Strontium Ferrite Research Articles

Complex oxide thin films: Pyrochlore, defect fluorite and perovskite model systems for structural, spectroscopic and catalytic studies

Well-ordered thin films of different defect fluorite and perovskite materials, namely lanthanum zirconate (La2Zr2O7), cerium zirconate (Ce2Zr2O7), lanthanum cerate (La2Ce2O7) and lanthanum strontium ferrite (La0.4Sr0.6FeO3), have been prepared by sputtering the respective powder targets onto vacuum-cleaved NaCl(0 0 1) single crystal facets. Characterization specifically also includes the sophisticated preparation of the initial target materials. For the defect fluorite materials, the target materials are the respective pyrochlore compounds, which reproducibly transform to the respective defect fluorite structures upon sputtering. At template temperatures of around 573 K, well-crystallized epitaxial thin films of the defect fluorite compounds La2Zr2O7 and Ce2Zr2O7 result, whereas for the perovskite compound post-annealing procedures at 973 K in air are necessary to obtain epitaxial ordering of orthorhombic structures. Epitaxial relations for the well-ordered defect fluorite thin films are determined to be La2Zr2O7(0 0 1)//NaCl(0 0 1) and Ce2Zr2O7(0 0 1)//NaCl(0 0 1), respectively. Structural and spectroscopic characterization of the films by (high-resolution) electron microscopy (HR-TEM), selected-area electron diffraction (SAED) and depth-profiling X-ray photoelectron spectroscopy (XPS) reveal that the structures and compositions of the initial target materials are well-preserved during the sputtering process. Following this preparation routine, access to well-ordered thin films of complex oxide materials with defined stoichiometry is therefore granted, which can subsequently be used as model systems for studies of their material’s or catalytic properties, as presented for simpler systems earlier. As a primary example, we show that for the defect fluorite thin films of Ce2Zr2O7 the defect fluorite-pyrochlore phase transformation can be observed at around 1123 K as detected by in situ electron diffraction.

The use of strontium ferrite in chemical looping systems

This work reports a detailed chemical looping investigation of strontium ferrite (SrFeO3−δ), a material with the perovskite structure type able to donate oxygen and stay in a nonstoichiometric form over a broad range of oxygen partial pressures, starting at temperatures as low as 250°C (reduction in CO, measured in TGA). SrFeO3−δ is an economically attractive, simple, but remarkably stable material that can withstand repeated phase transitions during redox cycling. Mechanical mixing and calcination of iron oxide and strontium carbonate was evaluated as an effective way to obtain pure SrFeO3−δ. In–situ XRD was performed to analyse structure transformations during reduction and reoxidation. Our work reports that much deeper reduction, from SrFeO3−δ to SrO and Fe, is reversible and results in oxygen release at a chemical potential suitable for hydrogen production. Thermogravimetric experiments with different gas compositions were applied to characterize the material and evaluate its available oxygen capacity. In both TGA and in-situ XRD experiments the material was reduced below δ = 0.5 followed by reoxidation either with CO2 or air, to study phase segregation and reversibility of crystal structure transitions. As revealed by in-situ XRD, even deeply reduced material regenerates at 900°C to SrFeO3–δ with a cubic structure. To investigate the catalytic behaviour of SrFeO3−δ in methane combustion, experiments were performed in a fluidized bed rig. These showed SrFeO3−δ donates O2 into the gas phase but also assists with CH4 combustion by supplying lattice oxygen. To test the material for combustion and hydrogen production, long cycling experiments in a fluidized bed rig were also performed. SrFeO3−δ showed stability over 30 redox cycles, both in experiments with a 2-step oxidation performed in CO2 followed by air, as well as a single step oxidation in CO2 alone. Finally, the influence of CO/CO2 mixtures on material performance was tested; a fast and deep reduction in elevated pCO2 makes the material susceptible to carbonation, but the process can be reversed by increasing the temperature or lowering pCO2.

Preparation and magnetic properties of Sm–Co doped strontium ferrite

:In this work, Sm–Co doped strontium ferrite, SrFe12-xSmxCoxO19 (x = 0·1–0·5) composites were prepared by adding Sm3+ and Co2+ cations into M-type strontium ferrite using one-step chemical coprecipitation method. The composites were characterized by FTIR and XRD. The magnetic properties of the samples were measured and analyzed by VSM. The results indicated that the average crystallite sizes located in a range of 29·7–32·9 nm. Lattice constants, a and c were close to 5·88 and 23·05 Å for the M-type strontium ferrite phase. The analyses of magnetic properties demonstrated that when x = 0·2 and 0·4, Sm3+ and Co2+ ions substituted Fe3+ ions at 12 k (↑) or 2a (↑) site. While, as x = 0·1, 0·3, and 0·5, Fe3+ ions at 12 k (↑) or 2a (↑) site were replaced by Sm3+ ions and Fe3+ ions at 4f2 (↓) site were substituted by Co2+ ions. Meanwhile, the proposed substitute mechanism in this work was expected to provide a good concept to modify directionally M-type strontium ferrite in order to extend its application field.

Effect of oxygen potential on Co solubility limit in La–Co co-substituted magnetoplumbite-type strontium ferrite

La–Co co-substituted magnetoplumbite-type (M-type) strontium ferrite is typically used as a base material for high-performance hard ferrite magnets. Generally, the unquenched orbital moment of Co2+, substituted for Fe3+, is thought to enhance magnetic anisotropy and then coercivity. La3+ is substituted for Sr2+ to compensate for the charge unbalance associated with Co2+ substitution. Even if the synthesis starts with equal amounts of Co and La, the Co contents of the resulting samples are generally lower than the La contents because of the partial reduction of Fe3+ to Fe2+. We studied how the oxygen potential suppresses the formation of Fe2+ and expands the Co solubility range in this system. Polycrystalline samples, synthesized under oxygen partial pressures of 0.2, 1.0, and 387 atm, were characterized by powder X-ray diffraction, wavelength-dispersive X-ray analysis, and magnetization measurements. We reveal that the application of higher oxygen pressure dramatically expands the Co solubility range, resulting in the enhancement in magnetic anisotropy. We successfully synthesized Sr-free LaFe11CoO19 with high magnetic anisotropy using the hot isostatic pressure (HIP) technique.

Ceramic and transport properties of halogen-substituted strontium ferrite

An attempt of fluorine and chlorine substitution for oxygen in strontium ferrite SrFeO3–δ was undertaken in search of a new means of influence on the oxide properties. An uncontrollable removal of halogen-ions from oxides during synthesis was detected. Nevertheless, residual halogens were found to provide a notable impact on ion conductivity and oxide stability under reducing conditions. More than twice higher ion conductivity relative to SrFeO3–δ was recorded in materials with nominal compositions SrFeO2.8–δF0.2, SrFeO2.6–δCl0.4, and SrFeO2.4–δCl0.6. The strong effect of halogen addition on ceramic properties of oxides was revealed. While SrFeO3–δ was known for its poor sinterability, adding 0.2 mol of fluorine or chlorine during the synthesis of SrFeO3–δ was shown to result in successful obtaining gas-tight ceramics.

Oxygen nonstoichiometry and thermodynamic quantities in solid solution SrFe1–xSnxO3–δ

The oxygen content (3–δ) variations in tin substituted derivatives SrFe1–xSnxO3–δ, where x = 0.05, 0.1, 0.17 and 0.25, of perovskite-like strontium ferrite, have been studied by coulometric titration measurements within oxygen partial pressure (pO2) range 10–19–10–2 atm at 800–950 °С. The obtained dependencies of (3–δ) from pO2 and temperature are used for calculations of partial molar thermodynamic functions of oxygen in the oxide structure. It is found that a satisfactory explanation of the experimental results can be attained within frameworks of the ideal solution model with ion and electron defects appearing in the result of oxidation and disproportionation of iron cations. The increase of the oxidation reaction enthalpy with tin content is consistent with the increase of the unit cell parameter, i.e., the stretch and relaxation of Fe-O chemical bonds.

Strontium ferrite as a three-way catalyst component

Ni-containing SrFeO3-δ/Al2O3 and Ce0.5Zr0.5O2-δ/Al2O3 catalysts were prepared and deposited onto surfaces of porous alumina oxide foams. The measured oxygen storage capacity values are 1448 and 1275 µmol-O/g for SrFeO3-δ and Ce0.5Zr0.5O2-δ powder, respectively (about 55% and 75% of their theoretical values). The experimental reactor using exhaust gas of a gasoline engine was assembled. Decrease of CO and other harmful components in case of Ni/SrFeO3-δ/Al2O3 coating is comparable to that of Ni/Ce0.5Zr0.5O2-δ/Al2O3 layer. After reactor conditions (700 °C during 15–20 min), microstructure of both catalysts remains unchanged. Strontium ferrite maintained an orthorhombic brownmillerite-type crystal structure, whereas nickel was detected in both metallic and oxide forms.

Microwave absorption properties of reduced graphene oxide strontium hexaferrite/poly(methyl methacrylate) composites

The key factors to consider when designing microwave absorber materials for eradication of electromagnetic (EM) pollution are absorption of incident EM waves and good impedance matching. By keeping these things in mind, flexible microwave absorber composite films can be fabricated by simple gel casting techniques using reduced graphene oxide (RGO) and strontium ferrite (SF) in a poly(methyl methacrylate) (PMMA) matrix. SF nanoparticles are synthesized by the well known sol–gel method. Subsequently, reduced graphene oxide (RGO) and SF nanocomposite (RGOSF) are prepared through a chemical reduction method using hydrazine. The structure, morphology, chemical composition, thermal stability and magnetic properties of the nanocomposite are characterized in detail by various techniques. The SF particles are found to be nearly 500 nm and decorated on RGO sheets as revealed by field emission scanning electron microscopy and transmission electron microscopy analysis. Fourier transform infrared and and Raman spectroscopy clearly show the presence of SF in the graphene sheet by the lower peak positions. Finally, ternary polymer composites of RGO/SF/PMMA are prepared by an in situ polymerization method. Magnetic and dielectric studies of the composite reveal that the presence of RGO/SF/PMMA lead to polarization effects contributing to dielectric loss. Also, RGO surrounding SF provides a conductive network in the polymer matrix which is in turn responsible for the magnetic loss in the composite. Thus, the permittivity as well as the permeability of the composite can be controlled by an appropriate combination of RGO and SF in PMMA. More than 99% absorption efficiency is achieved by a suitable combination of magneto-dielectric coupling in the X-band frequency range by incorporating 9 wt% of RGO and 1 wt% of SF in the polymer matrix.

Study on preparation and magnetic properties of Sr1-xGdxFe12-xCuxO19 (0.00 ≤ x ≤ 0.20) strontium ferrite prepared by solid phase method

ABSTRACTM-type strontium ferrites (Sr1-xGdxFe12-xCuxO19, x = 0.00, 0.05, 0.10, 0.15 and 0.20) doped with equivalent ions were synthesized via the solid phase method. The crystallographic structure, morphology and magnetic properties of samples were investigated using x-ray diffractometry (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM), respectively. The XRD results show that the pure M-type phase is successfully obtained when the Gd–Cu substitution amount x below 0.20. When the doping content is low, the typical hexagonal block structure appears in the magnet. When x is 0.05, the saturation magnetization (Ms), remanent magnetism (Mr) and coercivity (Hc) reach the maximum values of 51.25 emu/g, 565.50 Oe and 27.67 emu/g, respectively.

High Surface Area SOFC Electrode Materials Prepared at Traditional Sintering Temperatures

Solid oxide fuel cell (SOFC) electrode materials with surface areas up to 99 m2·g−1 were prepared at traditional sintering temperatures, 1050°C–1350°C, by sintering hybrid inorganic-organic materials in an inert atmosphere followed by calcination in air at 700°C. The electrode materials investigated were yttria-stabilized zirconia (YSZ), lanthanum strontium cobalt ferrite (LSCF), gadolinia doped ceria (GDC), and strontium titanate (STO). During sintering, an amorphous carbon template forms in situ and remains throughout the sintering process, aiding in the creation and preservation of mixed-metal-oxide nanomorphology. The carbon template is removed during subsequent calcination in air at 700°C, leaving behind a nanostructured ceramic. Phase stability, carbon template concentration, and specific surface area was determined for each mixed-metal-oxide. Final specific surface areas up to 83, 66, 95, and 99 m2·g−1 were achieved for YSZ, LSCF, GDC, and STO, respectively. The impact of high surface area YSZ on symmetrical YSZ-lanthanum strontium ferrite (LSF) cathode cell performance was evaluated in the temperature range of 550°C–800°C. Adding nanostructured YSZ decreased the electrochemical impedance by 45% at 550°C. The performance improvement lessened with increasing temperature, and at 800°C there was essentially no improvement. The findings reveal a promising approach to improving low temperature SOFC performance.

Structural and Magnetic Properties Evolution of Co-Nd Substituted M-type Hexagonal Strontium Ferrites Synthesized by Ball-Milling-Assisted Ceramic Process

M-type hexagonal Sr1−x Co x Nd x Fe12−x O19 (x = 0, 0.08, 0.16, and 0.24) has been synthesized by ball milling, followed by calcination in air. The calcined products have been characterized by x-ray powder diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectra, and vibrating sample magnetometry. XRD and SEM analyses confirm the formation of M-type Sr hexaferrite with platelet-like morphology when Sr1−x Co x Nd x Fe12−x O19 (x = 0, 0.08, 0.16, and 0.24) precursors are calcined at 950°C in air for 2.5 h. Lattice parameters “a” and “c” values of Sr1−x Co x Nd x Fe12−x O19 reflect a very small variation after doping of Nd3+ and Co2+ ions. Average crystallite size of Sr1−x Co x Nd x Fe12−x O19 sample, calcined at 1150°C, decreased obviously after doping of Co2+ and Nd3+ ions. This is because the bond energy of Nd3+-O2− is much larger than that of Sr2+-O2−. Magnetic characterization indicates that all the samples exhibit good magnetic properties. Substitution of Sr2+ and Fe3+ ions by Nd3+ and Co2+ ions can improve the specific saturation magnetizations and remanence of Sr1−x Co x Nd x Fe12−x O19. Sr0.84Co0.16Nd0.16Fe11.84O19, calcined at 1050°C, has the highest specific saturation magnetization value (74.75 ± 0.60 emu/g), remanence (45.15 ± 0.32 emu/g), and magnetic moment (14.34 ± 0.11 μ B); SrFe12O19, calcined at 1150°C, has the highest coercivity value (4037.01 ± 42.39 Oe). These magnetic parameters make this material a promising candidate for applications such as high-density magnetic recording and microwave absorbing materials.

Microwave-induced catalytic application of magnetically separable strontium ferrite in the degradation of organic dyes: Insight into the catalytic mechanism

Abstract A novel microwave (MW)-absorbing material, SrFe2O4, was prepared by sol-gel method and used to degrade organic dyes under MW irradiation. The MW-induced catalytic performances were evaluated in detail. SrFe2O4 showed high catalytic activity for organic dyes such as methyl violet (MV) and crystal violet (CV). The enhanced catalytic performance could be ascribed to the production of MW-induced active species, such as holes (h+), superoxide radicals ( O2−) and hydroxyl radicals ( OH) in the degradation process. A possible degradation mechanism and pathway was proposed in MW/SrFe2O4 system. Moreover, SrFe2O4 as an eco-friendly catalyst could be easily separated and recycled by a magnet. This work showed a great potential of SrFe2O4 particles for environmental purification of organic pollutants.

Niobium Doped Lanthanum Strontium Ferrite as A Redox‐Stable and Sulfur‐Tolerant Anode for Solid Oxide Fuel Cells

The Nb-doped lanthanum strontium ferrite perovskite oxide La0.8 Sr0.2 Fe0.9 Nb0.1 O3-δ (LSFNb) is evaluated as an anode material in a solid oxide fuel cell (SOFC). The effects of Nb partial substitution in the crystal structure, the electrical conductivity, and the valence of Fe ions are studied. LSFNb exhibits good structural stability in a severe reducing atmosphere at 800 °C, suggesting that high-valent Nb can effectively promote the stability of the lattice structure. The concentration of Fe2+ increases after Nb doping, as confirmed by X-ray photoelectron spectroscopy. The maximum power density of a thick Sc-stabilized zirconia (ScSZ) electrolyte-supported single cell reached 241.6 mW cm-2 at 800 °C with H2 as fuel. The cell exhibited excellent stability for 100 h continuous operation without detectable degeneration. Scanning electron microscopy clearly revealed exsolution on the LSFNb surface after operation. Meanwhile, LSFNb particles agglomerated significantly during long-term stability testing. Impedance spectra suggested that both the LSFNb anode and the (La0.75 Sr0.25 )0.95 MnO3-δ /ScSZ cathode underwent an activation process during long-term testing, through which the charge transfer ability increased significantly. Meanwhile, low-frequency resistance (RL ) mainly attributed to the anode (80 %) significantly increased, probably due to the agglomeration of LSFNb particles. The LSFNb anode exhibits excellent anti-sulfuring poisoning ability and redox stability. These results demonstrate that LSFNb is a promising anode material for SOFCs.

Synthesis, characterization and photocatalytic evaluation of strontium ferrites towards H2 production by water splitting under visible light irradiation

Renewable hydrogen production demands materials with adequate structural, textural, optical and photocatalytic properties for the splitting of the water molecule under visible light irradiation. In this research, the spinel, hexagonal and orthorhombic strontium ferrite crystalline phases (SrFe2O4, SrFe12O19 and Sr7Fe10O22) were synthesized, characterized and photocatalytically evaluated towards H2 production through water splitting. These materials show attractive features such as being inexpensive, environmentally friendly and present visible spectrum band gaps (1.5–2.5 eV). Strontium ferrites were synthesized from nitrate precursors by the modified Pechini's method at high temperatures. Characterization consisted in XRD, SEM, BET surface area, and UV–Vis spectroscopy. XRD results indicate the presence of pure phases with a crystal size of 15, 30 and 30 nm for the spinel, hexagonal and orthorhombic phases, while BET surface area were 18, 14 and 4 m2/g, respectively. Photocatalytic evaluation resulted in the spinel phase being the most active material producing 730 μmol H2/g h.

Absorption Characterization of Mn-Zr-Substituted La-Sr Hexaferrite Using Open-Circuit and Short-Circuit Approaches in 8.2–18 GHz Frequency Range

The present study reports on the microwave absorption characterization of Mn2+-Zr4+ substituted lanthanum strontium ferrites, Sr0.85La0.15(MnZr) x Fe12−2x O19, where x = 0.0, 0.25, 0.50, 0.75 and 1.0 in the X- and Ku-band. The synthesized ferrites are characterized with regard to their electromagnetic properties such as complex permittivity \( \left( {\varepsilon^{\prime} - j\varepsilon^{\prime\prime}} \right) \) and complex permeability \( \left( {\mu^{\prime} - j\mu^{\prime\prime}} \right) \) using vector network analysis in the 8.2–18 GHz frequency range. Real and imaginary parts of permittivity decrease with the increase in Mn-Zr concentration due to a reduction in electron hopping conduction and eddy current losses, respectively. Microwave permeability spectra are also affected by the doping. The amplitude of magnetic loss peak increases with the increase in doping except for the x = 1.0 composition. Two commonly used approaches, open-circuit and short-circuit, have been employed for the absorption analysis. The difference in the results of these two techniques is justified on the basis of the reflection mechanism. The presented experimental findings underline the potential of the synthesized compositions with Mn-Zr concentrations x = 0.25, 0.5 and 0.75 in the suppression of electromagnetic reflections and radar signatures.

Magnetic and Structural Properties of SrFe12−xCrxO19 (x = 0, 0.25, 0.5, 0.75, 1) Hexaferrite Powders Obtained by Sol–Gel Auto-Combustion Method

The crystalline structure and magnetic properties of chromium substituted hexagonal strontium ferrite, SrFe12−xCrxO19 (x = 0, 0.25, 0.5, 0.75, 1), nanoparticles have been investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), and vibrating sample magnetometer (VSM). These materials were prepared by the chemical sol–gel auto-combustion method. From the structural analysis, it was observed that the non-magnetic phase α-Fe2O3 appears after x ≥ 0.5 and remains to be a hexagonal magnetoplumbite phase for x < 0.5. Various parameters such as lattice constants (a and c) were calculated from the XRD data. The coercively H c initially decreases and then increases with increasing chromium concentration. The saturation magnetization exhibits an increase as the chromium content increases up to (x = 0.25) and then decreases with increase in the Cr content ion from 75.19 to 61.66 emu/g due to the substitution of Fe3+ ions by less magnetic Cr3+ ions. Furthermore, M r continually decreases with increase in the Cr content.

Physical and magnetic properties of (Ba/Sr) substituted magnesium nano ferrites

In the presented paper, strontium (Sr) and barium (Ba) nano ferrites were synthesized by citrate auto combustion method. The investigated samples are characterized by X-ray diffraction technique (XRD), field emission scanning electron microscopy, high resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. The structural properties of the obtained samples were examined by XRD analysis showing that the synthesized nanoparticles are in cubic spinel structure. The average crystallite sizes are in the range of 22.66 and 21.95 nm for Mg0.7Ba0.3Fe2O4 and Mg0.7 Sr0.3Fe2O4 respectively. The VSM analysis confirms the existence of ferromagnetic nature of Sr2+/Ba2+ substituted magnesium nano particles. Exchange interaction between hard (Sr/Ba) and soft (Mg) magnetic phases improves the structural and magnetic properties of nano ferrite particles. Rigidity modulus, longitudinal and shear wave velocities are predicted theoretically from Raman spectroscopy and structural data of the investigated spinel ferrite. The magnetic and structural properties of magnesium are enhanced by doping with barium and strontium nano particles. The saturation magnetization, remanent magnetization and coercivity reported on vibrating sample magnetometer curve illustrate the promising industrial and magnetic recording applications of the prepared samples.

Predicting Oxygen Vacancy Polaron Size from D-Orbital Splitting in ABO3 (A=La, Sr; B=Mn, Fe, Co) Perovskites

Mixed Ionic Electronic Conducting (MIEC) transition metal oxide (ABO3) perovskites are utilized for catalytic,1 energy conversion,2,3 and other applications. The oxygen ionic conductivity of these MIECs depends on their oxygen vacancy concentration and ionic mobility. In turn, the oxygen ion concentration and mobility both depend on the charge of the B-site atom,4 which itself depends on factors such as the A site La/Sr ratio, the crystal structure, etc. Previous calculations demonstrated that the Fe atom charge state distribution observed in strontium ferrite (SrFeO3-δ) is caused by differences in the d-orbital splitting of octahedral (Oh) versus square pyramidal (SP) Fe-O coordination (Figure 1).5 This d-orbital splitting ensures that the two electrons left behind when a charge-neutral oxygen vacancy forms in cubic SrFeO3-δ are transferred to the second nearest Fe neighbors,5 resulting in a large oxygen vacancy polaron size that produces strong oxygen vacancy interactions at high oxygen nonstoichiometry. As a result, the oxygen vacancy formation energy increases with oxygen nonstoichiometry in cubic SrFeO3-δ.5 Previous calculations also demonstrated that in orthorhombic lanthanum ferrite (LaFeO3-δ) the formation of a neutral oxygen vacancy produces a polaron that is localized to the oxygen-vacancy-adjacent Fe atoms. Hence, in orthorhombic LaFeO3-δ the oxygen vacancies do not interact with increasing oxygen nonstoichiometry.6 The objective of the present work was to examine whether d-orbital splitting could also explain the oxygen vacancy behavior of other MIEC perovskites. Hence, the impact of d-orbital splitting on oxygen vacancy polaron size and formation energies in lanthanum strontium manganite and lanthanum strontium cobaltite were modeled for the first time. Here, the charge redistribution caused by oxygen vacancy formation was studied for six different compositions: LaFeO3-δ, SrFeO3-δ, LaMnO3-δ, SrMnO3-δ, LaCoO3-δ, and SrCoO3-δ. The GGA+U method with PAW potentials implemented in VASP was utilized for all the structural energy calculations. First, the Hubbard-U parameter was calibrated to describe the charge on the B-site atom (U = 3 was selected for Fe, as done previously).6 The oxygen vacancy formation energy as a function of oxygen non-stoichiometry (δ) was then calculated at 0 K in vacuum by varying the size of the supercell, with oxygen vacancy interactions being enabled by the periodic boundary conditions. The calculated polaron size for neutral-oxygen-vacancy–containing ABO3 structures are summarized in Table 1.In LaMnO3-δ, Mn3+ has an [Ar] 3d4 electronic configuration in Oh coordination. After the formation of a neutral oxygen vacancy, the two Mn atoms adjacent to the oxygen vacancy go from Oh to SP coordination and the excess electrons left behind by the removed oxygen are pushed to the a1g level of the second nearest neighboring Oh-Mn (instead of remaining localized to the b1g level of the SP-Fe). This long range charge transfer results in large polaron size. This behavior is in contrast with that of LaFeO3-δ. However, LaMnO3-δ and SrFeO3-δ exhibit comparable polaron sizes since Fe4+ has an [Ar] 3d4 electronic- configuration. In SrMnO3-δ, Mn4+ has a [Ar] 3d3 electronic configuration in Oh coordination. Therefore, the formation of a neutral oxygen vacancy localizes the electrons to the oxygen vacancy adjacent Mn atoms, producing a small polaron, as explained by the d-orgbial splitting in Figure 1. This d-orbital splitting analysis indicates that the polaron size in LSM > LSF > LSC. Additional work aimed at calculating the oxygen vacancy formation energies of other ABO3 compositions are in progress.

Preparation of urchin-like strontium ferrites as microwave absorbing materials

The uniform urchin-like strontium ferrites (SrFe12O19) with a typical diameter of 3–4μm were prepared by a facile surfactant assisted hydrothermal method. A minimum reflection loss (RL) of −22.8dB was observed at 15.1GHz with a thickness of 3mm, and the effective absorption frequency (RL<−10dB) ranged from 12.4 to 18.0GHz, indicating the sound microwave absorption performance of the novel urchin-like SrFe12O19. The absorbing property of the urchin-like SrFe12O19 was better than that of the SrFe12O19 particles. The urchin-like structure of SrFe12O19 was responsible for the enhanced absorbing performances.

Properties characterization of tungsten doped strontium ferrites as cathode materials for intermediate temperature solid oxide fuel cells

Cobalt-free SrFe1-xWxO3-δ (SFW, x=0, 0.1, 0.2, 0.3) perovskite cathode materials were synthesized by the citrate-EDTA-nitrate combustion method and systematically characterized in terms of their phase structure, thermal sintering and expansion behavior, conductivity and catalytic activity on oxygen reduction reaction (ORR). All SFW powders exhibit cubic perovskite structure and the good chemical compatibility between SFW cathode and Ce0.8Sm0.2O2-δ (SDC) electrolyte is found. Introduction of W clearly inhibits the loss of lattice oxygen in temperature range of 400–900°C and decreases the thermal expansion coefficients. As the W-doping concentration increases, the thermal sinterabilty weakens with decreasing grain size, and the conductivity decreases from 118.2 to 6.1Scm−1 at 600°C. Among of SrFe1-xWxO3-δ-SDC composite cathodes, the SrFe0.8W0.2O3-δ-SDC cathode shows the smallest area specific resistance (ASR) value and the best ORR catalytic activity in range of 500–800°C. The anode-supported single cell based on SDC electrolyte and SrFe0.8W0.2O3-δ-SDC composite cathode generates high power density and low polarization resistance. At 700°C, the peak density and polarization resistance are 694mWcm−2 and 0.036Ωcm2, respectively.