Materials In Bulk Form Research Articles

Understanding How the Suppression of Insertion-Induced Phase Transitions Leads to Fast Charging in Nanoscale LixMoO2.

Fast-charging Li-ion batteries are technologically important for the electrification of transportation and the implementation of grid-scale storage, and additional fundamental understanding of high-rate insertion reactions is necessary to overcome current rate limitations. In particular, phase transformations during ion insertion have been hypothesized to slow charging. Nanoscale materials with modified transformation behavior often show much faster kinetics, but the mechanism for these changes and their specific contribution to fast-charging remain poorly understood. In this work, we combine operando synchrotron X-ray diffraction with electrochemical kinetics analyses to illustrate how nanoscale crystal size leads to suppression of first-order insertion-induced phase transitions and their negative kinetic effects in MoO2, a tunnel structure host material. In electrodes made with micrometer-scale particles, large first-order phase transitions during cycling lower capacity, slow charge storage, and decrease cycle life. In medium-sized nanoporous MoO2, the phase transitions remain first-order, but show a considerably smaller miscibility gap and shorter two-phase coexistence region. Finally, in small MoO2 nanocrystals, the structural evolution during lithiation becomes entirely single-phase/solid-solution. For all nanostructured materials, the changes to the phase transition dynamics lead to dramatic improvements in capacity, rate capability, and cycle life. This work highlights the continuous evolution from a kinetically hindered battery material in bulk form to a fast-charging, pseudocapacitive material through nanoscale size effects. As such, it provides key insight into how phase transitions can be effectively controlled using nanoscale size and emphasizes the importance of these structural dynamics to the fast rate capability observed in nanostructured electrode materials.

An Automatic Apparatus for Simultaneous Measurement of Seebeck Coefficient and Electrical Resistivity

A fully automated experimental system was designed for simultaneous measurement of the Seebeck coefficient and electrical resistivity of thermoelectric materials in bulk form. The system incorporates a straightforward and easily fabricated sample holder along with commercially available electronic instrument components. The sample holder showcases a compact design that utilizes two Peltier module heaters to induce sample heating and generate the required temperature gradient. System automation and control are achieved through the implementation of a LabView program. The Seebeck voltage and resistance of the sample (under specified temperature conditions) are determined using I–V measurements. The Seebeck voltage and resistance of the sample correspond to the intercept and slope of the I–V characteristic diagram in the four-point probe method, respectively. To verify the accuracy and reliability of the developed apparatus, a variety of experiments were performed on N-type and P-type bismuth telluride samples. The measurement results closely matched those obtained from commercial systems, with an overall data difference of less than 10% for both the Seebeck coefficient and resistivity measurements.

Room-temperature electric field-induced out-of-plane ferroelectric polarization in strain-free freestanding 2D SrTiO3 membranes

SrTiO3 (STO), a room-temperature paraelectric material in bulk form, has been a rich playground for emergent phenomena for decades. As an emerging material, great attention has been paid to freestanding 2D STO thin films. Recently, the room-temperature ferroelectricity has been unveiled in strained STO thin films; however, it remains an open question whether the strain-free freestanding 2D STO thin film is room-temperature ferroelectric or not. Here, we report the electric field-induced out-of-plane ferroelectric polarization in large-scale, freestanding, and strain-free 2D STO membranes at room temperature. High-resolution piezoresponse force microscopy measurements show that polarization in freestanding strain-free STO membranes can be switched by electric field and persist for an hour. The first-principles calculations suggest that the intrinsic defects such as oxygen vacancies could be linked to the observed spontaneous out-of-plane polarization in 2D STO membranes, which could be further enhanced by external electric field due to the induced symmetry breaking. Our work reports the unprecedented room-temperature ferroelectric polarization in strain-free freestanding 2D STO membranes, unlocking the great potential of freestanding 2D STO for the applications in novel electronic and logic-in-memory devices.

Patterned diamond anvils prepared via laser writing for electrical transport measurements of thin quantum materials under pressure.

Quantum materials exhibit intriguing properties with important scientific values and huge technological potential. Electrical transport measurements under hydrostatic pressure have been influential in unraveling the underlying physics of many quantum materials in bulk form. However, such measurements have not been applied widely to samples in the form of thin flakes, in which new phenomena can emerge, due to the difficulty in attaching fine wires to a thin sample suitable for high-pressure devices. Here, we utilize a home-built direct laser writing system to functionalize a diamond anvil to directly integrate the capability of conducting electrical transport measurements of thin flakes with a pressure cell. With our methodology, the culet of a diamond anvil is equipped with a set of custom-designed conducting tracks. We demonstrate the superiority of these tracks as electrodes for the studies of thin flakes by presenting the measurement of pressure-enhanced superconductivity and quantum oscillations in a flake of MoTe2.

Nanoscale ZnO/α‐Fe2O3 Heterostructures: Toward Efficient and Low‐Cost Photoanodes for Water Splitting

Nanoscale ZnO/α-Fe2O3 Heterostructures Among metal oxides, ZnO and α-Fe2O3 hold great potential for hydrogen production through photoelectrochemical water splitting (PEC WS). However, significant drawbacks related to these materials in bulk form hinder their efficiency and scalability. In article number 2100104, Letizia Liccardo, Elisa Moretti, Alberto Vomiero, and co-workers report morphology, doping, and nanostructuring as key parameters for boosting the overall performance in PEC WS, highlighting how ZnO/α-Fe2O3 composite systems represent a promising solution compared to single building blocks.

Coexistence of Superconductivity and Nontrivial Band Topology in Monolayered Cobalt Pnictides on SrTiO3.

As an intrinsically layered material, FeSe has been extensively explored for potentially revealing the underlying mechanisms of high transition temperature (high-Tc) superconductivity and realizing topological superconductivity and Majorana zero modes. Here we use first-principles approaches to identify that the cobalt pnictides of CoX (X = As, Sb, Bi), none of which is a layered material in bulk form. Nevertheless, all can be stabilized as monolayered systems either in freestanding form or supported on the SrTiO3(001) substrate. We further show that each of the cobalt pnictides may potentially harbor high-Tc superconductivity beyond the Cu- and Fe-based superconducting families, and the underlying mechanism is inherently tied to their isovalency nature with the FeSe monolayer. Most strikingly, each of the monolayered CoX's on SrTiO3 is shown to be topologically nontrivial, and our findings provide promising new platforms for realizing topological superconductors in the two-dimensional limit.

New Tools to Support the Risk Assessment Process of Nanomaterials in the Insurance Sector.

During the last decade, the use of nanomaterials, due to their multiple utilities, has exponentially increased. Nanomaterials have unique properties such as a larger specific surface area and surface activity, which may result in health and environmental hazards different from those demonstrated by the same materials in bulk form. Besides, due to their small size, they can easily penetrate through the environmental and biological barriers. In terms of exposure potential, the vast majority of studies are focused on workplace areas, where inhalation is the most common route of exposure. The main route of entry into the environment is due to indirect emissions of nanomaterials from industrial settings, as well as uncontrollable releases into the environment during the use, recycling and disposal of nano-enabled products. Accidental spills during production or later transport of nanomaterials and release from wear and tear of materials containing nanomaterials may lead to potential exposure. In this sense, a proper understanding of all significant risks due to the exposure to nanomaterials that might result in a liability claim has been proved to be necessary. In this paper, the utility of an application for smartphones developed for the insurance sector has been validated as a solution for the analysis and evaluation of the emerging risk of the application of nanotechnology in the market. Different exposure scenarios for nanomaterials have been simulated with this application. The results obtained have been compared with real scenarios, corroborating that the use of novel tools can be used by companies that offer risk management in the form of insurance contracts.

Strain Induced Anisotropic Magnetic Behaviour and Exchange Coupling Effect in Fe-SmCo5 Permanent Magnets Generated by High Pressure Torsion

High-pressure torsion (HPT), a technique of severe plastic deformation (SPD), is shown as a promising processing method for exchange-spring magnetic materials in bulk form. Powder mixtures of Fe and SmCo5 are consolidated and deformed by HPT exhibiting sample dimensions of several millimetres, being essential for bulky magnetic applications. The structural evolution during HPT deformation of Fe-SmCo5 compounds at room- and elevated- temperatures of chemical compositions consisting of 87, 47, 24 and 10 wt.% Fe is studied and microstructurally analysed. Electron microscopy and synchrotron X-ray diffraction reveal a dual-phase nanostructured composite for the as-deformed samples with grain refinement after HPT deformation. SQUID magnetometry measurements show hysteresis curves of an exchange coupled nanocomposite at room temperature, while for low temperatures a decoupling of Fe and SmCo5 is observed. Furthermore, exchange interactions between the hard- and soft-magnetic phase can explain a shift of the hysteresis curve. Strong emphasis is devoted to the correlation between the magnetic properties and the evolving nano-structure during HPT deformation, which is conducted for a 1:1 composition ratio of Fe to SmCo5. SQUID magnetometry measurements show an increasing saturation magnetisation for increasing strain γ and a maximum of the coercive field strength at a shear strain of γ = 75.

Structure and Tensile Strength of Pure Cu after High Pressure Torsion Extrusion

The microstructure and mechanical properties of rod-shaped samples (measuring 11.8 mm in diameter and 35 mm in length) of commercially pure (CP) copper were characterized after they were processed by high pressure torsion extrusion (HPTE). During HPTE, CP copper was subjected to extremely high strains, ranging from 5.2 at central area of the sample to 22.4 at its edge. This high but varying strain across the sample section resulted in HPTE copper displaying a gradient structure, consisting of fine grains in the central area and of ultrafine grains both in the middle-radius area and at the sample edge. A detailed analysis of the tensile characteristics showed that the strength of HPTE copper with its gradient structure is similar to that of copper after severe plastic deformation (SPD) techniques, typically displaying a homogeneous structure. Detailed analysis of the contributions of various strengthening mechanisms to the overall strength of HPTE coper revealed the following: The main contribution comes from Hall–Petch strengthening due to the presence of high and low angle grain boundaries in gradient structure, which act as effective obstacles to dislocation motion. Therefore, both types of boundaries should be taken into account in the Hall–Petch equation. This study on CP copper demonstrated the potential of using the HPTE method for producing high-strength metallic materials in bulk form for industrial use.

Structural investigation of low energy ion irradiated Al2O3

Abstract The present study is undertaken to investigate the ion irradiation impact on the structural properties of α-Al2O3 which is one of the potential insulator used for ITER-like reactors. 300 keV Ar6+ ion beam with varying fluence is used to irradiate Al2O3 polycrystalline material in bulk form. Structural damage to α-Al2O3 due to ion irradiation is investigated by grazing incidence XRD (GIXRD), micro-RAMAN and photoluminescence spectroscopy. Further, the surface morphology of irradiated and pristine samples is studied using atomic force microscopy. GIXRD data reveal the radiation induced damage at low fluence and structural improvement at higher fluence up to 1 × 1016 ions/cm2. Annealing effect is explained by temperature rise during irradiation and due to partial grain growth of Al2O3 at 1 × 1016 ions/cm2. It is important to note that there is no sign of amorphization even at the highest fluence 1 × 1016 ions/cm2 used in this study. The explanation is supported by the Raman characterization results. Detailed peak assessment of photoluminescence spectra is presented. Photoluminescence characterization results are corroborated with XRD results. The present study reveals the radiation hardness of polycrystalline Al2O3 material in the low energy ion regime.

Large tetragonality and room temperature ferroelectricity in compressively strained CaTiO3 thin films

Ferroelectricity and piezoelectricity are desirable for a variety of high-temperature applications such as actuators and sensors in heat engines, high-temperature manufacturing, and space technologies; however, the material candidates are currently limited. Here, we demonstrate that CaTiO3, the prototype perovskite mineral, abundantly found in the Earth, which as a nonpolar material in bulk form, becomes a high-temperature ferroelectric oxide under compressive strain when grown as a thin film. A strain-phase-temperature diagram of CaTiO3 films is created by growing films on various substrates with different in plane strains in order to map out the polar behavior for compressive and tensile strain. Using temperature dependent optical second harmonic generation analysis, we show that tensile strained films exhibit predominantly in-plane polarization with orthorhombiclike point group symmetry with a phase transition below room temperature. On the other hand, compressively strained CaTiO3 films exhibit a near-tetragonal unit cell with a c/a ratio of 1.03, larger than that of classic ferroelectric, e.g., BaTiO3 (c/a ∼ 1.01). These films exhibit a robust and switchable out-of-plane polarization at room temperature, with a ferroelectric transition temperature up to ∼800 K. Density functional theory calculations reveal that compressive strain gives rise to a large out-of-plane displacement of Ti-cations inside the TiO6 octahedral cages and is the major contributor to the calculated polarization of ∼9 µC/cm2. Given that nearly half of the perovskites exhibit the bulk symmetry of CaTiO3, compressive strain tuning of this perovskite family may prove to be a fertile ground for the discovery of strain-induced piezoelectrics and ferroelectrics at high-temperatures.

Study of the microstructure and texture heterogeneities of Fe–48wt%Ni alloy severely deformed by equal channel angular pressing

A Fe–48wt%Ni alloy was processed by severe plastic deformation using equal channel angular pressing process. A stacking of 9 sheets was introduced and pressed up to two passes into die with an inner angle of Φ = 90o and outer arc of curvature ψ = 17° at room temperature following route A. The same material in bulk form was also ECAPed up to one pass. The microstructure and the texture were investigated by means of electron backscattered diffraction and X-ray diffraction, respectively. To evaluate the mechanical response, Vickers microhardness was carried out. The given analyses concern the as-received sample, the peripheral and the central plates of the pressed stacks and the upper, the middle and the lower parts of the pressed bulk material. The deformation was heterogeneous, and variations in texture and microstructure, resulting from different efficiencies in the shearing process, were locally noted. For the stacks samples, the microstructure evolved from equiaxed grains of 9 μm with high fraction of high-angle grain boundaries (around 90%) to a heterogeneous fine grain structure with an average grain size of 3 μm after two passes. On the contrary, for the bulk sample, the evolution was to a banded structure after one pass. Results of mechanical property show that microhardness increased significantly from 147 Hv before deformation to mean values of 244 (after one pass) and 235 Hv (after two passes) for the bulk and stacked samples, respectively. The Hall–Petch effect and dislocation density were evaluated as most responsible in material strengthening.

Analysis of instrumented scratch hardness and fracture toughness properties of laser surface alloyed tribological coatings

The mechanical characteristics of ceramic matrix composite (CMC) coatings are widely different from the same materials in bulk form or the individual constituents and are very important to be assessed to carry out application oriented studies on CMC coatings with novel compositions. In the present work, a composite coating of TiB2, TiN and SiC is fabricated in-situ through a combination of high temperature chemical reaction and laser surface alloying. The formation of the surface layer is due to the laser-assisted chemical reaction followed by laser melting. A mixture of TiO2, SiO2, hBN and graphite in stoichiometric proportions is used as the precursor for the chemical reaction. The presence of all the reaction products in the CMC coatings developed is confirmed by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). A thorough evaluation of various mechanical properties achieved more insight into the CMC coatings developed. Hardness and fracture toughness of the coatings are measured with a scratch tester. The property evaluations are performed in a similar way for two more coatings fabricated with precursor mixtures containing more than a stoichiometric amount of SiC and hBN respectively. For comparison, a number of composites fabricated through various other routes are characterized afresh with the same set of techniques. Coatings formed with SiC in precursor show higher values of scratch hardness (14.37GPa), microhardness (24.37GPa) and fracture toughness (6.63MPa-m1/2).

(Invited) Oxygen Exchange Kinetics on Technological Versus Model Type Electrodes

The rate of the oxygen exchange at the gas solid interface is of key importance for the performance of solid oxide fuel cells (SOFCs), especially at low temperature, as well as for the overall durability of solid oxide electrolysis cells (SOECs), when operated at high current density. Hence, details of this process are of great technological importance. Whereas simple guidelines like; i) Combine an electrolyte material and an electro-catalyst in the electrode, ii) maximize triple phase boundary length per unit area, iii) maximize surface area of the electro catalyst, iv) use preferably a mixed conducting electro-catalyst, v) maximize ionic conductivity in both phases,... are well established in the field, and have lead to development of well performing electrodes, the rate limiting steps of the exchange process itself are not well established. It is very challenging to elucidate these on the “real” geometrically very complex composite systems. This has spurred interest in studying simpler model electrode systems [1,2,3], for example in the form of ultra-thin dense films (with a well-defined surface area) and also in trying to model electrode performance from a detailed geometrical description of the electrode structure and the material properties deduced from studies of the materials in bulk form. The study of thin film model electrode systems certainly lends insight into the details of the exchange process, but the approach is not without problems, as the model system may differ from the “real” nanostructured composite electrodes in a number of ways, e.g. with respect to surface structure/termination, chemical composition [4], purity and possibly strain, which may all affect the exchange process. In this paper we discuss challenges in predicting performance of real electrodes from studies on model systems. We shall compare trends observed on dense thin films of LSC (La0.5Sr0.5CoO3), LSCF (La0.58Sr0.4Co0.2Fe0.8O3- δ) and LSF (La0.6Sr0.4FeO3) with trends observed when these materials are used in porous composites. Also a model study of a multi-phase nano-structured electrode, prepared by infiltrating a porous CGO backbone structure with “LSC” will be presented, and it is shown that performance can be well accounted for when assuming exchange properties as deduced from LSC in bulk form. Finally, recent results obtained on two other types of simplified model electrode systems are presented. The first one is based on a dense composite of two phases (Fig. 1, A and B) and in the second type a mixed conducting material is studied with and without second phase surface decorations (Fig. 1, D). In both cases, the oxygen exchange is studied using conductivity relaxation (Fig. 1, C). The advantage of these geometries are that they are simpler than the real electrodes, e.g. the TPP can be accurately assessed, and the manufacturing route in many respects lies closer to the one applied on technological electrodes than does the ones typically applied when making ultra-thin films. On a LSF/CGO (La0.6Sr0.4)0.98FeO3-Ce0.9Gd0.1O1.95) composite system with different volume ratios between the two phases (0%, 30 %, 50 %, 70 % CGO) we find that the relaxation time for re-equilibrating the oxygen stoichiometry in the sample after a sudden change in oxygen activity reduces strongly the more CGO is added. This is not only an effect of the increased ionic conductivity in the composite (CGO has a higher ionic conductivity than LSF) but also the surface exchange rate is increased (by up to a factor of 25) relative to the one of the pure LSF sample. Effects of adding small amounts of second phase decorations on the surface on both the composites and the LSF end-member were studied. Results are compared to recent findings in literature [5] and the underlying mechanisms for the increased surface exchange rates are discussed.

Unsupported single-atom-thick copper oxide monolayers

Oxide monolayers may present unique opportunities because of the great diversity of properties of these materials in bulk form. However, reports on oxide monolayers are still limited. Here we report the formation of single-atom-thick copper oxide layers with a square lattice both in graphene pores and on graphene substrates using aberration-corrected scanning transmission electron microscopy. First-principles calculations find that CuO is energetically stable and its calculated lattice spacing matches well with the measured value. Furthermore, free-standing copper oxide monolayers are predicted to be semiconductors with band gaps ∼3 eV. The new wide-bandgap single-atom-thick copper oxide monolayers usher a new frontier to study the highly diverse family of two-dimensional oxides and explore their properties and their potential for new applications.

Structural factor in bending testing of fivefold twinned nanowires revealed by finite element analysis

In this study, we performed finite element method simulations to investigate the effect of the structure on the elastic response of Ag and Au nanowires (NWs) with a fivefold twinned crystal structure in bending tests. Two different models of a pentagonal NW were created: a ‘uniform model’ having an isotropic continuous structure and a ‘segmented model’ consisting of five anisotropic domains. Two asymmetrical mechanical test configurations were simulated: cantilevered beam bending and 3-point bending. The dimensions of the NW, the test configurations, as well as the force and the displacement ranges were based on the previously obtained experimental data. The results of the simulations demonstrated that the segmented model was stiffer than the uniform one in both of the bending tests. The effect was more pronounced for the cantilevered beam bending configuration. This fact should be taken into account in the interpretation of the increased measured Young’s modulus of pentagonal NWs in comparison to the elasticity of the same material in bulk form.

A review on preparation techniques for synthesis of nanocrystalline soft magnetic ferrites and investigation on the effects of microstructure features on magnetic properties

Soft magnetic materials have been used in many applications, i.e., electrical and electronic industries, due to their desirable electromagnetic characteristics. The performance of these materials in bulk form, where the size of grains is in micrometer scale, is only limited to a few megahertz frequencies due to their higher conductivity and domain wall resonance. Synthesizing the ferrite particles in nanometer scales before compacting them for sintering would be one way to solve using these materials at higher frequencies. The properties of ferrite depend mainly on the technique and conditions of preparation, which, in turn, affect the cation distribution over the tetrahedral and octahedral sites. Thus, the aim of this study was to introduce some methods used for synthesizing nanocrystalline soft magnetic ferrites. Furthermore, the microstructure features, i.e., grain sizes and porosities, which are influenced by the types of method used for preparation, playing key role on the magnetic properties of the sample, are also highlighted.

2D Transition Metal Oxides/Hydroxides for Energy‐Storage Applications

AbstractThe development of 2‐dimensional materials has expanded beyond the realm of graphene, and now includes inorganic 2‐dimensional transition metal oxides/hydroxides, which show promise for a wide range of applications. As an emerging class of nanoscale materials, they show unprecedented properties that are unattainable in their bulk lamellar systems, which can be attributed to their confined thickness compared to several tens of micrometer lateral dimensions. Such qualities make them viable candidates for battery and supercapacitor applications. There are a few challenges ahead for 2‐dimensional transition metal oxides/hydroxides, including the limited types of 2‐dimensional parent materials in bulk form, the controlled synthesis of 2‐dimensional nanostructures with nonlayered structures, and the ability to control the properties of layers by tuning the chemistry and nanoscopic features. This Focus Review will cover the research landscape of 2‐dimensional transition metal oxides/hydroxides, ranging from synthetic approaches, to understanding the properties that emerge at the single‐layer scale, to exploiting these properties in both new and existing technologies.

Enhancement and Tunability of Near-Field Radiative Heat Transfer Mediated by Surface Plasmon Polaritons in Thin Plasmonic Films

The properties of thermal radiation exchange between hot and cold objects can be strongly modified if they interact in the near field where electromagnetic coupling occurs across gaps narrower than the dominant wavelength of thermal radiation. Using a rigorous fluctuational electrodynamics approach, we predict that ultra-thin films of plasmonic materials can be used to dramatically enhance near-field heat transfer. The total spectrally integrated film-to-film heat transfer is over an order of magnitude larger than between the same materials in bulk form and also exceeds the levels achievable with polar dielectrics such as SiC. We attribute this enhancement to the significant spectral broadening of radiative heat transfer due to coupling between surface plasmon polaritons (SPPs) on both sides of each thin film. We show that the radiative heat flux spectrum can be further shaped by the choice of the substrate onto which the thin film is deposited. In particular, substrates supporting surface phonon polaritons (SPhP) strongly modify the heat flux spectrum owing to the interactions between SPPs on thin films and SPhPs of the substrate. The use of thin film phase change materials on polar dielectric substrates allows for dynamic switching of the heat flux spectrum between SPP-mediated and SPhP-mediated peaks.

HRTEM investigation of phase stability in alumina–zirconia multilayer thin films

Phase stability of nanostructured thin films can be significantly different from the stability of the same materials in bulk form because of the increased contribution from surface and interface effects. Zirconia (ZrO2), stabilized in tetragonal and cubic phases, is a technologically important material and is used for most high temperature applications. In literature, zirconia can be found to be stabilized in its high temperature phases down to room temperature via two routes, doping with divalent or trivalent cations and crystallite size controls. Apart from these, in the alumina/zirconia thin-film multilayer system, a constraining effect on the zirconia layers provides another route to stabilization of the tetragonal zirconia phase at room temperature. However, in such nanostructured geometries, at high temperatures, the small diffusion lengths involved can influence the phase stability. The present work deals with the high-resolution transmission electron microscope (HRTEM) studies of pulsed laser ablated alumina–zirconia thin-film multilayers in the as deposited state and annealed up to 1473 K at 2 × 10−5 mbar. Conventional techniques such as X-ray diffraction lack the ability to detect localized phase changes at nanometre length scales and also for the low volume fraction of newly formed phases. Cross-sectional HRTEM techniques have been successful in detecting and characterizing these interactions.