Lattice Imperfections Research Articles

Influence of Strain Rate on Barkhausen Noise in Trip Steel

This paper deals with Barkhausen noise in Trip steel RAK 40/70+Z1000MBO subjected to uniaxial plastic straining under variable strain rates. Barkhausen noise is investigated especially with respect to microstructure alterations expressed in terms of phase composition and dislocation density. The effects of sample heating and the corresponding Taylor–Quinney coefficient are considered as well. Barkhausen noise of the tensile test is measured in situ as well as after unloading of the samples. In this way, the contribution of external and residual stresses on Barkhausen noise can be distinguished in the direction of tensile loading, as well as in the transversal direction. It was found that the in situ-measured Barkhausen noise grows in both directions as a result of tensile stresses and the realignment of domain walls. The post situ-measured Barkhausen noise drops down in the direction of tensile load due to the high opposition of dislocation density at the expense of the growing transversal direction due to the prevailing effect of the realignment of domain walls. The temperature of the sample remarkably grows along with the increasing strain rate which corresponds with the increasing Taylor–Quinney coefficient. However, this effect plays only a minor role, and the density of the lattice imperfection expressed especially in terms of dislocation density prevails.

Simultaneous determination of the phase boundary thermal resistance and thermal conductivity in phase-separated TiO2 thin films

Phase boundaries, most often between crystalline and amorphous phases of a material, are ubiquitous lattice imperfections in thin films, particularly deposited by low temperature processes such as atomic layer deposition (ALD). Thermal resistances from these boundaries may impede phonon transport and reduce thermal conductivity. However, to date, there have been no reports on the direct measurement of the phase boundary thermal resistance, due in part to challenges associated with fabricating an atomically flat yet geometrically planar interface between different phases within a film. Here, we present a systematic study to simultaneously determine the phase boundary thermal resistance and thermal conductivity in a series of phase-separated titanium oxide (TiO2) thin films prepared via plasma-enhanced ALD (PEALD). A precise implementation of per-cycle plasma treatment enables extremely localized crystallization—i.e., plasma-assisted atomic layer annealing (ALA)—and the fabrication of crystalline/amorphous layered structures with their respective thicknesses systematically varied. Frequency-domain thermoreflectance measures the effective thermal conductivity of each film and its top and bottom interfaces, confirmed independently using time-domain thermoreflectance. We simultaneously solve a system of linear equations generated by applying a series resistor model to each film, yielding the phase boundary thermal resistance of 5.1 m2 K GW–1 between crystalline and amorphous TiO2, as well as the thermal conductivities of 5.1 and 2.1 W m–1 K–1 for crystalline and amorphous TiO2, respectively. The results suggest that the crystalline/amorphous phase boundary acts like a layer of crystalline (amorphous) TiO2 with equivalent thickness ∼26 (∼11) nm.

Lattice imperfections and high-harmonic generation in correlated systems

We study effects of lattice imperfections on high-harmonic generation from correlated systems using the Fermi–Hubbard model. We simulate such imperfections by randomly modifying the chemical potential across the individual lattice sites. We control the degree of electron–electron interaction by varying the Hubbard U. In the limit of vanishing U, this approach results in Anderson localization. For nonvanishing U, we rationalize the spectral observations in terms of qualitative k-space and real-space pictures. When the interaction and imperfection terms are of comparable magnitude, they may balance each other out, causing Bloch-like transitions. If the terms differ significantly, each electron transition requires a relatively large amount of energy and the current is reduced. We find that imperfections result in increased high-harmonic gain. The spectral gain is mainly in high harmonic orders for low U and low orders for high U.

DETERMINING THE PRESENCE OF PACKING DEFECTS IN PALLADIUM-BASED ALLOYS BY X-RAY DIFFRACTION METHOD

Alloys studied in this work are of interest for the needs of hydrogen energy, electronics, pharmacology. The aim of the research is to reveal regularities of defect subsystems development, which is necessary for development of processes to improve performance of metallic systems. Probability of the presence in palladium-based membrane alloys of two-dimensional lattice defects a ecting the structure-sensitive properties of materials - packing defects (PD) - are assessed. The analysis of lattice imperfections of Pd93.5In6.0Ru0.5 (numerical indices-mass.alloys was performed according to the results of X-ray di raction using synchrotron radiation (SR) of the Kurchatov Research Center. The dependence of probability of PD formation from the concentration and grade of the palladium doping element has been established.

Shell-Driven Localized Oxide Nanoparticles Determine the Thermal Stability of Microencapsulated Phase Change Material.

Not all encapsulation techniques are universally apt for every type of phase change material (PCM), highlighting the imperative for methodological precision. This study addresses the challenges of microencapsulated PCM (MEPCM) arising from the immiscible pairing of α-Al2O3 nanoparticles with Sn microparticles. The high-speed impact blending (HIB) dry synthesis technique is employed, facilitating large-volume production of Sn@α-Al2O3 MEPCMs. The resulting MEPCMs not only seamlessly endure 100 cycles of melting-solidification but also, with the strategic incorporation of a glass frit, exhibit remarkable thermal durability, withstanding up to 1000 melting-solidification cycles. Even under ultrafast thermal fluctuations, the α-Al2O3 shell remained resilient through 100 cycles. A marked reduction in supercooling is observed, which is attributed to the formation of SnO and SnO2 nanoparticles within the α-Al2O3 crystal lattice. The atomically resolved interface dynamics between SnO2 and α-Al2O3 play a pivotal role, lowering the energy barrier for Sn nuclei formation during solidification. This affects the accelerated Sn nucleation rate, effectively suppressing supercooling. Such insights offer a deeper understanding of the interplay between nanoscale crystal lattice imperfections and their implications for energy storage applications.

Hot-Dip Aluminizing of Flow-formed AISI 4140 Steel

Cold plastic deformations recognizably cause increasing lattice imperfections such as point defects and dislocations in the structure, which could then have an effect on diffusion characteristics of the material. In order to explore such an effect of flow forming, a flow formed AISI 4140 steel and an annealed 4140 steel were subjected to the HDA process in a molten Al7020 bath at 750◦C for 4 min, and a subsequent diffusion annealing was performed at 800◦C, and their coating characteristics such as coating thickness and hardness were compared. The results indicated that the coating thickness of the flow formed samples was higher (80◦m) than that of the annealed sample (50◦m) after the HDA process. Diffusion annealing increased the coating thickness of both samples five times, reaching 400◦m and 250◦m for the flow formed and the annealed samples, respectively. Comparing the measured thickness of the coatings revealed that flow forming accelerates diffusion during the HDA process, probably due to the defect structure induced by the flow forming. On the other hand, the coating hardness was in between 1000-1100 HV for both samples, implying that the initial condition of the sample does not have a remarkable effect on hardness after the HDA process.

Bottom Electrode Effects on Piezoelectricity of Pb(Zr0.52,Ti0.48)O3 Thin Film in Flexible Sensor Applications

Piezoelectric thin films grown on a mechanical, flexible mica substrate have gained significant attention for their ability to convert mechanical deformation into electrical energy though a curved surface. To extract the generated charge from the PZT thin films, bottom electrodes are typically grown on mica substrates. However, this bottom electrode also serves as a buffering layer for the growth of PZT films, and its impact on the piezoelectric properties of PZT thin films remains understudied. In this work, the effect of Pt and LaNiO3 bottom electrodes on the piezoelectric effect of a Pb(Zr0.52,Ti0.48)O3 thin film was investigated. It was observed that the PZT thin films on LNO/Mica substrate possessed weaker stress, stronger (100) preferred orientation, and higher remanent polarization, which is beneficial for a higher piezoelectric response theoretically. However, due to insufficient grain growth resulting in more inactive grain boundaries and lattice imperfections, the piezoelectric coefficient of the PZT thin film on LNO/Mica was smaller than that of the PZT thin film on a Pt/Mica substrate. Therefore, it is concluded that, under the current experimental conditions, PZT films grown with Pt as the bottom electrode are better suited for applications in flexible piezoelectric sensor devices. However, when using LNO as the bottom electrode, it is possible to optimize the grain size of PZT films by adjusting the sample preparation process to achieve piezoelectric performance exceeding that of the PZT/Pt/Mica samples.

Application of green-synthesized cadmium oxide nanofibers and cadmium oxide/graphene nanosheet nanocomposites as alternative and efficient photocatalysts for methylene blue removal from aqueous matrix.

For the first time, cadmium oxide (CdO) nanofibers (NFs) and graphene nanosheet (GNS)-doped CdO nanocomposites (NCs) have been synthesized by a simple green route using green tea (Camellia sinensis) extract, for subsequent application as photocatalysts for methylene blue (MB) removal from an aqueous matrix. In addition, the materials were tested as working electrodes for supercapacitors. The prepared samples were analyzed by FESEM, UV-Vis spectroscopy, FTIR, and X-ray diffraction (XRD). FESEM revealed that the obtained NPs and NCs show fiber-shaped nanostructure. FTIR confirmed the presence of biomolecules on CdO and carbon compounds on CdO/GNS, while XRD exhibited the cubic crystalline structure of obtained NPs and NCs. The Rietveld refinement using XRD data was performed to ascertain the crystallographic characteristics of the produced samples and look into lattice imperfections. UV-Vis spectroscopy evaluated the optical bandgap energies of CdO and CdO/GNS NCs. The CdO/GNS NCs demonstrated a fast cleavage of the dye molecule under UV irradiation, resulting in 97% removal in 120 min. In addition, CdO/GNS NCs showed remarkable chemical stability as an electrode material, with a high specific capacitance of 231 F g-1 at a scan rate of 25 mV s-1. These observed NCs characteristics are higher when compared to pristine CdO NPs. Finally, we found that the investigated NCs showed enhanced multifunctional properties, such as photocatalytic and supercapacitor characteristics, which can be useful in practical applications.

Enhanced Thermoelectric Performance in the SrTi0.85Nb0.15O3 Oxide Nanocomposite with Fe2O3-Functionalized Graphene.

Doped SrTiO3 is considered one of the potential thermoelectric (TE) candidates but its TE figure of merit, ZT needs to be improved for practical application of electricity generation from high-grade waste-heat. In the present work, enhanced TE performance has been realized for SrTi0.85Nb0.15O3 (STN) perovskite adopting the strategy of composite formation with Fe2O3-functionalized graphene (FGR). We have achieved a maximum electrical conductivity of 1.4 × 105 S m-1 for 1 wt % FGR added to STN, which is around 1185% larger than that of pristine STN. The presence of FGR in the STN matrix acts as a mobility booster of electrons, overcoming the effect of Anderson localization of electrons, which impedes the electron transport in STN. This is evident from the order of magnitude increase in weighted mobility of STN after FGR addition. Furthermore, the incorporation of FGR causes about a 34% decrease in the lattice thermal conductivity. The Debye-Callaway model demonstrates that the phonon-phonon Umklapp scattering is primarily responsible for reduced thermal conductivity. The presence of FGR sheets along the grain boundaries of STN, Fe2O3 nanoparticles, and lattice imperfections gives rise to the glass-like temperature-independent phonon mean-free-path, especially above Debye temperature. The maximum ZT ∼ 0.57 has been obtained at 947 K for the 1 wt % FGR sample, which is around 420% higher than that of pristine STN. Furthermore, we have fabricated a prototype of a four-legged n-type TE module, demonstrating one of the highest power outputs of 18 mW among reported oxide thermoelectrics.

Solvothermally silver doping boosting the thermoelectric performance of polycrystalline Bi2Te3

Bismuth telluride (Bi2Te3) is one of the most promising thermoelectric materials for commercial application at room temperature, but the thermoelectric performance of these materials still needs to be improved. In this study, we report a type of solvothermally Ag-doped Bi2Te3 microplate. By sintering these microplates into polycrystalline bulk materials, a high room-temperature figure of merit of 1 has been achieved. Based on comprehensive micro/nanostructural characterizations, we found that the solvothermally doped Ag in Bi2Te3 plays two main roles, namely as the dopants that effectively induce the point defects of AgBi and forming Ag2Te nanophases. AgBi can provide additional hole charge carriers, effectively adjusting the initially high electron carrier concentration in the system, while the Ag2Te nanophases effectively trigger energy filtering effect to maintain a high Seebeck coefficient, thereby contributing to a competitively high power factor of 25.5 μW cm−1 K−2 at 298 K. Simultaneously, a low thermal conductivity of 0.74 W m−1 K−1 is obtained due to the strong phonon scattering at various lattice imperfections, induced by the solvothermally Ag-doping, which include point defects, grain/phase boundaries, local lattice distortions, and dislocations. This work fills the gap in knowledge on the solvothermally Ag-doping mechanism in Bi2Te3 and provides guidance for the innovative design of high-performing inorganic thermoelectrics.

Annealing‐Induced Dynamic Strain Evolution in Cycled High‐Ni Layered Oxides Revealing Latent Lattice Imperfections (Adv. Energy Mater. 36/2023)

Annealing‐Induced Dynamic Strain Evolution in Cycled High‐Ni Layered Oxides Revealing Latent Lattice Imperfections (Adv. Energy Mater. 36/2023)

Annealing‐Induced Dynamic Strain Evolution in Cycled High‐Ni Layered Oxides Revealing Latent Lattice Imperfections

AbstractHigh‐Ni layered oxides undergo distinct chemomechanical evolution upon high‐voltage charging over 4.2 V, producing crystallographic defects and associated lattice distortions within the cathode particles. Due to the significant recovery of chemomechanical evolutions during discharging, lattice imperfections resulting from high‐voltage charging tend to remain hidden and pose challenges in their characterization at discharged states. Moreover, their impact on the structural stability and electrochemical performance of cathode materials remains elusive. In this study, synchrotron‐based X‐ray diffraction and Bragg coherent diffraction imaging (CDI) successfully detects subtle yet persistent lattice distortions in the discharged state after high‐voltage cycling. In situ Bragg CDI further reveals internal strain evolution within the particles upon mild heating, which can be correlated with hidden lattice imperfections and structural instability. Despite comparable strain distributions before heating, the particles cycled over 4.2 V show significant strain evolution and intraparticle domain deformation upon heating, whereas the particles cycled below 4.2 V release the internal strain and retain their shape. These results suggest that latent lattice imperfections formed during high‐voltage cycling can trigger detrimental microstructural degradation at high temperatures and critically deteriorate the structural stability of the high‐Ni layered oxide particles, thereby increasing the potential risk of degradation at elevated temperatures.

Realizing High Thermoelectric Performance in N‐Type Mg3(Sb, Bi)2‐Based Materials via Synergetic Mo Addition and Sb–Bi Ratio Refining

AbstractDeveloping thermoelectric (TE) performance is impeded by the compromise of TE parameters, resulting in inadequate conversion efficiency of heat to electricity. Herein, this work reports that Mo is a particularly effective additive in Mg3Sb2‐based alloys with significantly improved electronic transport via grain‐boundary engineering and band‐structure regulation synergy. In addition, phonon transport is simultaneously suppressed by employing multiple effects, lattice imperfection scattering, reduced phonon group velocity, and enhanced atomic disorder, leading to a minimum κlat = 0.52 W m−1 K−1 at 723 K. As a result, an outstanding ZT peak of ≈1.84 at 723 K and ZTav = 1.34 within 323–723 K are achieved in Mg3Sb2‐based alloys, and the corresponding fabricated single‐leg TE module shows an exceptionally high conversion efficiency of ≈12% under a hot‐side temperature of 450 °C. These results demonstrate the great potential for advancing mid‐temperature heat harvesting in Mg3Sb2‐based materials.

Probing the Free Volume in Polymers by Means of Positron Annihilation Lifetime Spectroscopy.

Positron annihilation lifetime spectroscopy (PALS) is a valuable technique to investigate defects in solids, such as vacancy clusters and grain boundaries in metals and alloys, as well as lattice imperfections in semiconductors. Positron spectroscopy is able to reveal the size, structure and concentration of vacancies with a sensitivity of 10-7. In the field of porous and amorphous systems, PALS can probe cavities in the range from a few tenths up to several tens of nm. In the case of polymers, PALS is one of the few techniques able to give information on the holes forming the free volume. This quantity, which cannot be measured with macroscopic techniques, is correlated to important mechanical, thermal, and transport properties of polymers. It can be deduced theoretically by applying suitable equations of state derived by cell models, and PALS supplies a quantitative measure of the free volume by probing the corresponding sub-nanometric holes. The system used is positronium (Ps), an unstable atom formed by a positron and an electron, whose lifetime can be related to the typical size of the holes. When analyzed in terms of continuous lifetimes, the positron annihilation spectrum allows one to gain insight into the distribution of the free volume holes, an almost unique feature of this technique. The present paper is an overview of PALS, addressed in particular to readers not familiar with this technique, with emphasis on the experimental aspects. After a general introduction on free volume, positronium, and the experimental apparatus needed to acquire the corresponding lifetime, some of the recent results obtained by various groups will be shown, highlighting the connections between the free volume as probed by PALS and structural properties of the investigated materials.

Synergistic effect of vanadium tungsten oxide nanocomposites for enhanced photocatalysis and photovoltaic performance

Tungsten oxide nanoparticles (PW NPs) and different wt% (5, 10, 15, and 20) of vanadium tungsten oxide nanocomposites (VW NCs) namely VW1, VW2, VW3 and VW4 were synthesized successfully by using hydrothermal approach. The synthesized VW NCs were characterized by X-ray diffraction, Scanning Electron Microscope, UV–visible spectroscopy and Keithley electrometer to analyze structure, morphology, optical, and electrical properties. For PWNPs and VW NCs, structural examination of the XRD pattern reveals the development of orthorhombic and monoclinic phases, respectively. The sponge-like porous morphology for VW NCs was obvious in SEM. The absorption spectrum of band gap energy decreased with increased VW concentrations. AFM pictures show rougher surfaces with more pits and troughs than PW NPs. Gradual increase in the conductivity due to the doping concentration reduces the lattice dislocations and imperfections in the prepared NCs. The current density curves were recorded to gauge the photostability of the photoanodes with different vanadium weight percentages. The results showed that VW4 NCs had the highest power conversion efficiency (PCE), which was around 5.02%, the highest short circuit current density (Jsc), which was 8.0(mA/cm2), and the highest open-circuit voltage, which was 0.8. This improved photovoltaic efficiency of VW NCs might be attributed to their ability to capture visible light, their greater dye-loading capacity, and their decreased photoelectron recombination. The methyl blue dye (MB) was successfully degraded using VW4 NCs with a rate constant of 0.0388/min. These VW NCs also exhibited better antibacterial activity in the inactivation of S. aureus than E. coli.

High-Resolution Characterization of Deformation Induced Martensite in Large Areas of Fatigued Austenitic Stainless Steel Using Deep Learning

This paper aims to demonstrate a novel technique enabling the accurate visualization and fast mapping of deformation-induced α′-martensite produced during cyclic straining of a metastable austenitic stainless steel, refined by reversion annealing to different grain sizes. The technique is based on energy and angular separation of the signal electrons in a scanning electron microscope (SEM). Collection of the inelastic backscattered electrons emitted under high take-off angles from a sample surface results in the acquisition of micrographs with high sensitivity to structural defects, such as dislocations, grain boundaries, and other imperfections. The areas with a high density of lattice imperfections reduce the penetration depth of the primary electrons, and simultaneously affect the signal electrons leaving the specimen. This results in an increase in the inelastic backscattered electrons yielded from the vicinity of α′-martensite, and a bright halo surrounds this phase. The α′-martensite phase can thus be separated from the austenitic matrix in SEM micrographs. In this work, we propose a deep learning method for a precise α′-martensite mapping within a large area. Various deep learning-based methods have been tested, and the best result measured by both Dice loss and IoU scores has been achieved using the U-Net architecture extended by the ResNet encoder.

The catalysis of Dy2O3 for RE-Mg based alloys hydrogen storage performance

The study used ball-milling to create composites of Mg90Ce5Y5 with various amounts of Dy2O3 catalyst (0, 2, 4, 6, 8 wt. %). The structures of samples were analyzed and it was found that the hydrogen storage mechanism involved the reactions: Mg + H2 ↔ MgH2 and DyH2 + H2 ↔ DyH3, and the stable phases CeO2, YH2, and DyH2 did not react. The catalyzed samples had better kinetics performance compared to the uncatalyzed ones. At 473 K, the composite with 8 wt. % catalyst absorbed 78.6% of the maximum hydrogen storage in 2 min, compared to 64.7% for Mg90Ce5Y5 alone, and desorbed 3 wt. % H2 in 49 min, 60% faster than Mg90Ce5Y5 (122 min). The Edes of hydrogen desorption for the 4 wt. % Dy2O3 catalyzed sample was reduced from 129.2 ± 1.2 to 102.2 ± 1.5 kJ/mol. There is no significant improvement in thermodynamic properties, with ΔH decreasing only slightly from 80.7 ± 1.8 to 76.0 ± 1.6 kJ/mol H2 because the presence of the extra reversible reaction. The outstanding kinetics of the Dy2O3 catalyzed samples is due to the in-situ formation of DyH2/DyH3, which provides more lattice imperfections, rich phase compositions and fine grains to improve diffusion and nucleation.

Spatial separation of spin currents in transition metal dichalcogenides

We theoretically predict spatial separation of spin-polarized ballistic currents in transition metal dichalcogenides (TMDs) due to trigonal warping. We quantify the effect in terms of spin polarization of charge carrier currents in a prototypical 3-terminal ballistic device where spin-up and spin-down charge carriers are collected by different leads. We show that the magnitude of the current spin polarization depends strongly on the charge carrier energy and the direction with respect to crystallographic orientations in the device. We study the (negative) effect of lattice imperfections and disorder on the observed spin polarization. Our investigation provides an avenue towards observing spin discrimination in a defect-free time reversal-invariant material.

Robustness of the Skyrmion Phase in a Frustrated Heisenberg Antiferromagnetic Layer against Lattice Imperfections and Nanometric Domain Sizes

By employing GPU-implemented hybrid Monte Carlo simulations, we study the robustness of the skyrmion lattice phase (SkX) in a frustrated Heisenberg antiferromagnetic (AFM) layer on a triangular lattice with a Dzyaloshinskii–Moriya interaction in the external magnetic field against the presence of lattice imperfections (nonmagnetic impurities) and lattice finiteness. Both features are typical of experimentally accessible magnetic materials and require theoretical investigation. In the pure model of infinite size, SkX is known to be stabilized in a quite wide temperature-field window. We first study the effects of such imperfections on the SkX stability and compare them with those in the nonfrustrated ferromagnetic counterpart. The partial results of this part appeared in the conference proceedings [M. Mohylnaand M. Žukovič, Proceedings of the 36th International ECMS International Conference on Modelling and Simulation, ECMS, 2022]. We further look into whether SkX can also persist in finite clusters, i.e., zero-dimensional systems of nanometric sizes. In general, both the presence of magnetic vacancies as well as the finiteness of the system tend to destabilize any ordering. We show that in the present model, SkX can survive, albeit in a somewhat distorted form, in the impure infinite system up to a fairly large concentration of impurities, and, in the pure finite systems, down to sizes comprising merely tens of particles. Distortion of the SkX phase due to the formation of bimerons, reported in the ferromagnetic model, was not observed in the present frustrated AFM case.

Structural, optical and electrical properties of sulphide based heterojunction thin film

This contribution presents structural, optical and electrical properties of PbS/CdS thin films at different annealing temperatures. The XRD analysis reveal the existence of three different crystalline phases, i.e., the beta-CdS, hawleyite-CdS and greenockite-alpha-CdS phases within the CdS-shell of PbS/CdS core shell thin film. The dislocation density and microstrain decreased with annealing temperatures indicating a decrease in lattice imperfections and formation of high quality film and it can be attributed to the increase in grain size of the film with increase in annealing temperatures. Optical studies placed the band gaps at 1.25eV-1.75eV. About the same magnitude were found from electrical conductivity test. The samples exhibited high conductivity with increasing annealing temperatures which resulted to the bumping of electrons from the valence band to the conduction band. The high absorbance suggest that the film could be coated on collectors to enhance solar energy collection. The achieved band gaps placed the PbS/CdS fabricated with the presented method as a good material for solar photovoltaic applications.