High-resolution Transmission Electron Microscopy Characterization Research Articles

Positron annihilation lifetime spectroscopy of FeCr and FeCrAl oxide dispersion strengthened (ODS) alloys

Positron annihilation lifetime spectroscopy (PALS) was performed to characterize the interfacial nanostructure between the oxide particles and ferritic matrix in two types of oxide dispersion strengthened (ODS) alloys: FeCr dispersed with Y-Ti oxide nanoparticles and FeCrAl dispersed with Y-Al oxide nanoparticles. The spectra were precisely fitted and decomposed into three components with two trapping rates. Bulk lifetime approached the theoretical value of 107 ps by taking into account the positron trapping at two types of defects. Structural characterization was performed by coincidence Doppler broadening and high-resolution transmission electron microscopy (HRTEM), revealing that two-trapping defect clusters were closely associated with Y-Ti or Y-Al oxide particles. Trapping was not detected in the non-ODS alloys. Based on Kuramotos advanced theoretical work, the shorter annihilation lifetime (179–194 ps) could be ascribed to positron trapping at vacancies and divacancies localized under the misfit dislocations associated with Y-Ti or Y-Al oxide particles. The longer annihilation lifetime (301–323 ps) could be attributed to Ar-filled gas bubbles precipitated at the oxide particle-matrix interfaces. These results of PALS analyses were consistent with the HRTEM characterization.

The Construction of a Lattice Image and Dislocation Analysis in High-Resolution Characterizations Based on Diffraction Extinctions.

This paper introduces a method for high-resolution lattice image reconstruction and dislocation analysis based on diffraction extinction. The approach primarily involves locating extinction spots in the Fourier transform spectrum (reciprocal space) and constructing corresponding diffraction wave functions. By the coherent combination of diffraction and transmission waves, the lattice image of the extinction planes is reconstructed. This lattice image is then used for dislocation localization, enabling the observation and analysis of crystal planes that exhibit electron diffraction extinction effects and atomic jump arrangements during high-resolution transmission electron microscopy (HRTEM) characterization. Furthermore, due to the method's effectiveness in localizing dislocations, it offers a unique advantage when analyzing high-resolution images with relatively poor quality. The feasibility of this method is theoretically demonstrated in this paper. Additionally, the method was successfully applied to observed edge dislocations, such as 1/6[211-], 1/6[2-11-], and 1/2[01-1], which are not easily observable in conventional HRTEM characterization processes, in electro-deposited Cu thin films. The Burgers vectors were determined. Moreover, this paper also attempted to observe screw dislocations that are challenging to observe in high-resolution transmission electron microscopy. By shifting a pair of diffraction extinction spots and superimposing the reconstructed images before and after the shift, screw dislocations with a Burgers vector of 1/2[011-] were successfully observed in electro-deposited Cu thin films.

First report on binder-free boehmite (AlOOH) based prototype, flexible, high-energy and high-power density solid-state symmetric supercapacitor

Supercapacitor has potential to accomplish the world-wide growing demand for clean and sustainable energy. It has virtue of both the battery and the conventional capacitor. The proper selection of electrode material is one of the deterministic factors for the excellent performance of supercapacitor. This work presents the first report on the boehmite phase of a polymorph of earth abundant and inexpensive aluminum based oxy hydroxide for energy storage application. The binder-free synthesis of AlOOH electrode has been carried out by the simple, user-friendly, and low-cost successive ionic layer adsorption and reaction (SILAR) technique at 300 K. The X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy characterizations are performed for the knowledge of crystal structure, chemical bonds, oxidation states, morphology and elements present in the sample, respectively. The electrochemical activities of electrode as well as fabricated solid-state symmetric device are disclosed by cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. The maximum specific capacitance of 726.5 F g−1 @ 5 mV s−1 has been achieved for AlOOH electrode with ~67 % cyclic stability for 6500 cycles. The assembled device exhibits an excellent specific capacitance of 314 F g−1 @ 5 mV s−1, energy density of 242 Wh Kg−1 and power density of 4701 W Kg−1 with ~80 % stability up to 6500 cycles. The device has ability to glow red, green, blue and white coloured light emitting diodes (LEDs) for 50, 52, 27 and 21 min respectively. Also, the fabricated device is successful for glowing a panel ‘MNNIT’ made up by 56 red coloured LEDs. These results indicate that this work can be beneficial for fulfilling the energy demand of future generation.

Interfacial oxygen atom modification of a PdSn alloy to boost oxygen reduction in zinc-air batteries

Modifying the surface of a catalyst with heteroatoms can regulate the interfacial atomic valence state and adjust the charge distribution, which is promising for obtaining desirable platinum carbon catalyst (Pt/C)-matched oxygen reduction reaction (ORR) catalytic performance. Here, we developed an efficient method to access O-rich crystalline interfacial-exposed palladium-tin alloy (111) crystal surfaces [Pd3Sn (111)] for highly efficient ORR via direct reduction of Pd/Sn metal salt species that are well dispersed in a nitrogen, phosphorus-doped carbonaceous (NPC) substrate. In addition to the other materials, preembedded Pd/Sn metal salt species in NPC control the release of metal sources upon reduction in the liquid phase, resulting in the grafting of an as-prepared PdSn alloy with many merits, such as efficient electron conduction, short-range crystallinity and increased crystal interface exposure. The presence of a considerable quantity of oxygen atoms at the interface of small-sized PdSn alloys on NPC substrates has been methodically verified by powder X-ray diffraction, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy characterizations. The PdSn-O sample exhibited excellent ORR activity, achieving an onset potential of ∼0.99 V and a half-wave potential of ∼0.88 V at 1600 rpm in O2-saturated 1.0 M KOH. Density functional theory simulations of pure Pd, Pd-O, the PdSn alloy and PdSn-O suggest that interfacial oxygen atom modification is responsible for the significantly improved ORR activity. The assembled zinc-air battery provides a high specific power of 218.9 mW cm−2 and a specific capacity of 810.6 mAh gZn-1. Our approach has the potential to stimulate the preparation of O-rich crystalline interfacial-exposed alloy compounds for other energy conversion applications.

Nitrogen-Doped Graphene Quantum Dot-Passivated δ-Phase CsPbI3: A Water-Stable Photocatalytic Adjuvant to Degrade Rhodamine B.

Inorganic halide perovskite CsPbI3 is highly promising in the photocatalytic field for its strong absorption of UV and visible light. Among the crystal phases of CsPbI3, the δ-phase as the most aqueous stability; however, directly using it in water is still not applicable, thus limiting its dye photodegradation applications in aqueous solutions. Via adopting nitrogen-doped graphene quantum dots (NGQDs) as surfactants to prepare δ-phase CsPbI3 nanocrystals, we obtained a water-stable material, NGQDs-CsPbI3. Such a material can be well dispersed in water for a month without obvious deterioration. High-resolution transmission electron microscopy and X-ray diffractometer characterizations showed that NGQDs-CsPbI3 is also a δ-phase CsPbI3 after NGQD coating. The ultraviolet-visible absorption spectra indicated that compared to δ-CsPbI3, NGQDs-CsPbI3 has an obvious absorption enhancement of visible light, especially near the wavelength around 521 nm. The good dispersity and improved visible-light absorption of NGQDs-CsPbI3 benefit their aqueous photocatalytic applications. NGQDs-CsPbI3 alone can photodegrade 67% rhodamine B (RhB) in water, while after compositing with TiO2, NGQDs-CsPbI3/TiO2 exhibits excellent visible-light photocatalytic ability, namely, it photodegraded 96% RhB in 4 h. The strong absorption of NGQDs-CsPbI3 in the visible region and effective transfer of photogenerated carriers from NGQDs-CsPbI3 to TiO2 play the key roles in dye photodegradation. We highlight NGQDs-CsPbI3 as a water-stable halide perovskite material and effective photocatalytic adjuvant.

Hydrothermal synthesis of rGO/ZnCoS composite for high performance asymmetric supercapacitor and HER applications

Here, we describe the successful hydrothermal synthesis of zinc cobalt sulfide (ZCS) and reduced graphene oxide/zinc cobalt sulfide (rZCS) nanoparticles (NPs). X-Ray diffraction (XRD), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (TEM) characterizations were used to test ZCS and rZCS NPs. Through the use of cyclic voltammetry (CV), galvanostatic charge and discharge (GCD), and electrochemical impedance (EIS), the electrochemical performance of the produced materials was examined. Electrochemical hydrogen evolution reactions and supercapacitor applications required the development of ZCS and rZCS electrodes. 10 mA cm−2 is driven by rZCS electrodes at 132 mV HER overpotential. High specific capacitance of 891 F g−1 at 1 A g−1 current density and 99 % cyclic stability are displayed by the rZCS electrode even after 5000 cycles. The specific capacitance of (181 F g−1 at 1 A g−1 current density), capacitance retention of (92 % after 10,000 cycles at 1 A g−1, and energy and power density of (29 Wh kg−1 and 2215 W kg−1), respectively, are recommended for asymmetric supercapacitors made from cells using rZCS and rGO. Given this, the produced electrodes material holds potential for use in energy storage technologies.

Three-Dimensional (3D) Ordered Macroporous Bimetallic (Mn,Fe) Selenide/Carbon Composite with Heterojunction Interface for High-Performance Sodium Ion Batteries.

Transition-metal selenides have captured significant research attention as anode materials for sodium ion batteries (SIBs) due to their high theoretical specific capacities and excellent electronic conductivity. However, volumetric expansion and inferior cycle life still hinder their practical application. Herein, a three-dimensional (3D) ordered macroporous bimetallic (Mn,Fe) selenide modified by a carbon layer (denoted as 3DOM-MnFeSex@C) composite containing a heterojunction interface is fabricated through selenizing a 3D ordered macroporous Mn-based Prussian Blue analogue single crystal. The 3DOM-MnFeSex@C exhibits hierarchically porous architecture with enhanced mass-transfer efficiency; MnSe and FeSe2 particles are encapsulated into macroporous carbon framework, which can significantly promote the electronic conductivity and maintain the structural integrity. The density functional theory calculation indicates that the heterojunction interface between MnSe and FeSe2 has been successfully engineered so that Na+ can be readily adsorbed and rapidly converted, thus promoting the reaction kinetics and extending the cyclic life. As expected, the 3DOM-MnFeSex@C composite delivers excellent rate performance (277.6 mA h g-1 at 10 A g-1), and prolonged cycling life (191.6 mA h g-1 even after 1000 cycles at 2 A g-1) as a sodium storage anode. The sodium storage mechanism of the composite was further investigated by in situ X-ray diffraction and ex situ high-resolution transmission electron microscopy characterization techniques.

Simple and Ingenious Manner To Build Li-Rich Layered Materials with Surface Layered/Spinel Heterostructures and Li Deficiencies

The voltage and capacity attenuation is one of the main bottlenecks limiting the commercialization of Li-rich layered materials, the introduction of a spinel structure and Li deficiencies into materials may mitigate or suppress these shortcoming. The Li-rich layered materials with surface layered/spinel heterostructures and Li deficiencies (LR-S) are prepared by a simple and ingenious manner; that is, the 10 wt % precursors are added to uptake the volatilize of Li ions, which produced from the Li-rich material (LR-0) in a high-temperature post-treatment process. The generation of the spinel structure and Li deficiencies in the LR-S sample are confirmed by the inductively coupled plasma mass spectrometry, X-ray diffraction, Raman spectra, and high-resolution transmission electron microscopy characterizations. The LR-S sample displayed excellent electrochemical performances; that is, the capacity retentions is 88.92% at 1 C after 200 cycles, and the voltage drop for each cycle is 1.91 mV, respectively. The reason is mainly attributed to the spinel structure serving as a pillar structure and the Li deficiencies modulating the oxidation products of the oxygen ion in the removal/uptake process.

Photocatalytic Degradation of Methylene Blue Using N-Doped ZnO/Carbon Dot (N-ZnO/CD) Nanocomposites Derived from Organic Soybean.

This study reports on successful synthesis of carbon dots (CDs), nitrogen-doped zinc oxide (N-ZnO), and N-ZnO/CD nanocomposites as photocatalysts for degradation of methylene blue. The first part was the synthesis of CDs utilizing a precursor from soybean and ethylenediamine as a dopant by a hydrothermal method. The second part was the synthesis of N-ZnO with urea as the nitrogen dopant carried out by a calcination method in a furnace at 500 °C for 2 h in an N2 atmosphere (5 °C min-1). The third part was the synthesis of N-ZnO/CD nanocomposites. The characteristics of CDs, N-ZnO, and N-ZnO/CD nanocomposites were analyzed through Fourier transform infrared (FTIR), UV-vis absorbance, photoluminescence (PL), high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), thermal gravimetry analysis (TGA), field-emission scanning electron microscopy energy-dispersive spectroscopy (FESEM EDS), X-ray photoelectron spectroscopy (XPS), and Brunauer-Emmett-Teller (BET) analysis. Based on the HR-TEM analysis, the CDs had a spherical shape with an average particle size of 4.249 nm. Meanwhile, based on the XRD and HR-TEM characterization, the N-ZnO and N-ZnO/CD nanocomposites have wurtzite hexagonal structures. The materials of N-ZnO and N-ZnO/CD show increased adsorption in the visible light region and low energy gap E g. The E g values of N-ZnO and N-ZnO/CDs were found to be 2.95 and 2.81 eV, respectively, whereas the surface area (S BET) values 3.827 m2 g-1 (N-ZnO) and 3.757 m2 g-1(N-ZnO/CDs) belonged to the microporous structure. In the last part, the photocatalysts of CDs, N-ZnO, and N-ZnO/CD nanocomposites were used for degradation of MB (10 ppm) under UV-B light irradiation pH = 7.04 (neutral) for 60 min at room temperature. The N-ZnO/CD nanocomposites showed a photodegradation efficiency of 83.4% with a kinetic rate of 0.0299 min-1 higher than N-ZnO and CDs. The XRD analysis and FESEM EDS of the N-ZnO/CDs before and after three cycles confirm the stability of the photocatalyst with an MB degradation of 58.2%. These results have clearly shown that the N-ZnO/CD nanocomposites could be used as an ideal photocatalytic material for the decolorization of organic compounds in wastewater.

Influence of V/III flux ratio on type-II InAs/GaSb superlattice for very-long wavelength

A study was conducted to develop InAs/GaSb type-II superlattices (T2SLs) for very long wavelength infrared detection. The results showed that the V/III flux ratio not only affected the magnitude of the strain but also greatly influenced the surface roughness. By further optimizing the V/III ratio, a sample with a surface roughness of 0.1 nm was obtained within the 5 × 5 μm2 range, and the full width at half maximum (FWHM) of the 0th order peak was only 22.1 arcsec. Through high-resolution transmission electron microscopy (HRTEM) characterization, InAs and GaSb could be effectively distinguished, and strain maps revealed that both interfaces were under large tensile strain. Furthermore, energy dispersive spectroscopy (EDS) testing revealed some degree of intermixing between different layers. Finally, a photoluminescence (PL) spectrum was used to measure the band gap and quality of the superlattice. The PL peak was determined to be 13.65 μm with an FWHM of 27 meV, laying the foundation for the subsequent preparation of a very long wavelength array detector.

Microstructure evolution and deformation behaviors of pulse electro-assisted deformation in M50 bearing steel

M50 bearing steel, as a typical high-carbon and high-alloy steel, exhibits low cold working capacity with high susceptibility to produce voids during the ring rolling process. In this paper, the deformation behavior and microstructure evolution during the pulse electro-assisted tensile of M50 bearing steel was studied. The results show that the maximum flow stress of the pulse electro-assisted deformation (PEAD) specimen decreases by 24.4% compared with that of the traditional deformation (TD) specimen. However, the section shrinkage rate of the PEAD specimen is increased by 32.2%, which is even higher than that of the hot deformation (HD) specimens. This indicates the positive role of pulse current on the increase of plasticity. Besides, a constitutive model of the PEAD process was established by fitting the experimental data. With the assistance of pulse current, the deformation resistance and work hardening are reduced, so the number of voids is significantly reduced due to the easier metal flow. Moreover, the energy barrier of recrystallized grain nucleation is decreased, while the nucleation rate is increased under pulse current. As a result, the occurrence of recrystallization leads to the reduction of average grain size from 4.51 μm to 1.62 μm. According to the high-resolution transmission electron microscopy (HRTEM) and electron backscatter diffraction (EBSD) characterizations, the dislocation density decreases by 24% under the applied current, which should be ascribed to the increase of atomic flux and lattice diffusion coefficient. This work provides a new idea for solving the plastic deformation problem of M50 bearing steel.

Size-dependent magnetic properties of Mn-Co-NiO based heterostructured nanoparticles

In this work, we investigate the synthesis, along with the structural and magnetic properties, of novel Mn-Co-NiO-based heterostructured nanocrystals (HNCs). The objective is to develop novel, well-structurally ordered inverted antiferromagnetic (AFM) NiO–ferrimagnetic (FiM) spinel phase overgrowth HNCs. Inverted HNCs are particularly promising for magnetic device applications because their magnetic properties are more easily controlled by having well-ordered AFM cores, which can result in magnetic structures having large coercivities, tunable blocking temperatures, and other enhanced magnetic effects. The synthesis of the HNCs is accomplished using a two-step process: In the first step, NiO nanoparticles are synthesized using a thermal decomposition method. Subsequently, Mn-Co overgrowth phases are grown on the NiO nanoparticles via hydrothermal nanophase epitaxy, using a fixed pH level (∼5.3) of the aqueous medium. This pH level was selected based on previous work in our laboratory showing that NiO/Mn3O4 HNCs of constant size have optimal coercivity and exchange bias when synthesized at a pH of 5.0. The crystalline structure and gross morphology of the Mn-Co-NiO-based HNCs have been analyzed using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques, respectively. Analysis using these techniques shows that the HNCs are composed of a NiO core and a CoMn2O4 overgrowth phase. Rietveld refinement of XRD data shows that the NiO core has the rocksalt (Fm3̄m) cubic crystal structure and the CoMn2O4 overgrowth has the spinel (I41/amd) crystal structure. Moreover, an increased relative amount of the CoMn2O4 overgrowth phase is deposited with decreasing NiO core particle size during the synthesis of the HNCs. The results from PPMS magnetization and high-resolution transmission electron microscopy (HRTEM) characterization of the Mn-Co-NiO-based HNCs are discussed herein.

CVD Approach to a Single Gallium Oxide Nanowire for Solar-blind UV Detector

Aims: Finding a simple, energy-saving and low-cost synthesis method to process gallium oxide nanowires by CVD for solar-blind detector. Background: Due to a bandgap of 4.5 ~ 4.9 eV and a high breakdown field strength of 8 MV/cm, gallium oxide (Ga2O3) has great application prospects in solar blind ultraviolet detection and highpower devices. Notably, Ga2O3 NWs are currently the key research objects of solar-blind UV detection materials because of the characteristics of efficient photocarrier separation and collection, and the bandgap width perfectly matches the energy of deep ultraviolet photons. Objective: To find a simple method to synthesize Ga2O3 nanowire with smooth surface and uniform diameter, and the prepared single gallium oxide nanowire UV detector has high photoelectric conversion efficiency. Methods: Ga2O3 NWs are prepared on the SiO2/Si substrate by chemical vapor deposition (CVD) approach at low reaction temperature with gold particles serving as the catalyst and gallium arsenide (GaAs) as a gallium source. Results: X-ray diffraction and Raman spectroscopy characterization indicate the crystal structure of NWs is β-Ga2O3, and scanning electron microscope (SEM) characterization proves that the NWs have a uniform diameter and smooth surface. Moreover, the high-resolution transmission electron microscopy (HRTEM) characterization shows that the material had high crystal quality. Moreover, the photoconductive solar-blind UV detector with a single Ga2O3 NW is prepared, showing the excellent performance of the high responsivity and external quantum efficiency. Conclusion: The effects of growth temperature and the size of gold catalyst on the morphology of β-Ga2O3 NWs have been investigated. The results show that with the reaction temperature is 625 °C and the diameter of Au catalyst is about 30~50 nm, it is more conducive to the formation of NMs with crystal structure, smooth surface and uniform diameter. The performance of the solar-blind UV photodetector shows that the device has higher sensitivity (R = 149.82 A/W), external quantum efficiency (EQE = 73206%), and response rate τrise = 0.66 s, and τdown = 0.45 s.

Wafer-scale detachable monocrystalline germanium nanomembranes for the growth of III-V materials and substrate reuse.

Germanium (Ge) is increasingly used as a substrate for high-performance optoelectronics, photovoltaics, and electronic devices. These devices are usually grown on thick and rigid Ge substrates manufactured by classical wafering techniques. Nanomembranes (NMs) provide an alternative to this approach while offering wafer-scale lateral dimensions, weight reduction, waste limitation, and cost effectiveness. Herein, we introduce the Porous germanium Efficient Epitaxial LayEr Release (PEELER) process, which consists of the fabrication of wafer-scale detachable Ge NMs on porous Ge (PGe) and substrate reuse. We demonstrate the growth of Ge NMs with monocrystalline quality as revealed by high-resolution transmission electron microscopy (HRTEM) characterization. Together with the surface roughness below 1 nm, it makes the Ge NMs suitable for growth of III-V materials. Additionally, the embedded nanoengineered weak layer enables the detachment of the Ge NMs. Finally, we demonstrate the wet-etch-reconditioning process of the Ge substrate, allowing its reuse, to produce multiple free-standing NMs from a single parent wafer. The PEELER process significantly reduces the consumption of Ge in the fabrication process, paving the way for a new generation of low-cost flexible optoelectronic devices.

Diffusion mechanism and electrochemical investigation of 1T phase Al–MoS2@rGO nano-composite as a high-performance anode for sodium-ion batteries

We report the electrochemical investigation of 5% Al doped MoS2@rGO composite as a high-performance anode for sodium (Na)-ion batteries. The x-ray diffraction (XRD), Raman spectroscopy and high-resolution transmission electron microscopy characterizations reveal that the Al doping increase the interlayer spacing of (002) plane of MoS2 nanosheets and form a stable 1T phase. The galvanostatic charge–discharge measurements show the specific capacity stable around 450, 400, 350, 300 and 200 mAhg−1 at current densities of 0.05, 0.1, 0.3, 0.5 and 1 Ag−1, respectively. Also, we observe the capacity retentions of 86% and 66% at 0.1 and 0.3 Ag−1, respectively, over 200 cycles with a consistent Coulombic efficiency of nearly 100%. The cyclic voltammetry, galvanostatic intermittent titration technique, and electrochemical impedance spectroscopy are used to find the kinetic behavior and the obtained value of diffusion coefficient falls in the range of 10−10 to 10−12 cm2s−1. Intriguingly, the in-situ EIS also explains the electrochemical kinetics of the electrode at different charge–discharge states with the variation of charge transfer resistance. Moreover, the post cycling investigation using ex-situ XRD and photoemission spectroscopy indicate the coexistence of 1T/2H phase and field-emission scanning electron microscopy confirm the stable morphology after 500 cycles. Also, the Na-ion transport properties are calculated for 1T Al–MoS2@rGO interface and Al–MoS2–MoS2 interlayer host structure by theoretical calculations using density functional theory.

Ammonium Hydroxide-Assisted Growth of Large-Scale Single-Crystalline Molybdenum Disulfide

Controllable, massive synthesis of low-dimensional materials is one of the important cornerstones for the development of new functional devices oriented to high-tech industries, and the key to realize the broad application prospects in the quantum realm. How to synthesize high-quality low-dimensional materials for the next-generation high-performance devices has been regarded as a significant research direction in the field of functional material preparation technology. Here, we developed a convenient method via a solution-assisted thin-film precursor to replace the conventional solid-state powder precursor by taking advantage of the soluble nature of molybdenum trioxide in ammonium hydroxide, which can remarkably reduce the supersaturation state of vapor reactants and thus reduce the nucleation density, beneficial for the growth of large-scale single-crystalline molybdenum disulfide monolayers. The effect of the precursor concentration on the growth process has been systematically investigated. High-resolution transmission electron microscopy and temperature-dependent optical characterizations have been performed to examine the crystal quality of the ammonium hydroxide-assisted synthesized samples.

Interface effect on superlattice quality and optical properties of InAs/GaSb type-II superlattices grown by molecular beam epitaxy

We systematically investigate the influence of InSb interface (IF) engineering on the crystal quality and optical properties of strain-balanced InAs/GaSb type-II superlattices (T2SLs). The type-II superlattice structure is 120 periods InAs (8 ML)/GaSb (6 ML) with different thicknesses of InSb interface grown by molecular beam epitaxy (MBE). The high-resolution x-ray diffraction (XRD) curves display sharp satellite peaks, and the narrow full width at half maximum (FWHM) of the 0th is only 30–39 arcsec. From high-resolution cross-sectional transmission electron microscopy (HRTEM) characterization, the InSb heterointerfaces and the clear spatial separation between the InAs and GaSb layers can be more intuitively distinguished. As the InSb interface thickness increases, the compressive strain increases, and the surface “bright spots” appear to be more apparent from the atomic force microscopy (AFM) results. Also, photoluminescence (PL) measurements verify that, with the increase in the strain, the bandgap of the superlattice narrows. By optimizing the InSb interface, a high-quality crystal with a well-defined surface and interface is obtained with a PL wavelength of 4.78 μm, which can be used for mid-wave infrared (MWIR) detection.

Tailoring bainitic transformation and enhancing mechanical properties of carbide-free bainitic steel via high-temperature ausforming

Ausforming, as a classical thermomechanical treatment process, provides a promising solution for evading the strength-ductility trade-off in carbide-free bainitic (CFB) steel. However, systematic investigations of the relationships among ausforming strain, bainitic transformation kinetics, bainitic morphology and RA features, together with their effects on mechanical properties were scarce in the literature. In the present work, high-temperature ausforming (HT-AF) at 650 °C with different strains was employed in a CFB steel. With the increase of deformation, the prior austenite grain (PAG) gradually flattens. The content of retained austenite (RA) at room temperature increases and RA morphology changes from “film and a small amount of block” to “block and a small amount of film”. Systematic high-resolution dilatometry and transmission electron microscopy (TEM) characterization were performed to reveal the effect of dislocation substructures on bainitic transformation kinetics. The results show that the microband structures reduce the transformed volume fraction and the maximum transformation rate. But the small deformation (10–25%) promotes the initial transformation rate, when the deformation increases to 35–45%, the bainite transformation kinetics is inhibited due to the high mechanical stability of deformed austenite. Given these results and the variation of bainitic morphology, the mechanism of microstructure evolution, especially the RA evolution is elucidated. Compared with the sample without ausforming, HT-AF improves the strength and ductility simultaneously, achieving outstanding mechanical properties (an ultimate tensile strength of 1578 MPa and uniform elongation of 11.1% in the AF45 + AT sample). This is attributed to a large amount of blocky RA providing the dislocation absorption and sustained TRIP effect, the former promotes the ductility and the latter effectively enhances the work hardening capacity.

HRTEM and XPS characterizations for probable formation of TiBxNy solid solution during sintering process of TiB2–20SiC–5Si3N4 composite

The microstructural characterization of spark plasma sintered (SPSed) TiB2–20SiC–5Si3N4 composite was investigated precisely by high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The in-situ phases generated during SPS process were studied using HSC chemistry software; the result shows the development of plausible reaction between Si3N4 and B2O3, forming h-BN. In addition, the formation of in-situ TiN phase and/or TiBxNy solid solution was confirmed by XPS analysis, attributed to the reaction between TiO2 and Si3N4. The intense interfaces were formed due to the coherency between hexagonal structures of TiB2, SiC, and BN phases as detected by HRTEM. Furthermore, weak interfaces were generated because of the formation of amorphous phases in the junctions. The presence of structural defects and distortions was demonstrated in both TiB2 and h-BN by inverse fast Fourier transform patterns, which can be considered as the main channels for the diffusion and transportation of boron atoms into the Si3N4 matrix for the formation of the in-situ h-BN phase.

Preparation and capacitive properties of nitrogen-doped graphene aerogels compatible with ionic liquid electrolyte

<p indent="0mm">Nitrogen-doped graphene aerogels for high performance supercapacitors were synthesized using guanidine phosphate, guanidine carbonate and guanidine hydrochloride as the nitrogen source by a facile and cost-effective hydrothermal method. X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy characterizations and nitrogen adsorption/desorption analysis indicate that these nitrogen doped graphene aerogels have poorly ordering and hierarchical porous structures. Especially, the nitrogen-doped graphene aerogel (HRGA) prepared using guanidine hydrochloride as the nitrogen source possesses a richer nitrogen content (9.7 at%), higher conductivity <sc>(11.2 S cm⁻¹),</sc> and larger specific surface area <sc>(257.6 m² g⁻¹).</sc> Thus HRGA delivers a large specific capacitance of <sc>176 F g⁻¹</sc> at a current density of <sc>1 A g⁻¹</sc> and an energy density of 70 Wh kg⁻¹ at a power density of <sc>0.875 kW kg⁻¹.</sc> The nitrogen-doped graphene aerogels with enhanced electrochemical performance provide a huge application potential in the energy storage field.