Self-assembled 3D La(OH)3 and La2O3 nanostructures: Fast microwave synthesis and characterization

Self-assembled three-dimensional (3D) urchin-like and flower-like La(OH)₃ nanostructures were successfully prepared for the first time via a facile and fast microwave-assisted solution-phase chemical method in 15 min. The obtained products were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The SEM results reveal that the urchin-like and flower-like La(OH)₃ nanostructures are ca. 3 μm and 6 μm in diameter, respectively. The urchin-like La(OH)₃ nanostructures are constructed by nanorods with diameters of about 300 nm and lengths of about 500 nm. The flower-like La(OH)₃ nanostructures are built from nanopetals about 100 nm thick. The effects of reaction time, microwave power, amount of tetraethyl ammonium bromide (TEAB), and surfactants on the preparation were systematically investigated. The possible formation mechanism of the 3D La(OH)₃ nanostructures was preliminarily discussed. Urchin-like and flower-like La₂O₃ nanostructures were obtained after calcining the La(OH)₃ nanostructures at 800 °C for 4 h. Urchin-like and flower-like La₂O₃:Eu³⁺ nanostructures were also prepared and their photoluminescence (PL) properties were investigated.     

The impedance spectroscopic study and dielectric relaxation in A(Ni1/3Ta2/3)O3 [A=Ba,Ca and Sr]

We present the results of impedance spectroscopic study with its analytical interpretations in the framework of electric modulus formalism for Barium Nickel Tantalate Ba(Ni_1/3Ta_2/3)O₃ (BNT), Calcium Nickel Tantalate Ca(Ni_1/3Ta_2/3)O₃ (CNT) and Strontium Nickel Tantalate Sr(Ni_1/3Ta_2/3)O₃ (SNT) synthesized by the solid-state reaction technique. The results of powder X-ray diffraction study reveal that BNT and SNT crystallize in cubic structure with lattice parameter a=4.07 Å and 3.98 Å respectively, whereas CNT crystallizes in monoclinic structure having lattice parameters, a=5.71 Å, b=13.45 Å and c=5.47 Å with β=118.3°. The logarithmic angular frequency dependence of the real part of complex dielectric permittivity and loss tangent as a function of temperature indicate significant dielectric relaxation in the samples, which have been explained by the Debye theory. The frequency dependence of the loss peak and the imaginary part of electrical modulus are found to obey the Arrhenius law. The relaxation mechanism of these samples is modeled by the Cole–Cole equation. This confirms that the polarization mechanism in BNT, CNT and SNT is due to the bulk effect arising in semiconductive grains. The scaling behavior of imaginary part of electric modulus M″ suggests that the relaxation describes the same mechanism at various temperatures but relaxation frequency is strongly temperature dependent. The normalized peak positions of tan δ/tan δ_m and M″/M″_m versus log ω for BNT, CNT and SNT do not overlap completely and are very close to each other. These indicate the presence of both long-range and localized relaxation. Due to their high dielectric constant and low loss tangent, these materials may find several technological applications such as in capacitors, resonators, filters and integrated circuits.     

Dielectric relaxation in complex perovskite oxide BaCo1/2W1/2O3

Polycrystalline BaCo_1/2W_1/2O₃ (BCW) is prepared by the solid-state reaction technique. The X-ray diffraction study of the compound at room temperature reveals the monoclinic phase. The field dependence of the dielectric constant and the conductivity are measured in the frequency range from 50 Hz to1 MHz and in the temperature range from 300 to 413 K. An analysis of the real and imaginary parts of the dielectric permittivity with frequency is performed. The frequency-dependent maxima in the imaginary impedance are found to obey an Arrhenius law with an activation energy=0.86 eV. The frequency-dependent electrical data are also analysed in the framework of the conductivity and modulus formalisms.     

Decay dynamics of Eu/Tb in solution-processed Y2O3

As new synthesis techniques are developed to create phosphors with smaller particles the physical processes that govern luminescence behaviors, like efficiency and decay rates, are influenced by the large surface-to-volume ratios of the nano-structured materials. Decay of lanthanide excited states, following pulsed laser excitation, is used to probe the affect of size-induced decay channels on the luminescent properties. We report results on the decay dynamics of europium and terbium excited states in solution-produced yttrium oxide. Radiative decay from the Eu³⁺⁵D₀ state is found to be well described by a single exponential function over a range of concentrations (0.01-2.0%). The Tb³⁺⁵D₄ decay exhibits a distribution of decay rates over the concentration range 0.01-5.0%. The distribution of decay rates is discussed using a probability density function based on the Kohlrausch decay function.     

Structural, ferroelectric and impedance spectroscopy properties of Y modified Pb(Fe0.5Nb0.5)O3 ceramics

Some ceramic samples of Pb_1−xY_x(Fe_0.5Nb_0.5)_1−x/4O₃ (PYFN) (0.00≤x≤0.08) were synthesized by a mixed oxide route. X-ray diffraction patterns of all the above samples confirm the formation of single phase material crystallizing in monoclinic structure. Dielectric properties (ε_r and tan δ) were analyzed in a wide temperature (30-350 °C) and frequency range (100 Hz-1 MHz). Ferroelectric properties of these compounds were confirmed from polarization (P-E hysteresis loop) measurements at room temperature. All the room temperature hysteresis loops of PYFN ceramics were well simulated using the ferroelectric capacitor model. Yttrium substitution resulted in notable enlargement of room temperature remnant polarization (2P_r). The 2P_r of PYFN (x=0.02) reaches to a large value (23 μC/cm²), which is nearly 5 times greater than that of PFN ceramic (4.6 μC/cm²). All the compounds exhibits negative temperature coefficient of resistivity (NTCR) type behavior as that of semiconductors. Dc conductivity (estimated via bulk resistivity) variation with temperature of all the samples follows Arrhenius type of electrical conductivity.     

Influence of colloidal templates on the impedance spectroscopic behaviour of Pr0.7Sr0.3Fe0.8Ni0.2O3 for solid oxide fuel cell applications

The creation of porous materials with three-dimensional periodicity has been identified as being of potential interest for increasing the overall performance of solid oxide fuel cells (SOFC). In this work, we have investigated the formation of pore systems in the nanometer scale by replicating colloidal templates. Templating methods have been used to prepare iron-nickel-based perovskite Pr_0.7Sr_0.3Fe_0.8Ni_0.2O₃ material with nanoporous microstructure. Polymethyl methacrylate (PMMA), polystyrene (PS) and polycarboxylate (PC) microspheres with different diameters were used as pore formers. These samples were synthesized and characterized by thermogravimetric analysis, inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray diffraction, transmission electron microscopy and field emission scanning electron microscopy. The polarization resistance of the materials was studied by Electrochemical Impedance Spectroscopy. The study demonstrated that templated porosity is maintained and highly influences on the impedance spectroscopic behaviour, being the material synthesized with policarboxylate microspheres the most interesting of the three used templates for SOFC applications.     

Oxygen exchange kinetics of La0.58Sr0.4Co0.2Fe0.8O3 at 600 °C in dry and humid atmospheres

The time-dependent degradation of the oxygen exchange kinetics of the solid oxide fuel cell cathode material La_0.58Sr_0.4Co_0.2Fe_0.8O_3 − δ (LSCF) is investigated at 600 °C. Special emphasis is placed on systematic long-term dc-conductivity relaxation measurements (t > 1000 h) in dry as well as in humidified atmospheres in order to obtain representative trends for the application of LSCF in intermediate-temperature SOFCs. The determination of the chemical surface exchange coefficient k_chem of oxygen is combined with investigations of the elemental surface compositions and depth profiles of fresh and degraded samples by X-ray photoelectron spectroscopy (XPS), providing further insight into the mechanisms of degradation. The slow decrease of k_chem by a factor of 2 during exposure of the sample to a dry O₂-Ar reference atmosphere for 1000 h at 600 °C can be ascribed to an enrichment of La and Sr in correlation with an elevated oxygen concentration within about 30-35 nm depth. The interpretation of the XPS core level spectra indicates the formation of SrO and La₂O₃ secondary phases in this zone. The subsequent treatment in a humidified atmosphere for 1000 h results in a pronounced initial decrease of k_chem by an additional factor of 10, followed by a time dependent decay of about 15% kh^− 1. A Sr-rich silicate layer of about 10 nm thickness is identified by XPS as the major cause of the degradation in humidified atmosphere. The evidence of Si-poisoning over the whole sample surface could also be confirmed by post-test SEM analysis. In addition, indications of a re-structuring of the sample surface during the degradation are shown. These results indicate, that with LSCF as a cathode in ambient (humid) air in SOFC stacks containing various Si-sources, such as glass or glass-ceramic seals, and thermal insulation materials a significant decrease of the surface oxygen exchange coefficient can occur, even at temperatures as low as 600 °C. In order to prevent a severe Si-induced degradation, dry air should be used as an oxidant. However, even in dry atmosphere a minor decrease of k_chem can occur during long-term operation due to changes in the relative cation and oxygen content at the surface.     

Nanocrystalline oxide (Y2O3, Dy2O3, ZrO2, NiO) coatings on BaTiO3 submicron particles by precipitation

Nanocoatings (5–20 nm) of different compounds on fine BaTiO₃ particles were obtained by means of precipitation processes. Homogeneous and smooth shells of Y(OH)CO₃ and Dy(OH)CO₃ were grown from nitrate solutions in the presence of urea. An irregular coating consisting of zirconia nanoparticles was produced from zirconyl nitrate solution using ammonia as a precipitating agent after adsorption of a polymeric polyelectrolyte on the BaTiO₃ surface. Composite particles with a peculiar morphology were obtained by inducing heterogeneous nucleation and growth of Ni(OH)₂ lamellae on the BaTiO₃ surface. The different shells can be transformed in a nanocrystalline coating of the corresponding oxide (Y₂O₃, Dy₂O₃, ZrO₂, NiO) by calcination at moderate temperatures (400–700 °C). The overall results indicate that precipitation from solution represents a versatile process to grow a second-phase layer on the surface of BaTiO₃ particles. This approach can be used as an alternative to mechanical wet mixing for controlled doping of ferroelectric materials and for the fabrication of composite materials with specific geometry of the two-phase assembly.     

Li ion mobility in TeO2---LiO0.5---LiX (X = F, Cl) glasses determined by Li NMR and impedance spectroscopy

NMR and impedance spectroscopies have been used to study the Li⁺ ion mobility in the TeO₂---LiO_0.5---LiX (X = F, Cl) glasses. Activation energies determined from the temperature dependence of dc conductivity data decrease as the lithium or halogen content increases. Activation energies deduced from the analysis of the ac conductivity data in the complex modulus formalism and corresponding to the true one-particle energy barrier of ionic motion according to the coupling model, have been compared to those deduced from NMR data (T₁⁻¹ relaxation time): a good agreement is obtained. Finally, the influence of the glass composition and ion distribution on Li⁺ ion mobility has been discussed.     

Complex impedance spectroscopy studies of (La0.70−xNdx)Sr0.30Mn0.70Cr0.30O3 (x≤0.30) perovskite compounds

The transport properties of Nd-doped perovskite materials (La_0.7−xNd_x)Sr_0.3Mn_0.7Cr_0.3O₃ (x≤0.30) were investigated using impedance spectroscopy techniques over a wide range of temperatures and frequencies. AC conductance analyses indicate that the conduction mechanism is strongly dependent on temperature and frequency. The DC conductance plots can be described using the small polaron hopping (SPH) model, with an apparent reduction of the polaron activation energy below the Curie temperature T_C. Complex impedance plots exhibit semicircular arcs described by an electrical equivalent circuit. Off-centered semicircular impedance plots show that the Nd-doped compounds obey to a non-Debye relaxation process. The conductivity of grains and grain-boundaries has been estimated. The activation energies calculated from the conductance and from time relaxation analyses are comparable. This indicates that the same type of charge carriers is responsible for both the electrical conduction and relaxation phenomena.     

Reaction kinetics of the Pt, O2(g)|c-ZrO2 system: precursor-mediated adsorption

A micro kinetic model of the Pt, O₂(g)|c-zirconia electrode/electrolyte system was developed in state space form (model M3). The oxygen adsorption/desorption process was modeled as a precursor-mediated surface reaction. The surface diffusion of atomic oxygen and the electrochemical reduction of atomic oxygen near the three-phase boundary (tpb) were considered. It was shown that the simulated charge-transfer behavior of M3 is significantly different from models with ordinary Langmuir kinetics (model M2). The electrochemical rate constant was estimated from selected experimental data as k₁₀=(6.05±0.25)·10⁶ m³/(mol·s). From experimental results it was concluded that only one adsorbed oxygen species is relevant for the dynamic behavior. In porous Pt electrodes binary gas phase diffusion of oxygen in O₂/N₂ gas mixtures becomes relevant at oxygen partial pressures below 10⁻³ atm. The general procedure for state and parameter estimation can be well adopted for the investigation of further reaction mechanisms.     

Optical spectroscopy of Ca3Sc2Si3O12, Ca3Y2Si3O12 and Ca3Lu2Si3O12 doped with Pr

The silicates Ca₃Sc₂Si₃O₁₂, Ca₃Y₂Si₃O₁₂ and Ca₃Lu₂Si₃O₁₂, both undoped and doped with Pr³⁺ ions, have been synthesized by solid-state reaction at high temperature. The luminescence spectroscopy and the excited state dynamics of the materials have been studied upon VUV and X-ray excitation using synchrotron radiation. All doped samples have shown efficient 5d-4f emission upon direct VUV excitation of 5d levels, but only Ca₃Sc₂Si₃O₁₂:Pr³⁺ shows luminescence upon interband VUV or X-ray excitation. The VUV excited emission spectra of Ca₃Y₂Si₃O₁₂:Pr³⁺ and Ca₃Lu₂Si₃O₁₂:Pr³⁺ show features attributed to emission from two distinct sites accommodating the Pr³⁺ dopant. The decay kinetics of the Pr³⁺ 5d-4f emission in Ca₃Sc₂Si₃O₁₂:Pr³⁺ upon VUV excitation across the band gap are characterized by decay times in the range 25-28 ns with no significant rise after the excitation pulse. They appear to be faster upon X-ray irradiation than for VUV excitation. Weak afterglow components are attributed to defect luminescence.     

Molten salt synthesis of Li1 + x (Ni0.5Mn0.5)1 − xO2 as cathode material for Li-ion batteries

Li_1 + x(Ni_0.5Mn_0.5)_1 − xO₂ cathode material for Li-ion batteries has been prepared by a molten salt method using Li₂CO₃ salt. The influences of synthetic temperature and time have been intensively investigated. It is easy to obtain materials with a hexagonal α-NaFeO₂ structure except broad peaks between 20° and 25°. Nickel in Li_1 + x(Ni_0.5Mn_0.5)_1 − xO₂ is oxidized to a trivalent state while manganese maintained a tetravalent state. It is found that the discharge capacities of all samples increase with cycling. The sample prepared at 850 °C for 5 h has a discharge capacity of 130 mAh g^− 1 between 2.5 and 4.5 V versus V_Li+/Li at a specific current of 0.13 mA cm^− 2 after 50 cycles at 25 °C.     

Mixed electronic and ionic conductivity of LaCo(M)O3 (M=Ga, Cr, Fe or Ni).: V. Oxygen permeability of Mg-doped La(Ga, Co)O3−δ perovskites

The LaGa_1−x−yCo_xMg_yO_3−δ solid solutions with rhombohedrally-distorted perovskite structure were ascertained to form in the concentration range of 0≤y≤0.10 at x=0.60 and 0≤y≤0.20 at x=0.35–0.40. Increasing cobalt content results in increasing electrical conductivity and thermal expansion of the perovskites. Thermal expansion coefficients of the LaGa_1−x−yCo_xMg_yO_3−δ ceramics were calculated from the dilatometric data to vary in the range of 12.4–19.8×10⁻⁶ K⁻¹ at 300–1100 K. Doping La(Ga,Co)O_3−δ solid solutions with magnesium leads to increasing oxygen nonstoichiometry, electronic and oxygen ionic conductivity. Oxygen permeation fluxes through LaGa_1−x−yCo_xMg_yO_3−δ membranes were found to be limited by the bulk ionic conduction and to increase with magnesium concentration, being essentially independent of cobalt content.     

第2系列ZrO2(Y2O3)薄膜的慢正电子研究

用慢正电子技术研究了在溅射时不加偏压,衬底加热300℃,纯Ar气氛下制备的用Y_{2O3稳定的ZrO2薄膜材料(简称YSZ薄膜),发现了YSZ薄膜在不同 深度处的缺陷分布情况,退火温度对YSZ薄膜缺陷有影响.简要讨论了致密、优质YSZ薄膜的 制备方法.}     

Stress-induced cubic-tetragonal transformation in partially stabilized ZrO2: Raman spectroscopy study

Regularities of the cubic-tetragonal transformation (C→t′) in partially stabilized zirconia were studied with Raman spectroscopy and high-temperature Raman spectroscopy techniques. New ‘low temperature’ mechanism of tetragonal nanoparticles formation in a volume of cubic solid solution was revealed in ZrO₂-Gd₂O₃ (Eu₂O₃) (6-8 mol%) single crystals. This mechanism includes nucleation of the tetragonal nanoparticles due to diffusionless C→t′ phase transformation at the first stage and gradual decrease of the stabilizer concentration inside t′-domains after subsequent low-temperature annealing. Predominant orientation of tetragonal domains due to the stress-induced C→t′ transformation was registered in ZrO₂-Gd₂O₃ (8 mol%) single crystals.     

Surface termination, composition and reconstruction of Fe3O4(001) and γ-Fe2O3(001)

We address a current controversy concerning the nature of the surfaces of Fe₃O₄(001) and γ-Fe₂O₃(001) grown on MgO(001) by molecular beam epitaxy. Despite recent claims to the contrary, we show that γ-Fe₂O₃(001) unambiguously exhibits a (1×1) surface net, in contrast to Fe₃O₄(001), which assumes a R45 reconstruction. In addition, we present high-energy-resolution Fe 2p and O 1s core-level photoelectron spectra obtained at both normal and grazing emission for γ-Fe₂O₃(001) and Fe₃O₄(001). These spectra show that the Fe₃O₄(001) surface has a higher Fe(III)/Fe(II) ratio than the bulk, and that the asymmetry in the O 1s line shape for Fe₃O₄(001) is due to final state effects rather than the presence of a surface oxygen or hydroxyl species.     

Oxygen permeability of La2Cu(Co)O4+δ solid solutions

Formation of the La₂Cu_1−xCo_xO_4+δ solid solutions with orthorhombic K₂NiF₄-type structure was found to be in the range of 0≤x≤0.30 at temperatures above 1270 K. Incorporating cobalt into the copper sublattice of lanthanum cuprate leads to increasing oxygen hyperstoichiometry and decreasing electrical conductivity. Thermal expansion coefficients of the La₂Cu_1−xCo_xO_4+δ (x=0.02–0.30) ceramics at 470–1100 K were calculated from the dilatometric data to vary in the range (12.2–13.2)×10⁻⁶ K⁻¹. Studying the dependence of oxygen permeation fluxes through La₂Cu(Co)O_4+δ on the membrane thickness demonstrated that the oxygen transport at the thickness values below 1 mm is limited by both surface exchange rate and bulk ionic conductivity. Oxygen permeability of the La₂Cu_1−xCo_xO_4+δ solid solutions was ascertained to increase with cobalt concentration at x=0.02–0.10 and to decrease with further dopant additions, indicating a participation of interstitial oxygen in the ionic transport.     

Phase transition and superconductivity of (Ba,Sr,La)2YCu3Oz (z>6.9)

A series of (Ba,Sr,La)₂YCu₃O_z (z>6.9) samples is studied to elucidate the relationship between structure, cation composition and oxygen content. The concentration of La plays an important role in the orthorhombic to tetragonal phase transition. The tetragonal structure is stabilized by increasing oxygen content and decreasing average ionic radius of cations on the Ba site. The phase transition is explained in terms of the random distribution of CuO₅ pyramids in the BaO–CuO_x slab. Superconductivity does not depend significantly on the phase transition.