Miodrag Mitrić

Mechanochemical synthesis of NiFe2O4 ferrite

A mixture of crystalline NiO and α-Fe2O3 powders in equimolar ratio was mechanochemically treated in a planetary ball mill up to 50 h of milling. The mechanochemical reaction leading to the formation of the NiFe2O4 spinel phase was followed by X-ray diffraction and magnetization measurements. The spinel phase was first observed after 10 h of milling and its formation was completed after 35 h of milling. The synthesized NiFe2O4 ferrite has a nanocrystalline structure with a crystallite size of about 6 nm. Powders obtained after 20, 35 and 50 h of milling, as well as the starting mixture without mechanochemical treatment were compacted by cold pressing and sintering. The specific electric resistivity of the compacts greatly depends on the milling time of the powder and decreases by four orders of magnitude as the milling time increases from 0 to 50 h.

Mechanochemical treatment of α-Fe2O3 powder in air atmosphere

Abstract Powder of α -Fe 2 O 3 was mechanochemically treated in a planetary ball mill in an air atmosphere. Structural changes were followed by X-ray diffraction analysis, magnetization measurements and differential scanning calorimetry after various milling times. It was found that complete transformation of α -Fe 2 O 3 to Fe 3 O 4 is possible during milling in an air atmosphere under appropriate milling conditions. Presumably, the decrease in the oxygen partial pressure during milling has a critical influence on promoting the dissociation of α -Fe 2 O 3 . Before nucleation of the Fe 3 O 4 phase, the crystallites of the α -Fe 2 O 3 phase are reduced to a minimal size accompanied by the introduction of atomic-level strain. Local modeling of a collision event, coupled with a classical thermodynamic assessment of the Fe 2 O 3 -Fe 3 O 4 system, were used to rationalize the experimental results. It is proposed that the mechanochemical reactions proceed at the moment of impact by a process of energization and freezing of highly localized sites of a short lifetime. Excitation on a time scale of ∼10 −5 s corresponds to a temperature rise of the order of (1–2)×10 3 K. Decay of the excited state occurs rapidly at a mean cooling rate higher than 10 6 K s −1 .

The ball milling induced transformation of α-Fe2O3 powder in air and oxygen atmosphere

Abstract The mechanochemical treatment of α -Fe 2 O 3 powder was done concurrently in air and oxygen atmospheres using a conventional planetary ball mill. The influence of the duration of milling and of the balls-to-powder mass ratio on the transformation of α -Fe 2 O 3 was investigated. Under appropriate milling conditions, α -Fe 2 O 3 completely transforms to Fe 3 O 4 , and for prolonged milling to the Fe 1− x O phase, either in air or oxygen atmosphere. Owing to the higher oxygen pressure, the start of the reaction in oxygen is delayed by ∼1 h in comparison with the reaction in air. The reverse mechanochemical reaction Fe 1− x O→Fe 3 O 4 → α -Fe 2 O 3 takes place under proper oxygen atmosphere. The oxygen partial pressure is the critical parameter responsible for the mechanochemical reactions. The balls-to-powder mass ratio has a considerable influence on the kinetics of mechanochemical reactions. Below the threshold value the reaction does not proceed or proceeds very slowly. Plausibly, three phenomena govern mechanochemical reactions: (i) the generation of highly energetic and localized sites of a short lifetime at the moment of impact; (ii) the adsorption of oxygen at atomically clean surfaces created by particle fracture; and (iii) the change of activities of the constituent phases arising from a very distorted (nanocrystalline) structure.

Corrosion Stability and Bioactivity in Simulated Body Fluid of Silver/Hydroxyapatite and Silver/Hydroxyapatite/Lignin Coatings on Titanium Obtained by Electrophoretic Deposition

Hydroxyapatite is the most suitable biocompatible material for bone implant coatings. However, its brittleness is a major obstacle, and that is why, recently, research focused on creating composites with various biopolymers. In this study, hydroxyapatite coatings were modified with lignin in order to attain corrosion stability and surface porosity that enables osteogenesis. Incorporating silver, well known for its antimicrobial properties, seemed the best strategy for avoiding possible infections. The silver/hydroxyapatite (Ag/HAP) and silver/hydroxyapatite/lignin (Ag/HAP/Lig) coatings were cathaphoretically deposited on titanium from ethanol suspensions, sintered at 900 °C in Ar, and characterized by X-ray diffraction, scanning electron microscopy, field emission scanning electron microscopy, attenuated total reflection Fourier transform infrared, and X-ray photoelectron spectroscopy. The corrosion stability of electrodeposited coatings was evaluated in vitro in Kokubos simulated body fluid (SBF) at 37 °C using electrochemical impedance spectroscopy. Bioactivity was estimated by immersion in SBF to evaluate the formation of hydroxyapatite on the coating surface. A microcrystalline structure of newly formed plate-shaped carbonate-hydroxyapatite was detected after only 7 days, indicating enhanced bioactive behavior. Both coatings had good corrosion stability during a prolonged immersion time. Among the two, the Ag/HAP/Lig coating had a homogeneous surface, less roughness, and low values of contact angle.

Nanomaterial with High Antimicrobial Efficacy—Copper/Polyaniline Nanocomposite

This study explores different mechanisms of antimicrobial action by designing hybrid nanomaterials that provide a new approach in the fight against resistant microbes. Here, we present a cheap copper-polyaniline (Cu-PANI) nanocomposite material with enhanced antimicrobial properties, prepared by simple in situ polymerization method, when polymer and metal nanoparticles are produced simultaneously. The copper nanoparticles (CuNPs) are uniformly dispersed in the polymer and have a narrow size distribution (dav = 6 nm). We found that CuNPs and PANI act synergistically against three strains, Escherichia coli, Staphylococcus aureus, and Candida albicans, and resulting nanocomposite exhibits higher antimicrobial activity than any component acting alone. Before using the colony counting method to quantify its time and concentration antimicrobial activity, different techniques (UV-visible spectroscopy, transmission electron microscopy, scanning electron microscope, field emission scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectrophotometry, and inductively coupled plasma optical emission spectrometry) were used to identify the optical, structural, and chemical aspects of the formed Cu-PANI nanocomposite. The antimicrobial activity of this nanocomposite shows that the microbial growth has been fully inhibited; moreover, some of the tested microbes were killed. Atomic force microscopy revealed dramatic changes in morphology of tested cells due to disruption of their cell wall integrity after incubation with Cu-PANI nanocomposite.

Interfacial Synthesis of Gold-Polyaniline Nanocomposite and Its Electrocatalytic Application.

Gold-polyaniline (Au-PANI) nanocomposite was prepared using a simple interfacial polymerization method, performed in an immiscible water/toluene biphasic system using tetrachloroaurate, AuCl4(-) as an oxidant. The formation of Au nanoparticles (AuNPs) or Au-PANI nanocomposite can be controlled to a certain degree by varying the ratio of initial Au(+) and aniline concentrations. Under optimal condition (HAuCl4/aniline ratio is 1:2), green dispersion of Au-PANI nanocomposite is produced in aqueous phase, whose morphology, structure and physicochemical properties are investigated in details. The nanocomposite shows granular morphology with mostly rodlike AuNPs embedded in polymer. It was found that polyaniline in the composite is in the conducting emeraldine salt form, containing high amount of Au (28.85 wt %). Furthermore, the electrical conductivity of the nanocomposite was found to be four-fold higher than that of the polymer itself. In addition, the nanocomposite powder, isolated from the as-prepared aqueous dispersion, can later be easily redispersed in water and further used for various applications. Moreover, the obtained Au-PANI nanocomposite showed excellent electrocatalytic performance toward the electrochemical oxygen reduction reaction (ORR), with high ORR onset potential and good selectivity. This makes it a promising candidate for a new class of Pt-free ORR catalyst.

The preferential site occupation and magnetic properties of GdxY2−xO3

Abstract The gadolinium-yttrium mixed oxides Gd x Y 2− x O 3 were synthesized over a wide range of concentrations: x = 0.10, 0.18, 0.41, 0.74 and 1.26. The X-ray diffraction data from the polycrystalline samples were taken at room temperature and refined using the Rietveld method. Gadolinium-yttrium oxides crystallize in the space group Ia 3. The cations occupy the six coordinated positions 8 b and 24 d . The anion occupies a general tetrahedrally coordinated position. At concentrations x ⩽ 0.41 gadolinium ion occupies exclusively 24 d , but at higher concentrations the occupation of position 8 b increases continuously. The magnetic susceptibility measurements showed the presence of the antiferromagnetic interaction in all samples. The magnetic moment in the sample x = 1.26 is significantly different from the magnetic moment of the free ion Gd 3+ . The Curie-Weiss temperature shows a nonlinear dependence on concentration.

An x-ray diffraction and magnetic susceptibility study of

Polycrystalline samples of the new semimagnetic semiconductor (x = 0.06, 0.12, 0.25, 0.39, 1.00, 1.40, 1.80) were obtained by ceramic technology. An x-ray diffraction experiment was performed on all of the samples at room temperature. These data provided the basis for Rietveld refinements. The refinements showed that ions preferentially occupy the 24d position. The lattice constants and the mean cation - anion distances decrease linearly with the -ion concentration. The inverse magnetic susceptibilities for all of the samples depend linearly on the temperature for . In this region only the lowest Kramers doublet is populated, and for that level the effective magnetic quantum number is deduced. The Curie - Weiss paramagnetic temperatures obtained from the low-temperature region depend linearly on the concentration x. Over the whole experimental region , the magnetic susceptibility deviates from the Curie - Weiss law, and its calculation requires inclusion of the first excited Kramers doublet. The effective magnetic quantum number and energy gap of the first excited doublet were deduced.

Structure properties and magnetic susceptibility of diluted magnetic semiconductor Y2-xHoxO3

Abstract The polycrystalline samples of Y 2− x Ho x O 3 (0.10 ≤ x ≤ 1.80) were synthesized by ceramic technology. The X-ray powder diffraction data were collected and the crystal structures were refined by the Rietveld method for the samples Y 2− x Ho x O 3 ( x = 0.00, 0.20, 0.40, 1.20, 1.80, 2.00). Holmium ions Ho 3+ were randomly distributed over two cationic sites 8b and 24d in the space group 1a3 in all refined structures. Cationanioncation bonds important for superexchange interaction were determined. Magnetic susceptibility measurements were done by the Faraday method in the temperature range 290 to 620 K and a behaviour in accordance with the Curie-Weiss law was obtained. The molar Curies constants linearly depend on concentration. The effective magnetic moments of Ho 3+ ions were smaller than the free ion value. The Curie-Weiss paramagnetic temperatures indicated antiferromagnetic interaction. The gram ion susceptibilities confirmed the random distribution of Ho 3+ ions over two crystallographic sites in the concentration range 0.20 ≤ x ≤ 2.00 and pointed to the random distribution of magnetic ions in the sample with x = 0.10. The nonlinearity of Curie-Weiss paramagnetic temperatures versus concentration occurred in the low concentration region ( x ≤ 0.40), indicating tendency of magnetic ions to form small clusters around cationic sites.

Cation ordering in cubic and monoclinic : an x-ray powder diffraction and magnetic susceptibility study

Mixed sesquioxides (x = 0.10, 0.20, 0.60, 1.00, 1.60 and 1.80) in the cubic (C) phase were obtained by precipitation and subsequent sintering. Cubic and were transformed into monoclinic (B) phases at 1400 K and 1600 K respectively. The transformation is reconstructive in character. All of the structures were refined using the Rietveld powder method, and the ions found to be randomly distributed. The molar magnetic susceptibilities at room temperature exceed the free-ion values, and are almost independent of the concentration for . For , the susceptibilities decrease with decreasing concentration of the magnetic ion . This behaviour is attributed to crystal-field and anisotropic exchange effects.