Iron oxide nanoparticles fabricated by electric explosion of wire: Focus on magnetic nanofluids

Nanoparticles of iron oxides (MNPs) were prepared using the electric explosion of wire technique (EEW). The main focus was on the fabrication of de-aggregated spherical nanoparticles with a narrow size distribution. According to XRD the major crystalline phase was magnetite with an average diameter of MNPs, depending on the fraction. Further separation of air-dry EEW nanoparticles was performed in aqueous suspensions. In order to provide the stability of magnetite suspension in water, we found the optimum concentration of the electrostatic stabilizer (sodium citrate and optimum pH level) based on zeta-potential measurements. The stable suspensions still contained a substantial fraction of aggregates which were disintegrated by the excessive ultrasound treatment. The separation of the large particles out of the suspension was performed by centrifuging. The structural features, magnetic properties and microwave absorption of MNPs and their aqueous solutions confirm that we were able to obtain an ensemble in which the magnetic contributions come from the spherical MNPs. The particle size distribution in fractionated samples was narrow and they showed a similar behaviour to that expected of the superparamagnetic ensemble. Maximum obtained concentration was as high as 5 % of magnetic material (by weight). Designed assembly of de-aggregated nanoparticles is an example of on-purpose developed magnetic nanofluid. Copyright © 2012 Author(s)

Carbon coated Nickel Nanoparticles in Polyacrylamide Ferrogels: Interaction with Polymeric Network and Impact on Swelling

Polyacrylamide ferrogels with embedded magnetic nanoparticles of metallic nickel (Ni) and nanoparticles of nickel coated with a carbon shell (Ni@C) were synthesized by radical polymerization in water. The effect of the carbon shell on the interaction of Ni and Ni@C nanoparticles with polyacrylamide matrix and on swelling ratio of the ferrogels has been studied. The deposition of carbon on the surface of Ni nanoparticles worsens their interaction with polyacrylamide but at the same time elevates the water uptake by ferrogels

Relative Oscillation Theory, Weighted Zeros of the Wronskian, and the Spectral Shift Function

We develop an analog of classical oscillation theory for Sturm-Liouville operators which, rather than measuring the spectrum of one single operator, measures the difference between the spectra of two different operators. This is done by replacing zeros of solutions of one operator by weighted zeros of Wronskians of solutions of two different operators. In particular, we show that a Sturm-type comparison theorem still holds in this situation and demonstrate how this can be used to investigate the finiteness of eigenvalues in essential spectral gaps. Furthermore, the connection with Krein's spectral shift function is established.Comment: 26 page

Electomagnetic absorption of composites based on epoxy resin and metallic iron nanoparticles

The heat losses originated from the electro-magnetic absorption in magnetic epoxy-based composites with embedded metallic iron nanoparicles were studied by Calvet microcalorimetry. Iron magnetic nanoparticles (MNPs) were synthesized by electrical explosion of wire (EEW) method; they were non-agglomerated, spherical in shape and had a weight average diameter 85 nm. Composites based on the cured epoxy-dian resin contained MNPs in weight content varying from 10% up to 70%. To study the heat loss in alterating magnetic field commercial Calvet microcalorimeter was equiped by two coils in the serial connection placed in the calorimeter cells; one of the coils contained a sample of composite the other was a reference. The electromagnetic adsorption was studied in the alternating magnetic field up to 1700 A/m in 67-214 kHz frequency range. The measured values of the specific power losses revealed linear dependence on iron MNPs content in composite and non-linear increasing function of the field frequency. © 2018 The Authors, published by EDP Sciences.This work is supported in part by the project for basic research of UD RAS. The assistance of Shchipanova T. A. in the measurements of hysteresis loops for composites is appreciated

Preparation of alumina nanoparticle suspensions with narrow particle size distribution

Dynamic light scattering (DLS) was applied to the study of the process of the preparing deaggregated water suspensions of alumina nanopowders with specific surface areas of 20-140 m2/g. Nanopowders were prepared by the electric explosion of wire and laser evaporation and, according to electron microscopy (TEM), consisted of nonagglomerated spherical nanoparticles with lognormal size distribution. According to DLS, nonsedimenting water suspensions of alumina nanoparticles, stabilized by sodium citrate at a 5 mM concentration, contain substantial fraction of aggregates. The dynamics of the change in the mean average size of aggregates under exhaustive ultrasound treatment of suspensions with 10 g/l concentration for 1.5-4 h by two types of ultrasonic processors was studied. It was shown that the mean average size of aggregates exponentially diminishes by 1.5-2 times and the fraction of individual particles in suspension enlarges from 45 to 85%. Sequentially centrifuging the suspension at 18000 g separates the remaining aggregates and results in suspensions of individual alumina nanoparticles. Particle size distributions in these suspensions obtained by TEM and DLS are the same within experimental error. © 2013 Pleiades Publishing, Ltd

Effect of particle diameter and packing density on heating of metallic iron particles in alternating magnetic field

The heat losses originated from the electro-magnetic absorption in mechanically packed metallic iron nano- and micro-sized magnetic particles (MPs) and in magnetic epoxybased composites with embedded MPs were studied by Calvet microcalorimetry. Nano-sized MPs with numerical average diameter 40 nm were synthesized by electrical explosion of wire method; micro-sized MPs with numerical average diameter 2 μm were synthesized by flame decomposition of the iron pentacarbonyl. The specific loss power (SLP) of re-magnetization of press-packed powdered samples and epoxy composites in magnetic field 1750 A/m at 214 kHz was measured as a function of the volume fraction of MPs in the sample. The results showed up that SLP depended both on particle size and on volume fraction, which meant the significant effect of magnetic interaction among particles on the heat losses in electromagnetic absorption. © Published under licence by IOP Publishing Ltd.Russian Foundation for Basic Research, RFBR: 18-08-00178Russian Academy of Sciences, RASThe study has been done with the financial support of RF State Assignment of IEP UB RAS as well as of the Russian Foundation of Basic Research, project 18-08-0017

Surface modified Ni nanoparticles produced by the electrical explosion of wire

Spherical nickel nanoparticles were prepared by the electrical explosion of wire. The as prepared nanoparticles were modified immediately after fabrication at room temperature in order to provide tunable surface properties with focus on the development of composites filled with nanoparticles. Following liquid modificators were used: hexane, toluene and the solution of polystyrene in toluene. In one case the surface modification by carbon was made in gas phase as a result of hydrocarbon injection. The average size of the nanoparticles was about 50 nm and unit cell parameters were close to 0.351 nm. Detailed characterization was done by X-ray diffraction, transmission electron microscopy, and magnetization measurements. Sphericity was also checked using microwave resonant absorption. © (2015) Trans Tech Publications, Switzerland.Electromagnetics of Russian Academy of Sciences;et al;Institute for Theoretical and Applied;Japan Society for the Promotion of Science;Lomonosov Moscow State University;Russian Foundation for Basic Researc

Tuning of Interfacial Interactions in Poly(Isoprene) Ferroelastomer by Surface Modification of Embedded Metallic Iron Nanoparticles

Zero-valent metallic iron nanoparticles (NPs) with modified surface were embedded in poly(isoprene) (PI) and the enthalpy of interfacial adhesion in resulted ferroelastomers was evaluated. Iron NPs were synthesized by the method of electrical explosion of wire (EEW) in inert gas. Modification of their surface was performed by the in situ liquid treatment of the active condensed NPs in the EEW installation. The enthalpy of mixing of poly(isoprene) with Fe NPs was determined using thermochemical cycle based on the isothermal calorimetry measurement of the enthalpy of dissolution of PI/Fe composites in chloroform at 25◦ C. Using these values the enthalpy of adhesion of PI to the surface of modified Fe NPs was evaluated using Langmuir-type isotherm. It was shown that the enthalpy of adhesion strongly depends on the properties of the surface of Fe NPs and its modification. It was the lowest in the case of oxidized Fe NPs and the highest for Fe NPs which surface was modified by pre-deposited polymeric shells. © 2021, ITMO University. All rights reserved.The research was supported by Russian Science Foundation grant 20-12-00031

Biorthogonal Quantum Systems

Models of PT symmetric quantum mechanics provide examples of biorthogonal quantum systems. The latter incorporporate all the structure of PT symmetric models, and allow for generalizations, especially in situations where the PT construction of the dual space fails. The formalism is illustrated by a few exact results for models of the form H=(p+\nu)^2+\sum_{k>0}\mu_{k}exp(ikx). In some non-trivial cases, equivalent hermitian theories are obtained and shown to be very simple: They are just free (chiral) particles. Field theory extensions are briefly considered.Comment: 34 pages, 5 eps figures; references added and other changes made to conform to journal versio

On the structure of eigenfunctions corresponding to embedded eigenvalues of locally perturbed periodic graph operators

The article is devoted to the following question. Consider a periodic self-adjoint difference (differential) operator on a graph (quantum graph) G with a co-compact free action of the integer lattice Z^n. It is known that a local perturbation of the operator might embed an eigenvalue into the continuous spectrum (a feature uncommon for periodic elliptic operators of second order). In all known constructions of such examples, the corresponding eigenfunction is compactly supported. One wonders whether this must always be the case. The paper answers this question affirmatively. What is more surprising, one can estimate that the eigenmode must be localized not far away from the perturbation (in a neighborhood of the perturbation's support, the width of the neighborhood determined by the unperturbed operator only). The validity of this result requires the condition of irreducibility of the Fermi (Floquet) surface of the periodic operator, which is expected to be satisfied for instance for periodic Schroedinger operators.Comment: Submitted for publicatio