Thermal relaxation of magnetic clusters in amorphous Hf_{57}Fe_{43} alloy

The magnetization processes in binary magnetic/nonmagnetic amorphous alloy Hf_{57}Fe_{43} are investigated by the detailed measurements of magnetic hysteresis loops, temperature dependence of magnetization, relaxation of magnetization and magnetic ac susceptibility, including a nonlinear term. Blocking of magnetic moments at lower temperatures is accompanied with the slow relaxation of magnetization and magnetic hysteresis loops. All of the observed properties are explained with the superparamagnetic behaviour of the single domain magnetic clusters inside the nonmagnetic host, their blocking by the anisotropy barriers and thermal fluctuation over the barriers accompanied by relaxation of magnetization. From magnetic viscosity analysis based on thermal relaxation over the anisotropy barriers it is found out that magnetic clusters occupy the characteristic volume from 25 up to 200 nm3 . The validity of the superparamagnetic model of Hf_{57}Fe_{43} is based on the concentration of iron in the Hf_{100-x}Fe_{43} system that is just below the threshold for the long range magnetic ordering. This work throws more light on magnetic behaviour of other amorphous alloys, too

Experimental determination of volumetric properties of biofuels under high pressure conditions and their modeling using saft and pc-saft equations of state

Kontinualni razvoj vazdušnog saobraćaja na svetskom nivou uslovljava veću potrošnju energije, što vodi do povećanja štetnog uticaja na životnu sredinu. U cilju smanjenja negativnog uticaja fosilnih goriva na životnu sredinu, raste interesovanje za primenom bioobnovljivih izvora energije. Alternativna biogoriva, tj. njihova implementacija predstavlja sve interesantniji izazov avio industrije u cilju odgovora na zahtev za smanjenje emisije štetnih gasova. Predmet istraživanja ove doktorske disertacije podrazumeva određivanje volumetrijskih i termodinamičkih karakteristika terpena. Termodinamičke karakteristike terpena na visokim pritiscima i različitim temperaturama su od izuzetnog značaja za efikasno funkcionisanje motora. Eksperimentalno su merene gustine čistih uzoraka četiri različita terpena (p-cimen, α-pinen, limonen i citral) na pritiscima p═(0,1-60) MPa i temperaturama T═(293,15-413,15) K. Dobijene eksperimentalne vrednosti gustina na visokim pritiscima modelovane su modifikovanom Tammann-Tait-ovom jednačinom i dalje korišćene za proračune koeficijenata izotermskog sabijanja i izobarskog toplotnog širenja, unutrašnjeg pritiska i razlike specifičnih toplotnih kapaciteta pri konstantnoj zapremini i konstantnom pritisku. Imajući u vidu da je za modelovanje volumetrijskih karakteristika pomoću kubnih jednačina stanja neophodno poznavanje kritičnih osobina, koje je veoma komplikovano odrediti za većinu biokomponenti, razvijene su SAFT i PC-SAFT jednačine. Modeli jednačina stanja, SAFT i PC-SAFT korišćeni su za korelaciju konačnih gustina na visokim pritiscima. Odabrani terpeni su pojedinačno okarakterisani sa pet parametara karakterističnih za svaki od pomenutih modela (broj segmenta, energija segmenta, prečnik segmenta, zapremina asocijacije i energija asocijacije). Rezultati su prikazani grafički i tabelarno, i pokazuju veoma dobro slaganje sa eksperimentalnim podacima. Volumetrijska i transportna svojstva (gustine, viskoznosti, indeksi refrakcije i brzine zvuka) čistih terpena kao i njihovih binarnih smeša (p-cimen+α-pinen; p-cimen+limonen; p-cimen+citral) određena su eksperimentalnim merenjem u temperaturnom opsegu (288,15-323,15) K i na atmosferskom pritisku . Na osnovu izmerenih vrednosti proračunate su dopunske zapremine, promene viskoznosti, promene izentropske kompresibilnosti i dopunske slobodne Gibs-ove energije aktivacije viskoznog toka i rezultati fitovani Redlich-Kister polinomom. Kako bi se objasnilo neidealno ponašanje smeša izračunate su i parcijalne molarne zapremine, dopunske parcijalne molarne zapremine, dopunske parcijalne molarne zapremine pri beskonačnim razblaženjima. Istraživanja urađena u ovoj doktorskoj disertaciji predstavljaju vrednu bazu eksperimentalnih podataka potencijalno važnih biogoriva. Izračunati termofizički, termodinamički i mehanički parametri čistih terpena i njihovih binarnih smeša pružaju dragocene informacije o njihovom ponašanju u uslovima povišenih pritisaka i temperatura. Detaljna analiza molekulskih interakcija na osnovu dobijenih parametra daje uvid u neidealno ponašanje odabranih sistema.Continuous development of air transport at the world level causes higher energy consumption, which leads to an increase in the environmental impact. In order to reduce the negative impact of fossil fuels on the environment, there is increasing interest in the use of bio-renewable energy sources. Alternative biofuels, i.e. their implementation represents an increasingly interesting challenge for the aviation industry to respond to the requirement to reduce greenhouse gas emissions. Naturally occurring terpenes in plants can potentially be used as additives to existing fuels. The subject of this doctoral dissertation involves determining the volumetric and thermodynamic characteristics of terpenes. The thermodynamic characteristics of the terpenes at high pressures and at different temperatures are of great importance for the efficient operation of the engine. The densities of pure samples of four different terpenes (p-cymene, α-pinene, limonene and citral) were experimentally measured at pressures p═ (0.1-60) MPa and temperatures T═ (293.15-413.15) K. The obtained experimental values of high-pressure densities were modeled by the modified Tammann-Tait equation and further used to calculate isothermal compressibility and isobaric thermal expansivity coefficients, internal pressure, and the difference of specific heat capacities at constant pressure and at constant volume. Bearing in mind that knowledge of critical properties, which is very complicated to determine for most biocomponents, requires the development of volumetric characteristics using cubic state equations, SAFT and PC-SAFT equations have been developed. State equation models, SAFT, and PC-SAFT were used to correlate finite densities at high pressures. The selected terpenes were individually characterized by five parameters characteristic of each of the mentioned models (number of segments, segment energy, segment diameter, association volume and association energy). The results are presented graphically and in tabular form, and show very good agreement with the experimental data. Volumetric and transport properties of of pure terpenes as well as their binary mixtures (p-cymen + α-pinene; p-cymen + limonene; p-cymen + citral) were determined in the temperature range (288.15-323.15) K and at atmospheric pressure. Based on the measured values, the additional volumes, viscosity changes, changes in isentropic compressibility, and the additional free Gibbs energy of activation of the viscous flow were calculated and the results fit with the Redlich-Kister polynomial. In order to explain the non-ideal behavior of mixtures, partial molar volumes, additional partial molar volumes, supplemental partial molar volumes at infinite dilutions were also calculated. The research done in this doctoral dissertation represents a valuable database of experimental data on potentially important biofuels. The calculated thermophysical, thermodynamic and mechanical properties of pure terpenes and their binary mixtures provide valuable information about their behavior under high-pressure and high-temperature conditions. A comprehensive analysis of molecular interactions based on the obtained properties provides insight into the non-ideal behavior of selected systems

Site-selective quantum correlations revealed by magnetic anisotropy in the tetramer system SeCuO3

We present the investigation of a monoclinic compound SeCuO3 using x-ray powder diffraction, magnetization, torque and electron-spin-resonance (ESR). Structurally based analysis suggests that SeCuO3 can be considered as a 3D network of tetramers. The values of intra-tetramer exchange interactions are extracted from the temperature dependence of the susceptibility and amount to ~200 K. The inter-tetramer coupling leads to the development of long-range antiferromagnetic order at TN = 8 K. An unusual temperature dependence of the effective g-tensors is observed, accompanied with a rotation of macroscopic magnetic axes. We explain this unique observation as due to site-selective quantum correlations

Size distribution of FeNiB nanoparticles

Two samples of amorphous nanoparticles FeNiB, one of them with SiO2 sheath around the core and one without, were investigated by transmission electron microscopy and magnetic measurements. The coating gives mean particle diameters of 4.3 nm compared to 7.2 nm for the uncoated particles. Magnetic measurements prove superparamagnetic behaviour above 160 K (350 K) for the coated (uncoated) sample. With use of effective anisotropy constant Keff – determined from hysteresis loops – size distributions are determined both from ZFC curves, as well as from relaxation measurements. Both are in good agreement and are very similar for both samples. Comparison with the size distribution determined from TEM pictures shows that magnetic clusters consist of only few physical particles

Analysing radionuclide content in soil samples and radiological risks in the clayey material surrounding of the “Zbegovi” deposit, Donje Crniljevo, Serbia

This paper presents the results of analyses of radionuclide content in the samples of the surrounding soil and clayey material of “Zbegovi” open-pit mine in Donje Crniljevo, Serbia. Samples from 78 sites were collected and prepared. The activity concentrations were determined for radionuclides: 238U, 232Th, 40K, 226Ra, and 137Cs. The mean values obtained are as follows: 23 Bqkg–1, 89 Bqkg–1, 372 Bqkg–1, 56 Bqkg–1, and 11 Bqkg–1, respectively. Concentrations of 238U, 40K, and 226Ra in the studied area do not deviate from the values obtained for the soil in Serbia. The concentration of 232Th in the studied area is slightly higher relative to average values for soil, and slightly lower compared to similar deposits of clayey material in the world. Measurements performed showed that the open-pit mine of clayey material is completely uncontaminated surface as far as 137Cs is concerned, while there are sites where measured 137Cs concentrations are significantly higher, which is due to topographic differences and inhomogeneous surface contamination of land after the Chernobyl accident. To assess the radiological risks in the observed area, the following indices were determined: absorbed dose rate, annual outdoor effective dose, absorbed dose for biota, excess lifetime cancer risk outdoors as well as external radiation hazard index. The mean value of the estimated absorbed dose rate in the given area amounts to 80.1 nGyh–1, and the annual outdoor effective dose ranges from 46.9 to 134 Sv. Absorbed dose rate for biota in the studied area is 1.31 10–4 Gyd–1. The mean excess lifetime cancer risk outdoors for the population is 3.8 10–4, and t he mean value of the external radiation hazard index obtained in this study is 0.48, which is consistent with the world average. A low dose of radiation will not pose a risk to the population and biota in the studied area

Magnetic excitations and electronic interactions in Sr2_2CuTeO6_6: a spin-1/2 square lattice Heisenberg antiferromagnet

Sr$_2$CuTeO$_6$ presents an opportunity for exploring low-dimensional magnetism on a square lattice of $S=1/2$ Cu$^{2+}$ ions. We employ ab initio multi-reference configuration interaction calculations to unravel the Cu$^{2+}$ electronic structure and to evaluate exchange interactions in Sr$_2$CuTeO$_6$. The latter results are validated by inelastic neutron scattering using linear spin-wave theory and series-expansion corrections for quantum effects to extract true coupling parameters. Using this methodology, which is quite general, we demonstrate that Sr$_2$CuTeO$_6$ is an almost realization of a nearest-neighbor Heisenberg antiferromagnet but with relatively weak coupling of 7.18(5) meV.Comment: 10 pages, 7 figure

Follow up of Cs-137 activity in the first 100 samples in the area of Kumodraz, Belgrade in the 2008 to 2011 period

This paper presents results of gamma spectrometric analyses of air samples, collected in the period from 2009 to 2011, at Kumodraz location. Cs-137 specific activity has been monitored for 4 years. Exposure rate of gamma background radiation has been showed, as well. Obtained results show that average annual effective doses are in the range from 1.01 to 1.19 mSv/y and lower than the worldwide average.I International Conference on Radiation in Various Fields of Research : RAD 2017 : proceedings book; April 25-27, 2012; Niš, Serbi

Follow up of Cs-137 activity in the first 100 samples in the area of Kumodraz, Belgrade in the 2008 to 2011 period

This paper presents results of gamma spectrometric analyses of air samples, collected in the period from 2008 to 2011, at Kumodraz location. Cs-137 specific activity has been monitored for 4 years. Exposure rate of gamma background radiation has been showed, as well. Obtained results show that average annual effective doses are in the range from 1.01 to 1.19 mSv/y and lower than the worldwide average.I International Conference on Radiation in Various Fields of Research : RAD 2017 : proceedings book; April 25-27, 2012; Niš, Serbi

Study of vacuum and freeze drying of bee honey

The aim of this research is to study the drying kinetics of vacuum-dried and freeze-dried bee honey produced from two different varieties: Sunflower honey (Helianthus Annuus L.) and Acacia honey (Robinia pseudo acacia L.). Vacuum drying treatments were carried out with the honey samples' initial temperatures of +25 degrees C, -20 degrees C, and -40 degrees C. Water content, total soluble solids, as well as the water activity of fresh and dried honey samples were determined. Freeze-drying of bee honey with initial sample temperature of -40 degrees C has resulted in shorter drying time (7-9 hours), moisture content (10%-12%), water activity (0.405-0.427 aW) and effective moisture diffusivity coefficient (8.26.10-7-9.51.0-7 m(2)/s). The high-performance liquid chromatography method was used when analyzing the impact that drying pre-treatments had on honey quality. The application of pre-treatments has led to an increase in hydroxy-methyl-furfural by 39-71%, and a decrease in diastase activity by 17-36%, all compared to fresh honey samples. The solutions of Verma model proved to be the best fit with the experimental results