Mehanohemijska sinteza nikal i nikal-cink feritnih prahova sa Nicolson-Ross analizom absorpcionih koeficijenata

The interest in finding new methods for the preparation of nickel ferrite (NiFe2O4) and nickel-zinc ferrite (NixZn1-xFe2O4) powders has recently increased, because the physical and chemical properties of these soft magnetic materials depend strongly on the preparation conditions. In this study, powder samples of ferrites were obtained by: 1) a classic sintering procedure (NixZn1-xFe2O4, x = 0.9) and 2) planetary mill synthesis (both NiFe2O4 and the NixZn1-xFe2O4). The mechano-chemical reaction leading to the formation of the spinel phase of NixZn1-xFe2O4 (x = 1 or 0.9) was monitored by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) analysis. The values of the real and imaginary parts of the 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 electromagnetic radiation (EMR) absorption coefficients were calculated for all three types of sample. It was concluded that the method of preparation and the final particle size influence the EMR absorption coefficient of nickel and nickel-zinc ferrites.U novije vreme povećan je interes za pronalaženje novih metoda za sintezu nikal-ferita (NiFe2O4) i nikal-cink-feritnih (NixZn1-xFe2O4) prahova, zbog činjenice da fizička i hemijska svojstva ovih 'mekih' magnetnih materijala u mnogome zavise od uslova pripreme. Dobijeni feritni prahovi, opisani u ovom radu, sintetisani su: 1) klasičnom procedurom sinterovanja (NixZn1-xFe2O4, x = 0,9 ili 2) sintezom u planetarnom mlinu (NiFe2O4 i NixZn1-xFe2O4). Prosečna veličina čestica dobijenih prvom metodom iznosi 3-5 μm, dok druga metoda daje čestice veličine 10-12 nm. Karakterizacija uzoraka praćena je skenirajućom elektronskom mikroskopijom (SEM), transmisionom elektronskom mikroskopijom (TEM), kao i difrakcionom analizom X-zracima (XRD). Realni i imaginarni delovi koeficijenata permitivnosti i permeabilnosti su mereni na dobijenim uzorcima nikal i nikal-cink-ferita u frekventnom opsegu 7-12 GHz. Na osnovu dobijenih rezultata, izračunati su EMR apsorpcioni koeficijenti za sve dobijene uzorke. Zaključeno je da je način pripreme, kao i dobijena veličina čestica, utiču na EMR apsorpcione koeficijente nikal i nikal-cink-ferita

Yttrium Orthoferrite Powder Obtained by the Mechanochemical Synthesis

Yttrium orthoferrite (YFeO3) powder was prepared by a mechanochemical synthesis from a mixture of Y2O3 and alpha-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 Mossbauer spectroscopy at room temperature confirms the superparamagnetic character of YFeO3 orthoferrite sample

Structural and electrical properties of the 2Bi(2)O(3) center dot 3ZrO(2) system

Powder mixtures of alpha-Bi2O3 (bismite) and monoclinic m-ZrO2 (baddeleyite) in the molar ratio 2:3 were mechanochemically and thermally treated with the goal to examine the phases, which may appear during such procedures. The prepared samples were characterized by X-ray powder diffraction, differential scanning calorimetry (DSC), electrical measurements, as well as scanning electron microscopy (SEM) and transmission election microscopy (TEM). The mechanochemical reaction leads to the gradual formation of a nanocrystalline phase, which resembles delta-Bi2O3, a high-temperature Bi2O3 polymorph. Isothermal sintering in air at a temperature of 820 degrees C for 24 h followed by quenching to room temperature yielded a mixture of ZrO2-stabilized beta-Bi2O3 and m-ZrO2 phases, whereas in slowly cooled products, the complete separation of the initial alpha-Bi2O3 and m-ZrO2 constituents was observed. The dielectric permittivity of the sintered samples significantly depended on the temperature. The sintered and quenched samples exhibited a hysteresis dependence of the dielectric shift, showing that the ZrO2-doped beta-Bi2O3 phase possess ferroelectric properties, which were detected for the first time. This fact, together with Rietveld refinement of the beta-Bi2O3/m-ZrO2 mixture based on neutron powder diffraction data showed that ZrO2-doped beta-Bi2O3 has a non-centrosymmetric structure with P $(4) over bar$2(1)c as the true space group. The ZrO2 content in the doped beta-Bi2O3 and the crystal chemical reasons for the stabilization of the beta-Bi2O3 phase by the addition Of m-ZrO2 are discussed. (c) 2008 Elsevier Inc. All rights reserved

Mechanochemical treatment of alpha-Fe2O3 powder in air atmosphere

Powder of alpha-Fe2O3 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 alpha-Fe2O3 to Fe3O4 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 alpha-Fe2O3. Before nucleation of the Fe3O4 phase, the crystallites of the alpha-Fe2O3 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 Fe2O3-Fe3O4 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 similar to 10(-5) s corresponds to a temperature rise of the order of (1-2) x 10(3) K. Decay of the excited state occurs rapidly at a mean cooling rate higher than 10(6) K s(-1). (C) 1998 Elsevier Science S.A. All rights reserved

Structure study of Bi4Ti3O12 produced via mechanochemically assisted synthesis

Nanosized bismuth titanate was prepared via high-energy ball milling process through mechanically assisted synthesis directly from their oxide mixture of Bi2O3 and TiO2. Only Bi4Ti3O12 phase was formed after 3 h of milling time. The excess of 3 wt% Bi2O3 added in the initial mixture before milling does not improve significantly the formation of Bi4Ti3O12 phase comparing to stoichiometric mixture. The formed phase was amorphized independently of the milling time, The Rietveld analysis was adopted to determine the crystal structure symmetry, amount of amorphous phase, crystallite size and microstrains. With increasing the milling time from 3 to 12 h, the particle size of formed Bi4Ti3O12 did not reduced significantly. That was confirmed by SEM and TEM analysis. The particle size was less than 20 nm and show strong tendency to agglomeration. The electron diffraction pattern indicates that Bi4Ti3O12 crystalline powder is embedded in an amorphous phase of bismuth titanate. Phase composition and atom ratio in BIT ceramics were determined by X-ray diffraction and EDS analysis. (c) 2007 Elsevier Ltd. All rights reserved

A Y2O3 : Yb nanoscale magnet obtained by HEBM: C-3i/C-2 site occupancies, size/strain analysis and crystal field levels of Yb3+ ions

A Y2O3: Yb nanoscale magnet that belongs to the cubic C-type or bixbyite structure family was synthesized by high energy ball milling ( HEBM). An as-prepared sample ( S1) was annealed at 650 degrees C ( S2) and 950 degrees C ( S3). Cation populations were determined by the refinement of site occupancies. It was found that in S1 and S2 Yb3+ ions occupy exclusively the 8b ( or C-3i) position, whereas in S3 a small amount of Yb3+ is also located on Wyckoff-site 24d ( or C-2). X-ray powder diffraction line broadening analysis was done by using the Rietveld method using regular TCH-pV functions ( isotropic effects) and symmetrized cubic harmonics ( anisotropic effects) for the refinement. The line broadening anisotropy decreases due to strain effects from S1 to S3, while the crystallite size anisotropy increases from S1 to S3. Transmission electron microscopy ( TEM) and Raman spectroscopy were used to define the homogeneity, microstructure and to locally probe the structure of the samples S1-S3. Magnetic susceptibility results in the 2-300 K temperature regions were analysed by applying the first-order perturbation theory. The mean energy gap between ground and excited crystal field levels ( E-i), and their effective magnetic numbers M-ieff, were determined. The analysis of the paramagnetic temperature shows the absence of clusterization of the magnetic ions

Characterization of partially inverse spinel ZnFe2O4 with high saturation magnetization synthesized via soft mechanochemically assisted route

ZnFe2O4 was prepared by a soft mechanochemical route from two starting combinations of powders: (1) Zn(OH)(2)/alpha-Fe2O3 and (2) Zn(OH)(2)/Fe(OH)(3) mixed in a planetary ball mill. The mechanochemical treatment provoked reaction leading to the formation of the ZnFe2O4 spinel phase that was monitored by XRD, TEM, IR and Raman spectroscopy. The spinel phase was first observed after 4 h of milling and its formation was completed after 18 h in both the cases of starting precursors. The synthesized ZnFe2O4 has a nanocrystalline structure with a crystallite size of about 20.3 and 17.6 nm, for the cases (1) and (2), respectively. In the far-infrared reflectivity spectra are seen four active modes. Raman spectra suggest an existence of mixed spinel structure in the obtained nanosamples. In order to confirm phase formation and cation arrangement, Mossbauer measurements were done. Estimated degree of inversion is about 0.58 for both starting mixtures. The magnetic properties of the prepared ZnFe2O4 powders were also studied. The results show that the samples have a typical superparamagnetic-like behavior at room temperature. Higher values of magnetization in the case of samples obtained with starting mixture (2) suggest somewhat higher degree of cation inversion. (C) 2014 Elsevier Ltd. All rights reserved

Spectroscopy investigation of nanostructured nickel-zinc ferrite obtained by mechanochemical synthesis

Nano crystalline samples of nickel zinc ferrite, Ni0.5Zn0.5Fe2O4 were prepared by mechanochemical route in a planetary ball mill starting from two mixtures of the appropriate quantities of the powders: case (1) oxide powders: NiO, ZnO and alpha-Fe2O3 in one case, and in the second case (2) hydroxide powders: Ni(OH)(2), Zn(OH)(2) and Fe(OH)(3). In order to monitor the progress of chemical reaction and confirm phase formation, powder samples obtained after 5 h and 10 h of milling were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman, IR and Mossbauer spectroscopy. It is shown that the soft mechanochemical method, i.e. mechanochemical activation of hydroxides, produces high quality single phase Ni0.5Zn0.5Fe2O4 samples in much more efficient way. From the IR spectroscopy of single phase samples it is obvious that energy of modes depends on the ratio of cations. The deconvolution of Raman spectra allows to separate contributions of different cations to a particular type of vibration and to estimate the degree of inversion. (C) 2014 Elsevier Ltd. All rights reserved