Ferrite Perovskite Research Articles

Optimization of Working Conditions for Perovskite-Based Gas Sensor Devices by Multiregression Analysis

Environmental degradation and resource depletion drive scientific research priorities to develop technologies for sustainable productive systems. Among them, chemical sensing technology plays a key role for regulating energetic, ecological, and productive efficiency by monitoring and controlling the industrial processes. Semiconducting metal oxide sensors are particularly attractive technology because of their simplicity in function, small size, and projected low cost. The aim of this work is to synthesize Ti-substituted lanthanum ferrite perovskite, LaFe0.8Ti0.2O3, in order to develop a resistive sensor device for monitoring carbon monoxide. Since sensor performances are affected by experimental factors, such as temperature, target gas concentration, and gas flow rate, the aim of the authors was to define the optimum working condition by performing multiple regression analyses. The investigated ranges of operating conditions were temperature from 300 to 480°C, carbon monoxide concentration from 100 to 200 ppm, and inlet-gas flow rate from 40 to 100 cm3/min. The results confirm that the applied systematic analysis is a powerful method for studying the direct and indirect effects of every experimental factor on sensor performance and for computing mathematical models with predictive ability, that are useful tools for defining the optimum chemiresistors’ operating conditions. In addition, mathematical models are able to be used as multiple-factor surface calibration for restive gas sensor devices.

Thermal conductivity in self-assembled CoFe2O4/BiFeO3 vertical nanocomposite films

The thermal conductivity of self-assembled nanocomposite oxide films consisting of cobalt ferrite (CFO) spinel pillars grown within a single-crystal bismuth ferrite (BFO) perovskite matrix is described as a function of the volume fraction of the spinel. Single phase BFO and CFO had cross-plane thermal conductivities of 1.32 W m−1 K−1 and 3.94 W m−1 K−1, respectively, and the thermal conductivity of the nanocomposites increased with the CFO volume fraction within this range. A small increase (∼5%) in thermal conductivity for the pure CFO phase in the AC-demagnetized state was observed, suggesting possible magnon contributions. Steady state gray-medium based variance-reduced Monte Carlo simulations show consistent trends with experimental data on the dependence of thermal conductivity with the CFO volume fraction.

Band alignment and electrocatalytic activity at the p-n La0.88Sr0.12FeO3/SrTiO3(001) heterojunction

Ferrite perovskites have exhibited promising p-type conductivity and oxygen evolution reaction (OER) activity. In this work, we investigate heteroepitaxial p-n junctions formed by La0.88Sr0.12FeO3 and n-SrTiO3(001). Sr substitution for La in LaFeO3 is shown to be effective for introducing p-type conductivity, lowering the optical bandgap, and enhancing electrocatalytic OER. A staggered, type-II band alignment with a large built-in potential within the LSFO forms due to the polar interface. This electronic structure facilitates charge transfer across the p-n junction and accounts for the strongly thickness-dependent extent of OER we observe.

Alkaline metal doped strontium cobalt ferrite perovskites as cathodes for intermediate-temperature solid oxide fuel cells

Perovskite oxides Sr0.9K0.1FexCo1-xO3-δ (SKFCx, x = 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0) are investigated as potential cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs) on Sm0.2Ce0.8O1.9 (SDC) electrolyte. The cubic phase of the SKFCx oxides is demonstrated by x-ray diffraction. The SKFCx cathode shows good compatibility with the SDC electrolyte up to 900 °C. Among the investigated compositions, SKFC0.1 displays the highest electrical conductivity of 443–146 S·cm−1 from 350 °C to 800 °C in flow air. The area specific resistances (ASRs) of the SKFCx (x = 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0) cathodes are 0.047, 0.058, 0.066, 0.101, 0.155 and 0.175 Ω cm2 at 650 °C in air on an SDC electrolyte. Among the five tested cathodes, SKFC0.1 exhibits the lowest area specific resistances between 550 °C and 750 °C, when tested on its symmetric cell configuration of cathode|SDC|cathode. The thermally stabilized cubic perovskite structure of the SKFC0.1 powder is demonstrated by high-temperature XRD. The average linear thermal expansion coefficient αL of SKFC0.1 is 18.9×10−6 K−1. A peak power density of 1643 mW·cm−2 is achieved on SKFC0.1|SDC|Ni-SDC anode supported fuel cell at 650 °C. These features, and excellent electrocatalytic activity and good stability, indicate the potential of alkaline metal doped strontium cobalt ferrite perovskites are promising cathode materials for IT-SOFCs.

Titanium-substituted ferrite perovskite: An excellent sulfur and coking tolerant anode catalyst for SOFCs

Efficient and direct utilization of hydrocarbon fuel in a solid oxide fuel cell (SOFCs) is highly desirable. However, the carbon deposition and contaminants poisoning issue should be overcome. In this work, we developed a titanium-doped (La,Sr)FeO3 ceramic oxide (LSFT) anode which integrates the advantage of high durability of titanate perovskite and super electrocatalytic activity of ferrite-based perovskite for methane-fueled SOFCs with H2S impurity. Its electrocatalytic activity and operational durability in H2 with various H2S contents and humidified CH4 fuel were systematically investigated. We found that the LSFT perovskite oxide anode showed acceptable activity and durability toward methane direct oxidation, and excellent coke and sulfur tolerance up to 750 ppm, making it a promising electrocatalyst for direct use of hydrocarbon fuels such as natural gas in high-temperature SOFCs.

Ferro- and magneto-electric characteristics in YFeO3−Y3Fe5O12 nanocomposites

The magnetoelectric (ME) nanocomposite material contain ferromagnetic phase of yttrium iron garnet (Y3Fe5O12) and ferroelectric phase of yttrium perovskite ferrite (YFeO3) with an equation (x)YFeO3 + (1 − x)Y3Fe5O12 (x = 0.0, 0.25, 0.50, 0.75, 1.0) is synthesized by sol–gel auto combustion method. The composite powder obtained by two phases of composite material sintered at 1100 °C for 6 h. The prepared samples were investigated by using powder X-ray diffraction (XRD) to study the structural properties, that confirm the existence of two phases of Y3Fe5O12 and YFeO3. The crystallite size of material decreased from 23 to 13 nm with increasing the concentration of YFeO3 in composite. The morphological investigation of samples was studied by field emission scanning electron microscopy that gives size, shape and distribution of grains in material and confirmed that material has nano-crystalline in nature. Saturation magnetization (Ms) studied by using vibrating sample magnetometer shows the decrement in its value from 23.9 to 0.48 emu/g with the increase in YFeO3 content. Maximum magnetoelectric coefficient (αME) of 10.711 mv/cm-Oe is observed for composition x = 0.50. Similarly, ferroelectric characteristics of sample such as dielectric constant and dielectric loss were measured in the frequency range 20 Hz to 10 MHz at room temperature. The value of dielectric constant is decreases with increasing frequency and ferroelectric hysteresis loop at room temperature is study by using P-E loop tracer it’s nature is unsaturated due to presence of Y3Fe5O12 phase.

Strontium-free rare earth perovskite ferrites with fast oxygen exchange kinetics: Experiment and theory

The Sr-free mixed ionic electronic conducting perovskites La0.8Ca0.2FeO3-δ (LCF82) and Pr0.8Ca0.2FeO3-δ (PCF82) were synthesized via a glycine-nitrate process. Crystal structure, phase purity, and lattice constants were determined by XRD and Rietveld analysis. The oxygen exchange kinetics and the electronic conductivity were obtained from in-situ dc-conductivity relaxation experiments at 600–800°C and 1×10−3≤pO2/bar≤0.1. Both LCF82 and PCF82 show exceptionally fast chemical surface exchange coefficients and chemical diffusion coefficients of oxygen. The oxygen nonstochiometry of LCF82 and PCF82 was determined by precision thermogravimetry. A point defect model was used to calculate the thermodynamic factors of oxygen and to estimate self-diffusion coefficients and ionic conductivities. Density Functional Theory (DFT) calculations on the crystal structure, oxygen vacancy formation as well as oxygen migration energies are in excellent agreement with the experimental values. Due to their favourable properties both LCF82 and PCF82 are of interest for applications in solid oxide fuel cell cathodes, solid oxide electrolyser cell anodes, oxygen separation membranes, catalysts, or electrochemical sensors.

Silver-modified manganite and ferrite perovskites for catalyzed gasoline particulate filters

Silver modified manganite (LaSrAgMn) and ferrite (LaSrAgFe) perovskites were synthesized with different Ag loadings and tested for the soot oxidation reaction at low oxygen concentrations (i.e. 1 vol%), as encountered in gasoline direct injection (GDI) engines application. The state and location of Ag determine the catalytic performances of perovskites. On ferrites, Ag is either partially incorporated on the structure or present on the surface as large agglomerates. Lattice Ag+ cations are not catalytically active. Furthermore, large surface Ag particles partially cover surface oxygen vacancies and block their ability to adsorb oxygen. On the other hand, small clusters of Ag and AgO as well as a surface Ag1.8Mn8O16 additional hollandite-type phase are stabilized on manganites surface, improving the reducibility and acting as active sites for soot oxidation.

Ruddlesden-Popper compounds (SrO)(LaFeO3)n (n = 1 and 2) as p-type semiconductors for photocatalytic hydrogen production

Here we report two Ruddlesden-Popper type ferrite perovskites (SrO)(LaFeO3)n (n=1 and 2) which demonstrate p-type semiconductivity. Their crystal structure, optical absorption and other physicochemical properties have been systematically explored. Our results show that both ferrites crystallize in tetragonal symmetry with structural lamination along c axis. Efficient photocatalytic hydrogen production has been achieved for both samples under full range and visible light illumination. Better performance is noticed for LaSrFeO4 with apparent quantum efficiency approaches 0.31% and 0.19% under full range and visible light illumination, respectively. The p-type semiconductivity is verified by their cathodic photocurrent as well as negative Mott-Schottky slop during Photoelectrochemical measurement. The relative lower activity for La2SrFe2O7 compared to LaSrFeO4 is likely due to its higher defect concentration which facilitates charge recombination. Both compounds exhibit anisotropic phenomenon for charge migrations according to theoretical calculations. Their p-type semiconductivity, strong visible light absorption, chemical inertness and high abundance of constituent elements signify promising applications in the field of solar energy conversion and optoelectronics.

Redox thermodynamics and phase composition in the system SrFeO3 − δ — SrMnO3 − δ

Perovskite oxides are considered promising redox materials for many fields of application, such as chemical looping processes for thermochemical air separation, oxygen pumping and fuel production, in particular considering a solar heat source. The large range of possible perovskite compositions can be extended by the synthesis of solid solutions between different perovskite phases. In this work, the solid solution formation in the system SrMn1−xFexO3−δ is evaluated, showing that SrFeO3−δ and SrMnO3−δ are miscible in any ratio investigated. Moreover, redox thermodynamics were studied using a van't Hoff approach. It has been found that in mixed manganese ferrite perovskites, the reduction of Fe4+ is preferred over the reduction of Mn4+, leading to an increase in redox enthalpy and entropy when both species are reduced. Our findings allow the targeted synthesis of perovskites with superior redox properties for applications in chemical looping processes, as the redox thermodynamics are adjustable via variation of the Fe content.

Reduction kinetics of lanthanum ferrite perovskite for the production of synthesis gas by chemical-looping methane reforming

The reduction kinetics of LaFeO3 oxygen carrier for the production of synthesis gas by chemical-looping methane reforming (CLMR) is investigated in the present work. The stoichiometric relationship between oxygen carrier and methane in chemical-looping process is established by temperature programmed surface reaction (TPSR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) characterization. The effects of reduction conditions, namely, gas to solid molar ratio, temperature and sequential redox reaction on the oxygen carrier conversion are investigated. The results from Hancock and Sharp method suggest that the phase boundary control is likely to be dominant. The power law model (PLM), shrinking core model (SCM), as well as nucleation and nuclei growth model (NNGM) are considered to interpret the reduction kinetics of LaFeO3 with methane to synthesis gas. The obtained results show that the 2-D NNGM best describes the experimental data providing parameters with adequate statistical fitting indicators. The model validity is also verified by the data from continuous flow reaction and sequential redox cycles to simulate chemical-looping methane reforming process. The apparent activation energy is estimated and compared with values from the literature data.

Niobium doped lanthanum calcium ferrite perovskite as a novel electrode material for symmetrical solid oxide fuel cells

Abstract Development of cost-effective and efficient electrochemical catalysts for the fuel cells electrode is of prime importance to emerging renewable energy technologies. Here, we report for the first time the novel La0.9Ca0.1Fe0.9Nb0.1O3−δ (LCFNb) perovskite with good potentiality for the electrode material of the symmetrical solid oxide fuel cells (SSOFC). The Sc0.2Zr0.8O2−δ (SSZ) electrolyte supported symmetrical cells with impregnated LCFNb and LCFNb/SDC (Ce0.8Sm0.2O2−δ) electrodes achieve relatively high power outputs with maximum power densities (MPDs) reaching up to 392 and 528.6 mW cm−2 at 850 °C in dry H2, respectively, indicating the excellent electro-catalytic activity of LCFNb towards both hydrogen oxidation and oxygen reduction. Besides, the MPDs of the symmetrical cells with LCFNb/SDC composite electrodes in CO and syngas (CO: H2 = 1:1) are almost identical to those in H2, implying that LCFNb material has similar catalytic activities to carbon monoxide compared with hydrogen. High durability in both H2, CO and syngas during the short term stability tests for 50 h are also obtained, showing desirable structure stability, and carbon deposition resistance of LCFNb based electrodes. The present results indicate that the LCFNb perovskite with remarkable cell performance is a promising electrode material for symmetrical SOFCs.

Cu-substituted lanthanum ferrite perovskites: Preparation, characterization and photocatalytic activity in gas-solid regime under simulated solar light irradiation

LaFeO3 perovskites, both bare and containing 5–40 mol% of Cu(II) substituting Fe(III), have been prepared by the citrate autocombustion synthesis method and used as heterogeneous photocatalysts for the degradation of 2-propanol in gas-solid regime under simulated solar light irradiation. Bare LaFeO3 resulted inactive, whereas Cu(II) containing perovskites oxidized 2-propanol to propanone, and eventually to CO2 and H2O. The photocatalytic activity of the Cu containing LaFeO3 perovskites increased by increasing the Cu amount up to 10 mol% and hence slightly decreased for higher copper contents. The samples were fully characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman and diffuse reflectance (DRS) spectroscopies, X-ray photoelectron spectroscopy (XPS) and photocurrent measurements. The photocatalytic activity was correlated with compositional, structural and morphological features.

Antimony doped barium strontium ferrite perovskites as novel cathodes for intermediate-temperature solid oxide fuel cells

Antimony was doped to barium strontium ferrite to produce ferrite-based perovskites with a composition of Ba0.5Sr0.5Fe1−xSbxO3−δ (x = 0.0, 0.05, 0.1) as novel cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The perovskite properties including oxygen nonstoichiometry (δ), mean valence of B-site, tolerance factors, thermal expansion coefficient (TEC) and electrical conductivity (σ) are explored as a function of antimony content. By defect chemistry analysis, the TECs decrease since the variable oxygen vacancy concentration is decreased by Sb doping, and σ decreases with x due to the reduced charge concentration of Fe4+ content. Consequently, the electrochemical performance was substantially improved and the interfacial polarization resistance was reduced from 0.213 to 0.120 Ωcm2 at 700 °C with Sb doping. The perovskite with x = 1.0 is suggested as the most promising composition as SOFC cathode material.

Linear magnetoelectricity at room temperature in perovskite superlattices by design

Discovering materials that display a linear magnetoelectric (ME) effect at room temperature is a challenge. Such materials could facilitate devices based on the electric field control of magnetism. Here we present simple, chemically intuitive design rules to identify a class of bulk magnetoelectric materials based on the ``bicolor'' layering of $Pbnm$ ferrite perovskites, e.g., ${\mathrm{LaFeO}}_{3}/{\mathrm{LnFeO}}_{3}$ superlattices, Ln = lanthanide cation. We use first-principles density functional theory calculations to confirm these ideas. We elucidate the origin of this effect and show it is a general consequence of the layering of any bicolor $Pbnm$ perovskite superlattice in which the number of constituent layers are odd (leading to a form of hybrid improper ferroelectricity). Our calculations suggest that the ME effect in these superlattices is larger than that observed in the prototypical magnetoelectric materials ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{BiFeO}}_{3}$. Furthermore, in these proposed materials, the strength of the linear ME coupling increases with the magnitude of the induced spontaneous polarization which is controlled by the La/Ln cation radius mismatch. We use a simple mean field model to show that the proposed materials order magnetically above room temperature.

Electrochemical Characterization of Ag-Y0.5Bi1.5O3 Composite Cathode on 8YSZ-Ce0.78Gd0.2Sr0.02O2-δ Electrolyte Using AC Impedance Spectroscopy

The electrochemical performance of Ag-Y0.5Bi1.5O3 (YSB) cathodes on Ce0.78Gd0.2Sr0.02O2-δ (GDCS) electrolyte with an epitaxial Y0.16Zr0.84O1.92 (8YSZ) thin film outer layer was estimated using electrochemical impedance spectroscopy (EIS). A series of bismuth oxides with different silver contents were prepared and measured by an impedance spectroscopy on symmetrical cells in the temperature range from 500 °C to 650 °C. The microstructural analyses of porous composite cathodes and bi-layer electrolyte with fluorite structures were investigated by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that silver in the cathode reduces the surface resistance which improves the efficiency of current collection from the cathode. Besides, the bi-layer electrolytes have lower activation energy than GDCS electrolytes. Without topping Pt or silver metallic working electrode, the 50-70 vol% of Ag powder mixed with a submicron size YSB powder exhibited much lower overpotential and higher exchange current density than Lanthanum Strontium Cobalt Ferrite perovskite cathodes below 650 °C.

Study of critical behavior of perovskite La0.8Ca0.2−xPbxFeO3 (x = 0.0, 0.1 and 0.2) compounds

The critical properties of ferrite perovskite La0.8Ca0.2−xPbxFeO3 (x=0.0, 0.1 and 0.2) compounds at the ferromagnetic–paramagnetic (FM–PM) transition have been analyzed. Experimental results revealed that all samples exhibit a second-order magnetic phase transition. The estimated critical exponents derived from the magnetic data using various techniques such as modified Arrott plot, Kouvel–Fisher method, and critical magnetization isotherms M (TC, H). The critical exponent values for all samples are in a close agreement with those predicted for the mean-field theory. The reason for this behavior may be long-range interactions between spins in this system.

Effect of Ce Doping on the Performance and Stability of Strontium Cobalt Ferrite Perovskites as SOFC Anode Catalysts

The bulk structure, surface characteristics, catalytic properties, button cell performance, and stability of perovskite-type oxides with the formula Sr1−xCexCo0.2Fe0.8O3−δ (x = 0.10, 0.15, and 0.20) were investigated as potential anode electro-catalysts for solid oxide fuel cells. In situ X-ray diffraction results indicated that the structural stability of the cerium-doped perovskites was enhanced with increasing cerium concentration. The catalysts with lower cerium concentration transformed from the initial cubic structure to a lower symmetry structure at high temperatures in a reducing environment. All the catalysts, however, were restored to the original cubic structure after re-oxidation. Electronic conductivity tests showed that the cerium-doped samples break down at higher temperatures, but display potential in terms of structural integrity and high conductivity at temperatures lower than 600 °C. The new materials showed better performance than Ni–YSZ in catalytic methane oxidation tests. The sulfur tolerance is significantly enhanced with the cerium-doped perovskites, showing no deactivation for up to 10 h of operation under 50 ppm H2S. X-ray photoelectron spectroscopy analysis on the poisoned samples suggests that the sulfur may exist on the surface primarily as strontium sulfate. The sample with a cerium loading of 15 % (x = 0.15) showed a particularly high sulfur tolerance during chronoamperometry tests and its button cell performance matched well with that of Ni–YSZ anode. Sr1−xCexCo0.2Fe0.8O3−δ (x = 0.10, 0.15, and 0.20) samples may have potential for low or intermediate temperature SOFC operation in which sulfur-containing hydrocarbon fuels are used, although they would not be stable at higher temperatures.

Bismuth Doped Lanthanum Ferrite Perovskites as Novel Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells

Bismuth is doped to lanthanum strontium ferrite to produce ferrite-based perovskites with a composition of La(0.8-x)Bi(x)Sr0.2FeO(3-δ) (0 ≤ x ≤ 0.8) as novel cathode material for intermediate-temperature solid oxide fuel cells. The perovskite properties including oxygen nonstoichiometry coefficient (δ), average valence of Fe, sinterability, thermal expansion coefficient, electrical conductivity (σ), oxygen chemical surface exchange coefficient (K(chem)), and chemical diffusion coefficient (D(chem)) are explored as a function of bismuth content. While σ decreases with x due to the reduced Fe(4+) content, D(chem) and K(chem) increase since the oxygen vacancy concentration is increased by Bi doping. Consequently, the electrochemical performance is substantially improved and the interfacial polarization resistance is reduced from 1.0 to 0.10 Ω cm(2) at 700 °C with Bi doping. The perovskite with x = 0.4 is suggested as the most promising composition as solid oxide fuel cell cathode material since it has demonstrated high electrical conductivity and low interfacial polarization resistance.

Phase Formation, Microstructure, Magnetic and Electrical Properties of (1-x)Mg0.7Zn0.3Fe2O4-xBa0.7Sr0.3TiO3 Ceramics Composite

(1-x)Mg0.7Zn0.3Fe2O4-(x)Ba0.7Sr0.3TiO3 ceramics composite with x = 0.0, 0.2, 0.4, 0.6 and 1.0 were prepared by the solid state reaction method. The spinel structure of the ferrite phase and perovskite structure of ferroelectric phase were confirmed by x-ray diffraction (XRD). SEM micrographs reveal compatibility of ferrite grains with ferroelectric grains. It can be seen size of grain decrease with increase Ba0.7Sr0.3TiO3 phase concentrations. The magnetic property can be identified from the magnetic permeability of the sample and the values were observed to be 0.53–0.83 N/A2 and its increases with the increasing of Mg0.7Zn0.3Fe2O4 phase contents. The electrical resistivity values were found to be in the range ∼0.06–52.99 MΩm. The dielectric constant was determined as a function of frequency (in the range 100–104 Hz). The results also reveal that for the lower concentrations of Mg0.7Zn0.3Fe2O4, ferroelectric behavior is noticed with a high electrical resistivity and dielectric constant.