Local Inhomogeneities Research Articles

Tuning of carrier concentration and superconductivity in high-entropy-alloy-type metal telluride (AgSnPbBi)(1-)/4In Te

High-entropy-alloy-type (HEA-type) compound superconductors have been drawing much attention as a new class of exotic superconductors with local structural inhomogeneity. NaCl-type (Ag,In,Sn,Pb,Bi)Te is a typical HEA-type superconductor, but the carrier doping mechanism had been unclear. In this study, we synthesized (Ag,In,Sn,Pb,Bi)Te with various In concentration using high-pressure synthesis: the studied system is (AgSnPbBi)(1-x)/4InxTe (x = 0-0.4). Single-phase samples were obtained for x = 0-0.3. A semiconductor-like temperature dependence of resistivity was observed for x = 0, while superconductivity appeared for the In-doped samples. The highest transition temperature (Tc) was 3.0 K for x = 0.3. The Seebeck coefficient decreases with increase of x, which suggests that In3+ generates electron carriers in (AgSnPbBi)(1-x)/4InxTe. Tuning of carrier concentration and superconducting properties of (Ag,In,Sn,Pb,Bi)Te would be useful for further investigation of exotic superconductivity in the HEA-type compound.

Active Region Extent Assessment with X-rays (AREA-X)

The development of semiconductor sensors for new particle tracking detectors places increasing limits on sensor characteristics such as uniformity, size and shape of inefficient areas and size of active compared to inactive sensor areas. Accurately assessing these relatively subtle effects requires either measurements in particle beams or the modification of samples to be used in dedicated laser test setups.Active Region Extent Assessment with X-rays (AREA-X) has been developed as an alternative method for the fast, efficient and precise study of the active area of a semiconductor sensor. It uses a monochromatic, micro-focused X-ray beam with a 10–20 keV energy range as provided by several synchrotron beam lines and uses the photo current induced in the sensor to measure the depth of the responsive sensor volume. It can be used to study local inhomogeneities or inefficiencies, the overall extent of the active sensor volume and its shape and its localised application, which makes the need to gather statistics over a large area unnecessary, allowing for fast readout, which enables studies of the sensor behaviour at a range of external parameters, e.g. temperature or applied bias voltage.This paper presents the measurement concept and technical setup of the measurement, results from initial measurements as well as capabilities and limitations of the method.

Formation and atomic structure of the (PbTi)O phase for epitaxial super‐tetragonal PbTiO3

AbstractLattice strain has been used to modify the properties of ferroelectric oxide films. Introducing strain in thin film is usually realized based on the lattice mismatch of the heterostructural materials, which form multilayered systems or composite structure. For the composite structure of PbTiO3/α‐PbO‐type (PTO/PO‐T) structure phase, the PO‐T structure phase plays the key role in introducing a high strain in the PTO phase and resulting in a giant polarization of the PTO phase. Here, we perform a detailed study on the PO‐T structure phase on atomic scale by means of high‐resolution (scanning) transmission electron microscopy. The atomic details, including cations and oxygen of the PO‐T structure phase, are revealed, which show the α‐PbO structure with parts of the Pb atoms being substituted by Ti atoms. Local inhomogeneity in Ti atom distribution is found in nanometer scale. Experimental investigations show that the PO‐T structure phase is formed on a layer of the PTO phase with a thickness of one or two unit cells, which grows covering the underneath La0.3Sr0.7MnO3 (LSMO) electrode layer. The formation mechanism of the PO‐T structure phase is discussed on the basis of the atomic details of the PO‐T/PTO/LSMO interfaces and the growth condition of the film.

Local mechanical properties and precipitation inhomogeneity in large-grained Al–Mg–Si alloy

Al–Mg–Si (6xxx series) alloys show excellent mechanical properties due to the precipitates formed during heat treatment. However, heat treatment of these alloys results in a soft precipitation free zone (PFZ) close to grain boundaries that weakens them and promotes fracture, and thereby reduces the ductility of the material. This study provides quantitative insights into the mechanical properties and underlying plasticity behavior of Al–Mg–Si (6xxx series) alloys through combined nanoindentation hardness measurements and in-depth characterization of the microstructure adjacent to the PFZ region and in the grain interior. Experimental nanoindentation, transmission microscopy (TEM) and electron channeling contrast imaging results confirm the weakening effect from PFZ by means of a reduced hardness close to grain boundaries. The nanoindentation hardness mapping also revealed an increase in hardness a few micrometers from the grain boundary with respect to the grain interior. Precipitate quantification from TEM images confirms that the hardness increase is caused by a locally higher density of precipitates. To the authors’ best knowledge, this harder zone has not been recognized nor discussed in previously reported findings. The phenomenon has important implications for the mechanical properties of large-grained (>100 µm) aluminium alloys.

Metal–organic framework-derived Co3O4 modified nickel foam-based dendrite-free anode for robust lithium metal batteries

For several decades, the promise of implementing of lithium (Li) metal anodes for Li batteries has been a ‘‘holy grail’’ for researchers. Herein, we have proposed a facile design of a MOF-derived Co3O4 nanoparticles modified nickel foam, i.e., Co3O4-NF, as a 3D host to achieve a uniform infusion of the molten Li. The molten Li was uniformly absorbed on the Co3O4-NF host only in 10 s due to its high Li lithiophilicity. The obtained Li-Co3O4-NF composite electrode shows high cycling stability in symmetric cells with low voltage hysteresis even at a high current density of 5 mA/cm2. The full cells of Li-Co3O4-NF/LiFePO4 can cycle for more than 500 cycles at 2C without obvious capacity decay. SEM after cycling and in situ optical microscope results suggest that the unique 3D host structure of the Li-Co3O4-NF anode plays key roles on suppressing the dendrite growth and decreasing the local current inhomogeneity. We believe this work might provide a new strategy for fabricating dendrite-free Li metal anodes and facilitate practical applications in Li batteries.

Orientation dependence of R2 relaxation in the newborn brain

In MRI the transverse relaxation rate, R2 = 1/T2, shows dependence on the orientation of ordered tissue relative to the main magnetic field. In previous studies, orientation effects of R2 relaxation in the mature brain's white matter have been found to be described by a susceptibility-based model of diffusion through local magnetic field inhomogeneities created by the diamagnetic myelin sheaths. Orientation effects in human newborn white matter have not yet been investigated. The newborn brain is known to contain very little myelin and is therefore expected to exhibit a decrease in orientation dependence driven by susceptibility-based effects. We measured R2 orientation dependence in the white matter of human newborns. R2 data were acquired with a 3D Gradient and Spin Echo (GRASE) sequence and fiber orientation was mapped with diffusion tensor imaging (DTI). We found orientation dependence in newborn white matter that is not consistent with the susceptibility-based model and is best described by a model of residual dipolar coupling. In the near absence of myelin in the newborn brain, these findings suggest the presence of residual dipolar coupling between rotationally restricted water molecules. This has important implications for quantitative imaging methods such as myelin water imaging, and suggests orientation dependence of R2 as a potential marker in early brain development.

The Influence of Local Microstructure Inhomogeneities on Local Drying Kinetics during Freeze-Drying.

Freeze-drying is a gentle drying technique to dry high value products, such as pharmaceuticals, without impacting the quality of the product. However, this method is very time and cost intensive. It is known that larger pores reduce the duration of primary drying due to facilitated mass transport. However, next to the pore size, other structural parameters exist whose influence on drying kinetics is still unknown. Therefore, the aim of this article is to investigate the influence of the microstructure (pore size, shape and orientation) on local primary drying kinetics. In the study, freeze-drying experiments on maltodextrin and sucrose solutions (c1 = 0.05 and c2 = 0.15 w/w) were carried out in a lyomicroscope. Two-dimensional images were recorded during the whole drying process and in the dry state and analyzed on the movement of the sublimation front, pore size, orientation and shape. Different microstructures were created by using different freezing parameters, namely two different cooling rates and solid concentrations. It could be shown that for pores with a high aspect ratio, the pore orientation was more important for the drying kinetics than the pore size, while for pores with a lower aspect ratio the pore size was the decisive parameter.

In-situ monitoring of Material Extrusion processes via thermal videoimaging with application to Big Area Additive Manufacturing (BAAM)

Material Extrusion (ME) is becoming one of the most promising class of Additive Manufacturing (AM) processes in the manufacturing panorama for its capability of realizing products with complex shapes, potentially in large dimensions, for a variety of applications and materials for different industrial sectors. Despite the significant advances in material range and machine capabilities, the lack of process stability and repeatability of ME still represents an obstacle for the adoption of this process on large industrial scales. Following the success of in-situ monitoring in other AM technologies, some studies are investigating the possibility of embedding in-situ sensors on commercial machines. Among them, thermal infrared monitoring systems have gained lots of interest for the crucial role of temperature evolution in final part quality and mechanical performance. This study presents a data-driven methodology for in-situ monitoring of thermal profiles. The proposed algorithm extracts, models, and compares the cooling history at different locations. By combining a tracking algorithm and automatic detection of region of interests and robust regression for functional data modeling, we propose a solution to study the cooling history of the 3D printed geometry as the process evolves, to draw general conclusions on the process and detect anomalous thermal profiles, which may entail lack of bonding or other flaws. This paper further proposes a novel approach to automatically detect local temperature inhomogeneities, also known as “hot” and “cold spots”, which are often symptomatic of critical defects in the 3D printed part. In-situ detection of hold- and cold-spots can be particularly useful for large scale products or functional products printing. The proposed methods can be applied to any ME process, but it is specifically applied to a real case study of Big Area Additive Manufacturing (BAAM), in collaboration with the Oak Ridge National Laboratory. Results show the effectiveness of all the proposed solutions and pave the way towards zero-defects production.

Physical properties enhancement of carbon fiber obtained from isotropic pitch doped by ultra-long carbon nanotubes

• DWCNT-doped isotropic petroleum pitch used successfully for carbon fiber formation • DWCNT doping leads to jump in both thermal and electrical conductivity of fibers • Inner cavities appear in the fibers at DWCNT content close to 1.0 wt. % • Hollow filaments formed at high DWCNT content paving way to hollow fiber technology Carbon fibers (CF) were formed from carbon nanotube-doped isotropic petroleum pitch. Ultra-long (the length of a single filament is more than 10,000 nm) double-walled carbon nanotubes (DWCNT) were used for doping in concentrations from 0.1 to 1.0 wt. %. The produced CF were investigated both in pristine and graphitized form. Physical properties (electrical resistivity, thermal conductivity coefficient), as well as X-ray diffraction (XRD) analysis and Raman spectroscopy of obtained DWCNT-doped CF were investigated. The influence of DWCNT concentration on the properties is nonlinear due to the influence on the CF molding mode and heterogeneity of the DWCNT distribution in the filament body. An increase in the DWCNT concentration, on the one hand, causes an enhancement in the thermal conductivity coefficient, and a decrease in the electrical resistivity of the fibers, on the other hand, it leads to local inhomogeneities formation in the material structure («gas bubbles»), as well as distortions of the CF outer surface. An increase in the DWCNT concentration also leads to an increase in CF average diameter and hollow filaments formation. This feature is paving the way to the development of novel technological methods to control CF properties and morphology, especially to feasible hollow CF composites manufacture.

Luminescence and photoconversion properties of micro-powder phosphors based on the Ce3+ and Mn2+ doped Ca2YMgScSi3O12 silicate garnets

This work is dedicated to study the synthesis processes and luminescence properties of micro-crystalline powder phosphors of Ce3+ and Mn2+ doped Ca2YMgScSi3O12 garnets (CYMSSG:Ce and CYMSSG:Ce,Mn) The photoluminescence emission spectra and decay kinetics confirm the formation of Ce3+ multicenters in the CYMSSG:Ce powder phosphor due to local inhomogeneity of the different dodecahedral positions and presence of the energy transfer between high-energy and low energy Ce3+ emission centers in this garnet. The effective Ce3+ to Mn2+ energy transfer process is observed in CYMSSG:Ce, Mn phosphors resulting in the redshift of the emission spectra in comparison with CYMSSG:Ce counterpart. We have shown that CYMSSG:Ce and CYMSSG:Ce, Mn powder phosphors can be used as an efficient photoconverter (PC) for creation of the high-power white LEDs (WLEDs). The prototypes of PC-WLEDs, based on 450 nm emitting blue LED and planar layers of CYMSSG:Ce and CYMSSG:Ce, Mn microcrystalline powder phosphors, embedded in the epoxy resin, were prepared and their photoconversion properties were investigated as well.

Mode Coupling Due to a Local Inhomogeneity in a Shallow-Water Acoustic Waveguide in a Broad Frequency Band

Mode Coupling Due to a Local Inhomogeneity in a Shallow-Water Acoustic Waveguide in a Broad Frequency Band

A novel method for estimating subresolution porosity from CT images and its application to homogeneity evaluation of porous media

We propose a new method, i.e., the statistical phase fraction (SPF) method, to estimate the total porosity and spatial distribution of local porosities from subresolution pore-dominated X-ray microtomography images of porous materials. The SPF method assumes that a voxel in a CT image is composed of either a single or a maximum of three pure phases of matter (solid, liquid and air). Gaussian function (GF) fitting is conducted on the basis that the summation of the area of each GF curve is equal to the total area covered by the CT histogram. The volume fraction of each phase corresponding to each GF is calculated based on the mean value of the GF, the area of the GF, and the CT numbers for pure phases. The SPF method is verified on three different types of components containing only air and solid phases, i.e., alumina ceramic and two sintered lunar regolith simulants with relatively homogenous and inhomogeneous microstructures. The estimated porosities of a total of 15 specimens (the total porosity ranges from 0 to 51%) via the SPF method show an average error of 3.11% compared with the ground truth. Spatial distribution of local porosities in the defined representative element volume is investigated for homogeneity evaluation. Results show that the local porosity inhomogeneity in the sintered FJS-1 specimens is more prominent than that in the sintered KLS-1 specimens.

Changing of LiNbO3 Raman spectra under external electric field applying

The results of investigating the effect of an external electric field on the Raman spectrum of the Y-cut of a lithium niobate single crystal are presented. A change in the intensity and position of individual lines of the Raman spectrum that is associated with a change in the parameters of the crystal lattice due to the reverse piezoelectric effect is found. A change in the intensity and a shift in the spectral lines caused by local inhomogeneities of the lithium niobate single crystal have been also observed. In order to determine what contribution each of these effects makes to the change in the Raman spectra, the principal components analysis has been used in the analysis of experimental data.

Low thermal conductivity in A-site high entropy perovskite relaxor ferroelectric

An A-site disordered high entropy perovskite (Pb1/6Ba1/6Sr1/6Ca1/6Na1/6Bi1/6)TiO3 (PBSCNBi) ceramic was prepared by a solid-state reaction method. XRD and scanning electron microscopy-energy dispersive x ray confirmed a single-phase tetragonal solid solution. Dielectric and hysteresis loop measurements showed relaxor ferroelectricity at room temperature; Curie Weiss fitting gives a Burns temperature (Tb) of 123 °C, and Vogel–Fulcher fitting gives a freezing temperature (Tf) of −67.24 °C, which confirms the room-temperature relaxor ferroelectricity of PBSCNBi. This is attributed to local chemical inhomogeneities in the high entropy ceramics. PBSCNBi also has a low thermal conductivity (1.15 W m−1 K−1 at room temperature) compared to all of its constituent simple perovskites (e.g., BaTiO3, PbTiO3, SrTiO3 CaTiO3, and Na1/2Bi1/2TiO3 in the range of 25–100 °C), which is attributed to the enhanced phonon scattering by both polar nanoregions and the mass contrast effect in the multi-element perovskite. This work demonstrates the great potential of making A-site high entropy ceramics with relaxor ferroelectric properties.

Molten-Salt Processed Potassium Sodium Niobate Single-Crystal Microcuboids with Dislocation-Induced Nanodomain Structures and Relaxor Ferroelectric Behavior.

We herein report a facile molten-salt synthetic strategy to prepare transparent and uniform Li, Ba-doped (K,Na)NbO3 (KNN) single-crystal microcuboids (∼80 μm). By controlling the degree of supersaturation, different growth modes were found and the single-crystal microcuboids were synthesized via island-like oriented attachment of KNN particles onto the growing surface. The distinct relaxor ferroelectric (RFE) properties were achieved in the single-crystal microcuboids, which were different from the normal ferroelectric (FE) properties found in their KNN ceramic counterparts prepared through a solid-state reaction using the same initial precursors. The RFE properties were realized by dislocation-induced nanodomain formation during oriented attachment growth of single-crystal microcuboids, which is different from the current strategies to derive the nanodomains by the local compositional inhomogeneity or the application of an electric field. The dislocations served as nucleation sites for ferroelectric domain walls and block the growth of domains. The KNN single-crystal microcuboids exhibited a higher effective piezoelectric coefficient (∼459 pm/V) compared to that of the bulk KNN ceramic counterpart (∼90 pm/V) and showed the broad diffuse maxima in the temperature dependence dielectric permittivity. The high maximum polarization (69.6 μC/cm2) at a relatively low electric field (30 kV/cm) was beneficial for energy storage applications. Furthermore, the KNN-based transparent, flexible pressure sensor directly monitored the mechanical motion of human activity without any external electric power. This study provides insights and synthetic strategies of single-crystal RFE microcuboids for other different perovskites, in which nanodomain structures are primarily imposed by their chemical composition.

Controllable tensile performance of additively manufactured Al–Fe alloy

Achieving both high strength and ductility is challenging in additive manufacturing of Al alloys using the laser powder bed fusion (L-PBF) process by altering the laser conditions to control the microstructural characteristics. The current work focuses on the influence of varying laser energy density (ED), which is adjusted by different sets of laser power (P) and scan speed (v), on the microstructure-tensile property relationships of the L-PBF-fabricated Al–2.5%Fe alloy under varying laser conditions (P = 204–230 W, v = 450–600 mm/s) with a fixed powder layer thickness (t = 30 μm) and hatch distance (h = 30 μm). With increasing ED (= P/v·t·h), a slight increase in the average size of the columnar α-Al grains was found in the experimental samples concurrently with a higher fraction of <001>-oriented grains. Nanosized particles of the Al6Fe phase with an increased tendency to interconnect were observed in the matrix inside the melt pool, and the θ phase formed within the submicron-sized cellular structure along the melt pool boundaries (MPBs). A low tensile ductility, induced by the local microstructural inhomogeneities across MPBs, was found in all specimens tensile-deformed along the building direction, which was characterized by a preference fracture along the MPBs. At higher applied ED, the tensile ductility of the experimental Al–2.5Fe alloy was significantly improved with only a slight decrease in tensile strength. The difference in concentration of Fe across the MPBs in the matrix was also moderately diminished, which is consistent with the reduced local difference in microhardness across the MPBs under higher ED conditions. Therefore, the present results demonstrate that local microstructural inhomogeneities across MPBs can be controlled by the laser conditions, thereby producing both high strength and good ductility in the L-PBF processed Al–Fe alloys.

Dendrite-free aqueous Zn-ion batteries via a multifunctional Se protective layer

The development of aqueous zinc-ion batteries (AZIBs) is hindered by severe corrosion, hydrogen evolution and dendritic growth problems of zinc anode. To solve this problem, electrodepositing Se on the surface of Zn anode is proposed to suppress side reactions and growth of dendrites. As expected, the [email protected] symmetric cell achieved a cycle life of more than 1000 h at the current density of 0.05 mA cm−2, and can stabilize with a low polarization (57 mV). When NixV2O5·nH2O was applied as cathode, this full cell achieved a long lifespan of 500 cycles with coulombic efficiency near 100%. The closely-arranged Se layer can act as an artificial solid electrolyte interphase SEI to limit the local inhomogeneity of zinc during electroplating/stripping, thereby suppressing the growth of zinc dendrites and also reducing the evolution of hydrogen.

Simulation of the Particle Transport Behaviors in Nanoporous Matter

The transport behaviors of proton into nanoporous materials were investigated using different Monte Carlo simulation codes such as GEANT4, Deeper and SRIM. The results indicated that porous structure could enhance the proton scattering effects due to a higher specific surface area and more boundaries. The existence of voids can deepen and widen the proton distribution in the targets due to relatively lower apparent density. Thus, the incident protons would transport deeper and form a wider Bragg peak in the end of the range, as the target materials are in a higher porosity state and/or have a larger pore size. The existence of voids also causes the local inhomogeneity of proton/energy distribution in micro/nano scales. As compared, the commonly used SRIM code can only be used to estimate roughly the incident proton range in nanoporous materials, based on a homogeneous apparent density equivalence rule. Moreover, the estimated errors of the proton range tend to increase with the porosity. The Deeper code (designed for evaluation of radiation effects of nuclear materials) can be used to simulate the transport behaviors of protons or heavy ions in a real porous material with porosity smaller than 52.3% due to its modeling difficulty, while the GEANT4 code has shown advantages in that it is suitable and has been proven to simulate proton transportation in nanoporous materials with porosity in its full range of 0~100%. The GEANT4 simulation results are proved consistent with the experimental data, implying compatibility to deal with ion transportation into homogeneously nanoporous materials.

Employing gadolinium micro-disks as temperature probes for magnetic resonance imaging

Magnetic resonance imaging (MRI) is a critical component in many medical procedures such as treating cancer during MRI guided thermal ablations and hyperthermia. These procedures require real-time, spatially and thermally accurate temperature maps. In this paper we demonstrate the effectiveness of using uniform gadolinium micro-disks dispersed within a tissue-mimicking medium as MRI temperature sensors. Disk shaped Gd microstructures passivated with chromium were fabricated using a sputtering method and embedded in a Ringer’s solution agar gel matrix. We correlate the nuclear magnetic resonance linewidth broadening of water hydrogen nuclei in a phantom containing these Gd micro-disks with the temperature dependence of the mass magnetization of Gd. We acquired MRI images of the phantoms using the gradient echo method, which is sensitive to local magnetic field inhomogeneities. The Gd micro-disks produce a strong, predictable correlation between the brightness of T2* weighted MRI images and phantom temperatures, indicating that these Gd micro-disks are a good candidate for use as MRI temperature sensors.

Automated slice-specific z-shimming for functional magnetic resonance imaging of the human spinal cord.

Functional magnetic resonance imaging (fMRI) of the human spinal cord faces many challenges, such as signal loss due to local magnetic field inhomogeneities. This issue can be addressed with slice-specific z-shimming, which compensates for the dephasing effect of the inhomogeneities using a slice-specific gradient pulse. Here, we aim to address outstanding issues regarding this technique by evaluating its effects on several aspects that are directly relevant for spinal fMRI and by developing two automated procedures in order to improve upon the time-consuming and subjective nature of manual selection of z-shims: one procedure finds the z-shim that maximizes signal intensity in each slice of an EPI reference-scan and the other finds the through-slice field inhomogeneity for each EPI-slice in field map data and calculates the required compensation gradient moment. We demonstrate that the beneficial effects of z-shimming are apparent across different echo times, hold true for both the dorsal and ventral horn, and are also apparent in the temporal signal-to-noise ratio (tSNR) of EPI time-series data. Both of our automated approaches were faster than the manual approach, lead to significant improvements in gray matter tSNR compared to no z-shimming and resulted in beneficial effects that were stable across time. While the field-map-based approach performed slightly worse than the manual approach, the EPI-based approach performed as well as the manual one and was furthermore validated on an external corticospinal data-set (N > 100). Together, automated z-shimming may improve the data quality of future spinal fMRI studies and lead to increased reproducibility in longitudinal studies.