TEM Observations Research Articles

Structural and luminescent behaviour of Dy(III) activated Gd3Al5O12 nanophosphors for white-LEDs applications

Single-phased cool white light emissive Gd3-xAl5O12:xDy3+ (x = 0.01–0.07) nanophosphors have been prepared via gel-combustion method. Diffraction data and Rietveld results have affirmed the development of a single cubic phase of all the prepared samples with space group Ia-3d (230). Furthermore, the crystallite size was calculated through Debye-Scherrer's formula and the Williamson-Hall method. In the EDX spectrum, the nonappearance of peaks analogous to the foreign elements affirms the pure synthesis of the considered phosphors. TEM observation reveals agglomerated porous crystalline material with non-uniform shapes and varying particle size in 20–80 nm range. Fourier Transform Infrared (FTIR) spectroscopy has been applied for the examination of chemical bonding with other vibrational characteristics. The luminescence study confirms the intense blue and yellow emission of synthesized phosphors at 482 nm and 576 nm respectively owing to 4F9/2→6HJ (J = 15/2, 13/2) transitions. Moreover, the detailed evaluation of the concentration quenching process established that dipole-quadruple interaction is answerable for the decrease in luminescence intensity after optimum dopant ion concentration. The chromatic analyses recommend that emission from the optimized nanophosphors lies in the region of cool white light. The outcomes of the present investigation propose that Gd3Al5O12:Dy3+ phosphors might be a potential candidate for white-LEDs applications.

Sub-micron grains and excellent mechanical properties of low-density Ti40Nb30Mo30-C refractory complex concentrated alloy reinforced with in-situ (Ti, Nb)C

A novel Ti40Nb30Mo30–C refractory complex concentrated alloy (RCCA) reinforced with in-situ (Ti, Nb)C carbide was fabricated via mechanical alloying (MA) and hot press sintering (HPS). The results showed that the alloy consisted of the BCC crystal-structured solid solution phase (TNM phase) and FCC crystal-structured carbide (Ti, Nb)C with submicron grain sizes of 0.79 μm and 0.62 μm, respectively. The ultra-fine microstructure benefitted from the inhabitation of in-situ (Ti, Nb)C on grain growth. The ab-initio calculations were conducted to study the formation process of in-situ composite carbide (Ti, Nb)C. Here, the lightweight alloy with a density of 7.04 g/cm3 exhibited promising compressive mechanical properties, such as yield strength of 2550 MPa at room temperature (RT), 1508 MPa at 800 °C, and 485 MPa at 1000 °C. According to the deformation microstructures at RT and 1000 °C, the dislocation-dominated deformation mechanism in TNM phase was invariable at different temperatures. A brittle-to-ductile translation of (Ti, Nb)C was found to happen at around 1000 °C supported by the stress-strain curve and TEM observation. The excellent mechanical properties of the alloy were attributed to the grain boundary strengthening and the strengthening effect of (Ti, Nb)C that hindered the movement of dislocations. The strategy that incorporating in-situ carbides into light RCCA system could broaden and inspire the design of lightweight and high-strength materials.

A Comparative Investigation of Mechanical Properties of TiB2/Cr Multilayer Film by Indentation

Alternating TiB2-dcMS and Cr-HiPIMS layers are used to fabricate TiB2/Cr multilayer films. Introducing a 5-nm-thick Cr interlayer deposited under a substrate bias of −60 V produces slight increases in both film hardness and elastic modulus. The TEM observation indicates that the Cr grains favor epitaxial growth on the TiB2 interlayer, forming a coherent TiB2/Cr interface. This improves hardness. Mechanic measurement by using AFM illustrates that the coherent interface increases the elastic modulus of the Cr up to ~280 GPa, which is significantly higher than bulk material.

A low-temperature prismatic slip instability in Mg understood using machine learning potentials

Prismatic slip in magnesium at temperatures T≲150 K occurs at ∼ 100 MPa independent of temperature, and jerky flow due to large prismatic dislocation glide distances is observed; this athermal regime is not understood. In contrast, the behavior at T≳150 K is understood to be governed by a thermally-activated double-cross-slip of the stable basal screw dislocation through an unstable or weakly metastable prism screw configuration and back to the basal screw. Here, a range of neural network potentials (NNPs) that are very similar for many properties of Mg including the basal-prism-basal cross-slip path and process, are shown to have an instability in prism slip at a potential-dependent critical stress. One NNP, NNP-77, has a critical instability stress in good agreement with experiments and also has basal-prism-basal transition path energies in very good agreement with DFT results, making it an excellent potential for understanding Mg prism slip. Full 3d simulations of the expansion of a prismatic loop using NNP-77 then also show a transition from cross-slip onto the basal plane at low stresses to prismatic loop expansion with no cross-slip at higher stresses, consistent with in-situ TEM observations. These results reveal (i) the origin and prediction of the observed unstable low-T prismatic slip in Mg and (ii) the critical use of machine-learning potentials to guide discovery and understanding of new important metallurgical behavior.

Mesoporous Calcium-Silicate Nanoparticles Loaded with Prussian Blue Promotes Enterococcus Faecalis Ferroptosis-Like Death by Regulating Bacterial Redox Pathway ROS/GSH.

Mesoporous calcium-silicate nanoparticles (MCSNs) are advanced biomaterials that have been used to control drug delivery for many years. Ultrasmall Prussian blue nanoparticles (UPBNPs) showed high peroxidase and catalase-like activities. This study evaluated the antibacterial and antibiofilm properties, mechanism and cytotoxicity of UPBNPs-MCSNs composites synthesized by both as precursors. UPBNPs-MCSNs were prepared and characterized. The antibacterial effect of UPBNPs-MCSNs was evaluated by the MTT assay and CFU counting method, and their biosafety was tested by CCK8. Then explore the antibacterial mechanism, including TEM observation of bacterial morphology, and detection of bacterial ROS, LPO and GSH levels. The antibiofilm activity of UPBNPs-MCSNs was tested by E. faecalis biofilm model in human roots. The roots were pretreated with materials and cultured with E. faecalis, and the survival of E. faecalis on the root canal wall was observed by SEM and CLSM. The results showed that UPBNPs-MCSNs had potent antibacterial and antibiofilm activities. They can aggregate on the dentin surface and significantly inhibit E. faecalis adhesion and colonization. Their antibacterial activity is as effective as NaClO and calcium hydroxide (CH), can significantly prolong the time of bacterial colonization than CH, but have lower cytotoxicity to normal cells. We found that UPBNPs-MCSNs trigger a like classic ferroptosis pathway in bacteria. UPBNPs-MCSNs can induce bacteria to produce ROS and LPO, and reduce GSH level. Moreover, we observed that the metal ions chelator and the antioxidant could block their antibacterial activity. These results reveal that UPBNPS-MCSNs have high antibacterial and antibiofilm, and can mediate the bacterial redox pathway ROS/GSH like the classical pathway of ferroptosis, providing a theoretical basis for them to develop into a safe and effective novel root canal disinfectant.

Changes in the microstructure and mechanical properties of Ti–6Al–4V alloys induced by Fe ion irradiation at a high He generation rate

As an attractive candidate structural material for nuclear applications, the irradiation resistance of high strength double-phase titanium alloys Ti–6Al–4V has received great attention. In this study, the damage dose dependence of defect evolution and hardening behavior at a high He generation rate (95 appm/dpa) for the Ti–6Al–4V alloys were investigated by TEM and nanoindentation. The Ti–6Al–4V specimen was pre-implanted with He ions with multi-energy at ambient temperature and subsequently irradiated with Fe ions at about 420 °C to obtain a He/dpa plateau region (400–1000 nm) with average damage doses of 1.5, 7.5 and 37.5 dpa. TEM observations show that the bubble density showed a strong damage dose dependence. When irradiated to 37.5 dpa, the bubble exhibited a bimodal size distribution with the critical radius of approximately 1 nm. Meanwhile, the precipitate length showed an increasing trend with increasing depth, which is due to the effect of injected-Fe ions rather than the damage dose. In addition, no He dependence was observed for the loops and the precipitates. Nanoindentation results indicate that the irradiation hardening became more significant with the increase of damage dose. The contribution of irradiation-induced defects to the hardening was estimated by dispersed barrier-hardening (DBH) model.

Effect of the processing route on the microstructure and mechanical behavior of superlight Mg-9Li-1Zn alloy via friction stir processing

In this study, the effect of the processing route using a friction stir processing (FSP) method on the microstructure and mechanical behavior of a Mg-9Li-1 Zn alloy was systematically investigated. In the FSP method, the odd-numbered (1st and 3rd) process directions and even-numbered (2nd and 4th) passes were alternated to distribute the strain throughout the whole processed zone uniformly. Consequently, the processed zone had a much more uniform microstructure and hardness distribution than the processed zone obtained using the conventional FSP method. Using this method, the grain size of a Mg-9Li-1 Zn sheet alloy was refined from ∼31 μm to ∼0.21 μm with uniformly distributed α and β phases. The processed alloy exhibited a high strength-ductility synergy with an ultimate tensile strength (UTS) of 220.1 MPa and total elongation of 70.0% at a strain rate of 10 −3 s − 1 , overwhelmingly higher than those of the base metal, 155.6 MPa in UTS and 36.0% in elongation. The in-situ SEM-DIC analysis and TEM observation demonstrated that such an outstanding ductility with moderate strength is caused by grain boundary sliding, the dominant deformation mechanism of the ultra-fine-grained sample after FSP. The processing route with reverse processing direction was proven to be efficient in producing the ultrafine grain size microstructure and improving the mechanical properties of superlight Mg-9Li-1 Zn alloy.

Origin of Rapid Coalescence and Active Unstable Fluctuation of Au Nanoparticles under TEM Observation: Electron Bombardment Versus Charge Buildup

Origin of Rapid Coalescence and Active Unstable Fluctuation of Au Nanoparticles under TEM Observation: Electron Bombardment Versus Charge Buildup

Direct TEM Observation of Vacancy‐Mediated Heteroatom Incorporation into a Zeolite Framework: Towards Microscopic Design of Zeolite Catalysts

AbstractIncorporating hetero‐metal‐atom, e.g., titanium, into zeolite frameworks can enhance the catalytic activity and selectivity in oxidation reactions. However, the rational design of zeolites containing titanium at specific sites is difficult because the precise atomic structure during synthesis process remained unclear. Here, a titanosilicate with predictable titanium distribution was synthesized by mediating vacancies in a defective MSE‐type zeolite precursor, based on a pre‐designed synthetic route including modification of vacancies followed by titanium insertion, where electron microscopy (EM) plays a key role at each step resolving the atomic structure. Point defects including vacancies in the precursor and titanium incorporated into the vacancy‐related positions have been directly observed. The results provide insights into the role of point defects in zeolites towards the rational synthesis of zeolites with desired microscopic arrangement of catalytically active sites.

Direct TEM Observation of Vacancy-Mediated Heteroatom Incorporation into a Zeolite Framework: Towards Microscopic Design of Zeolite Catalysts.

Incorporating hetero-metal-atom, e.g., titanium, into zeolite frameworks can enhance the catalytic activity and selectivity in oxidation reactions. However, the rational design of zeolites containing titanium at specific sites is difficult because the precise atomic structure during synthesis process remained unclear. Here, a titanosilicate with predictable titanium distribution was synthesized by mediating vacancies in a defective MSE-type zeolite precursor, based on a pre-designed synthetic route including modification of vacancies followed by titanium insertion, where electron microscopy (EM) plays a key role at each step resolving the atomic structure. Point defects including vacancies in the precursor and titanium incorporated into the vacancy-related positions have been directly observed. The results provide insights into the role of point defects in zeolites towards the rational synthesis of zeolites with desired microscopic arrangement of catalytically active sites.

Stable nanoparticles dispersion induced an unprecedented high strength in a bulk W-TiC alloy

The introduction of nano-scale second-phase particles with high thermal stabilities is expected to solve the dilemma of insufficient strength of metallic materials. In this work, an unprecedented high tensile strength of 1.59 GPa at room temperature is obtained in a representative W-TiC system. We focused on the structure and atomic bonding of TiC/W interface to explore the origin of its stability through high-resolution TEM observation and ab initio calculation. Results reveal that strong covalent bonding between W and C atoms and a low-energy semi-coherent interface with Nishiyama-Wassermanm orientation relationship between TiC and W matrix lead to highly stable and refined TiC particles (∼29 nm) that are responsible for a significant strengthening effect. The present study highlights the importance of interface bonding and stability to achieve fine dispersed nanoparticles for high strength in W materials, and paves the way for high-performance metallic materials through regulation of interface structure and atomic bonding.

Free surface entrainment of oxide particles and their role in ultrasonic treatment performance of aluminum alloys

Although the ultrasonic treatment of molten aluminum has been studied for long period, there is still much to be revealed for this process. Many studies have focused on the investigation of acoustic cavitation and streaming under the horn tip and their effects on the treatment efficiency. However, to the best of our knowledge, no attempt has been done to explain phenomena occurring near or on the melt free surface. Thus, the goal of this study is to investigate phenomena occurring at the free surface during ultrasound irradiation and clarify their possible influence on the ultrasound treatment performance. The results of high temperature and water model experiments reveal that ultrasound irradiation significantly promotes the formation of alumina particles on the melt free surface around sonotrode, and part of these particles can be entrained into aluminum melts. Furthermore, TEM observation results suggested that the entrained alumina inclusions can serve as nucleation sites for the primary Al3Zr compounds. Most importantly, the oxidation and entrainment of particles from free surface are likely to be controllable by the immersion depth of sonotrode into molten aluminum.

High temperature flow behavior and deformation mechanism of Nb-22.5Cr-2.5Mo alloy

The high temperature compression is performed by isothermal compression tests using Gleeble 3500 thermo-mechanical simulator at a constant strain rate for the hot-pressed Nb-22.5Cr-2.5Mo alloy. The deformation temperature ranges from 800 to 1200 °C and the strain rate ranges from 0.001 to 0.1 s−1. The high temperature flow behavior and deformation mechanism of the alloy are investigated based on the compression test data and TEM observation of the deformed microstructure. The results show that the ductile–brittle transition temperature of the alloy is about 800 °C, and the flow stress is sensitive to the deformation temperature and strain rate, in which the plastic deformation ability increases obviously with the increase of deformation temperature and the decrease of strain rate. Besides, the hyperbolic sine Arrhenius model with stress exponent of 3.131 and apparent activation energy of 402 kJ/mol is established for the peak flow stress. The value of apparent activation energy is higher than that of Nb metal (246 kJ/mol), which implies the existence of Laves phase NbCr2 makes the deformation of the alloy more difficult. Moreover, it is found that the deformation mechanisms in the Nbss matrix are mainly DRV controlled by the glide and climb of the dislocations and DRX formed by bulging nucleation mechanism, while the deformation mechanisms in the Laves phase NbCr2 particles are mainly twinning and synchroshear of the Shockley partial dislocation.

Direct environmental TEM observation of silicon diffusion-induced strong metal-silica interaction for boosting CO2 hydrogenation

Direct environmental TEM observation of silicon diffusion-induced strong metal-silica interaction for boosting CO2 hydrogenation

(Digital Presentation) Fabrication of Thick SiGe／Ge Multiple Quantum Wells on Ge-on-Si and Their Optical Properties

Recently, silicon photonics has been attracting increased attention toward the realization of on-chip optical interconnection, which requires high-efficiency light-emitting devices to be integrated on Si substrates. Although Ge is an indirect-band-gap semiconductor like Si, it is expected that strong direct-transition luminescence can be obtained from tensile-strain induced Ge-on-Si. We have previously reported that highly efficient room temperature EL emission can be obtained from strained Ge-on-Si p-i-n diodes[1]. Further increase in luminescence intensity and control of emission wavelengths can be made possible by introducing a quantum well structure in the active layer. Although the fabrication of high-quality strained SiGe layers on Ge is essential for the fabrication of quantum well structures, critical thickness limits the total growth thickness and numbers of strained layers, which makes device applications difficult. We have succeeded in fabricating a strained SiGe layer that is much thicker than the critical thickness by using a patterning method, and this result is very promising for the fabrication of SiGe/Ge multi-quantum well structure[2]. It also makes it possible to increase the number of layers of MQW structure. In this work, we fabricate relatively thick strained SiGe/Ge MQW structures on Ge-on-Si by utilizing the patterning method, and evaluate their crystallinity and optical properties.In experiments, the crystal growth was carried out with solid source molecular beam epitaxy. The Ge-on-Si was fabricated using the so-called two-step method. 40 and 500 nm thick Ge layers were subsequently grown on a Si(100) or Si(111) substrate at 350 and 600℃, respectively, followed by annealing at 800℃ for 10 min. Subsequently we carried out the patterning of the Ge-on-Si(111) substrate by photolithography process. 50nm thick Ge buffer layer ware grown on a Ge-on-Si(100) or Ge-on-Si(111), 6nm thick Si0.1Ge0.9 barrier layer and 10nm thick Ge well layer ware grown with 10-20 cycles at 350℃.Laser microscopic measurements showed no clear roughness on the surface. X ray diffraction measurements show periodic peaks originated from the multi layers of SiGe/Ge, which indicates that the sample has high crystallinity and abrupt interface between the Ge and SiGe layers. TEM observation indicated that no defect was found inside the crystal in the MQW structure. Photoluminescence (PL) measurements were carried out at room temperature and a PL peak originated from the quantum-confinement in the SiGe/Ge MQW was obtained. This PL peak wavelength was also found to shift depending on the Ge concentration of the SiGe. PL intensity of the peak was increased compared with Ge-on-Si without the SiGe/Ge MQW overgrowth. From these results, it is demonstrated that high quality SiGe/Ge multiple quantum structures can be fabricated on Ge-on-Si by utilizing the patterning method and strong room temperature PL was obtained from the MQW, indicating that SiGe/Ge MQW is promising for the applications to light-emitting devices integrated on the Si platform.[1] K.Yamada et al. Appl. Phys. Express 14 045504(2021) [2] Y.Wagatsuma et al. Appl. Phys. Express 14 025502(2021) Figure 1

Effects of Solidification Conditions on Microstructure and Properties of High-Entropy Alloys from the CoCrFeMnNi Family

Alloys from the CoCrFeMnNi family remain the most studied austenitic high-entropy alloys. In this study, four alloys, i.e., Cantor alloy, A3S (modified nonequiatomic Cantor composition), both “pure” or doped with carbon (200 wt. ppm) and niobium (1.3 wt.%), were investigated. Firstly, alloys were induction cast using a cold-crucible method. The obtained ingots were molten, and rapidly solidified by melt-spinning at two cooling rates to obtain “ribbons”, typical of such processing. The effects of the solidification rate and the presence of carbon and niobium on the microstructure and hardness were studied. All the studied alloys show an fcc structure. The lattice parameter of the fcc phase increases with the increasing cooling rate, and with the addition of niobium and carbon, which confirms at least a partial presence of these elements in solid solution. Yet, TEM observations revealed the formation of nanometric NbC precipitates. The microstructure of melt-spun ribbons consists of equiaxed grains of a few micrometers in size. The higher cooling rate led to a small decrease in the grain size and a slight increase in hardness. Moreover, the hardness of doped alloys can be further improved by annealing (500°C for 24 h) through NbC precipitation.

In Situ TEM Observation of the Atomic Transport Process during the Coalescence of Au Nanoparticles.

In practical applications, the coalescence of metal nanoparticles (NPs) is a major factor affecting their physical chemistry properties. Currently, due to a lack of understanding of the atomic-level mechanisms during the nucleation and growth stages of coalescence, the correlation between the different dynamic factors in the different stages of NP coalescence is unclear. In this study, we used advanced in situ characterization techniques to observe the formation of atomic material transport channels (Au chains) during the initiation of coalescence nucleation. We focused on the movement and migration states of Au atoms and discovered an atomic ordered arrangement growth mechanism that occurs after the completion of nucleation. Simultaneously, we used density functional theory to reveal the formation principle of Au chains. These findings improve our understanding of the atomic-scale coalescence process, which can provide a new perspective for further research on coalescence atomic dynamics.

Experimental Study at the Phase Interface of a Single-Crystal Ni-Based Superalloy Using TEM.

The single-crystal Ni-based superalloys, which have excellent mechanical properties at high temperatures, are commonly used for turbine blades in a variety of aero engines and industrial gas turbines. Focusing on the phase interface of a second-generation single-crystal Ni-based superalloy, in-situ TEM observation was conducted at room temperature and high temperatures. Intensity ratio analysis was conducted for the measurement of two-phase interface width. The improved geometric phase analysis method, where the adaptive mask selection method is introduced, was used for the measurement of the strain field near the phase interface. The strained irregular transition region is consistent with the calculated interface width using intensity ratio analysis. An intensity ratio analysis and strain measurement near the interface can corroborate and complement each other, contributing to the interface structure evaluation. Using TEM in-situ heating and Fourier transform, the change of dislocation density in the γ phase near the two-phase interface of the single-crystal Ni-based superalloy was analyzed. The dislocation density decreases first with the increase in temperature, consistent with the characteristics of metal quenching, and increases sharply at 450 °C. The correlation between the variation of dislocation density at high temperatures and the intermediate temperature brittleness was also investigated.

The aging behavior of Ni-coated carbon fibers and ZrC particles reinforced 2024Al matrix composites

In this paper, the aging behavior of Cf(Ni)-ZrC/2024Al composites with a quasi-network structure was studied carefully. The microstructure observation exhibits that the ZrC particles, carbon fibers, and Al matrix form a good interface bonding. The interfacial phase between the carbon fiber and Al matrix exhibits good thermostability during solution and aging treatment. With the addition of carbon fibers and ZrC particles, the composites only take 8 h to reach peak-aged condition while in monolithic 2024Al alloy, it needs 12 h. The peak-aged hardness of the composites is 136.4 HV, having 15.2% increments than 2024Al (118.4 HV). The yield, tensile strength, and strain of the peak-aged composites are 350.5 MPa, 536.4 MPa, and 14.8% respectively, which are improved by 56.5%, 45.8%, and 52.6% compared with 2024Al. The yield and tensile strength of composites are increased with time going by until they reach a peak aging state, agreeing well with that of hardness. The TEM observations of precipitated phases demonstrate the size and number of Al 2 Cu and Al 2 CuMg present a similar variation tendency. The improved tensile property of the composites is mainly due to pulled-out carbon fibers, quasi-network structure, outstanding interface bond, and precipitate strengthening. • The Cf(Ni)-ZrC/2024Al composites only take 8 h to reach to peak-aged condition, while the 2024Al needs 12 h. • The yield, tensile strength and strain of the composites are significantly improved compared with the 2024Al. • The improved strength is due to pulled-out carbon fiber, quasi-network structure, good interface bond and fine precipitate.

Investigation of the bactericidal mechanism of Penicilazaphilone C on Escherichia coli based on 4D label-free quantitative proteomic analysis

There is an urgent need to find new antibiotics to fight against the increasing drug resistance of microorganisms. A novel natural compound, Penicilazaphilone C (PAC), was isolated from a marine-derived fungus. It has displayed broad bactericidal activities against Gram-negative and Gram-positive bacteria. However, its bactericidal mechanism is still unknown. Herein, time-kill assays verified that PAC is a fast and efficient bactericidal agent. Furthermore, data from 4D label-free quantitative proteome assays revealed that PAC significantly influences over 898 proteins in Escherichia coli. Combining the results of biofilm formation, β-galactosidase measurement, TEM observation, soft agar plate swimming, reactive oxygen species measurement, qRT-PCR, and west-blotting, the mode of PAC action against E. coli was to block respiration, inhibit assimilatory nitrate reduction and dissimilar sulfur reduction, facilitate assimilatory sulfate reduction, suppress cysteine and methionine biosynthesis, down-regulate antioxidant protein expression and induced intracellular ROS accumulation, weaken bacterial chemotaxis, destroy flagellar assembly, etc., and finally cause the bacteria's death. Our findings suggest that PAC could have a multi-target regulatory effect on E. coli and could be used as a new antibiotic in medicine.