The thermal stability and structural site-distribution of Eu3+ ions in the red-emitting phosphors Ca9Eu2W4O24 and Sr9Eu2W4O24

The red-emitting phosphors Ca₉Eu₂W₄O₂₄ and Sr₉Eu₂W₄O₂₄ were synthesized by the solid-state reaction method. The crystal phases were characterized by X-ray powder diffraction. The photoluminescence excitation and emission spectra were investigated. The luminescence excitation and emission spectra confirm that the phosphors are efficiently excited by near UV light. The dependence of luminescence intensities on the heating temperatures was investigated. The Ca₉Eu₂W₄O₂₄ phosphor exhibits higher thermal stability than that of Sr₉Eu₂W₄O₂₄. The crystallographic sites for Eu³⁺ ions in Ca₉Eu₂W₄O₂₄ and Sr₉Eu₂W₄O₂₄ are investigated by the site-selective excitation spectra in the ⁵D₀ → ⁷F₀ wavelength region. It is identified that the Eu³⁺ ions occupy only M sites (statistically occupied by 0.5Eu and 0.5Ca) in Ca₉Eu₂W₄O₂₄ and, however, the Eu³⁺ ions can substitute both M sites (Eu³⁺ + Sr²⁺) and Sr²⁺ sites in Sr₉Eu₂W₄O₂₄. The luminescence spectra and the thermal stability are discussed on the basis of the crystal structure, Eu³⁺ site-distributions and the energy transfer.

Carbon coated Na7Fe7(PO4)6F3: A novel intercalation cathode for sodium-ion batteries

Electrode materials are being developed to realise sodium-ion batteries that can provide energy storage solutions. Here, we develop amorphous carbon coated Na₇Fe₇(PO₄)₆F₃, prepared by combining hydrothermal and solid state reaction methods, as an insertion electrode for sodium-ion batteries applications. Na₇Fe₇(PO₄)₆F₃ particles are surrounded by a thin layer (∼1.5–2 nm) of amorphous carbon. The Na₇Fe₇(PO₄)₆F₃/C composite cathode undergoes reversible sodium intercalation/de-intercalation with an average operational potential of ∼3.0 V (vs Na⁺/Na). This cathode has a capacity of 65 mA h g⁻¹ at 100 mA g⁻¹ current after 60 cycles and features twice higher capacity than that of an uncoated Na₇Fe₇(PO₄)₆F₃ sample. Therefore, the carbon-coated Na₇Fe₇(PO₄)₆F₃ composite presents feasible sodium intercalation/de-intercalation capacity, offering possibilities for developing a low cost, high performance sodium-ion battery positive electrode.

Lithium ferrite (Li0.5Fe2.5O4) nanoparticles as anodes for lithium ion batteries

Li0.5Fe2.5O4 nanoparticles of about 80 nm were synthesized through a hydrothermal method, followed by a solid state reaction between LiOH·H2O and Fe2O3. The Li0.5Fe2.5O4 nanoparticles exhibit a remarkable high capacity (up to 1124 mA h g-1), a good cycle stability (650 mA h g-1 after 50 cycles) and excellent coulombic efficiency. © 2014 the Partner Organisations.

Synthesis of Al-doped Mg2Si1-xSnx compound using magnesium alloy for thermoelectric application

Abstract Mg2Si1-xSnx thermoelectric compounds were synthesized through a solid-state reaction at 700 °C between chips of Mg2Sn-Mg eutectic alloy and silicon fine powders. The Al dopants were introduced by employing AZ31 magnesium alloy that contains aluminum. The as-synthesized Mg2Si1-xSnx powders were consolidated by spark plasma sintering at 650-700 °C. X-ray diffraction and scanning electron microscopy revealed that the Mg2Si1-xSnx bulk materials were comprised of Si-rich and Sn-rich phases. Due to the complex microstructures, the electrical conductivities of Mg2Si1-xSnx are lower than Mg2Si. As a result, the average power factor of Al0.05Mg2Si0.73Sn0.27 is about 1.5 × 10^-3 W/mK² from room temperature to 850 K, being less than 2.5 × 10^-3 W/mK² for Al0.05Mg2Si. However, the thermal conductivity of Mg2Si1-xSnx was reduced significantly as compared to Al0.05Mg2Si, which enabled the ZT of Al0.05Mg2Si0.73Sn0.27 to be superior to Al0.05Mg2Si. Lastly, the electric power generation from one leg of Al0.05Mg2Si and Al0.05Mg2Si0.73Sn0.27 were evaluated on a newly developed instrument, with the peak output power of 15-20 mW at 300 °C hot-side temperature.

Dielectric and impedance properties of Nd3/2Bi3/2Fe5O12 ceramics

The polycrystalline sample of Nd3/2Bi3/2Fe5O12 was prepared by a high- temperature solid-state reaction technique. Preliminary X-ray structural analysis exhibits the formation of a single-phase tetragonal structure at room temperature. Microstructural analysis by scanning electron microscopy shows that the sintered sample has well defined grains. These grains are distributed uniformly throughout the surface of the sample. Detailed studies of dielectric response at various frequencies and temperatures exhibit a dielectric anomaly at 400 A degrees C. The electrical properties (impedance, modulus and conductivity) of the material were studied using a complex impedance spectroscopy technique. These studies reveal a significant contribution of grain and grain boundary effects in the material. The frequency dependent plots of modulus and the impedance loss show that the conductivity relaxation is of non-Debye type. Studies of electrical conductivity with temperature demonstrate that the compound exhibits Arrhenius-type of electrical conductivity. Study of ac conductivity with frequency suggests that the material obeys Jonscher's universal power law.

IMPEDANCE CHARACTERISTICS of La3/2Bi3/2Fe5O12 CERAMICS

Polycrystalline La3/2Bi3/2Fe5O12 (LBIO) compound was prepared by a high-temperature solid-state reaction technique. The complex impedance of LBIO was measured over a wide temperature (i.e., room temperature to 500 C) and frequencies (i.e., 10(2)-10(6) Hz) ranges. This study takes advantage of plotting ac data simultaneously in the form of impedance and modulus spectroscopic plots and obey non-Debye type of relaxation process. The Nyquist's plot showed the presence of grain effects in the material at high temperature. The ac conductivity spectrum was found to obey Jonscher's universal power law. The dc conductivity was found to increase with rise in temperature. The activation energy of the compound was found to be 0.24 and 0.51 eV in the low and high-temperature region, respectively, for conduction process.

Structural and dielectric properties of relaxor Sr0.5Ba0.5Bi2Nb2O9 ceramic

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Characterization of the columbite precursor and (1-x)PMN-xPT powders prepared by Ti-modified columbite route

This study proposes to synthesize (1-x)PMN-xPT powders, where 0.10 < x < 0.45, using the T-modified columbite route. This methodology consists in the preparation of the MNT columbite precursor via the polymeric precursor method, followed by the solid state reaction with PbO to get the PMN-PT powders. It was verified that from 15 mol% of Ti, the MNT presents the coexistence of two main phases with different crystal symmetry: Rutile and Columbite. However, the synthesis of (1-x)PMN-xPT powders is not affected by this event. A detailed study of structural effects in MNT and PMN-PT powders as function of Ti content was made using the Rietveld method. It was also demonstrated that powders possess high chemical and microstructural homogeneity.

Microwave-hydrothermal synthesis of perovskite bismuth ferrite nanoparticles

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Schottky-type grain boundaries in CCTO ceramics

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Investigação de Er3+ nos dois sítios cristalográficos de Gd2SiO5 através da fotoluminescência resolvida no tempo

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Microstructural and electric properties of bismuth-layered structured ceramics - SrBi2(Ta1-xNbx)O-9

Ceramic samples of SrBi2(Nb1-xTax)O-9 (0 less than or equal to x less than or equal to 1) were prepared by the solid state reaction method in order to investigate their structural and electrical features as well as obtain useful information to improve the properties of SrBi2(Nb1-xTax)O-9 as a thin film. The X-ray diffraction patterns and the scanning electronic microscopy photomicrographs show no secondary phases but the formation of a solid-state solution for all the composition. The ac conductivity of the samples, measured at 25 degreesC and 100 kHz frequency, decreases with the increase of Ta content. Such results were explained by intrinsic conductivity of pure components.

Phase formation and characterization of BaBi2Ta2O9 obtained by mixed oxide procedure

Ferroelectric layefed-perovskite BaBi2Ta2O9 (BBT) has been prepared successfully by solid-state reaction. The influence of pressure and temperature/time annealing regime on the BBT phase formation was analyzed. The powders were characterized by thermal analysis and Xray diffraction and the sintered pellets by scanning electron microscopy. The crystalline BBT phase, free of secondary phases was obtained at 950 degreesC for 2 h. For an applied field strength of 380 kV/cm, a remnant polarization of 7.6 muC/cm(2) and an electric coercive field of 45.7 kV/cm were obtained. (C) 2004 Elsevier B.V. All rights reserved.

Mechanochemical synthesis of ceramic powders with perovskite structure

The mechanical activation is one of the most effective method for obtaining highly disperse system due to mechanical action stress fields form in solids during milling procedure. This effect results in changes of free energy, leading to release of heat, formation of a new surface, formation of different crystal lattice defects and initiation of solid-state chemical reaction. The accumulated deformation energy determines irreversible changes of crystal structure and consequently microstructure resulting in the change of their properties. Mechanochemical processing route has been developed recently for the production of intermetallic and alloy compounds. The intrinsic advantage of this process is that the solid-state reaction is activated due to mechanical energy instead of the temperature. It was shown that the chemical reactivity of starting materials could be improved significantly after mechanochemical activation and, subsequently, the calcination temperature was reduced. Besides, it was apparent that the mechanochemical treatment could enhance the reactivity of constituent oxides; however, the sintering process could not be avoided to develop the desired ceramics. A novel mechanochemical technique for synthesis of fine-grained perovskite structured powders has shown that it is possible to form perovskite at room temperature. The effect of milling on the formation of perovskite structure of barium titanate (BT), lead titanate (PT), PZT, PZN, magnesium niobate (PMN) and LM ceramic materials was analyzed. The dielectric properties of sintered ceramics are comparable with those prepared by other methods in the literature. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Sintering mechanisms of ZrO2 center dot MgO with addition of TiO2 and CuO

Substitutions of Ti and Cu in ZrO2.MgO (Z), cause transformation from monoclinic (m) to cubic (c) and tetragonal (t). According to the vacancy model and solid Solution formation models, neither CuO nor TiO2 cause zirconia stabilization, which derives front other phenomena. Data analysis by TMA using the CRH (constant rate of heating) method shows a solid state reaction of ZrO2.MgO.TiO2 (Z.TiO2) demonstrating a dominant mechanism of volume diffusion (n = 1). However, the sintering of ZrO2.MgO.CuO (Z.CuO) shows a viscous flow mechanism (n = 0), a similar phenomena to that of by sintering of glass. Transformations, such as: CuO to Cu2O at 1000 degreesC, ZrO2 (m) to ZrO2 (t) at 1100 degreesC and Cu2O (s) to Cu2O (l) at 1230 degreesC cause successive rearrangements of microstructure inside of region I (sintering process) and lead to interpretation errors when the Bannister equation is used. (C) 2003 Elsevier Ltd and Techna Group S.r.l. All rights reserved.