Aleksandar Nikolic

Magnetization enhancement in nanostructured random type MgFe2O4 spinel prepared by soft mechanochemical route

In this paper we report results of structural, spectroscopic, and magnetic investigations of MgFe2O4 nanoparticles prepared by soft mechanochemical synthesis. MgFe2O4 nanoparticles crystallize in Fd3¯m space group with mixed cation distribution and reduced percentage of Fe3+ at tetrahedral (8a) sites. Discrepancy in the cation distribution compared to that in the bulk Mg–ferrite is one of the highest known. X-ray line broadening analysis reveals crystallite size and strain anisotropy. The saturation magnetization, Msat=62 emu/g measured at 5 K is twice higher than that found in the bulk counterparts. Such high value of Msat is attributed to the low value of cation inversion parameter (δ=0.69), to the core/shell structure of the nanoparticles and to the surface/volume ratio. Mossbauer spectrum collected at room temperature reveals ferrimagnetic ordering between Fe3+ ions in 8a and 16d sites, while zero-field-cooled (ZFC) and field-cooled (FC) M(T) measurements were shown superparamagnetic state above 350 K.

Magnetization enhancement and cation valences in nonstoichiometric (Mn,Fe)3-δO4 nanoparticles

We present a study of the structural and magnetic properties of (Mn,Fe)3-δO4 nanoparticles synthesized by soft mechanochemistry using Mn(OH)2 × 2 H2O and Fe(OH)3 powders as starting compounds. The resulting nanoparticles with a composition of the (Mn,Fe)3-δO4 type are found to have a core/shell structure with different Mn/Fe ratios in the core and at the surface. XPS analysis points to valences of +2, +3, and +4 for Mn and +3 for Fe at the particle surface. Combined results of XRPD, Mossbauer spectroscopy, and EDX analysis suggest that there is a deviation from stoichiometry in the nanoparticle core compared to the shell, accompanied by creation of cation polyvalence and vacancies. The value of saturation magnetization, MS, of 73.5 emu/g at room temperature, is among the highest reported so far among nanocrystalline ferrite systems of similar composition.

Current source converter topologies for PMSG wind turbine applications

Different current source converter topologies for permanent magnet synchronous generator (PMSG) wind turbine applications are presented and analyzed in the paper. In high-power and medium-voltage range, current source converter has its natural advantages, such as simple topology, compact size, high power capability, excellent short circuit protection, and flexible power flow control. However, it also presents some challenges for control scheme development. In the paper two different approaches are shown. One utilizes phase control thyristors and active filter for harmonics compensation, while other uses PWM controlled SGCTs. By controlling the inverter modulation index at the highest possible level, the proposed control scheme keeps the dc link current at a minimum value with various wind speeds and therefore greatly reduces the converter power loss at lower power levels. Simulation results are given for both control schemes.

The change of crystal symmetry and cation ordering in Li–Mg ferrites

Abstract In this work we present results of synthesis and structural investigation of the spinels with general formula Li x Mg 1−2 x Fe 2+ x O 4 , where 0.00≤ x ≤0.50. Ultrafine powder of MgFe 2 O 4 ( x =0.00) was prepared by thermal decomposition of a mixture of complex compounds [Mg(AA) 2 ] and [Fe(AA) 3 ] at 500°C (AA=acetylacetone). The solid solutions of Li x Mg 1−2 x Fe 2+ x O 4 (0.10≤ x ≤0.45) were synthesized by conventional ceramic method. The samples with x ≥0.40 belong to ordered spinels with primitive cubic cell, space group P4 3 32 and the samples with x ≥0.35 are spinels with face-centered cubic cell, space group Fd-3m . Crystal structures of samples with x =0.00, 0.40 and 0.45 were refined by the Rietveld method using DBWS-9411 software package. In ordered spinels ( x =0.40, 0.45) Li + ions occupy both 4 b (preferentially) and 12 d octahedral sites. At tetrahedral 8 c positions Li + ions were not found (within the experimental error). Mg 2+ ions occupy octahedral 12 d and tetrahedral 8 c sites and Fe 3+ ions occupy all three different cationic sites. Cation ordering in 1:3 type ordered spinel (the occupancy ratio of the Wyckoff’s positions 4 b and 12 d ) of Li + and Fe 3+ at octahedral sites is not complete as in Li 0.5 Fe 2.5 O 4 . In MgFe 2 O 4 spinel, cation distribution in the tetrahedral sites was determined and given by occupation numbers N: N(Mg 2+ 8 a )=0.183(7) and N(Fe 3+ 8 a )=0.817(7).

Effect of cobalt doping level of ferrites in enhancing sensitivity of analytical performances of carbon paste electrode for simultaneous determination of catechol and hydroquinone

This work presents the simultaneous determination of catechol (CC) and hydroquinone (HQ), employing a modified carbon paste electrode (CPE) with ferrite nanomaterial. Ferrite nanomaterial was doped with different amount of cobalt and this was investigated toward simultaneous oxidation of CC and HQ. It was shown that this modification strongly increases electrochemical characteristics of the CPE. Also, electrocatalytic activity of such materials strongly depends on the level of substituted Co in the ferrite nanoparticles. The modified electrodes, labeled as CoFerrite/CPE, showed two pairs of well-defined redox peaks for the electrochemical processes of catechol and hydroquinone. Involving of ferrite material in the structure of CPE, cause increase in the potentials differences between redox couples of the investigated compounds, accompanied with increases in peaks currents. Several important parameters were optimized and calibration curves, with limits of detection (LOD) of 0.15 and 0.3µM for catechol and hydroquinone, respectively, were constructed by employing amperometric detection. Effect of possible interfering compounds was also studied, and proposed method was successfully applied for CC and HQ quantification in real samples.

Nanoscale Magnetic Behavior of C60 Thin Films in Earth Magnetic Field under Polarization Light Influences

Abstract Magnetic behavior of C60 thin films in the Earths magnetic field under polarization light influence is presented. Transformation of magnetic field for two fullerene thin films of different thickness is investigated. Two proton magnetometers were used for these measurements. Samples of 30 nm and 250 nm thickness illustrate a significant change of magnetic field intensity under the influence of polarization light, in range from 3.4 to 12.9 nT, for 200 measurement data per sample.

Energy management system for energy efficiency improvement in the industrial sector of the republic of Serbia

Main primary energy sources in Serbia are coal, oil and natural gas, oil shale and renewable energy sources. The share of high quality primary energy sources (oil and natural gas) is less than 1% of total geological reserves, while the rest of 99% consists of different types of coal, with highest share of lignite - over 95%. Estimated technically available renewable energy sources potential in Serbia is 5.65 Mtoe, where 1.054 Mtoe of biomass and 0.909 Mtoe of hidro-power are already used. Speaking of energy efficiency in final energy consumption sectors of the Republic of Serbia holds unenviable position comparing to EU countries. Since 2000 there is series of strategic and legal documents that are adopted and they are governing the operation of energy sector as well as implementation of energy policy at state level. The Law on energy efficiency was adopted in March 2013. It regulates implementation of energy management system within industrial sector, on the local level (cities and municipalities) as well as in public buildings sector. The purpose of secondary legislative is to define, in more details, terms for obtaining status of designated company within the system for energy management in industrial sector and in public companies, training programs for energy managers, as well as method of appointing energy managers in designated companies.

Energy efficiency in paper industry — Study of two cases

The paper presents two case studies in order to illustrate an efficient usage of energy in paper industry and to point out the benefits gained. Two different approaches for obtaining higher energy efficiency of the paper production process show that energy efficiency in paper industry must be considered as a multidisciplinary field. Differences in realization of paper machine systems for control and supervision are also presented in the paper.

Potentials for reducing primary energy consumption through energy audit in the packaging paper factory

Energy audit is a primary step towards a more rational use of primary energy sources in the energy-intensive process such as the production of packaging paper. By the adoption of the Law on efficient use of energy in 2013, energy audits have become mandatory for all industrial companies in Serbia. This paper presents the methodology and results of the one-day energy audit which was conducted in the Paper Factory - Belgrade (FHB). The measures to increase energy efficiency of the production process, together with the quantification of the savings in the primary energy consumption, are proposed based on the results of the study.

Spherical aromaticity of Jahn–Teller active fullerene ions

Density functional theory was applied to compute the nucleus-independent chemical shifts of fullerene (C60), the fullerene ion C6010+, and the Jahn–Teller active fullerene anion C60− and cation C60+. Positioning a 3He nucleus inside the cage of each of these fullerene species facilitates investigations of the substantial differences among them, 3He NMR chemical shifts can provide important data on the aromatic behavior of these molecular cages. Thus, we also calculated the NMR chemical shift of a 3He atom positioned at the center of each fullerene species investigated (C60, C6010+, C60−, and C60+). The data obtained revealed significant differences in the aromatic behavior of the C60 (moderately aromatic) and C6010+ (highly aromatic) species. The values of the nucleus-independent chemical shift parameters were also scanned along the intrinsic distortion path for the C60− and C60+ species. In both cases, antiaromatic character decreases with increasing deviation from high-symmetry structures to low-symmetry global minimum points, resulting in the antiaromatic C60− and weakly aromatic C60+.Graphical Abstract