Oxygen transport properties of dense and porous (La0.8Sr0.2)0.99Co0.8Ni0.2O3-δ

We have determined k_ex and D_chem for (La_0.8Sr_0.2)_0.99Co_0.8Ni_0.2O_3-δ by the use of electrical conductivity relaxation on a dense sample and by applying the ALS model to measured AC impedance spectrum on a porous electrode. Extracting k_ex and D_chem from the methods resulted in comparable values. k_ex and D_chem also agreed well with literature values on La_0.8Sr_0.2CoO_3-δ, indicating that nickel substitution does not change the oxygen transport properties. k_ex of the porous sample was further found to decrease with a five times higher rate than D_chem when measured by using an Electrochemical Impedance Spectroscopy (EIS) over several days.     

Magnetic and electrical properties of (Ni0.8Cu0.2)1−x Cox Mn0.02Fe1.9O4 ferrites

The effect of Co substitution on the magnetic and electrical properties of iron-deficient nickel–copper mixed ferrites containing a small quantity of manganese oxide was investigated. The presence of Co enhances the specific magnetization although the saturation magnetization falls a little due to decrement of density. The initial permeability changes linearly with the average grain size of the materials and shows fairly good thermal stability for higher Co concentration. An appreciable increment in DC resistivity along with decrement in dielectric loss factor at 100 MHz can also be obtained for higher Co concentration.     

Mechanical properties of La0.58Sr0.4Co0.2Fe0.8O3-δ membranes

In this paper, we report on the mechanical properties of a La_0.58Sr_0.4Co_0.2Fe_0.8O_3-δ perovskite material. We use ring-on-ring bending tests with disk-shaped samples and depth-sensitive micro-indentation. In particular, the temperature dependency of fracture stress and elastic behavior are addressed. The fracture load is measured to be ～ 40% higher at room temperature (RT) than at 800 °C, which is due to the ferro-elasticity of material at RT. The stiffness shows an increase of about 50% above 600 °C and 700 °C in vacuum and air, respectively. The effect is attributed to a rhombohedral to cubic phase transition, which is not fully reversible upon cooling. The changes in phase composition with temperature are also confirmed by in-situ high temperature XRD. The transition appears to be associated with a change of heat capacity.     

Location and associated hyperfine properties of μ+ in La2CuO4

The location of the positive muon used as a probe in highT _c systems is investigated using the unrestricted Hartree-Fock cluster procedure. Our calculations indicate that ⁺ is located in thea–c plane at a distance of 1.08 Å from the apical oxygen at a ⁺-O(a)-Cu angle of 25°. The hyperfine field at this site is also calculated. Our results show the importance of including the local contact and dipolar contributions to the hyperfine field which arise from the unpaired electron spin distribution in the vicinity of the muon.     

Rate limiting steps of the porous La0.6Sr0.4Co0.8Fe0.2O3−δ electrode material

The electrode reaction of porous La_0.6Sr_0.4Co_0.8Fe_0.2O_3?δ films deposited onto Ce_0.9Gd_0.1O_1.95 (CGO) was investigated by impedance spectroscopy within the temperature and oxygen partial pressure (pO₂) ranges of 500 ≤ T ≤ 700 °C and 10^? 4 < pO₂ < 1 atm, respectively, using Ar and He as gas carriers. The electrochemical impedance spectroscopy (EIS) measurements reveal a high frequency (HF) and a low frequency (LF) regions in the Nyquist plane. The high frequency (HF) region was fitted with a Warburg-type impedance element, and the low frequency (LF) region was reproduced with a resistance in parallel to a constant phase element. Both, the slight dependence of the polarization resistance (R_W) and the small variation of the apex frequency (f_v) of the HF Warburg-type element, on pO₂, suggest that this contribution corresponds to the oxygen diffusion in the bulk of the La_0.6Sr_0.4Co_0.8Fe_0.2O_3?δ electrode material. The variation of the polarization resistance of the LF region (R_rcpe) with pO₂ indicates that as T increases, the limiting step evolves from dissociative oxygen adsorption to oxygen gas diffusion in the pores of the mixed ionic/electronic conductor (MIEC) electrode.     

Effects of Zn on magnetic properties and pseudogap of optimally doped La2−xSrxCuO4

The effects of Zn substitution on the uniform (q = 0) magnetic susceptibility, χ(T), of optimally doped (x = 0.15) La_2?xSr_xCu_1?yZn_yO₄ sintered samples were investigated over a wide range of Zn contents (y). Non-magnetic Zn was found to enhance χ(T) systematically and depress T_c very effectively. We have extracted the characteristic pseudogap energy scale, ε_g, from the analysis of χ(T) data. Unlike T_c, ε_g was found to be fairly insensitive to the level of Zn substitution. This supports the scenario where the pseudogap phenomenon has non-superconducting origin. We have also analyzed the Zn-induced Curie-like enhancement of the χ(T) data using different models and discussed the various possible implications.     

Synthesis and optical properties of Co2+ and Ni2+ ions doped β-BaB2O4 nanopowders

Co²⁺ and Ni²⁺ ions doped β-BaB₂O₄ nanopowders have been prepared by co-precipitation method and their structural properties are studied by spectroscopic techniques. Powder XRD data reveals that the crystal structure belongs to monoclinic and the average crystallite size is calculated. Optical absorption spectra data reveal octahedral site symmetry for Co²⁺ and Ni²⁺ ions. Crystal field (Dq) and inter-electron repulsion (B and C) parameters are evaluated for Co²⁺ doped β-BaB₂O₄ nanopowders as Dq=960, B=900 and C=3850 cm^?1 and for Ni²⁺ doped β-BaB₂O₄ nanopowders, Dq=900, B=850 and C=3500 cm^?1. FT-IR spectra showed the characteristic vibrational bands related to BO₃ and BO₄ molecules. Photoluminescence spectra contain the emission bands in ultraviolet and blue regions.     

La0.8Sr0.2Ni0.4Fe0.6O3–Ce0.8Gd0.2O2–δ Nanocomposite as Mixed Ionic–Electronic Conducting Material for SOFC Cathode and Oxygen Permeable Membranes: Synthesis and Properties

Mixed ionic–electronic conducting nanocomposite La_0.8Sr_0.2Ni_0.4Fe_0.6O₃ (LSNF)–Ce_0.8Gd_0.2O_{2– δ} (GDC) was prepared via ultrasonic dispersion of nanocrystalline powders of perovskite and fluorite oxides in water with addition of surfactant, followed by drying and sintering up to 1300°C. Analysis of the real structure of nanocomposite (studied by XRD and TEM with EDX) and its surface composition (studied by XPS) revealed moderate redistribution of elements between phases favoring their epitaxy. Results of impedance spectroscopy, oxygen isotope exchange, O₂ TPD and H₂ TPR experiments revealed a positive effect of composite interfaces on the oxygen mobility and reactivity agreeing with the ambipolar transport behavior of MIEC composite. Preliminary testing of button-size cell with functionally graded LSNF–GDC cathode layer supported on thin YSZ layer covering Ni/YSZ cermet demonstrated high and stable performance, which is promising for its practical application.     

Microstructure and electrode properties of La0.6Sr0.4Co0.2Fe0.8O3-δ spin-coated on Ce0.8Sm0.2O2?δ electrolyte

Fine and uniform La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ powder was synthesized by a glycine–nitrate combustion process. La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ electrodes were prepared on dense Ce_0.8Sm_0.2O_2−δ electrolyte substrates using a spin-coating technique by sintering at 900–1,000 °C. The electrode properties of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ were investigated by electrochemical impedance spectroscopy and chronopotentiometry techniques with respect to preparation conditions and the resulting microstructures. The results indicate a significant effect of the microstructure on the electrode processes and polarization characteristics. The oxygen adsorption and dissociation process acted as a larger contribution to the overall electrode polarization R _p in magnitude compared with the charge transfer process due to relatively low porosity levels of the electrodes. It was detected that the grain size of the electrodes exhibited a crucial role on the electrocatalytic reactivity. At 800 °C, the electrode sintered at 950 °C attained a polarization resistance of 0.18 Ω cm², an overpotential of 27 mV at a current density of 200 mA cm⁻², and an exchange current density of 308 mA cm⁻².     

Magnetic and magneto-transport properties of (Ba0.8Sr0.2)2−xNdxFeMoO6

The variation in structural, magnetic and magneto-transport properties of the double perovskite system (Ba_0.8Sr_0.2)_2?xNd_xFeMoO₆ {0.0<X<0.5} induced by Nd³⁺ doping (electron doping) has been studied and compared. The samples were prepared by standard solid state reaction method in a reducing atmosphere. The parent compound showed a saturation magnetic moment value of 3.75 μ_B/f.u. at an applied field of 0.5 T and a change in magnetoresistance value up to 26% (77 K, 0.8 T). The Rietveld refinement of the X-ray diffraction data showed a continuous decrease in lattice parameters and Fe–Mo ordering with increasing Nd³⁺ doping. The Curie temperature was found to increase with Nd³⁺ doping (3 K per % of Nd) while the saturation magnetic moment values and magnetoresistance values were found to decrease. The observed variations in magnetic and magneto-transport properties of the system are explained on the basis of increasing antisite disorder defects and band filling effects induced by electron doping. We have observed the dominant role of band filling in determining the low field magnetoresistance of these systems.     

Phenomenological modeling of magnetic and magnetocaloric properties in rare earth doped La0.8Ca0.2MnO3

La_0.8Ca_0.2MnO₃ manganese oxides have been prepared by the conventional solid-state reaction. The samples crystallize in the orthorhombic structure with Pnma space group. A first-order magnetic phase transition from ferromagnetic to paramagnetic state is observed at the Curie temperature that is found to be around 241?K. The magnetic and magnetocaloric properties have been simulated using a phenomenological model. The maximum of magnetic entropy change is of order 8.2?J/kg?K for an applied magnetic field of 5?T, and the adiabatic temperature change is found to be of 4?K under a same magnetic field; this high value gives the sample the possibility of technologic application in the magnetic refrigeration area. The simulated results of magnetization and magnetic entropy change are compared with the experimental measurements. Finely, we found a good agreement between experimental and theoretical results.     

Oxygen ionic conductivity of La2NiO4+δ via interstitial oxygen defect

The ionic conduction properties of La₂NiO_4+δ were studied from oxygen permeation flux and defect-related transport properties. The effects of the applied oxygen chemical potential gradient and temperature on the oxygen permeability of La₂NiO_4+δ at various thickness are reported. The thermally activated oxygen permeation flux increased monotonically with increasing oxygen chemical potential gradient, yielding a maximum of 0.15 cc min^?1 cm^?2 under air/N₂ conditions for the 0.95 mm-thick La₂NiO_4+δ specimen at 900 °C. The oxygen ion conductivity of La₂NiO_4+δ was calculated as a function of temperature and oxygen partial pressure by differentiating the chemical diffusion equation for the oxygen permeation flux based on the dominant electronic transference number. In addition, the oxygen ion conductivity was extracted successfully by solving the Nernst–Einstein equation combining with the calculated self-diffusion coefficient of oxygen from the chemical diffusivity and thermodynamic enhancement factor from the equilibrium oxygen nonstoichoimetry of a La₂NiO_4+δ specimen, and a deviation of the OPP dependence of 1/6 power was observed.     

Synthesis and structure of Mg(Fe0.8Ga0.2)2O4−δ film materials

Features of the preparation of magnetic semiconductor films with the composition Mg(Fe_0.8Ga_0.2)₂O_4?δ on a 200-nm-thick silicon substrate are presented. Both a pure substrate surface and one with protective TiO _x layers with a crystallization temperature of 900–1000°C for 30 min were used. The effect of the protective layers on the formation of the surface morphology of the films and their magnetic properties is shown.     

Influence of γ-radiation on photoluminescence properties of YPO4:Eu3+,Ce3+ and YPO4:Dy3+,Ce3+ phosphors

Hexagonal YPO₄ phosphors doped with Eu³⁺/Dy³⁺ and co-doped with Ce³⁺ were synthesized by a hydrothermal route assisted using lauric acid as a capping agent. The prepared phosphors were characterized by transmission electron microscopy, infrared spectroscopy, powder X-ray diffraction and photoluminescence spectra. YPO₄: Eu³⁺ gives two red emission peaks at 587 and 610?nm corresponding to ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions, respectively. YPO₄: Dy³⁺ exhibits two emission peaks at 485?nm (blue) and 575?nm (yellow) corresponding to ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transitions, respectively. Ce³⁺ ions enhanced the emission intensity as a co-dopant in both phosphors. Moreover, the effect of γ-radiation in the dose range 5–300?kGy on the photoluminescence behaviour of YPO₄:Eu³⁺,Ce³⁺ and YPO₄:Dy³⁺,Ce³⁺ was also investigated. Quenching of emission intensity, after irradiation at 5 and 300?kGy, was observed in both the phosphors due to loss of excess energy through a non-radiative relaxation process.     

Effect of synthetic methods on electrochemical performances of VOPO4·2H2O supercapacitor

Ionics - In this paper, we studied the VOPO4·2H2O as an electrode material for supercapacitors. A systematic comparison of VOPO4·2H2O prepared by reflux and hydrothermal methods was...     

Electrical and thermoluminescence properties of γ-irradiated La2CuO4 crystals

Abstract

Measurements of the electrical properties of unirradiated as well as γ-irradiated La₂CuO₄ crystals were carried out at different temperatures in the frequency range of 0.1-100 kHz. Thermoluminescence (TL) studies were also performed on such crystals in the temperature range of 300-600K. The conductivity of the unirradiated La₂CuO₄ crystals were found to obey the power law frequency dependence at each measured temperature below the transition temperature (T_c = 450K). The activation energies for conduction and dielectric relaxation time have been calculated. The TL response and the dc resistance were found to increase with γ-irradiation dose up to 9-10 kGy. The results showed that the ferroelastic domain walls of La₂CuO₄ crystal as well as its TL traps are sensitive to γ-raditaion. This material can be used in radiation measurements in the range 225 Gy-10 kGy.     

Crystal structure and properties of FeSr2NdCu2O7+δ

The compound FeSr₂NdCu₂O₇₊ (Fe1212) was successfully synthesized by solid-state reaction. X-ray diffraction data indicate that the sample is of single phase. Rietveld whole-pattern-fitting method was used to refine the crystal structure of samples prepared under different annealing conditions using the tetragonal system with space group P4/mmm. Magnetization measurements indicate that magnetic susceptibility changes with temperature in a Curie-type manner. Resistance measurements indicate that these samples have a semiconductor-like behavior. PACS 61.10.Nz; 74.72.Jt     

Preparation of Hg0.8Cr0.2Ba2CuO4+δ under controlled oxygen and mercury partial pressures

Ba₂CuO₃/HgBa₂CuO_4+δ and Mn₃O₄/Mn₂O₃ couples were employed to control the mercury and oxygen partial pressures, respectively, during the synthesis of chromium doped Hg-1201 phase in the sealed quartz tube. Suitably adjusted conditions allowed us to prepare series of single phase samples of the final composition Hg_0.7Cr_0.2Ba₂CuO_4+δ determined by weight changes and EMA. The oxygen stoichiometry was modified by the oxygen partial pressure in the range of pO₂=0.05–1.59 atm. The X-ray diffraction and magnetic susceptibility study revealed linear dependence of the structural parameters and parabolic one of T_c vs. log pO₂. Multiphase samples, however, form for too high oxygen activity, namely for pO₂=5.16 atm.