Vladimir Babayan

Ultrathin MnO2 nanoflakes grown on N-doped carbon nanoboxes for high-energy asymmetric supercapacitors

We demonstrate the synthesis of ultrathin MnO2 nanoflakes grown on N-doped carbon nanoboxes, forming an impressive hierarchical MnO2/C nanobox hybrid with an average size of 500 nm, which exhibits an excellent electrochemical performance due to the unique structure, N-doping and strong synergistic effects between them. In addition, we also assembled a green asymmetric supercapacitor (ASC) using the as-synthesized MnO2/C nanoboxes as a positive electrode and the corresponding N-doped carbon nanoboxes as a negative electrode in a neutral aqueous electrolyte, aiming to further enhance its energy density by extending the operating potential. More significantly, our ASC device is able to reversibly cycle within a wide operating voltage of 2.0 V and delivers a maximum energy density of 39.5 W h kg−1 with superior cycling stability (∼90.2% capacitance retention after 5000 cycles). These intriguing results show that hollow nanostructures will be promising electrode materials for advanced supercapacitors.

Novel synthesis of core–shell urchin-like ZnO coated carbonyl iron microparticles and their magnetorheological activity

The overall stability (thermo-oxidation, sedimentation) of MR suspensions is a crucial problem decreasing their potential applicability in real life. In this study the unique functional coating of carbonyl iron (CI) particles with ZnO structures was presented in order to develop a new MR suspension based on the core–shell ZnO/CI urchin-like dispersed particles. The two-step synthesis provides the suitable core–shell particles with improved sedimentation and also thermo-oxidation stability. Moreover, due to the enhanced sedimentation stability core–shell based suspensions exhibit higher values of the yield stress than those of bare CI based suspensions at 20 wt% particle concentration. The suspension with 60 wt% particle concentration reaches values of the yield stress around 2.2 kPa at 272 mT. The excellent MR efficiency of the core–shell ZnO/CI based suspension at elevated temperatures was observed. Finally, the dimorphic particle based suspension was prepared when the ratio between the carbonyl iron and core–shell urchin-like particles was 1 : 1. The highest yield stress was obtained in the case of a dimorphic particle-based suspension due to the good magnetic properties of the bare carbonyl iron and mechanical gripping between core–shell ZnO/CI urchin-like particles.

The formation mechanism of iron oxide nanoparticles within the microwave-assisted solvothermal synthesis and its correlation with the structural and magnetic properties

Magnetic nanoparticles based on Fe3O4 were prepared by a facile and rapid one-pot solvothermal synthesis using FeCl3·6H2O as a source of iron ions, ethylene glycol as a solvent and NH4Ac, (NH4)2CO3, NH4HCO3 or aqueous NH3 as precipitating and nucleating agents. In contrast to previous reports we reduce the synthesis time to 30 minutes using a pressurized microwave reactor without the requirement of further post-treatments such as calcination. Dramatically reduced synthesis time prevents particle growth via Ostwald ripening thus the obtained particles have dimensions in the range of 20 to 130 nm, they are uniform in shape and exhibit magnetic properties with saturation magnetization ranging from 8 to 76 emu g(-1). The suggested method allows simple particle size and crystallinity tuning resulting in improved magnetic properties by changing the synthesis parameters, i.e. temperature and nucleating agents. Moreover, efficiency of conversion of raw material into the product is almost 100%.

The chemical stability and cytotoxicity of carbonyl iron particles grafted with poly(glycidyl methacrylate) and the magnetorheological activity of their suspensions

Carbonyl iron (CI) particles were grafted with poly(glycidyl methacrylate) (PGMA) using atom transfer radical polymerization. Compact coating of PGMA largely improved the chemical stability of the particles in an acid environment and thus reduced the common drawback of bare CI particles. Furthermore, due to possible medical applications of CI-polymer systems for magnetic drug targeting, an in vitro cytotoxicity test was performed using an NIH/3T3 cell line. The cell viability was evaluated by spectrometric assay (MTT). The results show that the prepared particles are not cytotoxic. Moreover, bare CI particles as well as synthesized core–shell particles were suspended in silicone oil, and the rheological behavior of MR suspensions was investigated in controlled shear rate mode under various magnetic field strengths. Dynamic yield stress as a measure of the rigidity of the created internal structures of the suspensions was determined using the Herschel–Bulkley model, which provided a reasonably good fit for rheological data. MR suspensions of PGMA-coated particles exhibited only slightly decreased yield stresses due to their negligibly-affected magnetic performance.

Graphene oxide modified by betaine moieties for improvement of electrorheological performance

Novel graphene oxide bearing betaine moieties as sulfobetaine (GO-SB), carboxybetaine (GO-CB) and carboxybetaine ester (GO-CBE) moieties were prepared in two simple fabrication processes based on silanization and a thiol–ene click-reaction. Betaine modified particles showed appropriate electrical conductivity for electrorheological applications and the electrorheological behavior was confirmed by on-line microscopic observation. Analysis of the dielectric data using the Havriliak–Negami model showed that the suspension bearing the carboxybetaine ester moiety (GO-CBE) in silicone oil had the highest dielectric relaxation strength and fastest relaxation time due to enhanced charge transfer caused by a specific carboxybetaine ester feature. Analysis of the rheological data of the prepared suspensions at various external fields by the power-law model revealed that the conductivity mechanism is responsible for the electrorheological effect. Development of the internal structures was considerably improved for the GO-CBE sample with a yield stress up to 97 Pa at 5 wt% GO-CBE particles in suspension.

Magnetic Iron Oxide Nanoparticles for High Frequency Applications

Magnetic properties of iron oxide nanoparticles synthesized by microwave solvothermal method are reported. The effect of the nucleating agent on the composition, size, morphology and microstructure of nanoparticles was studied with a view to their correlation with magnetic properties. It is shown that a soft magnetic material with high value of magnetization saturation and high-frequency dispersion of complex permeability can be obtained by an appropriate choice of the nucleating agent.

Electrically conductive polyaniline—A molecular magnet with the possibility of chemically controlling the magnetic properties

Different forms of an electrically conductive polymer, an organic semiconductor, namely, polyaniline, have been synthesized and characterized. The magnetization curves of the obtained forms have been analyzed. It has been found that, in the oxidized form, the material exhibits a magnetic hysteresis at room temperature. For polyaniline without a special doping with magnetic additives, this result has been obtained for the first time. The possibility of controlling the magnetic properties of the material by means of chemical treatment at the post-polymerization stage has been demonstrated.

Multicomponent Magnetic Particles With Controllable Electromagnetic Properties

In this paper, we focus on preparation and investigation of magnetic and dielectric properties of multicomponent particles with core-shell structure comprising of low-anisotropy MnZn ferrite (core) and polyaniline containing nanoparticles of noble metals (shell). The main advantage of such hybrid magnetic materials is that their electromagnetic properties can be effectively controlled by synthesis conditions. Developed materials can find their potential application as magnetic fillers in the preparation of electromagnetic wave absorbers, where high permeability, high magnetic loss, and optimum ratio between the permeability and permittivity are of high importance.

Magnetorheological characterization and electrospinnability of ultrasound-treated polymer solutions containing magnetic nanoparticles

In order to fabricate a magnetic nanofibrous membrane by electrospinning, it is necessary to follow a suitable method for incorporating nanoparticles into a polymer solution. Ultrasound treatment represents a very effective technique for distributing magnetic nanoparticles within polymer solutions. Adverse effects caused by sonication over time on the given nanofibrous membrane (polymer degradation and appearance of defects) were evaluated by using rotational (magneto)rheometry, transmission and scanning electron microscopy, and magnetometry. A magnetorheological approach was selected to estimate the optimal duration of sonication, and findings were experimentally verified. It was concluded that the processed nanofibrous membrane showed promise as an advanced magnetoactive device.

Heating Efficiency of Iron Oxide Nanoparticles in Hyperthermia: Effect of Preparation Conditions

The correlation between magneto-structural properties of iron oxide nanoparticles and their heating efficiency in ac magnetic fields is investigated. Nanoparticles with an average size of 13 nm and narrow size distribution are synthesized by coprecipitation of ferrous and ferric salts in an alkaline medium in which the salts ratio is systematically varied. To obtain nanoparticles with single-phase composition, the prepared samples are annealed at 200 °C. The lattice parameters of all nanoparticles deduced from X-ray diffraction analysis correspond to multiphase composition, i.e., to a mixture of magnetite and maghemite phases. However, low-temperature Mossbauer spectroscopy revealed complete transformation of magnetite to maghemite in annealed samples. The annealed samples show slightly higher value of specific loss power at a certain frequency and amplitude of ac magnetic field compared with other samples.