Assessment of the Sr-Mn-O system

A thermodynamic assessment of the Sr-Mn-O system is presented. The main practical relevance of this system is that it contains the perovskite phase SrMnO₃, which is the Sr-rich end member of the phase (La,Sr) MnO₃, that finds widespread use as a cathode material for solid oxide fuel cells (SOFCs) and has recently attracted a lot of attention due to its interesting giant magnetoresistive properties. The thermodynamic parameters are optimized by applying the CALPHAD method. The SrMnO_3−z phase exists in two modifications, a layered hexagonal modification at low temperatures and a perovskite modification at high temperatures. Both modifications show considerable oxygen deficiencies, which are modeled using the compound energy model. The sublattice occupation of the phases is (Sr²⁺) (Mn³⁺, Mn⁴⁺)(O²⁻, Va)₃. On reducing Mn⁴⁺ to Mn³⁺, oxygen vacanices are formed. The phase SrMn₃O_6−z also shows an oxygen deficiency, which is modeled in an identical way. The Ruddlesden-Popper phases Sr₂MnO₄ and Sr₃Mn₂O₇, and the phases Sr₇Mn₄O₁₅ and Sr₄Mn₃O₁₀ are modeled as stoichiometric phases. The ionic liquid is modeled using the two-sublattice model for ionic liquids. The stability and thermodynamic data on many of the phases in this system are poorly known. For this reason, some aspects of this assessment must be regarded as tentative.     

Cr doping effect in B-site of La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> on its phase stability and performance as an SOFC anode

La_0.75Sr_0.25Cr _y Mn_1−y O₃ (LSCM) (y = 0.0–0.6) composite oxides were synthesized by a complexing process of combining ethylene diamine tetraacetic acid (EDTA) and citrate. X-ray diffraction (XRD), temperature-programmed reduction, electrical conductivity, I–V polarization, and impedance spectroscopy were conducted to investigate the Cr doping effect of La_0.75Sr_0.25MnO₃ on its phase stability and electrochemical performance as a solid-oxide fuel cell (SOFC) anode. The chemical and structural stabilities of the oxides increased steadily with increasing Cr doping concentration, while the electrical conductivity decreased on the contrary. At y ≥ 0.4, the basic perovskite structure under the anode operating condition was sustained. a cell with 0.5-mm-thick scandia-stabilized zirconia electrolyte and La_0.75Sr_0.25Cr _y Mn_1−y O₃ anode delivered a power density of ∼15 mW·cm⁻² at 850°C.     

Synthesis and characterization of La0.8Sr0.2MO3−δ (M=Mn, Fe, or Co) cathode materials by induction plasma technology

In this work, nanopowders of perovskite cathode materials (La_0.8Sr_0.2MnO_3−δ, La_0.8Sr_0.2FeO_3−δ, and La_0.8Sr_0.2CoO_3−δ), for use in solid oxide fuel cells (SOFC), were successfully synthesized, using induction plasma techniques. Their compositions, structures, morphology, particle size distributions, and BET specific surface areas were determined for comparison with their counterparts prepared by the Pechini method and by the glycine-nitrate combustion (GNC) technique. The particle sizes of the plasma-synthesized powders are mostly around 63 nm. These plasma-synthesized powders are generally globular, their BET specific surface areas being about 26 m²g⁻¹, approximately twice those of powders prepared by the GNC and Pechini methods. These plasma-synthesized powders are readily reproducible and are not agglomerated. Their individual particle sizes and distributions are very independent of their composition.     

Glass-like thermal conductivity in ytterbium-doped lanthanum zirconate pyrochlore

Glass-like thermal conductivity is observed in (La_1?xYb_x)₂Zr₂O₇ (1/6 ? x ? 1/3), which exhibits great potential as a high-temperature thermal barrier coating material. In the pyrochlore-type La₂Zr₂O₇, the large 16c-site La³⁺ is weakly bonded by its surrounding 48f-site oxygen ions, and substitution of La³⁺ with smaller and heavier Yb³⁺ gives rise to a large atomic displacement parameter (ADP) of Yb³⁺ which behaves as a “rattler”, as evidenced by the X-ray diffraction refinement. The localized “rattling” of Yb³⁺ in the cation sublattice significantly scatters the heat-carrying phonons and lowers the thermal conductivity close to the amorphous limit in combination with the intrinsic oxygen vacancies in the anion sublattice. In contrast, substituting Yb³⁺ with the larger La³⁺ in Yb₂Zr₂O₇ does not result in as remarkable a decrease in the thermal conductivity as in Yb-doped La₂Zr₂O₇ due to the absence of rattling atoms. In this study, a resonant phonon scattering is proposed as a new approach to reduce the thermal conductivity of oxides which are important in various thermal engineering applications.     

Total conductivity,oxygen permeability and stability of perovskite-type oxide BaCo<Subscript>0.7</Subscript>Fe<Subscript>0.2</Subscript>Nb<Subscript>0.1</Subscript>O<Subscript>3−<Emphasis Type="Italic">δ</Emphasis></Subscript>

The total conductivity, oxygen sorption property, oxygen permeability and stability of pure perovskite-type oxide BaC_o0.7Fe_0.2Nb_0.1O_3−δ (BCFNO) in real operating conditions were investigated. Its total conductivity was measured to be 3.6 S·cm⁻¹ at 600°C. Though the total conductivity of the BCFNO membrane is much smaller than that of the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) membrane, the oxygen permeability of the BCFNO membrane is similar to that of the BSCFO membrane. SEM observation and EDX analysis of the BCFNO and BSCFO membranes indicated that no segregation of metal ions was found for the used BCFNO membrane while the original perovskite phase of BSCFO decomposed under the experimental condition. The experimental results of oxygen permeability and stability were consistent with the analysis on the oxygen sorption property of perovskites.     

Effects of Ce<Superscript>3+</Superscript> doping on the structure and magnetic properties of Mn-Zn ferrite fibers

Ce³⁺-doped Mn-Zn ferrite fibers were successfully prepared by the organic gel-thermal decomposition method from metal salts and citric acid. The composition, structure, and magnetic properties of these ferrite fibers were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). The results show that Mn_0.2Zn_0.8Fe_2−x Ce _x O₄ (x = 0–0.04) fibers are featured with an average grain size of 11.6–12.7 nm, with diameters ranging between 1.0 to 3.5 μm and a high aspect ratio (length/diameter). The Ce³⁺ ion doping has not resulted in crystal structural changes of the Mn-Zn ferrite phase and all the as-prepared ferrite fibers have a simple spinel phase structure, although this influences the morphologies of Mn_0.2Zn_0.8Fe_2−x Ce _x O₄ ferrite fibers possibly owing to the lattice distortion and internal-stress. Both the lattice constant and grain size increase slightly with the increase of the Ce³⁺ ion doping content. The soft magnetic properties of Mn-Zn ferrite fibers can be improved by a small amount of Ce³⁺ ion doping with an increase of the saturated magnetization and a decrease of the coercivity.     

Structural, magnetic and electrical transport properties for a series of La1−xSrxFe1−xMnxO3 (0.3 ≤ x ≤ 0.7) compounds

The structural, electrical transport and magnetic properties have been studied for compounds: La_1−xSr_xFe_1−xMn_xO₃ (0.3 ≤ x ≤ 0.7). The lattice parameter, a, first decreases with x, and followed by an increase when Sr²⁺ and Mn⁴⁺ was continuously doped. The cell parameters, b and c, slightly decrease with coupled substitution of Sr²⁺ for La³⁺ and Mn⁴⁺ for Fe³⁺. In the paramagnetic temperature range, formation of magnetic clusters is suggested; the sizes of clusters decrease with x up to 0.5, following that they increase sharply with continuing doping. The electrical behaviors of all specimens demonstrate insulators and the electrical resistivity increases with content of Mn⁴⁺ and Sr²⁺ ions doped. A variable range hopping model is suitable to describe electrical transport process for the compounds at low temperature. At high temperature the electrical transport process can be described by bipolaron model for all compounds.     

Multiphase magnetic state of Ca1 ? x La x MnO3 ? δ single crystals ( x = 0.03, 0.05, 0.07) containing oxygen vacancies

Magnetic properties (magnetization, dynamic and static susceptibility) and transport properties (resistance and magnetoresistance) have been studied in a temperature range of 2–600 K in magnetic fields to 90 kOe for single crystals of Ca_{1 − x} La _x MnO_{3 − δ} with a weak electron doping (x ≤ 0.07) grown in argon and oxygen atmospheres. The magnetic state of Ca_{1 − x} La _x MnO_{3 − δ} single crystals is multiphase. Below T = T _N(G) ∼ 110 K, in all the crystals there coexists an AFM G phase with an FM contribution and an AFM C phase. In crystals with x = 0.07, a transition from the paramagnetic phase into the AFM C phase occurs in part of their volume below T ∼ 130–150 K. In crystals with x = 0.05 annealed in oxygen, an anomaly of paramagnetic susceptibility is observed near T ^* ∼ 270 K, which is related to the formation of FM clusters near defects. At x = 0.05 and 0.07, AFM correlations are retained in the paramagnetic state (to 600 K). The differences in the magnetic and transport properties of single crystals grown in argon and oxygen are explained by the various content of oxygen vacancies and by their possible ordering. Original Russian Text ? N.N. Loshkareva, A.V. Korolev, T.I. Arbuzova, N.I. Solin, A.M. Balbashov, N.V. Kostromitina, 2007, published in Fizika Metallov i Metallovedenie, 2007, Vol. 103, No. 3, pp. 261–270.     

Thermodynamic reassessment of the BaO-B2O3 system

The BaO-B₂O₃ pseudobinary system is assessed. A two-sublattice ionic solution model, (Ba²⁺) _P (O²⁻, BO₃³⁻, B₄O₇²⁻, B₃O_4.5) _Q , is adopted to describe the liquid phase. All the solid phases are treated as stoichiometric compounds. A set of parameters consistent with most of the available experimental data on both phase diagram and thermodynamic properties is obtained by using CALPHAD technique. A comparison between the calculated results and experimental data as well as a previous assessment is presented.     

Thermodynamic reassessment of the BaO-B2O3 system

The BaO-B₂O₃ pseudobinary system is assessed. A two-sublattice ionic solution model, (Ba²⁺) _P (O²⁻, BO ₃ ³⁻ , B₄O ₇ ²⁻ , B₃O_4.5) _Q , is adopted to describe the liquid phase. All the solid phases are treated as stoichiometric compounds. A set of parameters consistent with most of the available experimental data on both phase diagram and thermodynamic properties is obtained by using CALPHAD technique. A comparison between the calculated results and experimental data as well as a previous assessment is presented.     

Electrochemical properties of carbon-coated LiFePO<Subscript>4</Subscript> and LiFe<Subscript>0.98</Subscript>Mn<Subscript>0.02</Subscript>PO<Subscript>4</Subscript> cathode materials synthesized by solid-state reaction

Olivine structured LiFePO4/C (lithium iron phosphate) and Mn2+-doped LiFe0. 98Mn0. 024/C powders were synthesized by the solid-state reaction. The effects of manganese partial substitution and different carbon content coating on the surface of LiFePO4 were considered. The structures and electrochemical properties of the samples were measured by X-ray diffraction (XRD), cyclic voltammetry (CV), charge/discharge tests at different current densities, and electrochemical impedance spectroscopy (EIS). The electrochemical properties of LiFePO4 cathodes with x wt. ％ carbon coating (x=3, 7, 11, 15) at γ=0. 2C, 2C (1C=170 mAh·g-1) between 2. 5 and 4. 3 V were investigated. The measured results mean that the LiFePO4 with 7 wt. ％ carbon coating shows the best rate performance. The discharge capacity of LiFe0. 98Mn0. 02PO4/C composite is found to be 165 mAh·g 1 at a discharge rate, γ=0. 2C, and 105 mAh·g-1 at γ=2C, respectively. After 10cycles, the discharge capacity has rarely fallen, while that of the pristine LiFePO4/C cathode is 150 mAh·g-1 and 98 mAh·g-1 at γ=0. 2 and 2C, respectively. Compared to the discharge capacities of both electrodes above, the evident improvement of the electrochemical performance is observed, which is ascribed to the enhancement of the electronic conductivity and diffusion kinetics by carbon coating and Mn2+-substitution.     

Development of proton conductors using pyrochlore-perovskite phase boundaries

The pyrochlore-perovskite binary systems La₂Zr₂O₇-SrZrO₃ and La₂Zr₂O₇-LaYO₃ were studied as potential high-temperature proton conductors. X-ray diffraction and direct current conductivity measurements were used to develop empirical relationships between structure and conductivity. The solubilities of Sr in La₂Zr₂O₇ and La in SrZrO₃ were low, less than 0.1 mol fraction. The solubility of Zr in LaYO₃ was at least 0.125 mol fraction, and the solubility of Y in La₂Zr₂O₇ was at least 0.25 mol fraction. Y-doped La₂Zr₂O₇ had the highest electrical conductivity, though no composition exceeded σ=3×10⁻⁴ S/cm at the target temperature of 600 °C. The effectiveness of Y as a dopant in La₂Zr₂O₇ was limited because Y substituted for both La on the A-site and Zr on the B-site. This paper was presented at the Fuel Cells: Materials, Processing, and Manufacturing Technologies Symposium sponsored by the Energy/Utilities Industrial Sector & Ground Transportation Industrial Sector and the Specialty Materials Critical Technologies Sector at the ASM International Materials Solutions Conference, October 13–15, 2003, in Pittsburgh, PA. The symposium was organized by P. Singh, Pacific Northwest National Laboratory, S.C. Deevi, Philip Morris USA, T. Armstrong, Oak Ridge National Laboratory, and T. Dubois, U.S. Army CECOM.     

VUV photoluminescence of La<Subscript>1−x</Subscript>Gd<Subscript>x</Subscript>PO<Subscript>4</Subscript>:Tb<Superscript>3+</Superscript> green phosphors synthesized by ultrasonic spray pyrolysis

(La_1−xGd_x)_0.94PO₄:Tb_0.06 (0 ≤ x ≤ 1.0) green phosphors were synthesized by an ultrasonic spray pyrolysis process. The annealed (La_1−xGd_x)_0.94PO₄:Tb_0.06 phosphors exhibited the formation of a single-phase oxide with the monoclinic monazite structure. Higher Gd³⁺ content yielded a smaller lattice parameter. The phosphors emitted green light due to the ⁵D₄→⁷F_J (J = 3, 4, 5, and 6) transition of Tb³⁺ at 489 nm, 543 nm, 585 nm, and 621 nm, respectively. The predominant peak at 543 nm was nearly a single peak, which was responsible for the green color emission. Among the (La_1−xGd_x)_0.94PO₄:Tb_0.06 phosphors, the strongest emission intensity was obtained for (La_0.25Gd_0.75)_0.94PO₄:Tb_0.06.     

Investigation on Oxygen Chemical Diffusion Properties of Perovskite Oxides (La1?x Pr x )0.6Sr0.4Co0.8Fe0.2O3?δ

Perovskite oxide samples of (La_1−x Pr _x )_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (x = 0.2, 0.4, 0.6, 0.8) are obtained by solid-state reaction method. The oxygen chemical diffusion properties of (La_1−x Pr _x )_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ are determined by electrical conductivity relaxation technique. The results show that the conductivity of (La_1−x Pr _x )_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ increases with the increase of oxygen partial pressure. The (La_1−x Pr _x )_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ samples have a high oxygen chemical diffusion coefficient, which decreases linearly with a decrease in temperature and an increase in Pr content. The oxygen chemical diffusion coefficient D _chem remains fairly constant at high PO₂. The oxygen chemical diffusion coefficient is the highest for (La_1−x Pr _x )_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ with x = 0.2, and attains a value of 9.41 × 10⁻⁵ cm² s⁻¹ at 600 °C. This shows the material’s promise as a cathode material for intermediate temperature solid oxide fuel cells.     

Optical properties of buffer and superconducting layers of the YBa2Cu3O7 ?δ/La1.75Ba0.25CuO4/La0.8Ba0.2MnO3 heterostructure for second-generation superconducting cables on an Ni5% W tape

An epitaxial YBa₂Cu₃O_{7 − δ}/La_1.75Ba_0.25CuO₄/La_0.8Ba_0.2MnO₃ heterostructure has been obtained on a biaxially textured tape of an Ni5% W alloy. It is shown that when films of a buffer layer of La_0.8Ba_0.2MnO₃ applied onto a tape of an Ni alloy in a reducing atmosphere are annealed in an oxygen atmosphere it is the compensation of oxygen vacancies that plays the dominant role in the formation of optical and magnetooptical properties of the layer, rather than stresses at the film-substrate interface. The shape of the reflection spectrum of the buffer layer after annealing corresponds to that of an La_0.8Ba_0.2MnO₃ single crystal, which indicates that the oxygen content is close to the stoichiometric one. The temperature dependences of the transverse Kerr effect and coefficient of light reflection of the films at a fixed wavelength can be used for the evaluation of T _c and the temperature of the optical response of the metal-insulator transition of the buffer layer on the basis of lanthanum manganite. Original Russian Text ? O.Yu. Gorbenko, A.R. Kaul’, Yu.P. Sukhorukov, E.A. Gan’shina, S.V. Samoylenkov, N.N. Loshkareva, A.V. Telegin, O.V. Melnikov, O. Stadel, 2007, published in Fizika Metallov i Metallovedenie, 2007, Vol. 104, No. 6, pp. 578–585.     

Reflection and refraction of acoustic waves at the insulator-ferromagnetic heusler alloy boundary

Reflection and refraction of longitudinal and transverse acoustic waves on a flat boundary between an insulator and a ferromagnetic crystal of a Heusler alloy Ni_{2 + x + y} Mn_{1 − x} Ga_{1 − y} have been considered in the region of premartensitic and martensitic phase transitions. The possibility is shown of using temperature to effectively controlling the angles of wave refraction and the coefficients of transformation of wave modes by changing strong acoustic crystalline anisotropy induced with approaching the point of the phase transition. The conditions of the appearance of critical angles and the angles of total internal reflection, as well as the effect of temperature on them, have been considered. The possibility has been analyzed of emission, in the vicinity of the phase transition, of a wave that propagates along the boundary into the bulk of the material. Based on the experimental data obtained by Trivisonno for the temperature dependences of the sound velocity in the Ni_{2 + x + y} Mn_{1 − x} Ga_{1 − y} crystal, the conclusions of the theory developed are illustrated by numerical calculations for a concrete structure, namely, Ni₂MnGa-quartz.     

Crystal microstructure, infrared absorption, and microwave electromagnetic properties of （La1-xDyx）2/3Sr1/3MnO3

The manganite perovskite polycrystal samples of (La_1−x Dy _x )_2/3Sr_1/3MnO₃ (x = 0, 0.1, 0.2, 0.35, and 0.5) doped with Dy were prepared by solid state reaction in atmosphere to measure their X-ray diffraction (XRD) patterns, scanning electric microscope (SEM) images, infrared absorption spectra, and microwave electromagnetic properties. The displacement of the XRD peaks of the samples was found, and the 2ϑ increases from 0.05° to 0.5°. The grains of undoped La_2/3Sr_1/3MnO₃ not only have the greatest size, but also the most regular shape. The size of the grains decreases as the Dy doping content increases from 0 to 0.5. The infrared absorption spectra of all samples were measured at room temperature. An absorption peak corresponding to the stretching vibration mode of Mn-O bonds appears within the range of 591–629 cm⁻¹. The absorption peak shifts from a higher frequency to a lower one with the decrease of the average ionic radius of A-site. The frequency dependence of microwave-absorbing properties, imaginary components of the complex magnetic permeability μ″ and dielectric permeability ε″ for all samples was measured at room temperature from 8 to 13 GHz. The results show that the loss of microwave absorption can be attributed to both the magnetic and electric losses. The increase of Dy content not only enhances the microwave absorption but also causes the displacement of the absorption peaks.     

Kinetics of forward extraction of Ti（IV） from H_2SO_4 medium by P_（507） in kerosene using the single drop technique

The kinetics of forward extraction of Ti(IV) from H2SO4 medium by P507 in kerosene has been investigated using the single drop technique.In the low concentration region of Ti(IV),the rate of forward extraction at 298 K can be represented by F(kmol·m-2·s-1)=10-5.07 [TiO 2 + ][H+]-1 [NaHA 2 ](o)·Analysis of the rate expression reveals that the rate determining step is(TiO)(i)2+ +(HA 2)(i)-[TiO(HA2)](i)+.The values of Ea,H±,S±,and G±298 are calculated to be 22 kJ·mol-1,25 kJ·mol-1,-218 J·mol-1·K-1,and 25 kJ·mol-1,respectively.The experimental negative S± values indicate that the reaction step occurs via SN2 mechanism.