are continuously being developed. In order to gain knowledge about modern materials, the investigation of high Cr and Ni austenitic steel oxidized in 100% steam at 700 °C for 500 h was performed. The morphology, the phase composition, and the chemical composition of the oxidation products were investigated through methods of advanced electron microscopy techniques. Moreover, precipitates present in bulk material were identified. The material developed a continuous and complex oxide scale, consisting of Fe2O3, Cr2O3, and spinel phases. Very fine MX, fine ε-Cu, and M23C6 precipitates were found in the bulk material. The creation of iron oxide is induced due to the coarse grain size of the material. Cr2O3 forms due to the internal oxidation process.