Lattice Dislocations Research Articles

High-pressure and high-temperature synthesis of stable SxCo3.6Ni0.4Sb12 skutterudite compounds

Compared to Te, Ni is found in abundance in the earth's crust. At the same time, the mechanical properties of Ni atom-substituted skutterudite are significantly improved. In this study, a series of S-filled Ni-substituted skutterudite compounds were synthesized by a high-pressure and high-temperature (HPHT) method in the synthesis pressure range of 1.0–3.0 GPa. The phase composition, microscopic morphology, and electrothermal transmission properties of the SxCo3.6Ni0.4Sb12 (x = 0, 0.05, 0.10, 0.20) samples were systematically characterized. Phase composition analysis showed that the introduction of S atoms into the intrinsic pores of skutterudite can improve the solid solution limit of Ni atoms in the Co sites of skutterudite. The filling limit of S increased with the synthetic pressure. Moreover, microscopic morphology analysis revealed that the filling of S atoms inhibited the growth of grains. A large number of lattice fringes in different directions were found in the sample, containing abundant microstructures such as lattice distortions and dislocation defects. Compared with the synthesized samples, S0.05Co3.6Ni0.4Sb12 synthesized at 1.0 GPa had a maximum room temperature power factor of 7.98 × 10−4 Wm−1K−2. The electrical properties of S0.05Co3.6Ni0.4Sb12 samples stored for 6 months without any protective measures were tested at room temperature. No obvious changes in performance were observed, which proved that the HPHT method can synthesize stable samples. After several thermal cycles, the electrical properties of the S0.05Co3.6Ni0.4Sb12 sample was tested for variable temperature, and it was found that the sample still had good stability. How the substitution of S atoms with Ni atoms reduces the lattice thermal conductivity can be explained by fitting the Callaway model. The S0.05Co3.6Ni0.4Sb12 sample synthesized at 1.0 GPa had a maximum zT value of 0.46 at the test temperature of 773 K, which decreased to 0.43 after multiple thermal cycles at the same temperature.

Deformation of Nanoglass under High-pressure Torsion

Metallic glasses are amorphous solids with metallic atomic bonds. Different from traditional crystalline metals, they have no long-range atomic order. Only short-range order on the scale of 0.5–1 nm can be detected. Because of the lack of a lattice dislocation slip system, as in crystalline alloys, metallic glasses exhibit some outstanding properties, such as high elastic strain, high hardness and high strength.

Structural, electrical, and optical properties of rare-earth Sm3+ doped SnO2 transparent conducting oxide thin films for optoelectronic device applications: Synthesized by the spin coating method

Various concentrations of Sm (1, 2, 3, 4, and 5 at%) doped SnO2 thin films were synthesized by the sol-gel spin coating method and investigated the structural, optical, and electrical properties. According to X-ray diffraction studies, all deposited films exhibited the polycrystalline rutile tetragonal structure. The average crystallite size decreased with Sm doping concentration in SnO2, while the dislocation density and lattice strain values were increased. Both XPS and Raman spectra confirm that Sm3+ enters the host SnO2. The higher average optical transmittance is above 86% in pure SnO2, while Sm: SnO2 films exhibit a decreasing trend with the increase of Sm doping concentration and reached up to 77% in 5 at% Sm: SnO2 film. The optical bandgap energy increased with the increase of Sm doping concentration up to 3 at % Sm: SnO2 film (4.23 eV) which is attributed to the Moss-Burstein (MB) effect and then slightly decreased for 4 at% and 5 at% Sm: SnO2 films. Further, the room temperature sheet resistance and resistivity values were found to decrease with the increase of Sm doping concentration up to 3 at % Sm: SnO2 film then it slightly increased in 4 and 5 at% Sm: SnO2 films. Additionally, the efficiency parameter figure of merit (φ) for all the deposited films was calculated.

Influence of poly(vinyl alcohol)/poly(N-vinyl-2-pyrrolidone) polymer matrix composition on the bonding environment and characteristics of Ag nanoparticles produced by gamma irradiation

A series of silver nanoparticles (AgNPs)-poly(vinyl alcohol)/poly(N-vinyl-2-pyrrolidone) (PVA/PVP) hydrogel nanocomposites were synthesized by gamma irradiation to examine the matrix's impact on properties of Ag nanocrystals important for strain engineering in functional materials. SEM analysis and the swelling studies revealed nanocomposite morphology and fluid transport properties. UV–Vis absorption characterization of AgNPs has enabled an understanding of the cluster size effects. The gel content indicated inter-crosslinking of the two components of the polymer matrix, especially for equivalent (1:1) PVA/PVP ratio, affecting the investigated AgNPs structural parameters. The lattice parameters and the interface stress, analyzed using XRD, have the lowest negative magnitude from the bulk lattice constant for the PVA/PVP (2:1) and (1:1) ratio. The lowest values of the lattice strain and dislocation density, as mutually dependent parameters, are obtained in AgNPs embedded in a PVA/PVP (2:1) and (1:1) matrix. The XPS and FTIR data show a positive shift of the O1s and a redshift of the CO – Ag vibration band indicating charge transfer to the vacant orbital of Ag nanocrystals. The N atoms of the pyrrolidine ring are not directly involved in the electronic interactions, but over the p-π conjugation with carbonyl O. In addition, the tested Ag-(PVA/PVP) hydrogel nanocomposites showed significant potential as a broad-spectrum antibacterial material.

Photonic topological insulator induced by a dislocation in three dimensions.

The hallmark of topological insulators (TIs) is the scatter-free propagation of waves in topologically protected edge channels1. This transport is strictly chiral on the outer edge of the medium and therefore capable of bypassing sharp corners and imperfections, even in the presence of substantial disorder. In photonics, two-dimensional (2D) topological edge states have been demonstrated on several different platforms2-4 and are emerging as a promising tool for robust lasers5, quantum devices6-8 and other applications. More recently, 3D TIs were demonstrated in microwaves9 andacoustic waves10-13, where the topological protectionin the latter isinduced by dislocations. However, at optical frequencies, 3D photonic TIs have so far remained out of experimental reach. Here we demonstrate a photonic TI with protected topological surface states in three dimensions. The topological protection is enabled by a screw dislocation. For this purpose, we use the concept of synthetic dimensions14-17 in a 2D photonic waveguide array18 by introducing a further modal dimension to transform the system into a 3D topological system. The lattice dislocation endows the system with edge states propagating along 3D trajectories, with topological protection akin to strong photonic TIs19,20. Our work paves the way for utilizing 3D topology in photonic science and technology.

Density-gradient mechanism of vortex plastic creep

The electromagnetic response of type-II superconductors exposed to a magnetic field is governed by the dynamics of the vortex matter. We found that there exists a force of non-Peach-K\"ohler type which acts on edge dislocations in vortex lattices and depends on the gradients of vortex density. This force is absent in atomic solids where the density of the crystal lattice is constant even in the presence of dislocations. By mapping the classic theory of dislocations onto vortex lattices, we find the critical current ${j}_{c}^{\text{pl}}$ and the activation energy ${U}_{\text{pl}}$ associated with the dislocation-mediated plastic creep. The latter proves to decrease with magnetic field as ${B}^{\ensuremath{-}3/4}$, in concordance with experimental data. Such behavior of ${U}_{\text{pl}}$ is just opposite to the case of elastic (collective) creep, where ${U}_{\text{el}}$ always grows with $B$. At high enough field ${U}_{\text{pl}}$ becomes less than ${U}_{\text{el}}$, thus plastic creep overwhelms the elastic one and becomes the dominant mechanism of vortex mobility. This explains the appearance of the ``fishtail'' (double-peak) shape of magnetization curves in high-${T}_{c}$ superconductors.

Martensitic Transition and Superelasticity of Ordered Heat Treatment Ni-Mn-Ga-Fe Microwires

The preparation of Ni-Mn-Ga and Ni-Mn-Ga-Fe master alloy ingots and microwires was completed by high vacuum electric furnace melt melting furnace and melt drawing liquid forming equipment, and the lattice dislocations and defects formed inside the microwires during the preparation process were corrected by stepwise ordered heat treatment. The micro-structure and phase structure were characterized using a SEM field emission scanning electron microscopy and an XRD diffractometer combined with an EDS energy spectrum analyzer; the martensitic phase transformation process of the microwires was analyzed using a DSC differential scanning calorimeter; and the superelasticity of the microwires was tested by a Q800 dynamic mechanical analyzer. The results indicate that Fe doping can refine the grain, transform the phase structure from parent phase to single 7M martensite, reduce the number of martensitic variants, and increase the mobility of the twin grain boundary interface. The MT phase transition temperature (MS) is substantially increased in the martensite transition (MT) process by the increase of the number of free electrons in its lattice. During the superelasticity (SE) test, both microwires displayed superior recover-ability of SE curves, and the Fe doping curves showed similar characteristics of “linear superelasticity”, showing higher critical stress values and complete SE in the experiment. The critical stress satisfies the Clausius-Clapeyron equation and exhibits higher temperature sensitivity than Ni-Mn-Ga microwires.

Structural, optical, dielectric and photovoltaic properties of Sn doped CdS films prepared with green synthesis route

Energy harvesting from renewable energy sources is one of the burning topics in today's research community. Solar cells are emerging as a potential candidate to harvest solar energy into electrical one. Here, we tested a series of pure and Sn-doped CdS as photoabsorbers in solar cells prepared by the sol-gel technique via the green synthesis route. The formation of a hexagonal wurtzite crystal structure of pure and Sn-doped CdS is confirmed using X-ray diffraction analysis. Doping of Sn caused a slight change in lattice parameters, crystallite size, d-spacing, and dislocation line density. UV–vis measurement revealed the direct bandgap nature of the material with bandgap value of 2.50 eV which reduced the 2.20 eV upon 3% Sn doping. Optical parameters, including extinction coefficient and refractive index, are measured. The film prepared with 3% of Sn doping has a high refractive index, i.e., 2.56. In addition, the imaginary and real parts of the dielectric constant are measured in the frequency range of 20Hz to 20 MHz, which exposed the presence of different relaxation phenomena in this composition. The solar cell prepared with 3% of Sn doping has a high value of current density (3.57 mA-cm−2), open circuit voltage (0.49 V), fill factor (0.654) and efficiency (1.14%). This solar cell is a stable cell due to the absence of liquid corrosion like in dye-sensitized solar cells.

Synthesis and characterization of Ca substituted Ni-Zn nanoferrites- microstructural, magnetic and dielectric analysis

A series of cubic spinel structured, nanocrystalline and monophasic samples of the composition CaxNi0.4Zn0.6−xFe2O4 (x = 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6) samples were synthesized via a chemical citrate precursor route, sintered at 700 °C. Raman and X-Ray Diffraction (XRD) investigations confirmed the single-phase cubic spinel configuration of the nanoferrites. A study of linearly fitted Williamson-Hall plots obtained using XRD data revealed the spinel structure of the prepared samples with the minute structural disorder wherein low values were estimated for lattice strain, ε, (0.003 × 10−3 −1.66 ×10−3) and dislocation density, δ, (0.665 × 10−3-1.73 × 10−3) for different Ca concentrations. The nanoferrites of Ca-doped Ni-Zn were found to be highly magnetic when subjected to a maximum applied field of 20 kOe under VSM investigations. Nanoferrite composition with x = 0.4 exhibited the maximum magneto-crystalline anisotropy constant (11452), remanent magnetization (22 emu/g), and coercivity (199.91 Oe) while maximum saturation magnetization (65.27 emu/g) and magnetic moment (2.756 μB) were found for the Ca0.1Ni0.4Zn0.5Fe2O4 composition. The samples were synthesized to explore their utilization in storage devices and high-frequency devices.

Hierarchical multiscale crystal plasticity framework for plasticity and strain hardening of multi-principal element alloys

The multi-principal element alloys (MPEAs) exhibit the unprecedented combinations of the excellent mechanical properties, especially high strength and good ductility. However, the accurate and reasonable models for describing the mechanical behavior of MPEAs are still scarce due to their distinctive serious lattice distortion effects, which limit the performance prediction. Here, we develop a new general framework by combining the atomic simulation, discrete dislocation dynamics, and crystal plasticity finite element method, to study the strain-hardening behavior for MPEAs, which achieves the influence of the complex cross-scale factors, including the lattice distortion at the nanoscale and the dislocation hardening at the microscale, on the plastic deformation. Compared with the classic crystal plasticity finite element, the bottom-up hierarchical multiscale model could couple the underlying physical mechanisms from the nano-micron-meso scales and captures the inhomogeneous strain field induced by the serious lattice distortion, thus showing the high accuracy and ubiquitous availability for MPEAs. The result shows that the prediction of the strain-stress curve in the polycrystal MPEAs agrees well with the experimental result at the quasi-static tension, which verifies the accuracy of the proposed method. In addition to the dislocation evolution, the heterogeneous strain distribution combined with the significant change from the orientation of some grains could be an important reason for the enhanced strength at the micron scale. The present work not only gives an insight into the relationship between the multiscale microstructure and strain hardening considering the mechanistic linkages of the lattice distortion, dislocation behavior, and grain structure, but also provides a general approach to physically predict the mesoscopic mechanical response in MPEAs.

Microstructure evolution and mechanical properties of two kinds of weathering steel welded joints

AbstractThe microstructural evolution and mechanical properties of welded joints for Q450NQR1 and Q450AWR5 alloys were investigated. The inhomogeneous microstructures of Q450NQR1 and Q450AWR5 welded joints can result in heterogeneous mechanical responses. Microhardness distributions of the two welded joints are not uniform with the highest hardness value in the FZ, followed by the HAZ, and the lowest hardness value in the BM. The tensile strength of Q450AWR5 welded joints is slightly higher than the Q450NQR1 welded joints. The tensile fracture occurred in the BM of both welded joints, probably due to the strengthening effect of bainite with high dislocation density and lattice distortion. The fatigue life is dominated by the welded defects inside the welded joint. The location of fatigue failure occurs mainly near the fusion zone and is limited by the welded defects, independent of the microstructure.

Stress partitioning between bcc and cementite phases discussed from phase stress and dislocation density in martensite steels

The present study investigated the tensile deformation behavior of quenched and tempered martensite steels at various austenitization and tempering temperatures using in situ neutron diffraction experiments. Phase lattice strains in the bcc and cementite (θ) phases and dislocation structures in the bcc phase were analyzed. The phase lattice strain in bcc (εphasebcc) became almost stagnant after yielding as the tempering temperature increased. The phase lattice strain in θ increased linearly with an increase in the flow stress, independent of the austenitization and tempering temperatures. The stress partitioning between bcc and θ was confirmed after the yielding of bcc, which contributed to the work hardening. The phase stresses of bcc and θ and their stress partitioning improve the mechanical properties of martensite steels, which can be summarized by the systematic changes in phase lattice strain and dislocation properties due to the austenitization and tempering temperatures.

Effects of partial substitution of Co by Ni on structural and transport properties of TiCoSb-based half-Heusler compound

Correlation amid structural and transport properties of TiCo(1−x)Ni x Sb, (x=0.00, 0.02, 0.04, 0.06) alloy have been investigated. Samples have been synthesized by solid state reaction method, followed by arc-melting. Minute amount of CoTi embedded phase has been revealed by in-depth structural characterization using Rietveld refinement. Unit cell volume decreases with increasing Ni in TiCo(1−x)Ni x Sb, indicate sucessful substitution. Lattice strain, crystalline size and dislocation density have been estimated from x-ray diffraction data. Temperature dependent resistivity (ρ(T)) measurements have been carried out down to 10 K. Transition from metallic to semiconducting behavior is observed in ρ(T) data for x=0.00, 0.02. ρ(T) for x=0.04 and 0.06 show metallic nature. Plausible explanation have been provided on the basis of position of Fermi surface and embedded phases.

Room-Temperature Ferromagnetism in Cu/Co Co-Doped ZnO Nanoparticles Prepared by the Co-Precipitation Method: For Spintronics Applications

In current work, pure ZnO and Zn0.96–xCu0.04CoxO (0 ≤ x ≤ 0.05) nanoparticles were synthesized by the co-precipitation method. Structural analysis and phase determination of the formed nanoparticles was carried out using X-ray diffraction (XRD) and Williamson–Hall plots. The hexagonal wurtzite structure was manifested by all the samples with divergent microstructures. The change in lattice parameters, bond length, dislocation density, and lattice strain indicates that Cu and Co were successfully incorporated. Average crystallite size was found to be in the range of 32.16–45.42 nm for various doping concentrations. Field emission scanning electron microscopy results exhibited that the surface morphology is an amalgam of spherical-like and hexagon-like structures. Spherical-shaped grains were homogeneous and evenly distributed all over the structure. Fourier transform infrared spectra indicated that the absorption bands were blue-shifted with increasing Co concentration. The UV–visible absorption spectra showed high absorption in the UV region and weak absorption in the visible region. An increase in the energy band gap for the maximum absorption peak was observed from 3.49 eV for ZnO to 3.88 eV for Zn0.91Cu0.04Co0.05O. The Burstein–Moss effect explained the noticed blue shift in absorption spectra and energy band gaps. The vibrating sample magnetometer study revealed the change in the diamagnetic behavior of pure ZnO to the ferromagnetic behavior of the prepared nanoparticles at room temperature for different doping concentrations. In the current study, we have developed the room-temperature ferromagnetism (RTFM) for Cu and Co co-doped ZnO nanoparticles. Since RTFM is the key objective for dilute magnetic semiconductors, therefore it can be served as the desirable expectant for spintronics applications with improved functionalities and device concepts.

Direct measurement of shear stress for dislocation transferring across {111} Σ3 grain boundary in aluminum bicrystal via in situ straining TEM

TEM in situ straining was applied to an Al bicrystal with a {111}Σ3 grain boundary. The activated slip systems in both grains were determined based on the dislocation line configurations and the crystallographic orientation of the applied load. The resolved shear stress for activation of the dislocation transfer at the GB, τgb, was directly evaluated from the dislocation motion and the simultaneously obtained stress-strain curve. The evaluated τgb value agrees with those obtained in a previous study on macroscopic tensile tests. From the geometry of the dislocation reaction, it was presumed that the pile-up lattice dislocations are dissociated into the lattice dislocations in the adjacent grain and the GB dislocation. It was determined that the activated slip system in the adjacent grain was dominated by local energetic and geometric conditions rather than the global condition such as the combination of Schmid low and applied stress.

Revealing the atomic-scale evolution of sessile disconnections on twin boundaries during deformation

Sessile disconnections on twin boundaries (TBs) are special interfacial defects which can not glide conservatively along TBs, playing an important role in plastic deformation of twin-containing metals. However, the detailed aspects of the evolution of sessile disconnections on TBs under stress are not yet fully understood, while the direct atomic-scale experimental evidence remains rare. By integrating the in situ transmission electron microscope tensile test, multichromatic complex analysis method and molecular dynamics simulation, we studied the evolution of sessile disconnections on TBs. Dislocation reaction analysis shows that sessile disconnections can serve as effective nucleation sites of twinning dislocations (TDs) which lead to the migration of TBs. Moreover, twin-twin intersections can also act as sinks for gliding lattice dislocations by transforming lattice dislocations into TDs along different twining systems. This work enriches the knowledge of the atomic-scale deformation mechanism of twin-containing metals.

Combined effects of stress and temperature on hydrogen diffusion in non-hydride forming alloys applied in gas turbines

Hydrogen plays a vital role in the utilisation of renewable energy, but ingress and diffusion of hydrogen in a gas turbine can induce hydrogen embrittlement on its metallic components. This paper aims to investigate the hydrogen transport in a non-hydride forming alloy such as Alloy 690 used in gas turbines inspired by service conditions of turbine blades, i.e. under the combined effects of stress and temperature. An appropriate hydrogen transport equation is formulated, accounting for both stress and temperature distributions of the domain in the non-hydride forming alloy. Finite element (FE) analyses are performed to predict steady-state hydrogen distribution in lattice sites and dislocation traps of a double notched specimen under constant tensile load and various temperature fields. Results demonstrate that the lattice hydrogen concentration is very sensitive to the temperature gradients, whilst the stress concentration only slightly increases local lattice hydrogen concentration. The combined effects of stress and temperature result in the highest concentration of the dislocation trapped hydrogen in low-temperature regions, although the plastic strain is only at a moderate level. Our results suggest that temperature gradients and stress concentrations in turbine blades due to cooling channels and holes make the relatively low-temperature regions susceptible to hydrogen embrittlement.

An enhanced thin-film resistance temperature detector and its application for catalyst layer surface temperature measurement inside PEMFC

Temperature detection is of vital significance for the thermal management of proton exchange membrane fuel cells (PEMFCs). In this work, a robust Au thin-film resistance temperature detector (RTD) was fabricated based on the micro-electro-mechanical system (MEMS) to achieve temperature monitoring inside PEMFC. Low-temperature heat treatment was carried out to promote the performance of RTD, and was confirmed to help achieve good linearity and small thermal hysteresis. Combined with X-ray diffusion (XRD) Rietveld refinement and morphology analysis, particularly the lattice strain and dislocation calculation, low-temperature heat treatment is supposed to help eliminate lattice defects and residual strains in the film. Subsequently, this prepared RTD was applied to measure cathode catalyst layer (CCL) temperature inside PEMFC under different relative humidity (RH) and current loads. The results show that current loads are positively related to temperature variation. The higher the current, the more Joule and irreversible heat are generated. The RH of anode and cathode has a more complex impact on temperature evolution, depending on the water concentration in the electrode sites, which is not only affected by RH, but also by the oxygen reduction reaction (ORR). Too high water contents in the electrode area would bring about flooding, while too low would result in membrane drying, so both of them cause different levels of ohmic and mass transport loss, hence inducing temperature variations. This work is supposed to benefit the research of RTD enhancement and thermal state inside PEMFC.

X-ray diffraction, magnetic measurements and Mössbauer spectroscopy of MgFe2O4 nanoparticles

Magnesium ferrite (MgFe2O4) was synthesized using the solution combustion method. The concentration of the fuel was varied and investigated its effect on spectroscopic, structural and magnetic properties of MgFe2O4 nanoparticles. The XRD pattern showed a decrease in the crystallite size and increase in the lattice strain, dislocation density and specific surface area upon increasing the fuel amount. The shift in the FTIR band (~434 cm–1) corresponds to the Fe–O vibration in the octahedral sites. The values of HC decreased while Mr values increased continuously with the increase of the fuel concentration. The Mössbauer spectra of MgFe2O4 nanoparticles measured with a high velocity resolution demonstrated a large number of magnetic sextets assigned to the tetrahedral (A) and octahedral [B] sites. These magnetic sextets were considered as a result of different Fe3+ local microenvironments depending on Mg2+ cations distributions among the neighboring (A) and [B] sites. The calculations of binomial distribution of the probabilities of the numbers of Mg2+ cations within the sphere of 3.7 Å around Fe3+ cations in both (A) and [B] sites demonstrated distributions of different probabilities for these sites in agreement with the Mössbauer spectra fits. The numbers of magnetic sextets and their hyperfine parameters assigned to the (A) and [B] sites were found slightly varied for MgFe2O4 nanoparticles prepared with different fuel concentrations.

Peritectic-eutectic transformation of intermetallic in Zn alloy: Effects of Mn on the microstructure, strength and ductility

Zn alloy containing peritectic intermetallic compound usually has fine grains, so they show relatively high strength. But the intermetallic compound is coarse, which is blamed for the brittleness. In this work, a strategy to transform the peritectic intermetallic phase of Zn-Fe alloy into eutectic intermetallic phase is proposed by adding Mn. In detail, Mn co-precipitates with Fe and Zn to form a (Fe, Mn)Zn13 phase, which provides nucleation substrate and increase the nucleation rate of the MnZn13 eutectic phase because of their low interatomic and interplanar spacing misfit according to the edge-to-edge matching model. The intermetallic compound formed via eutectic reaction is finer than that formed by peritectic reaction. Results show that the intermetallic in Zn-Fe-Mn is refined by 79% compared with that of Zn-Fe. Moreover, the MnZn13 phase forms a semi-coherent interface with negligible lattice misfit (0.09%) in (01¯11¯) orientation with Zn matrix, which is believed to decrease the intergranular fracture ratio. Consequently, Zn-Fe-Mn alloy has a compression yield strength, ultimate tensile strength and elongation of 135.98 MPa, 226.8 MPa, and 15.9%, respectively, which are 28%, 25% and 87% higher than those of Zn-Fe alloy. The strength is improved by dislocation shearing: lattice dislocations cut precipitates to advance through the alloy. The ductility is improved by significantly refined Zn grains, smaller second phase particles and their semi-coherent interfaces with the matrix.