Lattice Dislocations Research Articles

Probing an enhanced anisotropy Seebeck coefficient and low thermal conductivity in polycrystalline Al doped SnSe nanostructure

Recently, SnSe based thermoelectric materials attained much interest due to their environment friendly IV–VI semiconductor group. Herein, Al doped SnSe (Sn1−xAlxSe) specimens were prepared by combined ball milling and spark plasma sintering. High resolution transmission electron microscopy analysis of (Sn1−xAlxSe) nanostructure samples confirmed the existence of different crystal defects and dislocation induced by Al doping. Lower thermal conductivity (0.63 W/mK at 750 K) is observed for Sno.92Al0.08Se samples than pristine SnSe (1.14 W/mK at 750 K), which is mainly attributed to various crystal defects, such as lattice dislocation, stacking fault, grain boundary scattering, and excellent anharmonic bonding nature of SnSe. The maximum electrical conductivity is observed for the SAS-2 sample, which correlates well with the low activation energy of 0.20 eV. The minimal doping of Al (SAS-2) decoupled the strong interdependency of electrical and thermal transport properties, leading to a maximum ZT of 0.18 at 743 K. The Al doped SnSe (Sn1−xAlxSe) induced point defects in the sample, which provides a new strategy for waste heat recovery.

Realizing Plain Optimization of the Thermoelectric Properties in BiCuSeO Oxide via Self-Substitution-Induced Lattice Dislocations.

Seeking new strategies to tune the intrinsic defect and optimize the thermoelectric performance via no or less use of external doped elements (i.e., plain optimization) is an important method to realize the sustainable development of thermoelectric materials. Meanwhile, creating dislocation defects in oxide systems is quite challenging because the rigid and stiff ionic/covalent bonds can hardly tolerate the large strain energy associated with dislocations. Herein, taking BiCuSeO oxide as an example, the present work reports a successful construction of dense lattice dislocations in BiCuSeO by self-doping of Se at the O site (i.e., SeO self-substitution), and achieves plain optimization of the thermoelectric properties with only external Pb doping. Owing to the self-substitution-induced large lattice distortion and the potential reinforcement effect by Pb doping, high-density (about 3.0×1014 m-2) dislocations form in the grains, which enhances the scattering strength of mid-frequency phonon and results in a substantial low lattice thermal conductivity of 0.38 W m-1 K-1 at 823 K in Pb-doped BiCuSeO. Meanwhile, PbBi doping and Cu vacancy markedly improve the electrical conductivity while maintaining a competitively high Seebeck coefficient, thereby contributing to a highest power factor of 942 μW m-1 K-2. Finally, a remarkably enhanced zT value of 1.32 is obtained at 823 K in Bi0.94Pb0.06Cu0.97Se1.05O0.95 with almost compositional plainification. The high-density dislocation structure reported in this work will also provide a good inspiration for the design and construction of dislocations in other oxide systems.

Micromechanism of plastic deformation enhancement in ultrafine-grained Al-Cu-Zr alloy after annealing and additional deformation

A theoretical model which describes a micromechanism of plasticity enhancement in an ultrafine-grained Al-Cu-Zr alloy after annealing and additional deformation is suggested. Within the framework of the model, it was shown that nanoprecipitates of the secondary phase Al2Cu in the grain boundaries become the effective sources of the lattice dislocations in the presence of a large number of the grain boundary dislocations near the nanoprecipitates. The theoretical dependences of the flow stress on the degree of the plastic deformation demonstrate good qualitative and quantitative agreement with the experimental data. The emission of the lattice dislocations from nanoprecipitates provides higher plasticity compared to the emission of the lattice dislocations from the triple junctions of the grain boundaries. Keywords: ultrafine-grained materials, aluminum alloys, nanoprecipitates, annealing, severe plastic deformation by high-pressure torsion, dislocations, grain boundaries.

Влияние состояния границ зерен на эффект пластификации в ультрамелкозернистом сплаве Al-0.4Zr

The influence of small additional deformation by cold rolling (CR) on the microstructure and mechanical properties of ultrafine-grained (UFG) Al–0.4Zr alloy structured by high pressure torsion (HPT) has been studied. The results are compared with the application of small additional deformation by HPT, which, after low-temperature annealing, leads to a substantial increase in ductility (plasticization effect) with small decrease in strength. It is shown that, in contrast to the additional deformation of HPT, the deformation of CR after intermediate low-temperature annealing leads to a sharp drop in plasticity to ~2%, while the strength increases to ~275 MPa. The key role of the nonequilibrium state of grain boundaries in the manifestation of the plasticization effect in the UFG Al–0.4Zr alloy is revealed. A new approach is proposed for simultaneously increasing the strength and ductility of the UFG Al–0.4Zr alloy due to a small additional deformation by CR without intermediate annealing. As a result of this approach, a significant increase in strength by ~30% (ultimate tensile strength ~223 MPa) was achieved with a simultaneous increase in ductility up to ~26%, which is associated with an increase in the strain hardening rate due to an increase in the density of lattice dislocations in the UFG structure with nonequilibrium grain boundaries. The strain rate sensitivity and strain hardening coefficients have been determined for the UFG Al–0.4Zr alloy in various states.

Микромеханизм повышения пластичности в ультрамелкозернистом сплаве Al-Cu-Zr после отжига и дополнительной деформации

A theoretical model which describes a micromechanism of plasticity enhancement in an ultrafine-grained Al-Cu-Zr alloy after annealing and additional deformation is suggested. Within the framework of the model, it was shown that nanoprecipitates of the secondary phase Al2Cu in the grain boundaries become the effective sources of the lattice dislocations in the presence of a large number of the grain boundary dislocations. The theoretical dependences of the flow stress on the degree of the plastic deformation demonstrate good qualitative and quantitative agreement with the experimental data. The emission of the lattice dislocations from nanoprecipitates provides higher plasticity compared to the emission of the lattice dislocations from the triple junctions of the grain boundaries.

Optimal design of heat dissipation modules for high-power LED based on the Taguchi method

The semiconductor component of InGaN-based blue light-emitting diodes (LED) emits white light when combined with a yellow phosphor mixture. However, owing to the lattice dislocations and defect points in GaN, it exhibits a high thermal resistance, which leads to heat accumulation and an increase in temperature. This is problematic as overheating causes LED to produce dark spots and lines and reduces the luminous flux and optical power of high-power LED. In this study, we propose a variety of optimal structures for heat-transfer modules and apply the proposed architectures in the assembly of high-power LED. First, the high-power LED substrate was coated with a film of aluminum nitride. Then, copper fins were connected to the vacant spaces in the circuit boards to increase the surface area of the heat-transfer region. The Taguchi method was used to identify the optimal substrate thickness, fin arrangement, and fin depth for the effective heat dissipation in a 12 W high-power LED. A dielectric layer was grown on the surface of the aluminum nitride film to serve as a passivation layer to insulate the patient. The passivation layer reduces the physical damage caused by thermal stress, thereby improving the service life and characteristics of heat-transfer modules. The proposed design not only yields a stable LED substrate (with low thermal stress) but also induces reliable heat transfer.

Enhanced Density of States Facilitates High Thermoelectric Performance in Solution-Grown Ge- and In-Codoped SnSe Nanoplates

SnSe single crystals have gained great interest due to their excellent thermoelectric performance. However, polycrystalline SnSe is greatly desired due to facile processing, machinability, and scale-up application. Here, we report an outstanding high average ZT of 0.88 as well as a high peak ZT of 1.92 in solution-processed SnSe nanoplates. Nanosized boundaries formed by nanoplates and lattice strain created by lattice dislocations and stacking faults effectively scatter heat-carrying phonons, resulting in an ultralow lattice thermal conductivity of 0.19 W m-1 K-1 at 873 K. Ultraviolet photoelectron spectroscopy reveals that Ge and In incorporation produces an enhanced density of states in the electronic structure of SnSe, resulting in a large Seebeck coefficient. Ge and In codoping not only optimizes the Seebeck coefficient but also substantially increases the carrier concentration and electrical conductivity, helping to maintain a high power factor over a wide temperature range. Benefiting from an enhanced power factor and markedly reduced lattice thermal conductivity, high average ZT and peak ZT are achieved in Ge- and In-codoped SnSe nanoplates. This work achieves an ultrahigh average ZT of 0.88 in polycrystalline SnSe by adopting nontoxic element doping, potentially expanding its usefulness for various thermoelectric generator applications.

Microstructure and properties of Cu-Al-Ni-Mn-Y alloy with precipitation and effective strengthening by aging treatment

The Cu-11Al-3Ni-2.5Mn-1Y alloy prepared by hot-press sintering was subjected to aging treatment at different temperatures of 350 °C, 400 °C, 450 °C, 500 °C and 550 °C, respectively. The phase composition, microstructure, mechanical and damping properties were characterized. The results show that the alloys after the ageing treatment are mainly composed of 18 R martensite and 2 H martensite. The precipitation phase begins to appear and the best comprehensive properties are obtained at the aging temperature of 450 °C. The hardness and strength values reached 347.6 HV and 642.6 MPa, respectively. The strengthening mechanisms are mainly involved in lattice distortion and dislocation increment effect caused by solution strengthening, shearing and hindering effect of precipitated phases and dislocations, fine grain strengthening effect caused by grain size reduction, and the increase of large angle grain boundary. Simultaneously, the room temperature damping reached 0.0723, which is 93 % higher than the original sample of 0.38. The improvement of damping properties is closely related to the changes in the number and size of martensite, the interfacial damping effect caused by precipitated phases, and the pinning and unpinning reaction between dislocations and precipitated phases. Therefore, alloys with both high mechanical and high damping properties can be obtained by proper aging treatment. In addition, the increase in phase transition temperature of high temperature damping properties also provides some guidance for the exploration and development of this alloy in the high temperature field.

Effect of σ-Phase on the Strength, Stress Relaxation Behavior, and Corrosion Resistance of an Ultrafine-Grained Austenitic Steel AISI 321

This paper reported the results of research into the effect of Equal Channel Angular Pressing (ECAP) temperature and 1-h annealing temperature on mechanical properties, stress-relaxation resistance, and corrosion resistance of austenitic steel AISI 321L with strongly elongated thin δ-ferrite particles in its microstructure. The formation of α′-martensite and fragmentation of austenite grains takes place during ECAP. Ultrafine-grained (UFG) steels demonstrate increased strength. However, we observed a reduced Hall–Petch coefficient as compared with coarse-grained (CG) steels due to the fragmentation of δ-ferrite particles. UFG steel specimens were found to have 2–3 times higher stress-relaxation resistance as compared with CG steels. For the first time, the high stress-relaxation resistance of UFG steels was shown to stem from a internal stress-relaxation mechanism, i.e., the interaction of lattice dislocations with non-equilibrium grain boundaries. Short-time 1-h annealing of UFG steel specimens at 600–800 °C was found to result in the nucleation of σ-phase nanoparticles. These nanoparticles affect the grain boundary migration, raise strength, and stress-relaxation resistance of steel but reduce the corrosion resistance of UFG steel. Lower corrosion resistance of UFG steel was shown to be related to the formation of α′-martensite during ECAP and the nucleation of σ-phase particles during annealing.

Understanding Tribocorrosion of Aluminum at the Crystal Level

Aluminum (Al) and its alloys are excellent corrosion-resistant materials due to their sustained passivity in neutral aqueous solutions. However, tribocorrosion remains a major threat to the integrity of Al, during which corrosion and wear work synergistically to accelerate material degradation. In this work, a combined experimental and computational investigation was carried out using Al single crystals to develop a crystal-based tribocorrosion modeling framework that accounts for the effects of lattice reorientation and dislocations on surface corrosion. Specifically, the mechanical, corrosion, and tribocorrosion properties of Al (100), (110), and (111) single crystals were measured experimentally, followed by characterization of lattice rotation and dislocation density via electron backscattered diffraction (EBSD). Unlike the mechanical and corrosion properties that are orientation-dependent, the tribocorrosion rate was found to be insensitive to the initial orientations of the crystals. Using the experimental results as inputs and validations, a multiphysics finite element model was developed that successfully predicted the depassivation and repassivation currents during tribocorrosion by mapping the local corrosion kinetics as a function of passivation state, crystallographic orientation, and dislocation density. It was found that lattice rotation, rather than dislocations, dominates the overall tribocorrosion behavior. Finally, the decrease of orientation-dependence during tribocorrosion was explained in terms of the coupling between the un-rotated lattice in the unworn region and the rotated lattice from the worn region.

Interaction of an edge dislocation with a 〈110〉 tilt boundary in nickel: molecular dynamics simulation

The interaction of a lattice edge dislocation with a 〈110〉 tilt boundary in nickel was studied by the molecular dynamics method in the case when the dislocation glide plane is parallel to the grain tilt axis. The dependence of the local threshold stress on the grain misorientation angle is obtained. It is found that with increasing misorientation angle, the local threshold stress increases, but the growth rate for low- and high-angle boundaries is different: for high-angle boundaries, this growth is slower. When studying the mechanism of overcoming the low-angle 〈110〉 tilt boundary by an edge dislocation in the considered orientation, it was found that the dislocation, in fact, does not pass through the boundary, but exchanges places with a grain-boundary dislocation, which are ordinary perfect edge dislocations in the low-angle 〈110〉 tilt boundaries. When studying the interaction of a dislocation with high-angle boundaries, a peculiarity was noticed, which consists in pushing two or three grain-boundary dislocations out of the boundary at once.

The temperature-independent paramagnetic susceptibility peak at zero magnetic field in non-topological WSe2 single crystal

The temperature-independent paramagnetic susceptibility peaks were observed in the non-topological WSe2 single crystal. This is contrary to the proposed explanation that the temperature-independent paramagnetic peak in topological materials originates from the spin texture of the topological surface state. Weak ferromagnetic characteristic was observed from magnetization measurement from 300 to 5 K. The high resolution transmission electron microscope images combined with no detectable magnetic impurity was found in our sample indicate that the ferromagnetism originates from lattice dislocations at different layer planes and axes. The temperature-independent paramagnetism susceptibility peak should originate from the ferromagnetism induced by lattice dislocation.

Exploring the anneal hardening phenomenon in nanocrystalline Pt-Ru alloys

Pt and its alloys have important applications in the field of catalysis or in the jewellery industry. Especially in the latter case sufficient strength is necessary to ensure wear resistance. We thus explore here the potential of severe plastic deformation on the strengthening behaviour of two Pt-Ru alloys. Depending on the Ru content and the deformation conditions hardness values of up to 6.5 GPa could be achieved. A further hardness increase to more than 7 GPa can be obtained by adequate recovery annealing. Indentation with different tip geometries indicates that the specimens undergo significant strain softening after the heat treatment. The results further show that the anneal hardening effect occurs only below a specific threshold grain size, with the extent increasing towards smaller grains. This can be rationalized by the increasing role of grain boundaries for mechanical properties at the submicron scale. Considering different possible origins of the hardness increase and bearing in mind that indentation techniques sample the strength at rather large plastic strains, the results seem to be best explained by annihilation of lattice dislocations and grain boundary defects, complicating the absorption of dislocations at the grain boundaries after the heat treatments.

Investigation on corrosion behavior and mechanism of carbon steel under CaCO3 deposit in CO2-flooding environment

Under-deposit corrosion (UDC) is common in the wellbore and oil gathering and transmission systems, leading to system failure in severe cases. This study investigated the corrosion behavior and mechanism of carbon steel (L245N) under different thickness scales through weight loss experiment, potential dynamic polarization-scanning test, electrochemical impedance spectroscopy, scanning electron microscope, and energy-dispersive X-ray spectroscopy. The results show that a corrosion product film is formed on the surface of matrix covered with CaCO3 deposits. It has a specific protective effect on L245N steel, and however, the protective effect is limited. At 30°C, apparent pitting corrosion occurs on the metal matrix surface. The uniform corrosion rate of L245N steel decreases with an increase in the deposit layer thickness. The mechanism investigation results show that pitting corrosion under scale is caused by the autocatalytic effect in the activated zone and complex salt lattice dislocation. There are three reaction stages in the UDC process. Additionally, the anode process of under-deposit corrosion is controlled by activation and dissolution, and the cathodic process is mainly controlled by diffusion. However, the entire reaction process is controlled by the cathode.

Plastic deformation of superionic water ices

Due to their potential role in the peculiar geophysical properties of the ice giants Neptune and Uranus, there has been a growing interest in superionic (SI) phases of water ice. So far, however, little attention has been given to their mechanical properties, even though plastic deformation processes in the interiors of planets are known to affect long-term processes, such as plate tectonics and mantle convection. Here, using density functional theory calculations and machine learning techniques, we assess the mechanical response of high-pressure/temperature solid phases of water in terms of their ideal shear strength (ISS) and dislocation behavior. The ISS results are well described by the renormalized Frenkel model of ideal strength and indicate that the SI ices are expected to be highly ductile. This is further supported by deep neural network molecular dynamics simulations for the behavior of lattice dislocations for the SI face-centered cubic (fcc) phase. Dislocation velocity data indicate effective shear viscosities that are orders of magnitude smaller than that of Earth's lower mantle, suggesting that the plastic flow of the internal icy layers in Neptune and Uranus may be significantly faster than previously foreseen.

Achieving enhanced toughness of a nanocomposite coating by lattice distortion at the variable metallic oxide interface

Ceramic coatings are in general a kind of brittle material because they are predominantly made up of ionic crystals that avoid dislocation motion caused by lattice distortion. In this regard, a remarkable toughened ZrO2/MgO nanocomposite coating is obtained by the plasma electrolytic oxidation (PEO) process and in-situ synthesized ZrO2 with quantitative control approach. It is revealed that the toughening behavior of the ZrO2/MgO coating is related to the coordination and diversion of lattice distortion at the metallic oxide interface, which induces distinct dislocation motion at the interface. The semicoherent interface between m-ZrO2 and MgO is verified to act as a buffer to realize toughening of the nanocomposite coating through dislocation slipping induced by lattice coordinated distortion. Simultaneously, significant interfacial lattice distortion transfer and dislocation pinning are discovered at the semicoherent interface between t-ZrO2 and MgO, which are beneficial to toughness enhancement of the nanocomposite coating. The results indicate that the toughening effect occurs along with dislocation slipping and pinning caused by lattice distortion of the ZrO2/MgO semicoherent interface, which enables the toughness of novel nanocomposite coating to reach 2.7 times of the traditional PEO coating.

Enhancement of structural, linear and non-linear optical properties of Ti1−xCa2xO2 solid solutions

Structural and morphological analyses of the Ti1−xCa2xO2 solid solutions and their complete linear and non-linear optical properties were investigated. Ti1−xCa2xO2 (0≤ x ≤0.5) powders were successfully synthesized via solid-state reaction route. The XRD patterns confirmed that the pure and doped samples exhibited anatase TiO2 phase. The crystallite size of TiO2 decreased to 82 nm after Ca doping. Furthermore, the dislocation density and lattice deformation are increased with increasing calcium concentration. SEM analysis revealed that Ti1−xCa2xO2 (0≤ x ≤0.5) powders have a granular structure and with doping, atom aggregations take on softer shapes. Energy dispersive x-ray Analysis (EDX) showed the appropriate composition for the ions depicted in the structure. Using spectrophotometric data, linear and non-linear optical parameters were obtained and associated with the Ca content. The band gap of TiO2 has shifted from 3.11 to 2.8 eV with dopant concentration. As the percentage of Ca increased, the dielectric constant and optical conductivity also increased. The dispersion energy, plasma frequency, non-linear refractive index and Sellmeier’s parameters were also calculated and discussed with respect to Ca doping. The optical results obtained revealed the potential of the Ti1−xCa2xO2 (0≤ x ≤0.5) samples for optical and optoelectronic applications.

Enhancement of the photoconversion efficiency of Sb2S3 based solar cell by overall optimization of electron transport, light harvesting and hole transport layers

In thin-film photovoltaic, Sb2S3 is a leading absorber material due to its broad-band optical response and excellent electrical properties, with the highest reported efficiency in the planar Sb2S3 configuration being 8 %. By simulations, optimized parameters for each component of the n-i-p FTO/ETL/Sb2S3/HTL/Au planar heterojunction device, as well as a photoconversion efficiency (PCE-η) of 28.64 % under an AM 1.5G spectral irradiance, have been predicted. In this report, we systematically optimized the electron transport layer (ETL), light harvesting layer (Sb2S3), and hole transport layer (HTL) separately based on theoretically predicted values in order to understand the effect of each component and optimize solar cell performance. The optimized results showed that simply optimizing the ETL, Sb2S3 layer, and HTL resulted in achieving the efficiency ∼4.11 % and to achieve the theoretically predicted device performance, a precise control of intrinsically formed traps, the presence of surface defect states, and lattice dislocations in Sb2S3, are important as these factors are found to be the main factors that enhance charge carrier recombination, slow charge transfer across the interface, and charge carrier mobility.

Transgranular highly localized deformation and fracture mechanism in ductile CrFeNi medium-entropy alloys

The fracture evolution and damage mechanism of an engineering candidate, CrFeNi medium-entropy alloy (MEA), which exhibits superior tensile properties with the ultimate tensile strength of ∼1.03 GPa and ductility of ∼55%, were studied. This work highlights the effect of defect evolution, including microcrack, geometric necessary dislocation (GND) density, lattice rotation and dislocation configuration, on fracture evolution. The result reveals that the fracture mechanism of the studied alloy is dominated by the coalescence of microcracks, which initiate at a certain distance from the main crack tip. It is distinct compared to common ductile metals and is attributed to a transgranular highly localized deformation behavior which provides potential sites for microcrack initiation. The microstructural characteristics of the microcrack embryo are evaluated and clarified to be the localized damage accumulation with a high density of dislocations at the crystal-defect scale. It can result in elevated internal stress locally, providing the driving force for microcrack initiation. In addition, the mechanical model based on distributed dislocation technique (DDT) demonstrates that the transgranular highly localized deformation leads to a crack propagation acceleration effect and the deterioration of crack propagation resistance. By relating the defect evolution and fracture performance of the CrFeNi MEA, it provides new insight on the toughening design of ductile metals.

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.