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Yttrium orthoferrite (YFeO3) powder was prepared by a mechanochemical synthesis from a mixture of Y2O3 and α-Fe2O3 powders in a planetary ball mill for 2.5 h. The obtained YFeO3 powder sample was characterized by X-ray diffraction (XRD), Raman and infrared spectroscopy. The average crystallite size calculated by the Scherrer equation was 12 nm. The Mössbauer spectroscopy at room temperature confirms the superparamagnetic character of YFeO3 orthoferrite sample. [Project of the Serbian Ministry of Education, Science and Technological Development, Grant no. III 45003, Grant no. III 45015, Grant no. III 45018]

Two ferrite ceramic materials, MgFe2O4 and MnFe2O4, were successfully fabricated by a conventional sintering of nanosized powders (at 1373 K for 2 h) synthesized by soft mechanochemical route. The particle size and morphology of powders were studied using X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD analysis was carried out for the determination of phase purity, crystal structure and average crystallite size of sintered ferrites. Both mechanosynthesized ferrite samples show mean crystallite sizes in the nm-range. Over the frequency range of 100 Hz to 1 MHz, impedance spectra of prepared ferrite ceramics are investigated at and above room temperature. Changes in the impedance plane plots with temperature have been discussed and correlated to the microstructure of materials. An equivalent circuit model is applied to explore the electrical parameters (resistance and capacitance) associated with grains and grain boundaries. Complex impedance analysis indicates the dominance of grain boundary effects which control the overall electrical behaviour of studied ferrites. The decrease in grain boundary resistance with temperature suggests a thermally activated conduction mechanism. [Projekat Ministarstva nauke Republike Srbije, br. III43008 i br. III45003]

The goal of this study was to prepare CaO.ZnO catalyst which contain small amount of K2CO3 and analyze its activity for biodiesel synthesis. Catalyst was prepared using the following procedure: CaO and ZnO (molar ratio of 1:2), water and K2CO3 (in various amounts) were mechanochemically treated and after milling heated at 700 oC in air atmosphere for obtaining mixed CaO•ZnO/xK2CO3 oxides (x = 0, 1, 2 and 4; mole of K2CO3 per 10 mole of CaO). All the samples were characterized by X-ray diffraction (XRD), inductively coupled plasma (ICP), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), infrared spectroscopy (FTIR), scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS), particle size laser diffraction (PSLD) distribution, solubility measurement of Ca, Zn and K ions in methanol as well as by determination of their alkalinity (Hammett indicator method). Prepared CaO•ZnO/xK2CO3 composite powders were tested as catalysts for methanolysis of sunflower oil at 70ºC using molar ratio of sunflower oil to methanol of 1:10 and with 2 mas% of catalyst based on oil weight. The presence of K2CO3 in prepared samples was found to increase the activity of catalyst, and that such effect is caused by homogeneous-heterogeneous catalysis of biodiesel synthesis. [Projekat Ministarstva nauke Republike Srbije, br. 45001]

The interest in finding new methods for preparation of nickel ferrite (NiFe2O4) and nickel-zinc ferrite (NixZn1-xFe2O4) powders has recently increased, due to the fact that physical and chemical properties of these soft magnetic materials depend strongly on the preparation conditions. In this paper, powder samples of ferrites were obtained by: 1) classic sintering procedure (NixZn1-xFe2O4, x = 0.9) and 2) planetary mill synthesis (both NiFe2O4 and NixZn1-xFe2O4). Mechanochemical reaction leading to the formation of NixZn1-xFe2O4 (x = 1 and 0.9) spinel phase was monitored by SEM, TEM, and XRD. Values of the real and imaginary parts of permittivity and permeability were measured for the obtained nickel and nickel-zinc ferrite samples in the 7-12 GHz frequency range. Based on the obtained results, the EMR absorption coefficients were calculated for all three sample types. It has been concluded that the method of preparation and the final particle size influence the EMR absorption coefficient of nickel and nickel-zinc ferrites.

Nano-sized titanium monoxide (TiO) powder was prepared by mechanochemical synthesis. A mixture of commercial Ti and TiO2 (rutile) powders with the molar ratio of 1:1 was milled in a planetary ball mill for 5, 10, 20, 30 and 60 min under argon atmosphere. The final single-phase titanium monoxide sample was characterized by X-ray diffraction (XRD), magnetic measurements using a superconducting quantum interference device magnetometer (SQUID) and thermogravimetric analysis (TGA). The temperature dependency of the magnetic susceptibility is characterized by significant contribution of Pauli paramagnetism due to conduction electrons.

Powder mixtures of α-Bi2O3 containing 2, 5 and 10 mole % of TiO2, ZrO2 or HfO2 were homogenized, heated at 820ºC for 24 h and quenched in air. X-ray powder diffraction technique was used to characterize the prepared samples. In all cases metastable Bi2O3 polymorphs, γ-Bi2O3 or β-Bi2O3, are found as single or major phases. Addition of Ti4+ ions stabilizes γ-Bi2O3 polymorph, while either Zr4+ or Hf4+ ions stabilize β-Bi2O3 polymorph. In the samples with 2 and 5 mole % of TiO2 the presence of even two γ-Bi2O3 phases (Bi12TiO20 compound and a very low Ti-doped γ-Bi2O3) was established. Similarly, in the sample with 2 mole % of HfO2 two β-Bi2O3 phases were found. Phase composition of prepared samples, values of unit cell parameters and the appearance of two polymorphs with identical crystal structure but different unit cell parameters are discussed and compared with known data.

Nanosized perovskite YTiO3 with the mean crystallite size of 18 nm was synthesized for the first time by mechanochemical treatment. The mechanochemical solid state reaction between commercial Y2O3 powder and mechanochemically synthesized TiO powder in molar ratio 0.5:1 was completed for 3 h in a high-energy planetary ball mill in argon atmosphere. The heating in vacuum at 1150 °C for 12 h transforms nanosized YTiO3 to a well-crystallized single-phase perovskite YTiO3. Both samples were characterized by X-ray diffraction (XRD) and thermogravimetric (TGA/DTA) analyses, as well as superconducting quantum interference device magnetometer (SQUID) measurements.

A powder mixture of α-Bi2O3 and HfO2, in the molar ratio 2:3, was mechanochemically treated in a planetary ball mill under air, using zirconium oxide vials and balls as the milling medium. After 50 h of milling, the mechanochemical reaction led to the formation of a nanocrystalline δ-Bi2O3 phase (fluorite-type solid solution Bi0.78Hf0.59Zr0.63O3.61), with a crystallite size of 20 nm. The mechanochemical reaction started at a very beginning of milling accompanied by an accumulation of ZrO2 arising from the milling tools. The samples prepared after various milling times were characterized by X-ray powder diffraction and DSC analysis. The electrical properties of the as-milled and pressed Bi0.78Hf0.59Zr0.63O3.61 powder were studied using impedance spectroscopy in the temperature range from 100 to 700°C under air. The electrical conductivity was determined to be 9.43×10-6 and 0.080 S cm-1 for the temperatures of 300 and 700°C, respectively.

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Correlation between high-temperature electrical response and microstructural properties of dense hydroxyapatite with average grain size from micrometer to nanometer level in heating/cooling cycle was investigated. Grain interior and grain boundary contributions to overall conductivity were determined. Nanostructured hydroxyapatite exhibited enhanced grain interior conductivity, with significantly higher activation energy (∼2.4 eV) than that of coarsed microstructures (∼1.7 eV). This difference in activation energies is explained by the possible influence of dehydroxylation during materials fabrication procedure, which affected the amount of OH − ions as the main charge carriers. Grain boundary conductivity was similar for all microstructures, with activation energy above ∼2 eV, implying the OH − ions migration across grain boundaries. Electrical response in cooling cycle confirmed the trends found during heating. [ABSTRACT FROM AUTHOR]

In this study, a correlation between densification, microstructure, and electrical properties of BaTiO3 (BT) ceramics prepared from ultrasonically de-agglomerated BT powders has been analyzed. BT powders with the same crystal structure (tetragonal) and stoichiometry, but with different average particle size, were used to prepare sintered ceramics. Densification and electrical properties of the sintered BT ceramics were correlated to the average particle size of the powders. It was found that a decrease in average particle size improved the densification process as well as the ceramics' microstructure. The impedance measurements were done up to 320°C in order to separate grain (bulk) and grain boundary contributions. Bulk resistance was more or less the same, independent on the average particle size of the powder. On the contrary, grain boundary resistance increased with decreasing average particle size of the powder. [ABSTRACT FROM AUTHOR]

Calcium-containing perovskites CaTiO 3 , CaMnO 3 , CaZrO 3 and Ca 2 Fe 2 O 5 were synthesized by calcination of mechanochemically treated stoichiometric mixtures of CaCO 3 and either TiO 2 , MnO 2 , ZrO 2 or Fe 2 O 3 powders. X-ray diffraction analysis revealed a pure single perovskite phase for all four samples after calcination. This study proved that the pure perovskite phase exhibited low or no catalytic activity at 60 °C for the methanolysis of sunflower oil. However, at 165 °C FAME yield of more than 90% was reached after 2 h using CaTiO 3 , CaMnO 3 and CaZrO 3 as catalyst. Contrary to inactive pure perovskites at 60 °C, the CaTiO 3 and Ca 2 Fe 2 O 5 samples containing a very small amount of “free” CaO also showed methanolysis activity at 60 °C. This fact can be used to explain some contradictory statements reported in literature related to the perovskites as catalyst for biodiesel synthesis at mild temperatures. [ABSTRACT FROM AUTHOR]

Potassium based ceramic materials composed from leucite in which 5% of Al is exchanged with Fe and 4% of hematite was synthesized by mechanochemical homogenization and annealing of K2O-SiO2-Al2O3-Fe2O3 mixtures. Synthesized material was characterized by X-ray Powder Diffraction (XRPD) and Scanning Electron Microscopy coupled with Energy Dispersive X-ray spectroscopy (SEM/EDX). The two methods are in good agreement in regard to the specimen chemical composition suggesting that a leucite chemical formula is K0.8Al0.7Fe0.15Si2.25O6. Rietveld structure refinement results reveal that about 20% of vacancies exist in the position of K atoms. [ABSTRACT FROM AUTHOR]