Nanostructured Grains Research Articles

One pot synthesis and characterisation of two dimensional tin doped strontium oxide nanostructured electrode materials for electrochemical supercapacitor applications

ABSTRACT In this research work, we report a facile one pot synthesis of Sn doped SrO (Sr1-xSnxO1-δ; where x = 0.1, 0.2, 0.3 and 0.4) based two dimensional nanoparticles. The physico-chemical properties of the prepared materials were characterized by XRD, EDAX, SEM and TEM. XRD confirmed that the prepared samples are cubic. The presence of appropriate atomic percentage of elements was predicted by EDAX. SEM and TEM data confirmed the presence of nanostructured grains in the samples. The electrochemical techniques such as, cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) were employed to characterize the reproducible performance of the Sn doped SrO nanostructured electrode materials in 1 M of H2SO4 medium. Among the different samples studied, Sr0.6Sn0.4O1-δshowed a maximum specific capacitance of 56.49 Fg-1 at a scan rate of 5 mV. Hence, Sn doped SrO can be good electrode material for electrochemical supercapacitor applications.

A study on the Microstructures and the Suppression of Grain Growth in Rapidly Solidified Fe-Si-B(-Cu-Zr-Ca) Base Nanostructured Alloys

Magnetic induction and resonance wireless power charging generally operates in a high frequency range, and its efficiency is degraded by the generation of heat at high frequency. This makes it necessary to develop new soft magnetic materials capable of maintaining their characteristics even at high frequency. The purpose of this study is to minimize the heat generated by the eddy currents when charging, and to improve the charging efficiency under an alternating magnetic field. In this study, Fe-Si-B alloys containing Cu, Zr and Ca elements were melt-spun to prepare amorphous ribbons. The amorphous ribbons were heat treated to crystallize nanograins, The structure was analyzed by TEM and EELS, As a result, it was found that Zr was distributed mainly at the grain boundaries after heat treatment, whereas Ca was uniformly distributed only along the grain boundaries. It could be concluded that the Ca and Zr elements effectively suppressed the grain growth, and thus maintained the very fine nanograin structure.

Transport properties of AgPb16SbTe18 prepared by the inclusion of nano AgSbTe2 into PbTe matrix

For the present investigation, AgPb16SbTe18 material was prepared by the inclusion of AgSbTe2 nanopowder into PbTe matrix, employing mechanical alloying route. This material was characterized for phase formation, microstructure, and transport properties. The microstructure showed densely populated grains with a grain size ~ 80 nm which plays an important role in controlling the transport properties. A remarkable enhancement in Seebeck coefficient (~ − 500 µV at 450 K) value of AgPb16SbTe18 sample was observed which is attributed to the phonon scattering and the potential barrier, encountered by the charge carriers at the nanostructured grain boundaries. The other important findings are enhancement in electrical conductivity and reduction in thermal conductivity which directly enhance the figure of merit of the material and make it promising for thermoelectric power generator application in the mid-temperature zone.

Nano-Dual-Phase Metallic Glass Film Enhances Strength and Ductility of a Gradient Nanograined Magnesium Alloy.

Magnesium (Mg) alloys are good candidates for applications with requirement of energy saving, taking advantage of their low density. However, the fewer slip systems of the hexagonal‐close‐packed (hcp) structure restrict ductility of Mg alloys. Here, a hybrid nanostructure concept is presented by combining nano‐dual‐phase metallic glass (NDP‐MG) and gradient nanograin structure in Mg alloys to achieve a higher yield strength (230 MPa, 31% improvement compared with the reference base alloy) and larger ductility (20%, threefold higher than the SMAT‐H sample), which breaks the strength–ductility trade‐off dilemma. This hybrid nanostructure is realized by surface mechanical attrition treatment (SMAT) on the surface of a crystalline Mg alloy, and followed by physical vapor deposition of a Mg‐based NDP‐MG. The higher strength is provided by the nanograin layer generated by SMAT. The larger ductility is a synergistic effect of multiple shear bandings and nanocrystallization of the NDP‐MG, inhibition of crack propagation from the SMATed nanograined structure by the NDP‐MG, and strain‐induced grain growth in the SMATed nanograin layer. This hybrid nanostructure design provides a general route to render brittle alloys stronger and ductile, especially in hcp systems.

Microstructure Transformation in Different Regions of Zr61.4Cu27.8Al4Ni6Y0.8 Bulk Metallic Glass Induced by Rapid Laser Welding

An experimental study is performed on the microstructure transformation induced by rapid laser welding in the different regions of Zr61.4Cu27.8Al4Ni6Y0.8 bulk metallic glass (BMG). The laser input energy has a remarkable influence on the microstructure and microhardness of the Zr-based BMGs. The microstructures in different regions of as-welded joints are diverse. Crystallization happens in heat-affected zone (HAZ) with crystalline dendritic phase of CuZr, which results in the deterioration of the microhardness to 434.9 ± 17.1 HV in HAZ, much lower than that of parent material as 517 ± 5.0 HV. Meanwhile, the element profiles indicate that intragranular segregation of Cu element occurs during the crystallization process. In the center of molten zone (MZ), the structure is fully amorphous and its hardness is close to that of parent material. In the region transiting from MZ to HAZ, some nano-grains with an average size of 20–50 nm are identified. In addition, several flower-like nanostructured grains of Y2O3 phase with a size ranging from 50 to 80 nm are formed in MZ’s matrix. The nanocrystallization is believed to be responsible for the enhancement of the Vickers hardness up to 560.1 ± 8.4 HV in this region. Rapid laser processing induces three kinds of microstructures in Zr61.4Cu27.8Al4Ni6Y0.8 bulk metallic glasses, i.e., the severe crystallization of CuZr phase in heat affected zones (HAZ), the nano-grains in the region transiting from molten zone (MZ) to HAZ, and the fully amorphous structure in the center of MZ, which consequently results in the diverse microhardness in spatial distribution.

Fine-grain-embedded dislocation-cell structures for high strength and ductility in additively manufactured steels

Metals manufactured with either a fine-grained structure or a nano-grained structure possess the ultrahigh strength at the expense of ductility. Therefore, materials that combine high strength and ductility should be immediately manufactured for a wide variety of practical applications. In this study, we report a bulk stainless steel with fine grains embedded by dislocation cells using additive manufacturing (AM) methods, which can efficiently and economically manufacture complex structures to achieve the desired mechanical properties. Compared with conventionally cast and forged 316L steels, the average yield strength and tensile elongation of AM 316L steels are 170% and 45% higher, respectively, attributed to a synergistic deformation of the fine-grain-embedded dislocation-cell structures. High strength is expected from a block of dislocation cells and solute atoms to the planar slip of dislocations, whereas high ductility depends upon the slip of fine-grain boundaries associated with the altered shape of an embedded dislocation cell. Moreover, as deformation proceeds, the complex interlaced dislocation cell network structure and deformation twinning can further enhance work hardening as well as tensile elongation. This study proposes a fine-grain-embedded dislocation-cell structure to achieve high-strength-and-ductility materials.

Design of high-manganese nanostructured austenitic steel with particle swarm optimization

ABSTRACT Austenitic steels with high manganese content exhibit various deformation mechanisms depending on their stacking fault energies. Here we present a novel approach of combining the algorithm of particle swarm optimization with thermodynamic modeling, to design high-manganese steels with desired stacking fault energies. Implementation of this method is demonstrated for nanostructured austenitic steels, where composition, grain size, and temperature are used as the tunable parameters. The results show that for a given value of fault energy, the proposed method yields multiple solutions, which may offer valuable guidelines for designing the alloy. In contrast with the coarse-grained steels, the design parameters of the nanostructured alloys are shown to depend on the austenitic grain size.

Hall-petch relationship and heterogeneous strength of CrCoNi medium-entropy alloy

In this work, we mainly focused on single phase face-centered cubic (FCC) type CrCoNi medium-entropy alloy (MEA), fabricated the homogeneous and heterogeneous grain structure through heavily deformation and subsequent annealing treatment, then investigated the Hall-petch relationship and heterogeneous strength of CrCoNi MEA. The experimental results show that, the large friction stress of fully-recrystallized CrCoNi MEA resulted from the severe lattice distortion, short-time partial-recrystallized annealing and abundant thermal twinning facilitated the heterogeneous structure with multiple gain size. The heterogeneous structure (i.e. partial-recrystallized structure) exhibited a superb strength-ductility synergy in comparison with nano-grained structure and/or ultrafine-grained structure in CrCoNi MEA. The results would provide the comprehensive understanding of CrCoNi MEA, as well as the essentials for further development and applications of FCC type multicomponent alloys.

Numerical modelling of the potential inside the cylindrical shaped nanocrystallite metal oxide semiconductors and its effect on gas sensor response

The surface of metal oxide semiconductors exhibits great gas sensing response when the size of the grains becomes much smaller, i.e. in the range of the Debye length. In the presented work, the gas sensing properties of nanocrystalline cylindrical shaped metal oxide semiconductors have been studied. The effect of crystal size and surface states on gas sensor response has been investigated. The electric potential inside the grains has been calculated by solving the Poisson’s equation coupled with electroneutrality condition. The relationship between the electric potential and the size of the nanograins has been established as a function of available surface states, surface temperature and occupied surface states. It has been found that the critical value of available surface state density increases with an increase in the size of nanograins. Total carrier concentration inside the grain volume has been simulated as a function of available surface state density and grain size. Eventually, the sensitivity of the gas sensor has been simulated and compared for different sizes of nanocrystals. From the simulated studies, it is clear that the smaller cylindrical-shaped nanostructured grains could result in better gas sensing performances. Presented model has universal application for all metal oxides (both n-type and p-type) based semiconductors and can be extended for both oxidising and reducing gases with essential modifications.

Probing nanoscale structure and strain by dark-field x-ray microscopy

Abstract

Nanostructured monoclinic Cu2Se as a near-room-temperature thermoelectric material.

Searching for new-type, eco-friendly, and Earth-abundant thermoelectric materials, which can be used as an alternative to the high-cost bismuth telluride, is important for near-room-temperature applications. In this work, nanostructured monoclinic Cu2Se with a low carrier concentration has been synthesized by a wet mechanical alloying process combined with spark plasma sintering. Such a low carrier concentration, which originates from the effectively suppressed Cu deficiencies during the fabrication process, induces a relatively low electrical conductivity and carrier thermal conductivity. Besides, the nanostructured grains combined with point defects and phonon resonance enhance the phonon scattering to induce a low lattice thermal conductivity without sacrificing the electrical transport properties. As a result, our nanostructured monoclinic Cu2Se obtains a figure of merit of 0.72 at 380 K with good thermal stability. This work indicates that nanostructured monoclinic Cu2Se is a promising near-room-temperature thermoelectric material.

A study on the surface severe plastic deformation and corrosion behavior of an interstitial free steel

In the present work, a Ni-stabilized interstitial free (IF) steel was subjected to severe plastic deformation (SPD) in order to attain nanograin structure. To do this, a different technique so-called brushing was used. This technique is very cost-effective, simple and practical. For brushing process, different brushes made of brass, stainless steel and low carbon steel were used. The processing parameters such as brush rotational speed, different shapes and dimensions of the brush wire, the amount of brushing and applied load were adjusted so that to attain the desirable structure. The surface and microstructural evolutions were characterized by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The thickness of the nanostructure surface layer formed by brushing was about 40–50 μm with a maximum microhardness of about 540 Vickers. The formed nanostructure layer had a grain size of 30–50 nm. In order to study the effect of nanograin structure on the corrosion behavior, the samples, brushed under different conditions, were subjected to corrosion testing and the related polarization curves were obtained using 3% NaCl solution. The comparison of the polarization curves of brushed and non-brushed samples indicated that the peak of polarization curve of brushed sample, exhibited a larger potential difference and a lower current density comparing to not-brushed case. The larger potential difference and the lower current density mean greater corrosion resistance and lower corrosion rate. A quantitative comparison between the current density of the two samples revealed a 55% increase in corrosion resistance for the brushed sample as compared to not-brushed counterpart.

Tuning nanostructure using thermo-mechanical processing for enhancing mechanical properties of complex intermetallic containing CoCrFeNi2.1Nbx high entropy alloys

Possibilities of tailoring microstructure and mechanical properties of complex intermetallic containing CoCrFeNi2.1Nbx (x = 0.2, 0.4) high entropy alloys (HEAs) using thermo-mechanical processing were investigated in this work. For this purpose, the homogenized HEAs containing complex Laves phases distributed in the FCC matrix were successfully severely cold-rolled up to 90% reduction in thickness. The cold-rolled HEAs showed a nano lamellar microstructure with fragmented Laves phase particles arranged preferentially along the rolling direction. The microstructure of the Nb0.4 HEA revealed intense deformation zones surrounding the hard Laves phase particles, owing to their higher volume fraction and larger size as compared to those in the Nb0.2 HEA. Upon annealing at 800 °C, the lamellar deformed FCC phase in the two HEAs was transformed into an equiaxed nano-grained structure having fine dispersion of nano precipitates. However, the Nb0.4 showed a decidedly lower recrystallized grain size than the Nb0.2, resulting from the smaller deformed grain size and particle stimulated nucleation. The annealed Nb0.2 showed remarkable strength-ductility combination featured by tensile strength ~1080 MPa and ductility ~21%, originating from the combined effect of a high volume of ductile FCC matrix, remarkable grain refinement, and precipitation hardening. In stark contrast, the annealed Nb0.4 showed complete absence of tensile elongation, likely due to a low volume fraction of the ductile FCC matrix and a high volume fraction of the intermetallic phases. The present study emphasized the critical role of microstructural tailoring for achieving outstanding strength-ductility synergy in complex intermetallic containing CoCrFeNi2.1Nbx HEAs.

VO2 film with small hysteresis width and low transition temperature

In this paper, we investigate the vanadium dioxide (VO2) film as optical switching deposited on Al doped zinc oxide (AZO) glass substrates by DC reactive magnetron sputtering. Field-emission scanning electron microscopy reveal that the deposited VO2 film consists of irregular nanostructured grains. The optical tests by means of UV–visible spectrophotometer show that the deposited VO2 film exhibits excellent optical switching properties with a very small phase-transition hysteresis width of 2.9 °C as well as a low phase-transition temperature of about 48 °C. More interesting, the semiconductor-to-metal transition is readily induced by Joule heat produced by electrical current through the AZO conductive layer, which is more efficient and convenient compared with the traditional heating of the sample by heating plate. This work is of crucial importance for potential applications of VO2 film in compact optical switching devices.

Substrate Dependence in the Formation of Au Nanoislands for Plasmonic Platform Application

In this work, the influence of the various substrates on Au nanoisland formation has been studied. Nanostructures were obtained via annealing of thin Au films. In order to determine nanoisland formation mechanisms, correlation between an initial film thickness and temperature of formation, shapes, and dimensions of nanostructures was examined. For the surface morphology studies, nanograin structure, and chemical composition analysis, SEM, HR TEM, and EDS measurements were performed, respectively. Morphology studies showed that the temperature at which nanostructures form varies for different substrates, which indicates high impact of the substrate material on the nanostructure formation. In the case of silicon substrate, besides the phenomenon of spinodal dewetting, the effect of eutectics on the nanostructures was additionally taken into consideration.

Two‐Step Nitriding Behavior of Pure Iron with a Nanostructured Surface Layer

A two‐step gas nitriding treatment (280 °C for 2 h plus 550 °C for 6 h) is designed for pure iron with a nanostructured surface layer induced by surface mechanical attrition treatment (SMAT). The microstructure and surface properties of the SMAT sample after two‐step nitriding treatment are investigated in comparison with those of the SMAT sample after single‐step nitriding treatment (550 °C for 6 h). Experimental results show that a much thicker compound layer (up to 25 μm in thickness) is generated on the surface of the SMAT sample after two‐step nitriding treatment, in which the mean grain size of the Fe2‐3N phase is below 25 nm. The enhanced nitriding kinetic may be attributed to the formation of a large number of Fe(N) solid solutions and a small amount of dispersed Fe2‐3N particles on the SMAT sample after first low‐temperature nitriding, which enhances the thermal stability of nanostructured iron grains. Due to the enhanced nitriding kinetic and refined nitride structure, the SMAT sample after two‐step nitriding treatment displays higher surface hardness and improved wear resistance relative to its single‐step nitrided counterpart.

Nanostructured copper matrix composite with extraordinary strength and high electrical conductivity produced by asymmetric cryorolling

A novel method for fabricating nanostructured copper matrix composite with extraordinary strength and high electrical conductivity using casting, homogenization, and asymmetric cryorolling has been proposed. Microstructural observations were performed by optical microscopy (OM) and scanning electron microscopy (SEM) equipped by energy dispersive spectroscopy (EDS). A nanostructured (grain size < 100 nm) copper matrix composite due to the occurrence of discontinuous dynamic recrystallization (DDRX) and particle stimulated nucleation (PSN) mechanisms with a uniform dispersion of copper oxide particles achieved after 96% asymmetric cryorolling. The yield strength, ultimate tensile strength, and hardness of the 96% deformed copper matrix composite increased from 31.7 MPa, 206.2 MPa, and 29.4 HB (for the annealed sample) to 836.6 MPa, 841.7 MPa, and 103.4 HB, demonstrating 2539%, 308%, and 252% enhancement, respectively. The significant increase in strength and hardness was attributed to severe strain hardening, remarkable grain refinement, uniform dispersion of particles, and effective load transfer. At the same time, the improvement on yield and ultimate tensile strength did not cause serious deterioration in electrical conductivity so that the 96% deformed composite exhibited a high conductivity of 82.10% IACS (international annealed copper standard). Consequently, the copper matrix composite exhibited a good balance in strength and electrical conductivity. With increasing the thickness reduction, the number, and depth of dimples and the sharpness of tearing edges decreased and the failure mode changed from typical ductile fracture (for annealed, 30% and 60% rolled samples) to a combination of shear ductile and brittle fracture (for 90% and 96% deformed samples).

Synthesis and characterization of ZnO nanocrystal line for gas sensor application

In this research, nanostructure of ZnO film has been fabricated by spraying 0.2M concentration of Zinc salt solution on glass substrate. The Zinc solution was prepared by solves 4.389908g of the Zinc salt in 100 mL of isopropyl alcohol C3H9O. In order to obtain the best optimization in their optical properties via the annealing effect, these films were treated thermally at different temperatures (RT, 200, 300 and 400). Then AFM and XRD tests are employed to examine the structure. According to X-ray diffraction patterns, the structure of all ZnO films is polycrystalline with hexagonal wurtizte structure and preferential orientation in the (002) direction. It also turns out that the annealing process has led to increase the high of the peaks of the x-ray pattern tests which means the structure has been improved. As for the surface morphology, the Atomic Force Microscope (AFM) test results are shown there is uniform distribution of homogeneous with columnar nanostructure grains. The band gap varies between (3.2-3.37 eV) and tendency to rise with annealing temperature have been observed through the optical measurement. The sensing tests showed that linear response with increasing the gas concentration and, the annealing treatment gave rise to the sensitivity and more selectivity for NO2 gas. Moreover, the best sensing performance in term of sensitivity and selectivity has been absorbed by ZnO film that was treated by 300 oC annealing temperature.

A high strength and high electrical conductivity Cu-Cr-Zr alloy fabricated by cryogenic friction stir processing and subsequent annealing treatment

A bulk Cu-0.55Cr-0.2Zr alloy with structure of ultra-fined grains (UFGs), nanograins (NGs) and nano precipitates in grain boundaries (GBs) was fabricated by cryogenic friction stir processing (CFSP) followed with annealing treatment. A combination of high strength, pronounced electrical conductivity, and good microstructure thermal and mechanical stability was achieved.

Formation mechanism of graphite nanospheres in W-C-Co system under high current pulsed electron beam irradiation

Understanding the formation mechanism of graphite nanospheres (GNS) in W-C-Co system under high current pulsed electron beam (HCPEB) irradiation is necessary for its application on surface modification of WC-Co cemented carbide. With this aim, XRD, SEM, EDS and TEM were used to investigate the effect of HCPEB irradiation on surface microstructure and phase composition. Results showed the micro-WC grains and Co were melted and transformed into nano-grained composite structure in the surface layer ∼1.29 μm. Two general scales of GNS were discerned in two types of element composition ambiences, i.e. ∼100 nm in Co-rich region and ∼50 nm in Co-poor region. Thermodynamic analysis suggested that they were the results of different routes in the non-equilibrium melting and re-solidification process.