High-resolution Transmission Electron Microscopy Observations Research Articles

Microstructure and mechanical behavior of metallic glass fiber-reinforced Al alloy matrix composites.

Metallic glass-reinforced metal matrix composites are an emerging class of composite materials. The metallic nature and the high mechanical strength of the reinforcing phase offers unique possibilities for improving the engineering performance of composites. Understanding the structure at the amorphous/crystalline interfaces and the deformation behavior of these composites is of vital importance for their further development and potential application. In the present work, Zr-based metallic glass fibers have been introduced in Al7075 alloy (Al-Zn-Mg-Cu) matrices using spark plasma sintering (SPS) producing composites with low porosity. The addition of metallic glass reinforcements in the Al-based matrix significantly improves the mechanical behavior of the composites in compression. High-resolution TEM observations at the interface reveal the formation of a thin interdiffusion layer able to provide good bonding between the reinforcing phase and the Al-based matrix. The deformation behavior of the composites was studied, indicating that local plastic deformation occurred in the matrix near the glassy reinforcements followed by the initiation and propagation of cracks mainly through the matrix. The reinforcing phase is seen to inhibit the plastic deformation and retard the crack propagation. The findings offer new insights into the mechanical behavior of metal matrix composites reinforced with metallic glasses.

High-Resolution Transmission Electron Microscopy (HRTEM) Study of Stacking Irregularity in Fe-Rich Chlorite From Selected Hydrothermal Ore Deposits

AbstractThe structures of Fe-rich chlorite and berthierine and the formation mechanisms of 7 Å–14 Å interstratified minerals were not previously fully understood owing to the difficulties in analyzing them by X-ray diffraction (XRD). The present study characterizes Fe-rich chlorites in quartz veins of epithermal to xenothermal vein-type ore deposits without later structural modifications, based on high-resolution transmission electron microscopy (HRTEM) along with XRD examination and chemical analysis. Samples have a wide range of Fe/(Fe+Mg) ratios from 0.38 to 0.98 and tetrahedral Al substitution for Si from 0.94 to 1.44 atoms per formula unit (apfu). The variation in Fe content nearly parallels the tetrahedral Al content. The formation temperatures estimated by chlorite geothermometry range from 190°C to 320°C. In HRTEM, most of the samples showed interstratification between 7 Å, 14 Å, and/or (in some samples) smectite layers. Chlorites with relatively low Fe contents (Fe/(Fe+Mg) ≈ 0.4) were characterized by mostly 14 Å periodicity with the polytype IIbb. In contrast, interstratification of 7 Å and 14 Å layers predominated with increasing Fe content and the proportion of 7 Å layers exceeds 80% in Fe-rich samples with Fe/(Fe+Mg) > 0.9. The 7 Å component layer approximated Fe-rich berthierine based on the chemical composition. Layer stacking structures in the Fe-rich samples were complex, and characterized by disorder of 7 Å and 14 Å layers, differences in the polarity of the tetrahedral sheets, variations of the slant of the octahedral sheets, and positional disorder between octahedral and tetrahedral sheets involving the hydrogen bonding, as indicated from HRTEM observations along the Yi directions of the phyllosilicates. The complex stacking structures observed in Fe-rich samples suggest that irregularity was controlled by neither the Fe/(Fe+Mg) ratio nor the formation temperature; stacking was controlled by kinetic factors in the process of mineral precipitation under disequilibrium conditions.

Diethylene Glycol-Mediated Synthesis of Nano-Sized Ceria (CeO2 )Catalyst

Nano-crystalline particles of CeO2 have been synthesized by a low temperature chemical precipitation method. The precursor materials used in this research were Ce(NO3)3.6H2O, NaOH and diethylene glycol as surfactant. X-ray powder diffraction results showed that face centered cubic CeO2 nanoparticles with crystalline size in nanometer scale were formed. Scanning electron microscopy measurement showed that by increasing the calcinations temperature the crystallite size decreases. The particle size of CeO2 was around 20 nm as estimated by X-ray powder diffraction technique and direct high-resolution transmission electron microscopy observation. High-resolution transmission electron microscopy studies showed the size of CeO2 particles increase from 10-90 nm by increasing the ratio of diethylene glycol surfactant. The sharp peaks in Fourier transform infrared spectrum determined the purity of CeO2 nanoparticles and absorbance peak of ultraviolet–visible spectroscopy spectrum showed the small bandgap energy of 3.26 ev.

Mono-Disperse CaWO4 Microsphere with Hierarchical Structures: Room Temperature Synthesis and its Optical Properties

In this report, an inorganic salt assisted micro-emulsion method was reported for the preparation of mono-disperse CaWO4 microspheres with hierarchical structures. The as-obtained microspheres are composed of tiny nanorods, which have been proved by the field emission scanning electronic microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) observations. The formation of these hierarchical mono-disperse microspheres is considered to be the synergistic effect of both surfactant and the inorganic salt. Based on the time-dependent experiments, a probable formation mechanism based on surface adsorption and oriented attachment has been proposed. The photoluminescence properties of the microspheres were investigated. The generality of this synthetic method for other metal tungstates were also carefully investigated, which indicates that the morphologies of the final product not only depend on the synthetic conditions, but also depend on the growth habit of the materials.

Microstructure analysis of chemically synthesized wurtzite-type CdS nanocrystals

Microstructure of chemically synthesized wurtzite-type CdS nanocrystals have been investigated by X-ray diffraction (XRD) peak profile analysis by applying different forms of Williamson–Hall (WH) method viz., uniform deformation model (UDM), uniform stress deformation model (USDM) and uniform deformation energy density model (UDEDM), and transmission electron microscope (TEM) observations. The WH methods show the average crystallite size to be about 10 nm. Strain, stress and energy density of the nanocrystals are found to be 1.18 × 10 −2, 0.43 GPa and 2.27 kJ m −3, respectively. High-resolution TEM (HRTEM) results show the nanocrystals to be in spherical shape with an average crystallite size of 10 nm, thereby complementing the size estimation by WH methods. Further, HRTEM observations reveal the presence of edge dislocations and twin boundaries within the nanocrystals.

Nano- and Submicrometer-Sized Spherical Particle Fabrication Using a Submicroscopic Droplet Formed Using Selective Laser Heating

Detailed characterization of products by pulsed laser melting in liquid was performed for TiO2 particles obtained at different laser fluences. The size, crystal structure, and inner structure of obtained spherical particles depended on the irradiating laser fluence. Single crystalline submicrometer spherical TiO2 particles of 200 nm were obtained using nanosecond pulsed laser irradiation at 100 mJ cm–2 pulse–1 onto raw particles dispersed in ethanol using cross-sectional high-resolution transmission electron microscopy observation. At higher laser fluence (e.g., 225 mJ cm–2 pulse–1), large submicrometer spheres with strain and twin structures 500 nm in diameter and nanometer spheres of 70 nm were simultaneously observed. The formation of such bimodal size distribution of obtained particles was explained based on the phase transition fluence curves deduced by Mie theory and the optical absorption increase induced by the formation of nonstoichiometric TiO2 particles. Thus, with the appropriate laser fluence...

Mo Nanocontacts Manifest Flowlike Thinning and Yield Poorly Conducting Single-atom Contacts

High-resolution transmission electron microscopy observations of the breaking of Mo nanocontacts revealed flow-like plastic deformation of the contacts. Single-atom contacts of Mo, formed as a resu...

Hybrid nanostructured aluminum alloy with super-high strength

Methods to strengthen aluminum alloys have been employed since the discovery of the age-hardening phenomenon in 1901. The upper strength limit of bulk Al alloys is ~0.7 GPa by conventional precipitation strengthening and increases to >1 GPa through grain refinement and amorphization. Here we report a bulk hybrid nanostructured Al alloy with high strength at both room temperature and elevated temperatures. In addition, based on high-resolution transmission electron microscopic observations and theoretical analysis, we attribute the strengthening mechanism to the composite effect of the high-strength nanocrystalline fcc-Al and nano-sized intermetallics as well as to the confinement effect between these nano phases. We also report the plastic deformation of nano-sized intermetallics and the occurrence of a high density of stacking faults and twins in fcc-Al after low-strain-rate deformation at room and high temperatures. Our findings may be beneficial for designing high-strength materials for advanced structural applications. An international team of scientists create an aluminum alloy that is twice as strong as those made by conventional techniques. Zhi Wang from IFW Dresden in Germany and Tohoku University in Japan and co-workers from China, Germany, Japan and Austria have developed a technique that allows the microstructure of aluminum alloys to be carefully controlled, enabling them to create hybrid structures consisting of nanoscale intermetallic compounds in an aluminum matrix. Previous attempts to create strong alloys included making an aluminum-based material that is non-crystalline, like glass, or one that is made up of crystalline nanograins. But these approaches lead to materials that are unsuitable for use at high temperatures. In contrast, the aluminum alloy created by the researchers exhibits high strength at both ambient and elevated temperatures. We report a bulk hybrid nanostructured Al alloy with super-high strength at both room and elevated temperatures. The strengthening mechanisms were clearly elucidated, which are mainly attributed to the composite structure and confinement effect between the nano phases. The confining effect can effectively suppress the premature brittle fracture of the nano-intermetallic phases. The microstructural strategy and strengthening mechanisms in this work may be beneficial for the scientific community in understanding and designing high-strength materials.

Fine structure of tooth enamel in the yellowing human teeth: SEM and HRTEM studies.

The aim of this study was to clarify an influence of the fine structure of human tooth enamel to the yellowing teeth. Sound maxillary first premolars of 15-50-year-old females that were extracted for the orthodontic treatment were used as the test samples. The tooth enamel sections of these teeth that prepared by ion polishing were observed by scanning electron microscopy (SEM). Furthermore, the fine structure of substance filling the inter-rod spaces was analyzed by high resolution transmission electron microscopy (HRTEM). In white tooth, the inter-rod spaces were observed at the width of about 0.1 μm, while in yellow tooth, the inter-rod spaces were not clearly observed by SEM. HRTEM observations revealed for the first time that the inter-rod spaces were filled with fine particles of poorly crystallized hydroxyapatite in the yellow tooth. In yellow tooth, it was considered that the color of the inner dentin was recognized due to the decrease of light scattering by filling the tooth enamel inter-rod spaces. The generation of particles in the tooth enamel inter-rod spaces was considered to be caused by the long-time progression of calcification. These results suggested that the change in fine structure, filling in inter-rod spaces of tooth enamel, was related to progression of calcification in the inter-rod spaces with advancing age and one of the factors of yellowness of human tooth.

Nanostructure and strain properties of core-shell GaAs/AlGaAs nanowires

GaAs/AlGaAs core–shell nanowires (NWs) were grown on Si(111) by Ga-assisted molecular beam epitaxy via the vapor–liquid–solid mechanism. High-resolution and scanning transmission electron microscopy observations showed that NWs were predominantly zinc-blende single crystals of hexagonal shape, grown along the [111] direction. GaAs core NWs emerged from the Si surface and subsequently, the NW growth front advanced by a continuous sequence of (111) rotational twins, while the AlGaAs shell lattice was perfectly aligned with the core lattice. Occasionally, single or multiple stacking faults induced wurtzite structure NW pockets. The AlGaAs shell occupied at least half of the NW’s projected diameter, while the average Al content of the shell, estimated by energy dispersive x-ray analysis, was x = 0.35. Furthermore, molecular dynamics simulations of hexagonal cross-section NW slices, under a new parametrization of the Tersoff interatomic potential for AlAs, showed increased atom relaxation at the hexagon vertices of the shell. This, in conjunction with the compressively strained Al0.35Ga0.65As shell close to the GaAs core, can trigger a kinetic surface mechanism that could drive Al adatoms to accumulate at the relaxed sites of the shell, namely along the diagonals of the shell’s hexagon. Moreover, the absence of long-range stresses in the GaAs/Al0.35Ga0.65As core–shell system may account for a highly stable heterostructure. The latter was consolidated by temperature-dependent photoluminescence spectroscopy.

Synergistic effects of TiH2 and Pd on hydrogen desorption performances of MgH2

In the present work, co-additives of Pd and TiH2 synergistically enhance the dehydrogenation performance of MgH2. The volumetric release measurements on 2MgH2–TiH2–0.1Pd composite revealed that the onset temperature of the dehydrogenation (150 °C) was greatly lowered by 150 °C from that of pristine MgH2. X-ray diffraction (XRD) analyses and high-resolution transmission electron microscopy (HR-TEM) observations found that Pd decomposed the MgH2 through yielding the Mg6Pd phase. The destabilizations of MgH2 in 2MgH2–0.1Pd and 2MgH2–TiH2–0.1Pd were confirmed by calculating the slopes of Van't Hoff plots of those composites based on pressure–composition isotherms. Those composites contain two significant plateaus in P–C isotherms of dehydrogenation, one represents the 2MgH2–0.1Pd release in initial stage, the other belongs to the self-decomposition of MgH2. TiH2 did not involve in the destabilization of MgH2 in the composite. However, it remarkably reduced the activation energy of the ternary system in dehydrogenation and improved its kinetics along with Pd. The activation energy value (77 kJ mol−1) of 2MgH2–TiH2–0.1Pd was found drastically lower than those of 2MgH2–0.1Pd (254.6 kJ mol−1) and 2MgH2–TiH2 (154.2 kJ mol−1). It exhibits that the synergistic effect by co-additives of TiH2 and Pd on dehydrogenation kinetics are more remarkable than any individual one.

Enhanced Lithium Storage in Co3O4/carbon Anode for Li-ion Batteries

In this work, Co3O4 as an anode material for lithium-ion batteries is prepared by using molten-salt method that combines with carbon coating approach to prevent the cracking structure resulting from the Li+ insertion/deinsertion during the cycling process. Co3O4 and Co3O4/C are systematically characterized using X-ray diffraction, high-resolution transmission electron microscopy (HRTEM), thermogravimetric analysis and electrochemical testing. HRTEM observations demonstrate a thick layer of amorphous carbon that bound completely around Co3O4 particles. The electrochemical testing results indicate that the performances of Co3O4/C electrode is improved with a discharge capacity of ∼ 1164mAhg−1 at 0.1C rate compared with bare Co3O4 which continuously decreases and reaches ∼ 556mAhg−1 after 100 cycles. The results show that the carbon coating improves the battery performance and the overall consistency as compared to pure Co3O4 in terms of capacity and cyclability.

Synthesis and photocatalytic properties of multi-morphological AuCu3-ZnO hybrid nanocrystals

Noble metal–semiconductor hybrid nanocrystals represent an important class of materials for many potential applications, especially for photocatalysis. The utilization of transition metals to form alloys with noble metals can not only reduce the preparation costs, but may also offer tunable optical and catalytic properties for a broader range of applications. In this study, we report on the solution synthesis of AuCu3–ZnO hybrid nanocrystals with three interesting morphologies, including urchin-like, flower-like and multipod-like nanocrystals. In the synthetic strategy, Au–Cu bimetallic alloy seeds formed in situ are used to induce the heteroepitaxial growth of ZnO nanocrystals on the surface of bimetallic alloy cores; thus different types of morphologies can be achieved by controlling the reaction conditions. Through high-resolution transmission electron microscopy observations, well-defined interfaces between ZnO and AuCu3 are observed, which indicate that ZnO has a (0001) orientation and prefers to grow on AuCu3 {111} facets. The as-prepared hybrid nanocrystals demonstrate morphology- and composition-dependent surface plasmon resonance (SPR) absorption bands. In addition, much higher photocatalytic efficiency than pure ZnO nanocrystals is observed for the hybrid nanocrystals in the degradation of methylene blue. In particular, the multipod-like AuCu3–ZnO hybrid nanocrystals show the highest catalytic performance, as well as more than three times higher photocurrent density than the pure ZnO sample. The reported synthetic strategy provides a facile route to the effective combination of a plasmonic alloy with semiconductor components at the nanoscale in a controlled manner.

Facile synthesis of Au@SiO2 core–shell nanoparticles with multiple Au nanodots by a reverse microemulsion (water-in-oil) method

Abstract A water-in-oil (W/O) microemulsion was prepared by mixing cyclohexane and HAuCl4 aqueous solution at the presence of a nonionic surfactant Triton X-100 and 1-hexanol. By successively adding sodium borohydride (NaBH4), ammonium hydroxide (NH3·H2O) and tetraethyl orthosilicate (TEOS) to this microemulsion, Au@SiO2 hybrid nanoparticles were produced which have been fully characterized by transmission electron microscopy (TEM) and high resolution TEM (HR-TEM) observations, UV–vis measurements and thermo gravimetric analysis (TGA). It was found that in each Au@SiO2 hybrid nanoparticle, the silica shell can encapsulate multiple Au nanodots. The parameters of the Au@SiO2 hybrid nanoparticles including the thickness of the silica shell, the number of encapsulated Au nanodots and the size of the nanoparticles can be easily tuned by changing the volume of TEOS and the reaction time before adding TEOS, respectively. These interesting core–shell nanoparticles provide ideal candidates for a variety of applications in materials science, biology and catalysis.

Synthesis and Characterization of α-Fe2O3 Nanoparticles by Simple Co-Precipitation Method

In the past decade, magnetic nanomaterials have attracted much attention due to their physical properties and technological applications. In this work, α-Fe2O3 nanoparticles were first synthesized via a simple co-precipitation method using iron chloride hexahydrate (FeCl3.6H2O) as precursor and ammonia solution as precipitator. The samples were then characterized by high resolution transmission electron microscopy (HRTEM), field effect scanning electron microscopy (FESEM), X-ray diffraction (XRD) and electron dispersive spectroscopy (EDS) Fourier transform infrared spectroscopy (FTIR) and UV-Vis spectrophotometer. XRD pattern showed that the iron oxide nanoparticles exhibited alpha-Fe2O3 (hematite) structure innanocrystals. The α- Fe2O3 nano-powders with uniform size were prepared when the samples calcined at 500 °C, and the lowest particle size was found to be 30 nm by XRD technique and direct HRTEM observation. The surface morphological studies from SEM depicted sphere-like shaped particles with formation of clusters by increasing annealing temperature. The EDS spectrum showed peaks of iron and oxygen free of impurity with fewer elements. The sharp peaks in FTIRspectrum determined the purity of Fe2O3 nanoparticles and absorbance peak of UV-Vis spectrum showed the small bandgap energy of 2.58 ev.

Sulfur-Doped Carbon with Enlarged Interlayer Distance as a High-Performance Anode Material for Sodium-Ion Batteries.

S-doped carbon is investigated as a high-performance anode material for sodium-ion batteries. Due to the introduction of a high-content of S atoms, the as-obtained S-doped carbon shows an enlarged interlayer distance. As an anode, a high specific capacity of up to 303 mAh g-1 is achieved, even after 700 cycles at 0.5 A g-1.

Multiple-layered structure of obelisk-shaped crystalline nano-ZnO prepared by sol–gel route

Zinc oxide nanopowders were synthesized by the simple sol–gel method from an ethanol solution of zinc nitrate hexahydrate. Structural and surface morphological investigations were carried out using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR) and ultraviolet–visible (UV–Vis) spectrophotometry analyses. XRD pattern showed that the zinc oxide nanoparticles exhibited hexagonal wurtzite structure. A multiple-layered structure of obelisk-shaped ZnO nanoparticles was achieved after calcinations. The average particle size of ZnO was around 20 nm as estimated by direct HRTEM observation. The size of sphere-like shaped ZnO nanoparticles was measured in the range of 20–80 nm and the size of pyramid-like shaped annealed samples was achieved in the range of 40–100 nm with less agglomeration. The energy dispersive spectroscopy spectrum showed peaks of zinc and oxygen. The sharp peaks in FTIR spectrum determined the Zn–O stretching and absorbance peak of UV–Vis spectrum showed the wide bandgap energy of 3.35 eV.

Oxidation behavior of β-Nb precipitates in Zr–1Nb–0.2Bi alloy corroded in lithiated water at 360 °C

The oxidation behavior of β-Nb precipitates in Zr–1Nb–0.2Bi alloy corroded in lithiated water with 0.01M LiOH at 360°C/18.6MPa was investigated by high-resolution transmission electron microscopy (HRTEM). From the HRTEM observation and the analysis for the oxide layer, the oxidation behavior of β-Nb precipitates corroded in lithiated water in early stage is as follows: β-Nb precipitates are first oxidized to NbO2, then part of it is further oxidized to Nb2O5 in some specific crystallographic orientations. The reason for accelerated corrosion of Zr–Nb alloys in lithiated water is discussed from the rapid oxidation of β-Nb precipitates.

Three-component mixed-layer illite/smectite/kaolinite (I/S/K) minerals in hydromorphic soils, south China

To understand clay mineral transformations in hydromorphic conditions in the red earth sediments in Xuancheng, south China, clay mineralogy was investigated using X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The XRD results indicated that clay minerals in the hydromorphic soils were illite, kaolinite, smectite, vermiculite, and mixed-layer illite/smectite and illite/smectite/kaolinite. Changes of the kaolinitic reflections under the various conditions suggested that the kaolinitic phase is a mixed-layer structure having kaolinite layers randomly interstratified with illite and smectite layers. HRTEM observation showed that 10 A illite layers interstratified with both 15 A smectite layers and 7 A kaolinite layers in clay particles, confirming the occurrence of illite/smectite/kaolinite (I/S/K) three-component mixed-layer clays. The lattice fringes of the I/S/K clays appeared corrugated and vanishing, and also exhibited variable thickness along a lattice fringe, which were consistent with changes from illite to smectite, from smectite to kaolinite, and from illite to kaolinite, respectively. Hydromorphic conditions in the Xuancheng soils led simultaneously to the direct transformation of illite to kaolinite and the transformation of illite to smectite to kaolinite in the pedogenic processes, and the formation of I/S/K three-component mixed-layer clays as intermediate products of these processes.

High Resolution TEM Observation of Nanocrystalline Silicon Fabricated by High Pressure Torsion (HPT)

High Resolution TEM Observation of Nanocrystalline Silicon Fabricated by High Pressure Torsion (HPT)