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arXiv2018

3D Crystallographic Alignment of Alumina Ceramics by Application of Low Magnetic Fields

Non-cubic crystals exhibit anisotropic physical and functional properties. Microscopic crystallites as constituents of polycrystalline materials are randomly oriented, thus polycrystalline ceramics lack the anisotropic properties of their monocrystalline counterparts. We propose a technology that exploits the synergy between magnetic alignment and colloidal ceramics processing, and enables to independently tune the orientation of grains in different sample regions by infinitesimal magnetic fields (<10 millitesla). The grain pivot mechanism enables the emulation of single crystals, as well as the creation of large complex-shaped ceramic elements with designed crystallographic landscapes and spatially and directionally tuned properties. Ultra-high magnetic response arises from magnetic shape anisotropy of platelet-shaped seed crystallites coated with small amounts of iron oxide nanoparticles. To control crystallographic growth directions during subsequent annealing procedures, the seeds are dispersed and aligned in a matrix of chemically identical, but much finer spherical particles. The degree of crystalline orientation in accordingly prepared aluminum oxide exceeds 99%. This technology opens an avenue to remarkably improve structural and functional properties of ceramic elements employed in numerous industrial applications.

physics.app-ph↗

arXiv2019

Stabilization of binary NiO-YSZ nanocolloids for direct inkjet deposition

Water-based inks, containing nanometric NiO and YSZ particles in 66/34 vol. % ratio, are produced by colloidal stabilization of a binary dispersion obtained via continuous hydrothermal synthesis at supercritical conditions, i.e. 280 bar and 400 C. The method yields single-crystal particles with diameter < 10 nm for both phases in a single-step process, achieving a highly mixed composite. Two different approaches are applied to formulate inks printable with piezoelectric printheads, i.e. an electrostatic and an electrosteric stabilization path. The use of the electrosteric dispersant results in colloids with superior stability > 200 days, more uniform thin films and finely nanostructured porous cermet films with thickness below 500 nm, after reducing NiO to Ni. Particles coarsening to 50-150 nm is obtained at 1000 C, accompanied by a shrinkage of ca. 43 % in thickness without the formation of cracks or delamination of the zirconia substrates.

physics.app-ph↗

arXiv2019

Comparison of two early warning systems for regional flash flood hazard forecasting

The anticipation of flash flood events is crucial to issue warnings to mitigate their impact. This work presents a comparison of two early warning systems for real-time flash flood hazard forecasting at regional scale. The two systems are based in a gridded drainage network and they use weather radar precipitation inputs to assess the hazard level in different points of the study area, considering the return period (in years) as the indicator of the flash flood hazard. The essential difference between the systems is that one is a rainfall-based system (ERICHA), using the upstream basin-aggregated rainfall as the variable to determine the hazard level, while the other (Flood-PROOFS) is a system based on a distributed rainfall-runoff model to compute the streamflows at pixel scale. The comparison has been done for three rainfall events in the autumn of 2014 that resulted in severe flooding in the Liguria region (Northwest of Italy). The results obtained by the two systems show many similarities, particularly for larger catchments and for large return periods (extreme floods).

physics.ao-ph↗

arXiv2021

Simulation of self-piercing rivetting processes in fibre reinforced polymers: Material modelling and parameter identification

This paper addresses the numerical simulation of self-piercing rivetting processes to join fibre reinforced polymers and sheet metals. Special emphasis is placed on the modelling of the deformation and failure behaviour of the composite material. Different from the simulation of rivetting processes in metals, which requires the modelling of large plastic deformations, the mechanical response of composites is typically governed by intra- and interlaminar damage phenomena. Depending on the polymeric matrix, viscoelastic effects can interfere particularly with the long-term behaviour of the joint. We propose a systematic approach to the modelling of composite laminates, discuss limitations of the used model, and present details of parameter identification. Homogenisation techniques are applied to predict the mechanical behaviour of the composite in terms of effective anisotropic elastic and viscoelastic material properties. In combination with a continuum damage approach this model represents the deformation and failure behaviour of individual laminae. Cohesive zone elements enable the modelling of delamination processes. The parameters of the latter models are identified from experiments. The defined material model for the composite is eventually utilised in the simulation of an exemplary self-piercing rivetting process.

physics.app-ph↗

arXiv2018

Photocatalytic activity, optical and ferroelectric properties of Bi0.8Nd0.2FeO3 nanoparticles synthesized by sol-gel and hydrothermal methods

In this study, the effects of synthesis method and dopant Neodymium ion on the ferroelectric properties and photocatalytic activity of bismuth ferrite were studied. BiFeO3 (BFO) and Bi0.8Nd0.2FeO3 (BNFO) nanoparticles were prepared through a facile sol-gel combustion (SG) and hydrothermal (HT) methods. The as-prepared products were characterized by X-ray powder diffraction (XRD), Furrier transform infrared spectroscopy (FTIR) and transmission electron microscope (TEM) images. Both nanophotocatalysts have similar crystal structures, but the SG products have semi-spherical morphology. On the other hand, HT samples have rod-like morphology. TEM results indicated that the morphology of products was not affected by the doping process. The thermal, optical and magnetic properties of nanoparticles were investigated by thermogravitometry and differential thermal analysis (TG/DTA), UV-vis spectroscopy, and vibrating sample magnetometer (VSM). The ferroelectric properties of BNFO nanoparticles were improved compared to the undoped bismuth ferrite. The photocatalytic activity of as-synthesized nanoparticles was also evaluated by the degradation of methyl orange (MO) under visible light irradiation. The photocatalytic activity of nanoparticles prepared via sol-gel method exhibited a higher photocatalytic activity compared to powders obtained by hydrothermal method. Also substitution of Nd into the BFO structure increased the photocatalytic activity of products.

physics.app-ph↗

arXiv2018

The Quantum Field Of A Magnet Shown By A Nanomagnetic Ferrolens

It has been more than two hundred years since the first iron filings experiment, showing us the 2D macroscopic magnetic imprint of the field of a permanent magnet. However, latest developments in modern nanomagnetic passive direct observation devices reveal in real-time and color a more intriguing 3D dynamic and detailed image of the field of a magnet, with surprising new findings, that can change our perspective for dipole magnetism forever and lead to new research. This research is a continuation of our previous work (DOI: 10.1016/j.jmmm.2017.12.023). The magnetostatic fields were under our scope and examined with the aid of the ferrolens. We are presenting experimental and photographical evidence, demonstrating the true complex 3D Euclidian geometry of the quantum field of permanent magnets that have never been seen before and the classic iron filings experiment, apart of its 2D limitations, fails to depict. An analysis of why and what these iron filings inherent limitations are, giving us an incomplete and also in some degree misguiding image of the magnetic field of a magnet is carried out, whereas, as we prove the ferrolens is free of these limitations and its far more advanced visualization capabilities is allowing it to show the quantum image with depth of field information, of the dipole field of a permanent magnet. For the first time the domain wall (i.e. Bloch or Neel wall) region of the field of a magnet is clearly made visible by the ferrolens along with what phenomenon is actually taking place there, leading to the inescapable conclusion, novel observation and experimental evidence that the field of any dipole magnet actually consists of two distinct and separate toroidal shaped 3D magnetic bubbles, each located at either side of the dipole around the exact spatial regions where the two poles of the magnet reside.

physics.app-ph↗

arXiv2018

Uniform and Nonuniform Precession of a Nanoparticle with Finite Anisotropy in a Liquid: Opportunities and Limitations for Magnetic Fluid Hyperthermia

We focus on an in-depth study of the forced dynamics of a ferromagnetic single-domain uniaxial nanoparticle placed in a viscous fluid and driven by an external rotating magnetic field. The process of conversion of magnetic and mechanical energies into heat is a physical basis for magnetic fluid hyperthermia that is very promising for cancer treatment. The dynamical approximation allows us to establish the limits of the heating rate and understand the logic of selection of the system parameters to optimize the therapy. Based on the developed analytical and numerical tools, we analyze from a single viewpoint the synchronous and asynchronous rotation of the nanoparticle or/and its magnetization in the following three cases. For the beginning, we actualize the features of the internal magnetic dynamics, when the nanoparticle body is supposed to be fixed. Then, we study the rotation of the whole nanoparticle, when its magnetization is supposed to be locked to the crystal lattice. And, finally, we realize the analysis of the coupled motion, when the internal magnetic dynamics is performed in the rotated nanoparticle body. In all these cases, we describe analytically the uniform mode, or synchronous rotation along with an external field, while the nonuniform mode, or asynchronous rotation, is investigated numerically.

cond-mat.mes-hall↗

arXiv2018

Measurement independent magnetocaloric effect in Mn-rich Mn-Fe-Ni-Sn(Sb/In) Heusler alloys

We report a systematic study on the magneto-structural transition in Mn-rich Fe-doped Mn-Fe-Ni-Sn(Sb/In) Heusler alloys by keeping the total valence electron concentration (e/a ratio) fixed. The martensitic transition (MT) temperature is found to shift by following a proportional relationship with the e/a ratio of the magnetic elements alone. The magnetic entropy change across MT for a selected sample (Mn49FeNi40Sn9In) has been estimated from three different measurement methods (isofield magnetization (M) vs temperature (T), isothermal M vs field (H) and heat capacity (HC) vs T). We observed that though the peak value of magnetic entropy change changes with the measurement methods, the broadened shape of the magnetic entropy change vs T curves and the corresponding cooling power (~140 Jkg-1) remains invariant. The equivalent adiabatic temperature change ~ -2.6 K has been obtained from indirect measurements of temperature change. Moreover, an exchange bias field ~ 783 Oe at 5 K and a magnetoresistance of -30% are also obtained in one of these materials.

cond-mat.mtrl-sci↗

arXiv2019

The nature of transport and ferromagnetic properties of the GaAs structures with the Mn δ-doped layer

We reveal the nature of transport and ferromagnetic properties of the GaAs structure with the single Mn δ-doped layer. To modify the properties of the structure the electrically active radiation defects are created by irradiation with helium ions. The investigations show that the transport properties of the structure are determined by two parallel conduction channels (the channel associated with hole transport in the valence band and the channel associated with electron transport in the Mn impurity band) and that the ferromagnetic properties are determined by the electrons localized at the allowed states within the Mn impurity band. The results also help to understand the features of structures with the Mn δ-layer nearby the quantum well.

physics.app-ph↗

arXiv2018

Building traps for skyrmions by the incorporation of magnetic defects into nanomagnets: pinning and scattering traps by magnetic properties engineering

In this work we have used micromagnetic simulations to report four ways to build traps for magnetic skyrmions. Magnetic defects have been modeled as local variations in the material parameters, such as the exchange stiffness, saturation magnetization, magnetocrystalline anisotropy and Dzyaloshinskii-Moriya constant. We observe both pinning (potential well) and scattering (potential barrier) traps when tuning either a local increase or a local reduction for each one of these magnetic properties. It is found that the skyrmion-defect aspect ratio is a crucial parameter to build traps for skyrmions. In particular, the efficiency of the trap is compromised if the defect size is smaller than the skyrmion size, because they interact weakly. On the other hand, if the defect size is larger than the skyrmion diameter, the skyrmion-defect interaction becomes evident. Thus, the strength of the skyrmion-defect interaction can be tuned by the modification of the magnetic properties within a region with suitable size. Furthermore, the basic physics behind the mechanisms for pinning and for scattering is discussed. In particular, we discover that skyrmions move towards the magnetic region which tends to maximize its diameter; it enables the magnetic system to minimize its energy. Thus, we are able to explain why skyrmions are either attracted or repelled by a region with modified magnetic properties. Results here presented are of utmost significance for the development and realization of future spintronic devices, in which skyrmions will work as information carriers.

physics.comp-ph↗

arXiv2019

Efficient calculation of the mutual inductance of arbitrarily oriented circular filaments via a generalisation of the Kalantarov-Zeitlin method

In this article, we present a new analytical formulation for calculation of the mutual inductance between two circular filaments arbitrarily oriented with respect to each other, as an alternative to Grover [1] and Babic [2] expressions reported in 1944 and 2010, respectively. The formula is derived via a generalisation of the Kalantarov-Zeitlin method, which showed that the calculation of mutual inductance between a circular primary filament and any other secondary filament having an arbitrary shape and any desired position with respect to the primary filament is reduced to a line integral. In particular, the obtained formula provides a solution for the singularity issue arising in the Grover and Babic formulas for the case when the planes of the primary and secondary circular filaments are mutually perpendicular. The efficiency and flexibility of the Kalantarov-Zeitlin method allow us to extend immediately the application of the obtained result to a case of the calculation of the mutual inductance between a primary circular filament and its projection on a tilted plane. Newly developed formulas have been successfully validated through a number of examples available in the literature, and by a direct comparison with the results of calculation performed by the FastHenry software.

physics.class-ph↗

arXiv2019

Synergistic Effects of Nanoparticle Heating and Amoxicillin on H. Pylori Inhibition

We report the design and development of a dual-functional magnetic nanoparticle platform for potential treatment of H. pylori infection. We show that an ultralow concentration of Mn0.3Fe2.7O4@SiO2 nanoparticles subjected to a moderate AC magnetic field, without bulk heating effect, can deposit heat locally and effectively inhibit H. pylori growth and virulence in vitro. When coupled with antibiotic amoxicillin, the dual-functional amoxicillin loaded Mn0.3Fe2.7O4@SiO2 further decreases the bacteria survival rate by a factor of 7 and 5, respectively, compared to amoxicillin treatment and nanoparticle heating alone. The synergistic effect can be partially attributed to the heating induced damage to the cell membrane and protective biofilm, which may increase the permeability of antibiotics to bacteria. Our method provides a viable approach to treat H. pylori infection, with the potential of reducing side effects and enhancing the efficacy for combating drug resistant strains.

physics.app-ph↗

arXiv2019

Versatile Hall magnetometer with variable sensitivity assembly for characterization of the magnetic properties of nanoparticles

A Hall magnetometer with variable sensitivity is constructed to measure the magnetic properties of magnetic nanoparticles manufactured by different methods. This novel magnetometer can also be used to measure bulk materials and samples in liquids. The magnetometer is constructed with two commercial Hall-effect sensors in an acrylic structure, which serves as the support for a micrometer and the circuit board with the sensors. For operation, the magnetometer it acquires a complete magnetization curve in a few minutes. If has a magnetic moment sensitivity of 1.3*10-9 Am2 to sensitivity of 493 mV/mT, the sensitivity can be adjustable in the range of 10 to 493 mV/mT. Its performance is tested with magnetic nanoparticles. As an application example, we estimate the mean diameter of the nanoparticles using the magnetic curves. The results are compared with those obtained by other techniques, such as transmission electron microscopy (TEM), X-ray diffraction (XRD) and dynamic light scattering (DLS). The magnetization results are also compared with those obtained by independent commercial magnetometers, which reveals errors of approximately 0.31 Am2/kg (i.e., 0.6%) in the saturation region.

physics.ins-det↗

arXiv2019

Current-driven domain wall dynamics in ferrimagnets: micromagnetic approach and collective coordinates model

Theoretical studies dealing with current-driven domain wall dynamics in ferrimagnetic alloys and, by extension, other antiferromagnetically coupled systems as some multilayers, are here presented. The analysis has been made by means of micromagnetic simulations that consider these systems as constituted by two subsystems coupled in terms of an additional exchange interlacing them. Both subsystems differ in their respective gyromagnetic ratios and temperature dependence. Other interactions, as for example anisotropic exchange or spin-orbit torques, can be accounted for differently within each subsystem according to the physical structure. Micromagnetic simulations are also endorsed by means of a collective coordinates model which, in contrast with some previous approaches to these antiferromagnetically coupled systems, based on effective parameters, also considers them as formed by two coupled subsystems with experimentally definite parameters. Both simulations and the collective model reinforce the angular moment compensation argument as accountable for the linear increase with current of domain wall velocities in these alloys at a certain temperature or composition. Importantly, the proposed approach by means of two coupled subsystems permits to infer relevant results in the development of future experimental setups that are unattainable by means of effective models.

physics.app-ph↗

arXiv2018

Dynamic homogenization of composite and locally resonant flexural systems

Dynamic homogenization aims at describing the macroscopic characteristics of wave propagation in microstructured systems. Using a simple method, we derive frequency-dependent homogenized parameters that reproduce the exact dispersion relations of infinitely periodic flexural systems. Our scheme evades the need to calculate field variables at each point, yet capable of recovering them, if wanted. Through reflected energy analysis in scattering problems, we quantify the applicability of the homogenized approximation. We show that at low frequencies, our model replicates the transmission characteristics of semi-infinite and finite periodic media. We quantify the decline in the approximation as frequency increases, having certain characteristics sensitive to microscale details. We observe that the homogenized model captures the dynamic response of locally resonant media more accurately and across a wider range of frequencies than the dynamic response of media without local resonance.

physics.app-ph↗

arXiv2018

Microstructural constitutive model for polycrystal viscoplasticity in cold and warm regimes based on continuum dislocation dynamics

Viscoplastic flow of polycrystalline metallic materials is the result of motion and interaction of dislocations, line defects of the crystalline structure. In the microstructural (physics-based) constitutive model presented in this paper, the main underlying microstructural processes influencing viscoplastic deformation and mechanical properties of metals in cold and warm regimes are statistically described by the introduced sets of postulates/axioms for continuum dislocation dynamics (CDD). Three microstructural (internal) state variables (MSVs) are used for statistical quantifications of different types/species of dislocations by the notion of dislocation density. Considering the mobility property of dislocations, they are categorized to mobile and (relatively) immobile dislocations. Mobile dislocations carry the plastic strain (rate), while immobile dislocations contribute to plastic hardening. Moreover, with respect to their arrangement, dislocations are classified to cell and wall dislocations. Cell dislocations are those that exist inside cells/subgrains, and wall dislocations are packed in (and consequently formed) the subgrain walls/boundaries. Therefore, the MSVs incorporated in this model are cell mobile, cell immobile and wall immobile dislocation densities. The evolution of these internal variables is calculated by means of adequate equations that characterize the dislocation processes dominating material behavior during cold and warm monotonic viscoplastic deformation. The constitutive equations are then numerically integrated; and the constitutive parameters are determined/fitted for a widely used ferritic-pearlitic steel (20MnCr5).

cond-mat.mtrl-sci↗

arXiv2019

On the Emergence of Negative Effective Density and Modulus in 2-phase Phononic Crystals

In this paper we report metamaterial properties including negative and singular effective properties for what would traditionally be considered non locally resonant 2-phase phononic unit cells. The negative effective material properties reported here occur well below the homogenization limit and are, therefore, acceptable descriptions of overall behavior. The material property combinations which make this possible were first revealed by a novel level set based topology optimization process which we describe. The optimization process revealed that a 2-phase unit cell in which one of the phases is simultaneously lighter and stiffer than the other results in dynamic behavior which has all the attendant characteristics of a locally resonant composite including negative effective properties far below the homogenization limit. We investigate this further using the Craig-Bampton decomposition and clarify that these properties emerge through an interplay between the fundamental internal modeshape of the unit cell and a rigid body mode. Through explicit numerical calculations on 1-D, 2-phase unit cells, we show that negative effective properties only appear for the specific material property combination mentioned above. Furthermore, we provide a proof which supports this conclusion. The concept is also shown to hold for 2-D unit cells where we show that an appropriately designed hexagonal unit cell made of 2 material phases exhibits negative effective shear modulus and density in an appropriate frequency regime in which it also exhibits negative refraction. An important conclusion of this paper is that the class of unit cells expected to result in negative properties can be expanded beyond the classic unit cell (three-phase unit cells with an explicit locally resonant phase) to include topologically simpler 2-phase unit cells as well.

physics.app-ph↗

arXiv2018

Stress in a polymer brush

We study the stress distribution in a polymer brush material over a range of graft densities using molecular dynamics (MD) simulations and theory. Flexible polymer chains are treated as beads connected by nonlinear springs governed by a modified finitely extensible nonlinear elastic (FENE) potential in MD simulations. Simulations confirmed the quartic variation of the normal stress parallel to the substrate, within the bulk of the brush, as predicted in our previous work, for low graft densities. However, in the high graft density regime, the Gaussian chain elasticity assumption is violated by finite extensibility effects (force-extension divergence) and the restriction to binary interaction among monomers is insufficient. This motivated us to extend a semi-analytical strong stretching mean field theory (SST) for polymer brushes, based on Langevin chains and a modified Carnahan-Starling equation of state to model monomer interactions. Our extended theory elucidates the stress and monomer density profiles obtained from MD simulations, as well as reproduces Gaussian chain results for small graft densities. A good agreement is observed between predictions of MD and Langevin chain SST for monomer density profile, end density profile and stress profile in high graft density regime, without fitting parameters (virial coefficients). Quantitative comparisons of MD results with various available theories suggest that excluded volume correlations may be important.

physics.app-ph↗