Properties of Cr-substituted M-type barium ferrites prepared by nitrate–citrate gel-autocombustion process

The Cr-substituted M-type barium hexaferrites, BaFe_12−xCr_xO₁₉, with x=0.0–0.8$x=0.0–0.8$ have been successfully prepared by nitrate–citrate auto-combustion process using citric acid as a fuel/reductant and nitrates as oxidants. The resulting precursors were calcined at 1100 °C for 1 h and followed by sintering at 1200 °C for 12 h in oxygen atmosphere. The ferrites were systematically investigated by using powder X-ray diffractometer (XRD), magnetic hysteresis recorder, Mössbauer spectrometer, and scanning electron microscope (SEM). The XRD data show the formation of pure magnetoplumbite phase without any other impurity phases. Both a and c lattice parameters calculated by the Rietveld method systematically decrease with increasing Cr content. The effects of Cr³⁺ ions on the barium ferrites were reported and discussed in detail. The site preference of Cr³⁺ and magnetic properties of the ferrites have been studied using Mössbauer spectra and hystereses. The results show that the magnetic properties are closely related to the distributions of Cr³⁺ ions on the five crystallographic sites. The saturation magnetization systematically decreases, however, the coercivity increases with Cr concentration. The magnetization and Mössbauer results indicate that the Cr³⁺ ions preferentially occupy the 2a, 12k, and 4f_VI sites. The average size of hexagonal platelets obtained by SEM photographs tends to decrease with respect to Cr content.     

Structural analysis of cobalt titanate nanoparticles obtained by sol–gel process

In this work we report the synthesis of nanocomposites based on nanoparticles of cobalt titanate and titanium dioxide in their anatase crystalline phase by a sol–gel process. The synthesized nanoparticles of titanate vary from 1 to 6 nm in size. They are embedded in the anatase matrix, and they were obtained from TiO₂ monoliths doped with Co²⁺. The formation of cobalt titanate nanoparticles showed a linear dependence on the cobalt concentration. The cobalt titanate nanocrystals are very stable even at temperatures higher than 1000 °C. The crystalline structures of the samples were examined using high-resolution transmission electron microscopy and X-ray diffraction. Molecular simulation methods were utilized for a better understanding and for improving the analytical data interpretation of the experimental results. PACS 61.16.Bg; 79.60.Jv; 61.46.+w; 61.50.Ah     

Magnetic behavior of MnCo2O4+δ spinel obtained by thermal decomposition of binary oxalates

Manganese cobaltites MnCo₂O_4.62 and MnCo₂O_4.275 having a spinel structure were studied by measuring magnetization, AC susceptibility and by XANES spectroscopy. These compounds were synthesized by decomposition of the binary oxalate Mn_1/3Co_2/3C₂O₄·2H₂O in air at 220 and 500 °C, respectively. It was found that the differences in magnetic characteristics of these cation-deficient spinels are due mainly to variations in the degree of oxidation of manganese. It was shown that the complex oxide MnCo₂O_4.62 formed right after decomposition of the binary oxalate contains about 5×10⁻⁴ mass% metallic cobalt, which determines the dependence of magnetic susceptibility χ on the magnetic field at 300 K. The magnetic transition peculiar to the stoichiometric spinel MnCo₂O₄ at 183 K decreases to 167.5 K for MnCo₂O_4.275 and 67.5 K for MnCo₂O_4.62.     

Electroluminescence properties of In-doped Zn2SiO4 thin films prepared by sol–gel process

The effect of In doping on the electroluminescence (EL) properties of Zn₂SiO₄:In thin films was investigated. In-doped Zn₂SiO₄ thin films were deposited on BaTiO₃ substrates and their EL properties were characterized in this study. X-ray powder diffraction patterns of In-doped Zn₂SiO₄ powders revealed a single phase of Zn₂SiO₄ for In concentrations up to approximately 1.5 mol%, whereas a secondary phase of In₂O₃ was observed for In concentrations in the range of 2–10 mol%. The maximum luminance of thin film electroluminescent (TFEL) devices varied significantly with the amount of In doping. The highest luminance with blue emission was obtained when 2 mol% In was doped. The blue emission of In-doped Zn₂SiO₄ thin film may be related to the In substitution for Zn. The 2 mol% In-doped Zn₂SiO₄ thin film exhibited blue emission with CIE color coordinates of x=0.208 and y=0.086.     

Magnetic characteristics of MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles obtained by glycine–nitrate synthesis

The magnetic properties of magnesium–iron spinel (MgFe₂O₄) powdered nanoparticles obtained by glycine–nitrate synthesis are investigated by X-ray phase analysis and the NMR method. According to the results of X-ray phase analysis, the average size of the crystalline part of nanoparticles of the powder under investigation is 45 ± 4 nm. Magnetization J is determined using the formula J = (B/μ0)–H, where B and H are the induction and strength of the magnetic field in the sample, which are measured by the NMR method. The magnetic characteristics of MgFe₂O₄ are as follows: specific saturation magnetization J_sat = 17.52 A m²/kg, specific residual magnetization J_r = 5.73 A m2/kg, coercive force H_c = 4600 A/m, and magnetic moment P_sat = 371 × 10^–20 A m² in the magnetic saturation state and P_r = 121 × 10^–20 A m² in the residual magnetization state.     

Novel behavior in the M–H and field cooled (FC) curves of the MnFe2O4 nanoparticles synthesized by the coprecipitation method

MnFe₂O₄ nanoparticles were prepared by a coprecipitation chemical method. The average size of the obtained nanoparticles was about 30 nm. The hysteresis measured at T=300 K clearly shows ferromagnetic order at room temperature while that measured at T=450 K shows superparamagnetic behavior. The difference in the magnetization curves in the field increasing cycle and field decreasing cycle at higher temperatures (450 K or higher) is very unusual. In this case, a hysteresis in magnetization in higher magnetic fields with an opening up of the magnetization curve was observed. In this work, the effect of temperature and time of application of the applied field on the magnetization behavior was studied.     

The effect of Fe substitution on the electrical and thermal conductivity and thermopower of Ca3(FexCo1−x)4O9 synthesised by a sol–gel process

The effect of Fe substitution for Co on direct current (DC) electrical and thermal conductivity and thermopower of Ca₃(Co_1−xFe_x)₄O₉ (x = 0, 0.05, 0.08), prepared by a sol–gel process, was investigated in the temperature range from 380 down to 5K. The results indicate that the substitution of Fe for Co results in an increase in thermopower and DC electrical resistivity and substantial (14.9–20.4% at 300K) decrease in lattice thermal conductivity. Experiments also indicated that the temperature dependence of electrical resistivity ρ for heavily substituted compounds Ca₃(Co_1−xFe_x)₄O₉ (x = 0.08) obeyed the relation lnρ ∝ T^−1/3 at low temperatures, T < ~55K, in agreement with Mott’s two-dimensional (2D) variable range hopping model. The enhancement of thermopower and electrical resistivity was mainly ascribed to a decrease in hole carrier concentration caused by Fe substitution, while the decrease of thermal conductivity can be explained as phonon scattering caused by the impurity. The thermoelectric performance of Ca₃Co₄O₉ was not improved in the temperature range investigated by Fe substitution largely due to great increase in electrical resistivity after Fe substitution.     

Optimizing after-glow luminescent parameters of Sr<Subscript>4</Subscript>Al<Subscript>14</Subscript>O<Subscript>25</Subscript>:Eu<Superscript>2+</Superscript>, Dy<Superscript>3+</Superscript>, by sol–gel method and combustion process

Strontium aluminates are viewed as a promising persistent luminescent materials. There are many researches on strontium aluminates, including SrAl₂O₄, Sr₄Al₁₄O₂₅. Between these two phases, Sr₄Al₁₄O₂₅ shows much better properties than SrAl₂O₄. The traditional way to synthesize Sr₄Al₁₄O₂₅ is the solid state reaction. However, it exists few problems, especially non-homogeneous product. As a result, there are two methods chosen to make homogeneous precursor. One is sol–gel method, the other is combustion with Urea as a fuel. Boric acid is added as a flux in both method. In this study, combustion process is found to be a better way for synthesizing Sr₄Al₁₄O₂₅. We change the temperature, synthetic method. The samples are finely grinded and used for XRD analysis, photoluminescence measurement, and after-glow decay curve to figure out the optimizing luminescent parameters.