Lattice Images Research Articles

Deformation and energy absorption characteristics of additively-manufactured polymeric lattice structures — Effects of cell topology and material anisotropy

Additively manufactured lightweight lattice structures are being widely studied, one aspect being their energy absorption characteristics under large deformation, because their load–deformation responses can be adjusted by specifically tailoring the geometry of constituent cells. In this study, a newly-proposed hybrid structure (HS), which combines the geometrical features of a traditional primarily axial-deformation dominated octet cell and a primarily bending-dominated rhombic dodecahedron (RD), is designed and fabricated via Fused Deposition Modelling. To ascertain whether the geometrical hybrid enhances the energy absorption performance, the quasi-static compressive responses of such lattices are examined and compared with those of the constituent structures, i.e. the octet and RD. It is noted that the layer-wise additive manufacturing process affects the isotropy of the lattices, as it introduces angle-dependent strut material properties. To study this, the mechanical responses of lattice samples compressed along the rise (printing) and transverse directions are compared. Energy absorption efficiency criteria are adopted to identify the onset of the densification phase, and to evaluate how closely they approximate an ideal energy absorber. Finite element models are also established to study the effect of cell topology and loading direction on the resulting deformation modes and failure patterns. Compression tests along the rise direction show that the proposed novel hybrid structure displays a high stiffness and strength comparable to the octet, as well as a relatively stable post-yield stress–strain behaviour similar to that of an RD. The study demonstrates that the octet and HS topologies are significantly affected by the direction of compression, which alters the stress level and changes the deformation mode. The reason for this is analysed by examining deformation at the cell level, and this is substantiated by FE simulation of compression of cell assemblies, and CT scan images of actual lattices.

Cleavage of Calcitic CaCO3 during Dissolution in Aqueous Solution.

Calcite cleavage was observed using a device to accelerate the formation and dissolution of calcitic CaCO3 by acidification in aqueous solutions. Scanning electron microscopy showed that crystals formed within 1 day and dissolution began at 3 days of aging. In the X-ray diffraction pattern, calcite was predominantly formed but an amorphous phase did not exist, even after 12 days of aging. In the diffraction pattern and lattice images obtained using a series of high-resolution transmission electron microscopy analyses, cleavages mainly occurred in the {101̅1} plane and some cleavages were observed through the {101̅4} plane.

Microstructure of Milled Polyacrylonitrile-Based Carbon Fiber Analyzed by Micro-Raman Spectroscopy and TEM.

Milled polyacrylonitrile (PAN)-based Carbon Fibers (mPCFs) were prepared from PAN-based carbon fibers by using a ball milling process. The resulting structural changes in the mPCFs were analyzed by correlating the analytical results obtained by X-ray diffraction (XRD) and Raman spectroscopy and verified by transmission electron microscopy (TEM) lattice images and diffraction patterns. The crystallite size La calculated from the XRD measurements decreased as the milling time was increased to 12 h and then decreased as the milling time was further increased to 18 h. The La of both partially-milled Carbon Fiber (pmCF) and milled Carbon Fiber (mCF) calculated from the Raman spectroscopy data continuously increased as the milling time increased. The difference may be because XRD measured the entire sample regardless of pmCF and mCF, while Raman spectroscopy was limited to measuring the surface and differentiated pmCF and mCF. As the ball milling time increased, the fiber surface was firstly broken by the impact energy of the balls, decreasing crystallinity, while the La inside the unbroken fibers increased.

Multislice electron ptychography enables lattice vibration-limited resolution and linear phase-contrast imaging in thick samples

Multislice electron ptychography enables lattice vibration-limited resolution and linear phase-contrast imaging in thick samples

Stress relaxation of three dimensional textured AlN films on sapphire substrate by rapid thermal annealing

1000 nm high c-axis orientation and three dimensional textured AlN films were deposited on sapphire by reactive ion assisted sputtering and the rapid thermal annealing (RTA) was carried out at 700–900 °C for 5 min to release the film stress. Transmission electron lattice images show that the relationship of the AlN/sapphire interface is [0002]AlN//[0006]sapphire and [11-20]AlN//[1100]sapphire. The Fourier filtered images of the near substrate region suggest a thin buffer layer near the substrate, in which the atomic arrangement changes from matching sapphire lattice to AlN lattice. The buffer layer was disappeared after 900 °C RTA annealing. The Raman spectra shows the in-plane tensile stress of deposited AlN films is released by the RTA. The E2 (high) peak of the sample annealed at 900 °C shifts by 4.4 cm−1 compared to the as-deposited sample, which illustrates the tensile stress relaxation in the annealing process. In order to prevent the occurrence of peeling caused by the long heating time, this work proposed multiple-cycle rapid thermal annealing to anticipate stress relief.

Micrometre-scale strain mapping of transistor arrays extracted from undersampled atomic-resolution images

Strain maps extracted from atomic resolution images have the ultimate spatial resolution, but have a field of view limited by the sampling necessary to resolve atomic lattices. This has typically confined strain maps to dimensions less than ∼100 nanometers. To extend the field of view beyond this limit, we apply a modified geometric phase analysis to undersampled images of atomic lattices (i.e. with a pixel size too large to resolve atomic lattices). To reduce the effects of environmental and instrumental instabilities, the images were obtained by aligning series of rapid annular dark field scanning transmission electron microscopy acquisitions. We demonstrate that for undersampled images, a geometric phase analysis can still be performed on aliased frequencies and, as long as the appropriate scaling matrix is applied, provide accurate atomic displacement measurements at large scale. Experimental challenges related to the increased effects of scanning errors as the magnification is lowered are examined. Although such errors are found to significantly reduce geometric phase signals, it was still possible to produce strain maps for arrays of up to sixteen 20nm-technology transistors, corresponding to a field of view exceeding one micrometer.

HRTEM investigations on nano precipitates in Custom 475 maraging stainless steel

In this work, nano precipitates in stainless maraging steel (Custom 475) were investigated using high-resolution transmission electron microscopy. In this steel, aging induces three transformation products, NiAl-B2 phase, R phase and austenite, which contribute various hardening effects dependent on the aging temperature. In the steel aged at 480 °C, precipitation of NiAl-B2 phase was the primary reaction and led to slow incremental hardening. No hardening effect was observed in the steel aged at 650 °C despite the presence of austenite of over 25 vol% and coarse B2 and R phases. Efficient hardening was obtained by aging at 520 °C, and the maximum hardness (601 HV) was achieved when the steel was aged for 4 h. This high strength is attributed to the formation of both B2 and R nano precipitates. Moreover, lattice images and diffractograms clarified the crystal structure of the R phase and its orientation relationship with the martensite matrix. This work provides comprehensive insights into precipitation behaviors in Custom 475 maraging steel.

Super-Resolution Based on Generative Adversarial Network for HRTEM Images

As an important tool to study the microstructure and properties of materials, High Resolution Transmission Electron Microscope (HRTEM) images can obtain the lattice fringe image (reflecting the crystal plane spacing information), structure image and individual atom image (which reflects the configuration of atoms or atomic groups in crystal structure). Despite the rapid development of HTTEM devices, HRTEM images still have limited achievable resolution for human visual system. With the rapid development of deep learning technology in recent years, researchers are actively exploring the Super-resolution (SR) model based on deep learning, and the model has reached the current best level in various SR benchmarks. Using SR to reconstruct high-resolution HRTEM image is helpful to the material science research. However, there is one core issue that has not been resolved: most of these super-resolution methods require the training data to exist in pairs. In actual scenarios, especially for HRTEM images, there are no corresponding HR images. To reconstruct high quality HRTEM image, a novel Super-Resolution architecture for HRTEM images is proposed in this paper. Borrowing the idea from Dual Regression Networks (DRN), we introduce an additional dual regression structure to ESRGAN, by training the model with unpaired HRTEM images and paired nature images. Results of extensive benchmark experiments demonstrate that the proposed method achieves better performance than the most resent SISR methods with both quantitative and visual results.

Microstructure variations in carbon fiber reinforced plastics under ultraviolet-ray irradiation

Carbon fiber reinforced plastics (CFRPs) were irradiated with ultraviolet rays (UVR), and its structural variations were observed by high-resolution transmission electron microscopy. The lattice images of the resin, fibers, and fiber/resin interface regions in CFRPs and their spatial frequencies were investigated before and after UVR irradiation. In the fiber regions, the layer spacing of graphite structures increased after UVR irradiation. In the fiber/resin interface regions, the resin and the fiber adhered tightly at the atomic level before UVR irradiation. The anisotropic interface phases, in which atom-pairs having wide spacings were aligned along the direction perpendicular to the interface planes, were formed. After UVR irradiation, the tight interface adhesion was maintained, whereas the roughness of the interface shape increased. The effects of UVR irradiation on the mechanical properties of CFRPs were revealed from the atomistic structural variations in the resin, the fibers, and the fiber/resin interfaces.

Resolving atomic SAPO-34/18 intergrowth architectures for methanol conversion by identifying light atoms and bonds

The micro-structures of catalyst materials basically affect their macro-architectures and catalytic performances. Atomically resolving the micro-structures of zeolite catalysts, which have been widely used in the methanol conversion, will bring us a deeper insight into their structure-property correlations. However, it is still challenging for the atomic imaging of silicoaluminophosphate zeolites by electron microscopy due to the limits of their electron beam sensitivity. Here, we achieve the real-space imaging of the atomic lattices in SAPO-34 and SAPO-18 zeolites, including the Al–O–P atoms and bonds, by the integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM). The spatial distribution of SAPO-34 and SAPO-18 domains in SAPO-34/18 intergrowths can be clearly resolved. By changing the Si contents and templates in feed, we obtain two SAPO-34/18 catalysts, hierarchical and sandwich catalysts, with highly-mixed and separated SAPO-34 and SAPO-18 lattices respectively. The reduced diffusion distances of inside products greatly improve the catalytic performances of two catalysts in methanol conversion. Based on the observed distributions of lattices and elements in these catalysts, we can have a preliminary understanding on the correlation between the synthesis conditions and structures of SAPO-34/18 intergrowth catalysts to further modify their performances based on unique architectures.

Effects of Perpendicular Magnetic Field Annealing on the Structural and Magnetic Properties of [Co/Ni]2/PtMn Thin Films

In this study, [Co/Ni]2/PtMn thin films with different PtMn thicknesses (2.7 to 32.4 nm) were prepared on Si/SiO2 substrates. The post-deposition perpendicular magnetic field annealing (MFA) processes were carried out to modify the structures and magnetic properties. The MFA process also induced strong interlayer diffusion, rendering a less sharp interface between Co and Ni and PtMn layers. The transmission electron microscopy (TEM) lattice image analysis has shown that the films consisted of face-centered tetragonal (fct) PtMn (ordered by MFA), body-centered cubic (bcc) NiMn (due to intermixing), in addition to face-centered cubic (fcc) Co, Ni, and PtMn phases. The peak shift (2-theta from 39.9° to 40.3°) in X-ray diffraction spectra also confirmed the structural transition from fcc PtMn to fct PtMn after MFA, in agreement with those obtained by lattice images in TEM. The interdiffusion induced by MFA was also evidenced by the depth profile of X-ray photoelectron spectroscopy (XPS). Further, the magnetic properties measured by vibrating sample magnetometry (VSM) have shown an increased coercivity in MFA-treated samples. This is attributed to the presence of ordered fct PtMn, and NiMn phases exchange coupled to the ferromagnetic [Co/Ni]2 layers. The vertical shift (Mshift = −0.03 memu) of the hysteresis loops is ascribed to the pinned spins resulting from perpendicular MFA processes.

Investigation of opto-electronic properties and morphological characterization of magnesium niobate ceramics synthesized by two-stage process

Orthorhombic magnesium niobate is known to exhibit exceptional dielectric and photoluminescence properties. With the intention to evaluate these properties, MgNb2O6 (MN2) was synthesized at 900 °C for 6 h through a two-stage process. The phase examination was done by XRD and Raman spectroscopy. XRD analysis confirmed orthorhombic crystal structure with average crystallite size of 41 nm, approximately. The d-value and (hkl) plane corresponding to highest intensity was examined 2.9486 Å and (311), respectively. Raman spectroscopic analysis clarified various vibration bands of MN2 ceramics. The grain size was assessed close to 87 nm from FESEM analysis considering the grain distribution plot. The microstructure analysis by HRTEM reveal nanoparticles of average size of 41.83 nm. The clear lattice image and prominent spots in SAED confirm crystallinity of prepared ceramics. The PL result of the prepared ceramics at 900 °C indicated blue emission spectrum near 442 nm. The material prepared at 900 °C exhibited band gap of 2.65 eV using diffuse reflectance spectroscopic analysis. The dielectric constant (εr) and tanδ were also investigated where εr was measured 27.8 and 20.8 for material prepared at 800 and 900 °C, respectively. Low tanδ (dielectric loss) was also noted in present investigation.

Unexplored MBE growth mode reveals new properties of superconducting NbN

Accessing unexplored conditions in crystal growth often reveals remarkable surprises and new regimes of physical behavior. In this work, performing molecular beam epitaxy of the technologically important superconductor NbN at temperatures greater than 1000$^\circ$C, higher than in the past, is found to reveal persistent RHEED oscillations throughout the growth, atomically smooth surfaces, normal metal resistivities as low as 37$\mu\Omega$-cm and superconducting critical temperatures in excess of 15 K. Most remarkably, a reversal of the sign of the Hall coefficient is observed as the NbN films are cooled, and the high material quality allows the first imaging of Abrikosov vortex lattices in this superconductor.

Perpendicular magnetic anisotropy induced by La2/3Sr1/3MnO3–YBaCo2O5+δ interlayer coupling

Heterostructure with a symmetry-mismatched interface provides a promising playground for the exploration of emergent phenomena. Herein, we report a systematic investigation on La2/3Sr1/3MnO3/YBaCo2O5+δ (LSMO/YBCO) grown on SrTiO3, a heterostructure formed by perovskite oxides of different symmetry. A high-resolution lattice image shows the formation of high-quality perovskite LSMO and A-site cation-ordered oxygen-deficient double perovskite YBCO, without any signatures of atomic reconfiguration at the interface. Surprisingly, the YBCO-buffered LSMO exhibits perpendicular magnetic anisotropy (PMA), though bare LSMO film is in-plane anisotropic. The PMA is robust, appearing even when the thickness of YBCO is only one unit cell. The typical anisotropy constant is ∼4 × 106 erg cm−3. X-ray absorption spectroscopy analysis reveals a preferential occupation of the orbital compared with , which is confirmed by density functional theory calculations. This orbital reconstruction accounts for the PMA. The formation of a covalent bond between Mn and Co caged by different oxygen polyhedrons, an octahedron and a square pyramid, respectively, stabilizes the orbital reconstruction, resulting in anomalous spin orientation.

Studies of structural, electrical and optical properties of MgNb2O6-Mg4Nb2O9 nanocomposite for possible opto-electronic applications

Abstract MgNb2O6 (MN2)-Mg4Nb2O9 (M4N2) nanocomposite was successfully synthesized by a two-stage process at 1100 °C. The prepared nanocomposite was characterized by XRD, Raman spectroscopy, EDX, FESEM and HRTEM. Band gap, photoluminescence and dielectric behaviour were also studied. XRD confirmed orthorhombic and hexagonal crystal structure of MN2 and M4N2 phase, respectively. The crystallite size of MN2 and M4N2 was found equal to 43.52 and 41.38 nm, approximately. Raman spectroscopy and EDX analysis were performed to investigate molecular bonds and elemental composition, respectively. HRTEM confirmed nanoparticles of average size 44 nm. Clear lattice image and prominent spots in SAED pattern confirmed the crystallinity of prepared material. Grain size was estimated between 0.77 and 1.12 µm by FESEM analysis. A high bandgap (Eg) was found equal to 3.44 eV from diffuse reflectance spectroscopy. Photoluminescence spectroscopy revealed emission near 391.4 nm which was basically a prominent violet emission. Dielectric constant of prepared ceramics was measured 9.8 at 110 MHz and low dielectric loss was also observed. Excellent optical and dielectric properties may contribute towards certain opto-electronic applications.

Remarkable bactericidal traits of a metal-ceramic composite coating elated by hierarchically structured surface.

SummaryA ceramic-based coating with a hierarchical surface structure was synthesized via solid-state reaction enabled by a double cathode glow discharge technique. This innovative coating comprises two distinct layers, specifically an outer layer with a well-aligned micro-pillar array and a dense inner layer. Both are composed of a face-centered cubic Cu(Co,Ni,Fe) solid solution phase together with a spinel-type Fe(Al,Cr)2O4 oxide. This coating exhibits superhydrophobicity and, yet, a very strong adhesion to water, i.e., the so-called “rose petal effect”. This coating also exhibits highly efficient antibacterial ability against both Staphylococcus aureus and Escherichia coli bacteria under both dark and visible light conditions. The excellent antibacterial property originates from the synergistic effects through the release of Cu ions coupled with photothermal activity upon light activation.

HRTEM image-stitching for measurement of distances

AbstractVery accurate measurement of distances in the order of several µm is demonstrated on a single crystal Si sample by counting the lattice fringes on stitched high resolution TEM/STEM images. Stitching of TEM images commonly relies on correspondence points found in the image, however, the nearly perfect periodic nature of a lattice image renders such a procedure very unreliable. To overcome this difficulty artificial correspondence points are created on the sample using the electron beam. An accuracy better than 1% can be reached while measuring distances in the order of 1 µm. A detailed description of the process is provided, and its usability for accurately measuring large distances is discussed in detail.

Lattice resolution of vibrational modes in the electron microscope

The combination of aberration correction and ultra high energy resolution with monochromators has made it possible to record images showing lattice resolution in phonon modes, both with a displaced collection aperture and more recently with an on –axis collection aperture. In practice the objective aperture has to include Bragg reflections that correspond to the observed lattice image spacings, and the specimen has to be sufficiently thick for adequate phonon scattered intensity. There has been controversy as to whether the images with the on axis detector are really a consequence of lattice resolution in a phonon mode or just a transfer of information from an image that was formed by elastically scattered electrons. We present results of calculations based on a theory that includes the possibility of dynamical electron diffraction for both incident and scattered electrons and the full phonon dispersion relation. We show that Umklapp scattering from the second Brillouin Zone back to the first Brillouin Zone is necessary for lattice resolution with the on axis detector and that it is therefore reasonable to attribute the lattice resolution to the phonon scattering.

Te-rich Bi2Te3 thin films by electron−beam deposition: Structural, electrical, optical and thermoelectric properties

Tellurium-rich Bi2Te3 thin films are deposited by electron−beam evaporation technique at 300 K. These as-deposited thin films are further annealed at 100 °C, 200 °C and 300 °C for 1 h at a pressure of 3 × 10−4 Pa. X-ray diffraction (XRD) patterns of as-deposited films distinctly show Te phase along with Bi2Te3. Peak intensity ratio suggests the polycrystalline nature of as-deposited Bi2Te3 films. On vacuum annealing Te-rich Bi2Te3 films exhibit improved crystallinity with a c-axis preferred orientation. In addition, structural features related to Te and Bi2Te3 composition change with Te fraction diminishing on annealing at 300 °C. From Raman spectral studies, the presence of distinct Te-rich regions, predominantly within the interlayers of Bi2Te3, are discerned. Te becomes structurally integrated within the quintuples of Bi2Te3 lattice as intergrown layers. Disordered planar structures, mostly concentrated on crystallite surfaces result from Te accumulations as evidenced in high-resolution transmission electron microscopy lattice images and energy dispersive X-ray spectroscopy mapping. These are consistent with the observations from XRD and Raman studies further confirming Te-rich Bi2Te3 characteristics. Electrical properties of Te-rich Bi2Te3 thin films exhibit n-type semiconductor behaviour. Seebeck coefficient for as-deposited film is ~ 32 μV/K, which increases to ~ 97 μV/K on 200 °C annealing. Resistivity increases from 1.39 × 10−4 Ωcm to 18.76 × 10−4 Ωcm and power factor changes from 7.4×10−4W/K2m to 27.17×10−4W/K2m going through a maximum at 200 °C upon systematic annealing. Te-rich Bi2Te3 thin films annealed at 200 °C exhibit high power factor (∼29×10−4W/K2m) for a wide range of temperature gradients ( ΔT from 30 °C to 165 °C).

HAADF-STEM block-scanning strategy for local measurement of strain at the nanoscale

Lattice strain measurement of nanoscale semiconductor devices is crucial for the semiconductor industry as strain substantially improves the electrical performance of transistors. High resolution scanning transmission electron microscopy (HR-STEM) imaging is an excellent tool that provides spatial resolution at the atomic scale and strain information by applying Geometric Phase Analysis or image fitting procedures. However, HR-STEM images regularly suffer from scanning distortions and sample drift during image acquisition. In this paper, we propose a new scanning strategy that drastically reduces artefacts due to drift and scanning distortion, along with extending the field of view. It consists of the acquisition of a series of independent small subimages containing an atomic resolution image of the local lattice. All subimages are then analysed individually for strain by fitting a nonlinear model to the lattice images. The method allows flexible tuning of spatial resolution and the field of view within the limits of the dynamic range of the scan engine while maintaining atomic resolution sampling within the subimages. The obtained experimental strain maps are quantitatively benchmarked against the Bessel diffraction technique. We demonstrate that the proposed scanning strategy approaches the performance of the diffraction technique while having the advantage that it does not require specialized diffraction cameras.