Papers by Keyword: Spinel Structure

Abstract: The effect of x mole ratio on crystal structure and characteristic of microwave absorption of ZnLa_xFe_(2-x)O₄ system (x = 0.0; 0.01and 0.02) synthesized by solid state reaction method has been studied. The series of ZnLa_xFe_(2-x)O₄ samples were prepared using ZnO (99.99%) and Fe₂O₃ (99.99 %) powders (Merck product), while La₂O₃ (local production) powders in mole ratio. The identification result of the XRD shows that all of samples are single phase in this stage, it has cubic spinel structure with space group F d-3 m. The SEM image of ZnLa_xFe_(2-x)O₄ samples appear that the increase of mole ratio, the particle size of the compound powder rapidly becomes bigger, homogeneous and not uniform powder with spherical in shape and particle size of 200-500 nm. The results of the VNA characterization shows that the increasing of mole ratio (x = 0.0; 0.01 and 0.02) will enhance the ability to absorb microwave from 90.35% upto 97.69%. Thus. the composition of x=0.02 (ZnLa_0.02Fe_1.98O₄) possess to be the best composition for microwave absorbing material.

Abstract: CoMn₂O₄ nanoparticles were synthesized by reverse micelle processing from the mixed precursor (consisting of Co (NO₃)₂ ·6H₂O and MnCl₂·4H₂O). The CoMn₂O₄ was prepared by mixing the aqueous solution at a molar ratio of Co : Mn = 1 : 2. The synthesized powders were calcined at 600°C for 2h. The average size and distribution of synthesized powders were in the range of 10-20nm and narrow, respectively. The average size of the synthesized powders increased with increasing water to surfactant molar ratio. The XRD diffraction patterns show that the phase of CoMn₂O₄ was spinel (JCPDS no.77-0471). The synthesized and calcined powders were characterized by thermogravimetry-differential scanning calorimeter (TG-DSC), X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM). The magnetic property of the powder was measured by Vibrating Sample Magneto-meter (VSM) at 298K. The effect of synthesis parameter, such as the molar ratio of water to surfactant, is discussed.

Abstract: The study of ceramic materials has attracted the attention of many researchers due to the possibility of their use in nanotechnology. The spinel ferrites form a large group of materials with a broad range of applications. Some examples include electronic devices such as high-frequency transformer cores, antenna rods, induction-tuners, among many others. However, when the ferritic materials display superparamagnetic behavior, their potential for biological applications like drug delivery, hyperthermia, resonance magnetic imaging and magnetic separation, become amazingly high. Therefore, the superparamagnetism is a characteristic strongly desired for spinel ferrites. Since this phenomenon is size-dependent, the methodologies to synthesize these materials has emerged as a crucial step in order to obtain the desired properties. In this regarding, several synthetic processes have been developed. For example, co-precipitation is a fast and cheap method to synthesize superparamagnetic spinel ferrites. However, methodologies involving microwave, ultrasound or polymers frequently result in these kind of materials. Therefore, this review brings a brief historic introduction about spinel ferrites as well as essential concepts to understand their structure and magnetic properties. In addition to this, recent advances in synthesis and applications of the superparamagnetic spinel ferrites are mentioned. Contents of Paper

Abstract: The polycrystalline ferrites of MgAlxFe2-xO4 (0.0≤x≤0.4) were prepared by the conventional solid state ceramic method. The specimens were sintered at 13500C and X-ray diffraction experiments were done at room temperature which showed single phased cubic spinel structure. The lattice parameters were determined from the XRD data using Nelson-Riley extrapolation method and found to decrease with increasing Al concentration obeying Vegard’s law. The cation distribution and oxygen position parameters have also been determined by refining the data using the RIETAN-2000 in the Rietveld method which reveals that the samples possess cubic symmetry corresponding to the space group Fd-3m. The X-ray density and bulk density of each sample were calculated using the lattice parameters. The porosity has been determined from X-ray density (ρx) and bulk density (ρB) and it changes monotonically with Al content.

Abstract: Three types of ferromagnetic nanostructures based on barium hexaferrite (BaFe₁₂O₁₉), lanthanum-strontium manganites with perovskite structure ((La,Sr)MnO₃) and materials with spinel structure (AFe₂O₄, A = Ni, Zn, Co, Mn, Fe) have been synthesized by precipitation from aqueous and nonaqueous solutions, by the sol-gel method and from microemulsions. Magnetic properties of the synthesized nanoparticles and films have been investigated. It was shown that the obtained nanoparticles exhibit superparamagnetic properties. It has been found that the synthesized nanoparticles have promise in hyperthermia of cancer cells. It has been shown that the films based on barium hexaferrite can have promise in the creation of nonlinear resonant microwave elements on the basis of high-Q dielectric resonators and ferromagnetic films.

Abstract: Three synthetic and stoichiometric 2:3 type spinels (Fd3m symmetry): MgAl2O4, MgFe2O4, NiAl2O4 with a different initial structural order in cation sublattice were investigated. Investigations by means of high temperature XRPD method at temperatures ranging from 25°C to 1100°C were carried out. Diffraction patterns at each temperature in isothermal conditions were measured. For each tested spinel changes in several temperature-dependent parameters were measured: oxygen positional parameter u(T), cell edge a0(T), cation site occupancies Occ(T) in tetrahedral and octahedral positions and a cation-anion distance in tetrahedral TO(T) and octahedral MO(T) positions. Temperatures of initial order-disorder transformation were determined on the basis of the course of these dependences. Basing on changes of the cell edge a0(T), the values of thermal expansion coefficient and for spinel before and after the beginning of order-disorder transformation, respectively, were calculated. The values of measured temperature-dependent parameters were used to calculate the degree of inversion x in the spinel structure defined as the number of 3+ cations in tetrahedral sites. In each case two methods of calculating the degree of inversion from experimental data were applied. The first method involved observing the changes in sites occupancy in the cation sublattice versus temperature, which resulted in a change of diffraction lines intensities. The second method was based on observing the changes in cation-anion distances in tetra- and octahedral coordination versus temperature. The results obtained by both methods were compared, discussed and advantages and disadvantages of each of them were presented. It was shown that when atomic scattering factors of cations 2+ and 3+ in the spinel structure differ significantly, the most precise method is the one based on changes in sites occupancies versus temperature. The method based on calculation of changes in cation-anion distances is recommended when atomic scattering factors of cations differ slightly but oxygen positional parameter and cation-anion distances changes significantly during order-disorder transformation like in normal spinel structure when effective ionic radii of 2+ and 3+ cations differ significantly.

Abstract: CoCr_xFe_2-xO₄ samples with x = 0.0 to 2.0 were prepared by air oxidation of aqueous suspension containing Co²⁺, Cr³⁺ and Fe³⁺ ions and characterized by x-ray diffraction. Positron lifetime and coincidence Doppler broadening (CDB) measurements indicated three distinct stages of positron trapping in defects. Initially the vacancy-type defects located at the A-sites (tetrahedral) trapped positrons but, with the substitution of Fe by Cr in low concentrations (x <= 0.7), positrons are trapped by defects at the B-sites (octahedral). Mossbauer spectroscopic results indicated the cationic distribution at B-sites to be stoichiometry-dependent and, till x = 0.7, the deficiency of Fe³⁺(B) ions was compensated by interchange of Fe³⁺(A) ions with Co²⁺(B) ions. Between x = 0.9 and 1.7, the substitution resulted in continued decrease of Fe³⁺(B) ions and the structure got fully transformed into a normal spinel configuration during x = 1.8 to 2.0.

Abstract: We report in this work about the inversion of the spinel structure of ZnFe₂O₄ induced by the substitution of Zn²⁺ by Ni²⁺ ions. Positron lifetimes were measured in Zn_1-xNi_xFe₂O₄ with different concentrations (x) of doped Ni²⁺ ions and a drastic change across x = 0.4 – 0.6 was observed, which is attributed to this transformation. The interchange of positions of the cations on doping leaves a fraction of them unoccupied and these vacancies act as positron trapping centres. Since Ni²⁺ is smaller in size than Zn²⁺, defects due to non-stoichiometry are less in NiFe₂O₄ than in ZnFe₂O₄. The increase in positron lifetime implies the trapping of positrons being shifted from A- to B-sites and is an indication of the transformation from inverse to normal spinel configuration. Coincidence Doppler broadening measurements supported these findings.

Abstract: Spinel Zn₂SnO₄ photocatalysts have been prepared by the solid state reaction. as-prepared samples were characterized by power X-ray diffraction, scanning electron microscope, UV-vis diffuse reflectance spectroscopy and photocatalytic activity measurement. The results show that the reaction temperature have significant effects on the Zn₂SnO₄ photocatalytic activity . It has also been found that the distribution of the cations in Zn₂SnO₄ crystal lattice changes with the increase of calcination temperature, causing the local structural fine adjustment that associated with the photocatalytic properties.

Abstract: P- and N-type thermoelectric iron oxides were developed. The p- and n-type thermoelectric iron oxides were based on delafossite-type CuFeO2 and spinel-type Fe3O4, respectively. The dimensionless figure of merit, ZT, of the bulk p- and n-type iron oxides were 0.15 and 0.10 at 1200K, respectively. The ZT values were improved by the introduction of nano-voids. The physical properties of these iron oxides are structurally unique because of the triangular, or “Kagome,” arrangement of FeO6 octahedra. The delafossite-type CuFeO2 becomes anti-ferromagnet at temperatures less than 20K. The inverse spinel-type Fe3O4 is a ferrimagnet　at room temperature. In both crystals, the iron ions are assumed to be in the high-spin state.