Nanocrystalline Research Articles

Toroidal seawater conductivity sensors and instrumentation for anti-submarine warfare

This paper describes the development of toroid-based seawater conductivity sensors and instrumentation for 0 to 60 mS/cm conductivity range and 50 bar hydrostatic pressure operational capability which is equivalent to 500 metres of ocean depth. Double core toroidal transformer concept was used and investigated for Mn-Zn and Fe based nanocrystalline cores, turns ratio and temperature dependence. Different excitation and sense coil turn numbers were fabricated by keeping the turn’s ratio 1:2, 1:4, 1:6 and 1:8. Optimum frequency of Mn-Zn based conductivity sensors with lower and higher turn numbers were observed for turn ratio 1:8 and 1:4 respectively at 120 kHz and obtained voltage responses of 952 and 1567 mV. Fe based Nano crystalline core with turn ratio 1:8 shows an optimum frequency of 70 kHz and giving a voltage difference of 830 mV. Two dimensional polynomials of order 2 were presented for temperature dependence of conductivity for Mn-Zn core based sensors. Sensor responses of Fe based nanocrystalline core sensors in ‘lower turn number’ configuration with same turn ratio is presented and observed linear temperature dependence compared to Mn-Zn based sensors. The mechanical design and frequency of operation of developed conductivity sensors gives inherent immunity to ASW band of frequency interference. The efficacy of conductivity sensor with measuring electronics and commercial standard sensors profiling was proved experimentally compared up to 500 metres in sea profiling and obtained a root mean square error of 0.42 for profile data.

Features of gas environment influence on the electrical properties of nano- and microcrystals ZnO

Nanocrystalline powder based on ZnO:Mn is used to create sensitive elements of gas sensors. It is shown that the nanocrystalline material has a structure similar, from an energy point of view, to the structure of micro-sized ZnO:Ag ceramics, which contains intercrystalline potential barriers formed by chemisorbed oxygen.Adsorbed water can have a significant effect on the electrical conductivity of sensors at operating temperatures below T = 100oC. When evacuated, flammable gases such as propane or butane influence electrical conductivity opposite to what is observed in a normal air atmosphere containing oxygen.

A universal chemical approach to the growth of self-assembled vanadium dioxide nanostructures

Vanadium dioxide (VO2) nanocrystalline materials are of great interest as for modern electronics and photonics as well as for energy saving technologies. Nevertheless, there still remains a challenge to realize their controllable synthesis, as evidenced by the low rate of implantation of existing technologies into mass production. Here, a hydrothermal reaction in a water-ethylene glycol (EG) solution has been coupled with the post-deposition annealing, yielding a range of phase-pure vanadium dioxide nanostructures on single-crystal r-sapphire substrate. A directed change in the viscosity of the precursor solution by using a change in the EG : H2O ratio makes it possible to control the nucleation rate and thus form VO2 isolated nanocrystals, as well as their ordered ensembles with regular distribution and uniform films on the substrate. The obtained nanostructures were extensively characterized by Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy combined with energy-dispersive X-ray spectroscopy. Continuous VO2 nanostructures demonstrated a metal-insulator transition (MIT) with a jump in resistivity of about 103–104, as well as a giant terahertz (THz) modulation depth up to 86 % (in the range from 0.3 to 2.0 THz), which confirms their high-quality and the possibility of their use in the creation of functional electronic and THz optical devices.

Permittivity measurement of nanocrystals ZnO:Mn at microwaves with using a biconical сavity

The measurement of the dielectric constant of nanocrystalline materials at microwave frequencies using a biconical resonator is under consideration. Obtaining ZnO and ZnO:Mn (2%) nanocrystals is carried out under non-equilibrium conditions by the method of ultrasonic aerosol pyrolysis. Estimates of the dielectric constant of these samples in the range of 8–12 GHz were obtained. Dielectric constant estimates are 8,3±0,2 and 12,4±0,2, respectively.

In Situ Formed Perovskite Nanocrystal Films Toward Efficient Circularly Polarized Electroluminescence

AbstractCircularly polarized electroluminescence (CPEL) is an active area of research because of its wide application in 3D displays and anti‐counterfeiting. However, it is not trivial to obtain a high asymmetry factor for highly efficient CPELs. Herein, a highly efficient CPEL is demonstrated with a superior asymmetry factor by using a chiral phosphine‐oxide‐passivated perovskite film as an emitter. An in situ‐formed perovskite nanocrystalline (NC) film is obtained by passivation of the chiral phosphine oxide. The as‐formed film exhibits much‐enhanced photoluminescence compared with the pristine film. More importantly, the NC film exhibits excellent chiral photoluminescence with a luminescence dissymmetry factor glum as high as 5.6 × 10−3. Electrically driven light‐emitting diodes based on the NC film exhibited superior device performance with a maximum brightness of 18,783 cd m−2 and a peak external quantum efficiency of 3.6%. More encouragingly, an unprecedented electroluminescence asymmetry factor gEL of up to 0.18 is achieved because of the chiral lattice distortion of the perovskite, which is the state‐of‐the‐art among hybrid materials.

Temperature effect on the transition between grain boundary migration and sliding

Grain boundary (GB) dynamics plays an important role in the mechanical and physical properties of nanocrystalline metals. In this study, we investigate the temperature effect on GB deformation transition using molecular dynamics simulations of [100] symmetric tilt GBs in Au bicrystals. Different deformation behaviours were revealed in GBs with the same structures at varied temperatures. GB sliding occurs as the predominant deformation mechanism at elevated temperatures, while GB migration dominates at low temperatures. The temperature-dependent critical stresses for GB migration and GB sliding were compared, revealing that temperature alters the critical GB misorientation at which GB migration transitions to GB sliding. Our study provides new insight into GB-mediated plastic deformation in nanocrystalline materials.

Augmentation of the photoluminescence intensity via the incorporation of Eu3+ ions into single-phase Ba2La4Zn2O10 nanocrystalline material for advanced photonic applications

Strong red-emission is observed in the new Eu3+ activated Ba2La4Zn2O10 nanocrystalline material series fabricated via solution combustion methodology. A tetragonal crystal framework withspace group I4/mcm (140) having irregularly shaped grains with sizes between 33 and 62 nm is formed. Morphological aspects are examined via scanning and transmission electron microscopy (SEM and TEM). On near-UV excitation, the photoluminescence spectrum presents a fair red emission at 16129 cm−1 wavenumbers consistent with the electronic transition 5D0→7F2. The energy transfer phenomena are also discussed. The highest luminous intensity is observed for 5.0 mol% of Eu3+ composition with 394 nm excitation. d-d exchanges authorized the presence of the concentration quenching phenomenon. Diffuse reflectance spectroscopy was taken into use to explore the energy band gap which lies in the semiconductor domain. The CIE coordinates lay in the reddish zone of the chromaticity plot, thus confirming their latent contention in pc-WLEDs and other advanced photonic applications.

Phase-field crystal simulation of the evolution of voids in polycrystalline Au–Al bonds with elastic interaction and the Kirkendall effect

In this paper, the modified phase-field crystal (MPFC) model is employed to study the evolution of Kirkendall voids in polycrystalline Au–Al bonds under mechanical loading. The MPFC model can simulate the structural evolution of deformed pure nanocrystalline materials considering instantaneous elastic interactions. Therefore, coupling a concentration field evolved by the Cahn-Hilliard equation with a density field evolved by the existing MPFC model makes it possible to study the phase transformation of a binary system in the presence of mechanical deformation. Using this modified model, we focus on the morphology evolution of the voids in Au–Al bonds under different mechanical loadings. The semi-implicit Fourier spectral algorithm is applied to solve the six-order coupling dynamics equations. The Kirkendall porosity of the diffusion couple is computed and analyzed. The effects of quasi-static and cyclic axial tensile loads on the morphology of voids, and porosity of the system are investigated. It is found that the morphology of Kirkendall voids that result from applying a quasi-static load is different from those that result from applying a cyclic load.

Substantial toughening by thick nanoscale amorphous intergranular films in nanocrystalline materials

Amorphous intergranular films (AIFs) have been proven in experiments to improve the damage tolerance of nanocrystalline materials. However, a quantitative study is still lacking. Molecular dynamics simulations were performed here to investigate the effect of CuNb AIFs on the fracture toughness of nanocrystalline Nb. In order to clarify the role of AIFs, a bicrystal Nb model with one straight symmetrical tilt grain boundary and a mode-I crack in one of the grains was constructed, in which the AIF effect was introduced by replacing the normal grain boundary with a CuNb AIF. Then, AIF thickness-dependent tensile deformation of the bicrystal Nb samples was simulated. The work-of-fracture, which is defined as the released strain energy due to the newly generated unit area in the crack during stretching, was employed to quantify the fracture toughness of the bicrystal systems. The results show that the fracture toughness of the AIF sample can be tripled due to the blunted crack tip and the relieved stress concentration at the crack tip as compared to the AIF-free one that exhibits a brittle crack propagation behavior. Also, the thicker the AIFs, the more pronounced this reinforcing effect. More importantly, it is found that there exists a critical AIF width of 1.7 nm, below which the crack will eventually break through the AIF, and above which the crack failed to do this. It is revealed that the enhanced fracture toughness originated from the transformation of brittle crack propagation to abundant dislocation emission from AIFs.

On the grain growth of CeO2 nanocrystals in AC/DC electrical fields

Processing of materials using electrical field presents an important research and engineering field. It allows to process materials at less demanding conditions, synthesize new phases and compositions. Herein, nanocrystals of CeO2 were synthetized and grown in-situ under AC and DC electrical fields. Main crystallographic properties like crystallite size, morphology texture and grain boundaries were significantly influenced by the electrical field up to 600 °C. Above this temperature, the thermally driven grain growth hindered the electrical field effect and led to a common behavior, isotropic particle shape and surface properties. This can be linked to the electrical and thermal force competition for the grain boundary development. To our knowledge such comprehensive description of the field effect on the growth of nanocrystals and its limitations has never been reported and can open a new door for processing of nanocrystalline materials.

Simple ePDF: A Pair Distribution Function Method Based on Electron Diffraction Patterns to Reveal the Local Structure of Amorphous and Nanocrystalline Materials.

Amorphous, glassy or disordered materials play important roles in developing structural materials from metals or ceramics, devices from semiconductors or medicines from organic compounds. Their local structure is frequently similar to crystalline ones. A computer program is presented here that runs under the Windows operating system on a PC to extract pair distribution function (PDF) from electron diffraction in a transmission electron microscope (TEM). A polynomial correction reduces small systematic deviations from the expected average Q-dependence of scattering. Neighbor distance and coordination number measurements are supplemented by either measurement or enforcement of number density. Quantification of similarity is supported by calculation of Pearson's correlation coefficient and fingerprinting. A rough estimate of fractions in a mixture is computed by multiple least-square fitting using the PDFs from components of the mixture. PDF is also simulated from crystalline structural models (in addition to measured ones) to be used in libraries for fingerprinting or fraction estimation. Crystalline structure models for simulations are obtained from CIF files or str files of ProcessDiffraction. Data from inorganic samples exemplify usage. In contrast to previous free ePDF programs, our stand-alone program does not need a special software environment, which is a novelty. The program is available from the author upon request.

Grain boundary strengthening in nanocrystalline Mo0.2CoNi medium-entropy alloys produced via high-pressure torsion

In this study, we produced nanocrystalline (NC) MoCoNi-based medium-entropy alloys (MEAs) with a single face-centered cubic (FCC) phase, which exhibit exceptional hardness through combining compositional and microstructural features of alloys. The alloys were prepared via powder metallurgy and high-pressure torsion, and the processing condition was optimized to achieve full densification and prevent grain growth in order to maximize the effect of grain boundary strengthening. Consequently, NC Mo0.2CoNi MEAs revealed the refined microstructures with a grain size of approximately 50 nm and outstanding hardness of 857 HV (corresponding to approximately 2.86 GPa in yield strength). This remarkable property, even compared to the single FCC phase alloys with similar grain size, is attributed to the existence of alloying elemental atoms of Mo with large atomic radius, which leads to severe lattice distortion allows to improve friction stress as well as the Hall-Petch coefficient. This study aims to discuss the potential of developing hard metallic materials by using grain boundary strengthening from alloys with large atomic size difference.

Generation of viable nanocrystalline structures using the melt-cool method: the influence of force field selection

ABSTRACT The influence of force field selection on the formation of nanocrystalline structures of pure copper using the Melt-Cool method within Molecular Dynamics (MD) was investigated. This is of utmost importance since the use of MD to investigate nanocrystalline materials is critical due to the ability to study on the appropriate length scale. The Melt-Cool simulation method involves annealing the starting single crystal structure to temperatures exceeding the melting point of the material, followed by rapid quenching and equilibration to room temperature. The heating and rapid quenching allows for the mixture of atoms to randomised orientations with realistic microstructures and randomised defects, such as interstitial or vacancy atoms. Due to the requirement of the mathematical model (force field) to predict the nanocrystalline structure, the influence of force field selection is of paramount importance – a major gap found in currently available literature. The current investigation was performed in two phases: initial investigation to understand influence of force field parameterisation on formation of nanocrystalline structures, followed with an investigation to enhance predictions of mechanical properties of nanocrystalline copper found in currently available literature. The initial phase demonstrated a clear dependence on force field selection for the formation of viable nanocrystalline structures. The second phase demonstrated that the most accurate force field for mechanical properties may not be the ideal selection for use with the Melt-Cool method. The most ideal force field selection when performing the Melt-Cool method with the goal of obtaining accurate mechanical properties for pure copper was determined.

Research on Magnetic-Thermal-Force Multi-Physical Field Coupling of a High-Frequency Transformer with Different Winding Arrangements

In order to clarify the magnetic-thermal-force changing rule of high-frequency transformers under different winding arrangements, this paper tests the magnetization and loss characteristics of nanocrystalline materials at different temperatures, and based on the magnetization and loss data, establishes a magnetic-thermal-force coupling calculation model of 15 kVA, 5 kHz nanocrystalline high-frequency transformers, and calculates and analyzes the magnetic flux density, loss and temperature rise distributions of high-frequency transformers with three different winding arrangements under no-load and short-circuit conditions, respectively. Through comparative analysis, it was found that under no-load conditions, the cross-transposition of winding has less influence on the magnetic flux of the high-frequency transformer core, but it can reduce the iron-core loss and transformer temperature rise. The cross-transposition of winding under short-circuit conditions can significantly reduce the leakage magnetic field strength of high-frequency transformers; complete cross-transposition weakens the high-frequency transformer losses and temperature rise better than partial cross-transposition. According to the winding current density and core leakage field distribution under short-circuit conditions, we calculated and analyzed the distribution of its the axial and radial electromagnetic forces. The results show that the axial electromagnetic force causes the winding to be squeezed from both ends to the middle, the radial electromagnetic force causes the primary winding to shrink inward and the secondary winding to expand outward, so cross-transposition can greatly reduce electromagnetic force and weakening the deformation of the winding. Therefore, high-frequency transformers of winding cross-transposed should be used in actual projects to reduce transformer temperature rise and improve efficiency and security. This research has theoretical significance for the multi-physical field coupling of high-frequency transformers and its structural design.

A unified explanation for variation of the grain growth exponent based on grain boundary migration kinetics

Based on molecular dynamics simulation on grain boundary migration, the large grain growth exponent extensively observed in high-purity nanocrystalline materials is proved to be ascribed to specific boundary migration kinetics determined. It is also found that temperature, topology of polycrystal and grain boundary type all affect the migration kinetics, which therefore causes the exponent to vary. The findings obtained in this work are capable of rationalizing various grain growth phenomena. In addition, triple junction drag against boundary migration has been evaluated and discussed.

A Review on the Adiabatic Shear Banding Mechanism in Metals and Alloys Considering Microstructural Characteristics, Morphology and Fracture

The current review work studies the adiabatic shear banding (ASB) mechanism in metals and alloys, focusing on its microstructural characteristics, dominant evolution mechanisms and final fracture. An ASB reflects a thermomechanical deformation instability developed under high strain and strain rates, finally leading to dynamic fracture. An ASB initially occurs under severe shear localization, followed by a significant rise in temperature due to high strain rate adiabatic conditions. That temperature increase activates thermal softening and mechanical degradation mechanisms, reacting to strain instability and facilitating micro-voiding, which, through its coalescence, results in cracking failure. This work aims to summarize and review the critical characteristics of an ASB’s microstructure and morphology, evolution mechanisms, the propensity of materials against an ASB and fracture mechanisms in order to highlight their stage-by-stage evolution and attribute them a more consecutive behavior rather than an uncontrollable one. In that way, this study focuses on underlining some ASB aspects that remain fuzzy, allowing for further research, such as research on the interaction between thermal and damage softening regarding their contribution to ASB evolution, the conversion of strain energy to internal heat, which proved to be material-dependent instead of constant, and the strain rate sensitivity effect, which also concerns whether the temperature rise reflects a precursor or a result of ASB. Except for conventional metals and alloys like steels (low carbon, stainless, maraging, armox, ultra-high-strength steels, etc.), titanium alloys, aluminum alloys, magnesium alloys, nickel superalloys, uranium alloys, zirconium alloys and pure copper, the ASB propensity of nanocrystalline and ultrafine-grained materials, metallic-laminated composites, bulk metallic glasses and high-entropy alloys is also evaluated. Finally, the need to develop a micro-/macroscopic coupling during the thermomechanical approach to the ASB phenomenon is pointed out, highlighting the interaction between microstructural softening mechanisms and macroscopic mechanical behavior during ASB evolution and fracture.

Plastic deformation-driven grain coarsening in nanocrystalline Gd2Zr2O7 ceramics by nanoindentation

Unique deformation mechanisms are responsible for the superior mechanical properties of nanocrystalline materials. However, unlike in nanocrystalline metals, questions remain regarding the underlying microstructural mechanisms governing plastic deformation in nanocrystalline ceramics. In the present study, nanoindentation and postmortem electron microscopy analysis were carried out on the transparent nanocrystalline Gd2Zr2O7 ceramic. Apart from shear bands led from grain boundary (GB) sliding, experimental observations confirm that deformation-driven grain coarsening is controlled by grain rotation accompanying the GB migration mechanism.

3D characterization of abnormal grain growth in nanocrystalline nickel

AbstractAbnormal grain growth, a pervasive phenomenon witnessed during the annealing of nanocrystalline metals, precipitates a swift diminution of the distinctive properties inherent to such materials. Historically, conventional transmission electron microscopy has struggled to efficiently procure comprehensive five‐parameter crystallographic information from a substantial number of grain boundaries in nanocrystalline metals, thus inhibiting a deeper understanding of abnormal grain growth behavior within nanocrystalline materials. In this study, we utilize a high‐throughput characterization method—three‐dimensional orientation mapping in the TEM (3D‐OMiTEM) to characterize the crystallographic five‐parameter character of grain boundaries with an area of over 3.4 × 106 nm2 in an abnormally grown nanocrystalline nickel sample. When coupled with existing theoretical simulation results, it is discerned that the grain boundary population shows a relatively large scatter when it is correlated to the calculated grain boundary energy; the grain boundaries of abnormally grown grains exhibit lower grain boundary energy compared to those that have not undergone abnormal growth. Merging high‐throughput grain boundary information obtained from three‐dimensional orientation mapping data with grain boundary properties derived from high‐throughput theoretical calculations following the concept of materials genome engineering will undoubtedly facilitate further advancements in comprehending and discerning the interfacial behaviors of crystalline materials.

A review Synthesis and luminescence characterize of emitting phosphor of garnet structure with effect of heating time

In this review, we deal with the structural properties of the garnet family of phosphor materials activated with trivalent lanthanide ions in the nano-crystalline materials. The interest is devoted the important garnet hosts: structure and luminescence spectroscopy are presented and discussed with particular meanings given to the possibility of achieving efficiently luminescence from trivalent lanthanide ions at the nano-level and to the potential and predicted technological applications of this class of materials.

Effects of P segregation on deformation mechanism in Ni-P nanocrystalline by atomic simulations

By using hybrid Monte Carlo-molecular dynamics (MC-MD) simulations, the segregation behavior of solute P atoms in the grain boundaries (GBs) of nanocrystalline (NC) Ni-P alloy was studied under different temperatures. The effects of P segregation on the mechanical properties and quantitative deformation mechanism of the Ni-P alloy were also investigated at high temperatures (800 K). The results show that the temperature strongly influences the segregation behavior of P atoms. Uniform P segregation occurs along GBs at 300 K, while at elevated temperatures (e.g., 1000 K), P atoms are concentrated at triple junctions (TJs), forming Ni-P precipitates. The uniform grain boundary segregation of P leads to Ni-P alloy with larger grain sizes and higher strength, while Ni-P precipitates at TJs result in alloy with smaller grain size, higher yield and flow stresses, which could be attributed to the Zener pinning effect. Moreover, quantitative analysis of the deformation mechanisms reveals that for NC Ni-P alloy with small grain size, Ni-P precipitates show significant effect on the competition between intracrystalline deformation and grain boundary deformation. A low percentage of elastic strain on the GBs was observed in the elastic deformation stage. The plastic deformation process of the non-segregated sample with grain size of 5 nm is dominated by grain boundary deformation, whereas for non-segregated samples of 10 and 15 nm, the strain is accommodated by both intragranular dislocation slip and grain boundary deformation. Importantly, Ni-P precipitates can reduce the degree of grain boundary deformation in alloys with small grains and thereby stabilize the GBs. The less grain boundary deformation caused by uniform grain boundary segregation of P and the more dislocation slip play a major role in improving the mechanical properties of the alloy with large grains. The results and findings of this study provide further insights into the segregation-induced strengthening mechanisms of NC Ni-P alloys.