High-voltage Transmission Electron Microscopy Research Articles

Formation mechanism of dislocation network of cell structure in cyclically deformed near-[formula omitted] copper single crystals

As the initiation and propagation of fatigue crack is strongly correlated to the dislocations in cell structures of metals, investigating the formation of dislocation cell structures is essential for understanding the fatigue process. Although the cell boundary evolves from the dislocation network, the formation process of the dislocation network of fatigued copper single crystal is not completely realized. In this study, the dislocation structures in near-[1‾11] copper single crystals cyclically deformed at 293 K were investigated to analyze the formation mechanism of the dislocation network. The dislocation networks were characterized using a high-voltage scanning transmission electron microscope, where the virtual-scanning transmission electron microscope technique (virtual-STEM) was employed to conduct the Burgers vector analysis of the dislocations. The low-dislocation-density network was consistent with the three perfect screw dislocations (b = a/2 [1‾01], a/2 [1‾10], and a/2 [011‾]) in the (111) primary slip plane. For the high-dislocation-density region, the perfect and partial dislocations coexisted in the network. As the dislocation density increased, the perfect dislocations dissociated into Shockley partial dislocations (b = a/6 [1‾21‾], a/6 [112‾], and a/6 [2‾11]). This study provides a reference for future applications of the virtual-STEM technique in the field of dislocation analysis.

Urany-Less Low Voltage Transmission Electron Microscopy: A Powerful Tool for Ultrastructural Studying of Cyanobacterial Cells.

Sample preparation protocols for conventional high voltage transmission electron microscopy (TEM) heavily rely on the usage of staining agents containing various heavy metals, most commonly uranyl acetate and lead citrate. However high toxicity, rising legal regulations, and problematic waste disposal of uranyl acetate have increased calls for the reduction or even complete replacement of this staining agent. One of the strategies for uranyless imaging is the employment of low-voltage transmission electron microscopy. To investigate the influence of different imaging and staining strategies on the final image of cyanobacterial cells, samples stained by uranyl acetate with lead citrate, as well as unstained samples, were observed using TEM and accelerating voltages of 200 kV or 25 kV. Moreover, to examine the possibilities of reducing chromatic aberration, which often causes issues when imaging using electrons of lower energies, samples were also imaged using a scanning transmission electron microscopy at 15 kV accelerating voltages. The results of this study demonstrate that low-voltage electron microscopy offers great potential for uranyless electron microscopy.

Characterization of dislocation microstructures in near-[formula omitted] single crystal copper using high-voltage scanning transmission electron microscopy

Metallic structures are prone to failure caused primarily by fatigue-related damage to the structural material. Fatigue behavior is strongly correlated with dislocation structures in metals. Therefore, investigating the formation of dislocation structures is essential for understanding the fatigue process. The dislocation structure in face-centered cubic crystals has a strong orientation dependence. However, the detailed characteristics and evolution process of dislocation structures of fatigued [1‾11] single crystal copper are still insufficient. This study investigated dislocation structures in near-[1‾11] single crystal copper cyclically deformed at room temperature under different constant plastic shear strain amplitude (γpl) to analyze the formation mechanism of dislocation structures and their relationship with fatigue behavior. Characteristic dislocation structures were characterized by adopting multiple planes using a high-voltage scanning transmission electron microscope. With increasing shear strain amplitude, a vein-like structure along the (1‾11) plane gradually developed into a wall structure. When γpl ≥ 1 × 10−3, deformation bands and cell bands were formed to accommodate the higher plastic strain. The state of cell-band development significantly affected the cyclic softening of the single crystal copper. A positive linear relationship existed between the shear stress amplitude and the reciprocal of the cell width.

Hydrogen sorption behaviour of Mg-5wt.%La alloys after the initial hydrogen absorption process

In our earlier study, it has been shown that trace Na additions can improve the reaction kinetics of Mg–5%La (wt.%) alloys during the first absorption. However, the subsequent hydrogen desorption/absorption process of the Mg–5%La after the first absorption has not been investigated. In this study, we have investigated the hydrogen sorption behaviour of the Mg–5%La alloy after the first absorption in terms of phase evolution, and lattice expansion properties during desorption as function of temperature using in-situ synchrotron Powder X-ray Diffraction (PXRD) and in-situ High Voltage Transmission Electron Microscopy (HVTEM). Two distinct phase evolutions, a continuous phase transformation of LaH3 → LaH2 + ½ H2 (from 250 °C) and decomposition of MgH2 → Mg + H2 (between 440 and 460 °C) were identified during the desorption. It is determined that this alloy is cyclable in the absence of Mg12La intermetallic during the subsequent absorption/desorption cycling after the first hydrogen absorption.

A Model for Dose Dependence of the Void Swelling in Electron-Irradiated Alloys

Understanding the void swelling dependence on irradiation dose for structural materials is critical for the design and operation of advanced nuclear reactors. Due to their easy accessibility in high-voltage transmission electron microscopes, electron beams have been frequently employed to investigate the void swelling mechanisms. Here, we build a general model to describe the radiation-induced swelling produced by energetic electrons. Based on this model, we develop a quantitative relation between void swelling and irradiation dose, which is in good agreement with experimental data. By extrapolating to high-dose swelling in electron-irradiated alloys, our model validation is consistent with available experiments. Furthermore, the model is well supported by our phase-field simulations.

In Situ Observation of Liquid Solder Alloys and Solid Substrate Reactions Using High-Voltage Transmission Electron Microscopy.

The complex reaction between liquid solder alloys and solid substrates has been studied ex-situ in a few studies, utilizing creative setups to “freeze” the reactions at different stages during the reflow soldering process. However, full understanding of the dynamics of the process is difficult due to the lack of direct observation at micro- and nano-meter resolutions. In this study, high voltage transmission electron microscopy (HV-TEM) is employed to observe the morphological changes that occur in Cu6Sn5 between a Sn-3.0 wt%Ag-0.5 wt%Cu (SAC305) solder alloy and a Cu substrate in situ at temperatures above the solidus of the alloy. This enables the continuous surveillance of rapid grain boundary movements of Cu6Sn5 during soldering and increases the fundamental understanding of reaction mechanisms in solder solid/liquid interfaces.

Domain switching dynamics in relaxor ferroelectric Pb(Mg1/3Nb2/3)O3–PbTiO3 revealed by time-resolved high-voltage electron microscopy

Ferroelectric domain dynamics in Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 single crystals have been studied by in situ biasing high-voltage transmission electron microscopy with a direct electron detection camera. We have achieved time-resolved recording of polarization switching in real space on a 2.5 ms time scale. The reversible response of micrometer-scale domains was observed by applying an electric field of 1 kV/mm. Detailed analyses on smaller sized domains 100–500 nm in size revealed that the domain switching initiated at a corner of a rectangular domain and propagated inward rapidly. The switching proceeded within 60 ms and the maximum switching rate, as fast as 6–8 μm/s, was observed. The domain switching kinetics was classified as two-dimensional nucleation and growth mode based on the Kolmogolov–Avrami–Ishibashi model.

Grain-boundary plane orientation dependence of faceting–roughening transition in Au grain boundaries under electron-beam irradiation

Grain boundaries act to absorb and annihilate point defects generated by high-energy particles for polycrystalline nuclear materials. Thus, understanding the effect of irradiation of high-energy particles on the grain-boundary structural change is prerequisite to develop nuclear materials with desirable properties. It is expected that such an effect would be different for different GB characters. To explore the probability, we examine grain boundaries with twist and tilt components with the same misorientation relationship of [1 0 0]/45o in Au as a model system at a high-voltage transmission electron microscope with an acceleration voltage of 1250 keV. It is observed that, under the same irradiation condition, the tilt GB undergoes roughening while the twist one became faceted.

Progress in environmental high-voltage transmission electron microscopy for nanomaterials.

A new environmental high-voltage transmission electron microscope (E-HVEM) was developed by Nagoya University in collaboration with JEOL Ltd. An open-type environmental cell was employed to enable in-situ observations of chemical reactions on catalyst particles as well as mechanical deformation in gaseous conditions. One of the reasons for success was the application of high-voltage transmission electron microscopy to environmental (in-situ) observations in the gas atmosphere because of high transmission of electrons through gas layers and thick samples. Knock-on damages to samples by high-energy electrons were carefully considered. In this paper, we describe the detailed design of the E-HVEM, recent developments and various applications. This article is part of a discussion meeting issue 'Dynamic in situ microscopy relating structure and function'.

Reaction science of layer-by-layer thinning of graphene with oxygen neutrals at room temperature

Controlling the layers and in-plane defects in multi-layer graphene is the key to exploiting the extraordinary properties of graphene. Layer-by-layer thinning under high-density oxygen neutrals was observed in situ during room-temperature reaction science using a remote oxygen plasma inside a high-voltage transmission electron microscopy. Even though the etching rate of graphene was higher at the edge than at the basal plane, etching started from both locations. Each etched layer was represented by the plasmon loss intensity in the electron energy loss spectrum. After exposure to the remote oxygen plasma, the electron diffraction pattern for the multi-layer graphene keep clear six-fold diffraction pattern. Raman spectroscopy revealed the formation of defects in multi-layer graphene. The nonthermal reaction of oxygen neutrals required for graphene etching was demonstrated by in situ transmission electron microscopy.

Electron-microscopic analyses on high-temperature fatigue crack growth mechanism in a Ni-based single crystal superalloy

The microscopic mechanism of high temperature (900 °C) fatigue crack growth in a single crystal Ni-based superalloy is investigated. Particular attention is focused to its relation to the cyclic misorientation markings (CMMs), formerly termed EBSD (electron backscattered diffraction) striation, left along the crack wake. Observation of a cross-sectioned crack tip is conducted by various electron microscopy i.e. SEM, EBSD, high-voltage TEM and STEM-EDS. A {100} fatigue crack subjected to a relatively high ΔK (40 MPam1/2) shows CMMs with an interval corresponding to crack growth rate (2–4 μm/cycle), consisting of heavily dislocated γ (Ni) phase and less dislocated γ’ (Ni3Al) phase. It is also revealed that local oxidation along the γ/γ’ interface at the crack tip existed at the current temperature/ΔK level. On the bases of these findings, a rational yet qualitative crack growth model that incorporated both brittle/ductile processes is proposed.

Diagnostic Electron Microscopy of Viruses With Low-voltage Electron Microscopes.

Diagnostic electron microscopy is a useful technique for the identification of viruses associated with human, animal, or plant diseases. The size of virus structures requires a high optical resolution (i.e., about 1 nm), which, for a long time, was only provided by transmission electron microscopes operated at 60 kV and above. During the last decade, low-voltage electron microscopy has been improved and potentially provides an alternative to the use of high-voltage electron microscopy for diagnostic electron microscopy of viruses. Therefore, we have compared the imaging capabilities of three low-voltage electron microscopes, a scanning electron microscope equipped with a scanning transmission detector and two low-voltage transmission electron microscopes, operated at 25 kV, with the imaging capabilities of a high-voltage transmission electron microscope using different viruses in samples prepared by negative staining and ultrathin sectioning. All of the microscopes provided sufficient optical resolution for a recognition of the viruses tested. In ultrathin sections, ultrastructural details of virus genesis could be revealed. Speed of imaging was fast enough to allow rapid screening of diagnostic samples at a reasonable throughput. In summary, the results suggest that low-voltage microscopes are a suitable alternative to high-voltage transmission electron microscopes for diagnostic electron microscopy of viruses.

Direct observation of the Ni stabilising effect in interfacial (Cu,Ni)6Sn5 intermetallic compounds

The polymorphic transformation in interfacial intermetallic compounds in three lead-free solder joints was investigated using in situ heating/isothermal/cooling observation techniques in high-voltage transmission electron microscopy (HVTEM). Here, the hexagonal η- to monoclinic η′-Cu6Sn5 polymorphic transformation was visualised systematically through zone-axis electron diffraction patterns and real-space imaging. Cu6Sn5 grains obtained in the as-reflowed solder joints that did not contain Ni (Sn–0.7Cu/Cu and Sn–3.0Ag–0.5Cu/Cu) show weak reflections in the diffraction patterns from the η′-Cu6Sn5. In Sn–0.7Cu–0.05Ni/Cu joints, no weak reflections were present in the diffraction patterns of the (Cu,Ni)6Sn5 grains indicating the presence of Ni prevented the η- to η′-Cu6Sn5 transformation. The movement of bend contours was also observed at around 186 °C in adjacent the grain boundaries in Cu6Sn5 but not in the (Cu,Ni)6Sn5. We also uncover the origin of the weak reflections and confirm the superstructure of monoclinic η′-Cu6Sn5 by obtaining atomic-resolution images within the areas from which the selected area diffraction patterns were obtained.

Properties of CuGa2 Formed Between Liquid Ga and Cu Substrates at Room Temperature

Ga and Ga-based alloys have received significant attention due to their potential application in the liquid state for low-temperature bonding in microelectronics. This study investigated the interfacial reactions between liquid Ga and pure Cu substrates at room temperature. The directional thermal expansion behaviour of the resulting CuGa2 was analysed by synchrotron x-ray powder diffraction with supporting observations of single crystal foils in high-voltage transmission electron microscopy. The mechanical properties of CuGa2 were evaluated by nano-indentation. CuGa2 was found to have advantages over other intermetallics that are present in assemblies made with current generation lead-free solders, including Ag3Sn, Cu6Sn5 and Cu3Sn. In addition to enabling lower process temperatures, solder alloys that form CuGa2 at the interface with Cu offer the possibility of providing more reliable connections in the very small joints that play an increasingly important role as the trend to miniaturisation of electronics continues.

In-Situ Observation of the Continuous Phase Transition in Determining the High Thermoelectric Performance of Polycrystalline Sn0.98Se.

We report a comprehensive in-situ phase-change study on polycrystalline Sn0.98Se via high-temperature X-ray diffraction and in-situ high-voltage transmission electron microscopy from room temperature to 843 K. The results clearly demonstrate a continuous phase transition from Pnma to Cmcm starting from 573 to 843 K, rather than a sudden transition at 800 K. We also find that the thermal-conductivity rise at high temperature after the phase transition, as commonly seen in pristine SnSe, does not occur in Sn0.98Se, leading to a high thermoelectric figure of merit. Density functional theory calculations reveal the origin to be the suppression of bipolar thermal conduction in the Cmcm phase of Sn0.98Se due to the enlarged bandgap. This work fills the gap of in-situ characterization on polycrystalline Sn0.98Se and provides new insights into the outstanding thermoelectric performance of polycrystalline Sn0.98Se.

Transmission Electron Microscope Using a Linear Accelerator.

High-voltage transmission electron microscopes (HVTEMs), which can visualize internal structures of micron thick samples, intrinsically have large instrument sizes because of the static voltage isolation. In this Letter, we develop a compact HVTEM, employing a linear accelerator, a subpicosecond beam chopper, and a linear decelerator. 100kV electrons initially accelerated by a static field are accelerated at radio frequency (rf) up to 500kV, transmitting through the sample and finally rf decelerated down to 200kV to be imaged through a 200kV energy filter. 500kV imaging, as well as subnanometer resolution at 200kV, have been demonstrated.

The effects of Ni on inhibiting the separation of Cu during the lithiation of Cu6Sn5 lithium-ion battery anodes

Sn-based lithium-ion battery (LIB) anodes have a lower risk of lithium dendrite formation and a higher storage capacity of 993 mAh g−1 vs. 372 mAh g−1 compared to carbon-based anodes. Alloying Sn with Cu can reduce the reaction stresses in the anode, and Cu6Sn5 is therefore a promising candidate material to replace carbon-based anodes. However, the separation of Cu during the second stage of the lithiation reaction results in slow kinetics and degrades the cyclability of the anodes. This study proposes an effective method to inhibit the separation of Cu via the addition of Ni. Ni occupies the Cu positions in the Cu6Sn5 crystal structures to form (Cu, Ni)6Sn5, and therefore alters the crystal structure of the anode, leading to the formation of superstructures. As a result, Ni partially blocks the diffusion pathways of Li and therefore inhibits the Cu separation reaction, while the superstructure provides additional Li storage sites to increase the capacity of the anodes. Ni also refines the grain size of Cu6Sn5, leading to faster kinetics. The reaction mechanisms of the modified anodes are confirmed by in-situ synchrotron X-ray powder diffraction and ex-situ high voltage transmission electron microscopy.

In-situ Synchrotron X-ray Powder Diffraction Characterisation of Cu6Sn5 Lithium-ion Battery Anodes

Sn-based anodes have been widely studied for applications in lithium-ion batteries due to their superior storage capacities. However, the large volume changes during lithiation and delithiation often result in poor cyclic performances. Intermetallic compounds (IMCs) combining Sn with an element that does not react with lithium are commonly used to reduce the stresses in these anodes. One such example, the IMC Cu6Sn5, has been proven to improve the cyclic performance of Sn as an anode. Sn-based anodes also have lithiation voltages around 0.4 V vs. Li/Li+, making them less prone to lithium dendrite growth and therefore intrinsically safer. This work investigates the lithiation and delithiation mechanisms of a Cu6Sn5 anode fabricated via direct growth of the Cu6Sn5 IMCs on the Cu current collector by melt-solid contact. In-situ synchrotron X-ray powder diffraction revealed the “real-time” crystallographic changes of the anode during the lithiation and delithiation processes, which are confirmed by high-voltage transmission electron microscopy. The study shows a reversible two-step reaction during cycling, and highlights the differences in the reaction mechanisms at higher lithiation/delithiation rates than those reported in the published literature. Figure 1

High-Voltage Scanning Transmission Electron Microscopy: A Tool for Structural Characterization of Micrometer-Thick Specimens

Herein, the advantages of high-voltage scanning transmission electron microscopy (STEM) as a tool for structural characterization of micrometer-thick specimens are reported. Dislocations introduced in a wedge-shaped Si crystal were clearly observed by bright-field STEM operating at 1 MV. Many of the dislocations were straight and parallel to the 〈110〉, 〈112〉 or 〈113〉 directions. The widths of the dislocations in the STEM images were almost constant at 13–16 nm (i.e., 4–5 pixels) in the thickness range between 1 and 7.5 µm. The latest high-voltage STEM instrumentation is thus useful for imaging crystal defects in micrometer-thick materials, and enables multi-scale fields of view from a few nanometers squared to over 100 µm2.

Characterisation of lithium-ion battery anodes fabricated via in-situ Cu6Sn5 growth on a copper current collector

Li-ion batteries (LIBs) are favoured in many applications due to their high energy density and good cyclic performance. However, some safety concerns remain with respect to the risk of fires and explosions with LIBs. Carbon-based anode materials in LIBs operate close to the Li metal reduction potential, therefore Li dendrites can grow during voltage fluctuations, causing short circuits which may lead to fires. The Cu6Sn5 anodes has the lithiation potential at about 0.4 V vs Li/Li+ and therefore less prone to Li metal plating. A new manufacturing method involving direct in-situ formation of Cu6Sn5 on a Cu current collector via melt-solid contact is proposed. This method combines the active material production and the anode fabrication into a single process. Lithiation and delithiation mechanisms of the anode produced are studied by in-situ synchrotron X-ray powder diffraction (XRPD) and ex-situ high voltage transmission electron microscope (HV-TEM). The in-situ XRPD study shows a reversible two-step reaction during cycling, and also reveals the differences in the reaction mechanisms at higher charge/discharge rates than those in published data.