Function Of Oxygen Partial Pressure Research Articles

Correlation of Surface Oxidation and Mg-Based Intermetallic Phases in Grain Boundaries of Al–Mg Alloys Containing Third Elements

In this study, the correlation of surface oxidation and Mg-based intermetallic phases in the grain boundary in Al-Mg alloys containing third elements was investigated. The experimental results were examined by phase diagrams plotted as a function of oxygen partial pressure determined by thermodynamic calculation. The addition of Si and Cu as third elements into the Al-7 mass%Mg alloy formed Mg-based secondary phases during solidification. The 1 mass% Cu addition formed three different types of Mg-based intermetallic compounds. From weight gains by oxidation, all samples exhibited their weight gains depending on time. The Si-added alloy showed a considerably lower weight gain and maintained a nearly constant weight, while the weight gain of the Al-7 mass%Mg-1 mass%Cu alloy was significantly greater than those of other alloys. MgO and MgAl₂O4- spinel were the main oxides that formed the oxide scale in all examined alloys. Si addition formed the multi-element oxide including Mg and Si.

Experimental Study and Thermodynamic Calculations in the CaO–Cu2O–FeO–Fe2O3–SiO2 System for Applications in Novel Copper-Based Processes

A combination of high-temperature experiments and thermodynamic modeling in the CaO–Cu2O–FeO–Fe2O3–SiO2 system provides the fundamental information necessary to design and improve processes such as WEEE recycling through black copper route, continuous converting of copper matte, copper dross smelting, slag cleaning, and valorization of copper slag. In the experiments, samples were equilibrated on a primary phase substrate or Fe metal foil, then rapidly quenched and studied using the Electron Probe X-ray Microanalysis. The FactSage software was used for calculations, combined with custom developed thermodynamic database. A systematic analysis is provided to select the fluxing strategy and increase the copper recovery in industrial processes. Slag liquidus and concentration of copper in slag were assessed as functions of oxygen partial pressure, temperature, Fe/SiO2 in slag, and wt% CaO in slag.

Effect of zinc doping on electrical properties of LaAlO3 perovskite

New solid solution with the general formula of LaAl1-xZnxO3-1/2x was prepared by a solid-state reaction route. According to XRD, the crystal structure of LaAlO3 is rhombohedral, while the solid solution possesses cubic symmetry. Homogeneity region of the solid solution LaAl1-xZnxO3-1/2x was narrow and limited to the maximum concentration of 5 mol. %. Computer simulations using crystalochemistry and density functional theory approaches showed that LaAlO3 has high energy barriers for O2–-ion transport (2.79 eV). These results are in good agreement with the low values of electrical conductivity obtained experimentally. The electrical conductivity of LaAl1-xZnxO3-1/2x was measured by impedance spectroscopy in the temperature range of 200–1000 °C. The partial substitution of Al3+ by Zn2+ was found to increase the electrical conductivity by ~2 order of magnitude. The electrical conductivity of doped phase LaAl0.95Zn0.05O2.975 as a function of oxygen partial pressure was measured, and the partial contributions (oxygen-ionic and electronic) were determined. It was found that the sample has mixed ionic and p-type electronic conductivity, while the electronic contribution increases with the rise of the temperature.

Defect structure and redox energetics of NdBaCo2O6-δ

The oxygen content in the double perovskite NdBaCo2O6-δ was measured as a function of oxygen partial pressure (1–10−5 atm) at different temperatures (623–1323 K) using thermogravimetry analysis (TG) and coulometric titration technique. The defect structure model of the oxide studied was proposed and successfully verified using the pO2 – T – δ data, yielding the enthalpies and entropies of the defect reactions involved. Standard enthalpy of formation from elements (ΔfH298∘) was measured for quenched NdBaCo2O6-δ samples with different oxygen content. Regarding the ΔfH298∘(6-δ) dependences, two important observations were made. First, the ΔfH298∘(6-δ) consists of two linear fragments with different slopes reflecting different modes of oxygen exchange, accompanied by either reduction of cobalt from Co+3 to Co+2 or from Co+4 to Co+3 in the ranges 5.0 ≤ 6-δ ≤5.5 and 5.5 < 6-δ < 5.7, respectively. Second, when corrected for the quenching effect arising from the oxygen vacancy disordering, the experimental ΔfH298∘(6-δ) values and those calculated using the partial molar enthalpy of oxygen, which was evaluated using the defect structure model, agree well with each other. This can be regarded as an independent verification of both the model and the related defect reaction thermodynamics.

Correlation between Fabrication and Operation Conditions for Ce1-XZrxO2-σ (x~0.2) Stability

CeO2 shows high oxygen storage capacity (OSC) by changing the valency of Ce depending on oxygen partial pressures. The addition of Zr into CeO2 (CZO) improves OSC further and has been used in three-way catalysts (TWCs). Recently, researchers applied CZO as an anode in solid oxide fuel cells (SOFCs). Because the operating temperature of SOFCs (~1000K) is higher than that of TWCs (~600K), the thermal stability of CZO becomes an issue. When CZO is fabricated in oxidizing condition, the distribution of metal atoms in CZO would be less stable under reducing operation condition. In this study, we studied the structural stability of CZO as a function of oxygen partial pressure for fabrication and operation conditions; the oxygen partial pressure was varied by changing the number of oxygen vacancies in the model structures. To find energetically favorable structures efficiently among the huge number of possible structures, we used genetic algorithm and lattice statics.

Influence of Nanoscale Multilayered Proton Conducting Electrolytes for the Faraday Efficient of Solid Oxide Electrolysers

Solid oxide fuel cells (SOFCs) have been developed over half century as highly efficient and clear power generation devices, and are expected to play an important role in the electrical grids for absorbing output fluctuations of renewable energy power sources by providing new function as solid oxide electrolytic cells and working as SOFC / SOECs. For two decades, replacing of oxygen ionic conductors in SOFC to proton conductors, which show higher ionic conductivity at intermediate temperatures (400 - 600℃), have been trying to increase fuel utilization, and reduce the cost of cells and stacks. Such proton conducting SOFCs are also expected to be work as proton conducting SOECs. Among many proton conductors discovered for almost four decades, proton conductors based on perovskite type oxides such as strontium zirconate, barium zirconate and barium cerate show sufficient proton conductivity for practical application. Recent reports show that related materials Y and Yb doped Ba(Ce, Zr)O₃ has excellent conductivity and chemical stability, and further improvement of proton conducting SOFC’s performance is achieved. However, such acceptor doped perovskite oxides exhibit not only proton conduction but also oxygen ionic, and electronic conduction. Among them, hole conduction under high oxygen partial pressure has non negligible impact on the performance of SOFC/SOECs because it exposes to air atmospheres when working as a generator and an electrolyser. The influences of leakage current caused by hole conduction are observed as low electro motive forces, and appeared as the decrease of faraday efficiency as the electrolyser. As developing progressed, such influence on proton conducting SOFC/SOECs recently have come to the attention. Onishi et al. reports that faraday efficiency is influenced by hole conductivity as a function of oxygen partial pressure, applied bias voltage, and over potential at air electrode, and their calculation suggests that if air and fuel electrodes over potential are sufficiently small for the SOEC operation, its faraday efficiency become quite low[1]. Therefore, suppressing of electronic conduction is essential problem for feasible proton conducting SOECs. However, since both holes and protons are introduced via oxygen vacancies, it is unavoidable to introduce holes, and no material that can achieve both high proton conductivity and low hole conductivity has been found at present. Introducing an interlayer to prevent leakage current is an effective method and many examples have been reported for oxygen ionic conductors. However, the adoption of this concept for proton conducting SOECs considering whole electrodes are remained as new challenges especially to material selecting, and reduction of electrode overpotential at air side. We propose nanoscale multilayered electrolyte includes both surface modification layer, which is thin and inserted to only improve air overpotential, and hole blocking layer to solve this challenge.This research focuses on the material selection for the hole blocking layer. The influence of interlayer with various hole and proton conductivities and thickness range of 100 nm to several micron meter for the hole current suppressing was numerically studied. In the research, the ionic and electronic current densities and oxygen partial pressure distribution inside the multilayered electrolyte were calculated to investigate the faraday efficiency in SOECs operation. In the calculation, according to the calculation method of Onishi et al., the faraday efficiency was evaluated for various interlayers with different proton and hole conductivities. Figure 1 shows typical example of the calculation assuming sufficiently small electrode overpotential (0.1 Ωcm²), and an interlayer with low proton and hole conductivities. As an interlayer, a proton conductor, which had lower proton and hole conductivities than 20 mol% yttrium doped barium zirconate (BZY20) was employed. In addition, the ratio of hole to proton conductivities was one tenth of BZY20. This ratio is referred from the most promising reported value [2]. Fig.1 clearly indicates that the faraday efficiency could be improved significantly only by the thin interlayer. For example, the 100 nm interlayer improved faraday efficiency to be over 80% from 51% of BZY single layer at applied voltage of 1.3 V, although the protonic current only reduced by 4.7%, and the efficiency reached over 90 % for 500 nm interlayer. However, protonic current decreased by 17%. Hence, this result suggest that optimal thickness exist for specific operation conditions.

Signature of Oxide-Ion Conduction in Alkaline-Earth-Metal-Doped Y3GaO6.

We have studied alkaline-earth-metal-doped Y3GaO6 as a new family of oxide-ion conductor. Solid solutions of Y3GaO6 and 2% −Ca2+-, −Sr2+-, and −Ba2+-doped Y3GaO6, i.e., Y(3–0.06)M0.06GaO6−δ (M = Ca2+, Sr2+, and Ba2+), were prepared via a conventional solid-state reaction route. X-ray Rietveld refined diffractograms of all the compositions showed the formation of an orthorhombic structure having the Cmc21 space group. Scanning electron microscopy (SEM) images revealed that the substitution of alkaline-earth metal ions promotes grain growth. Aliovalent doping of Ca2+, Sr2+, and Ba2+ enhanced the conductivity by increasing the oxygen vacancy concentration. However, among all of the studied dopants, 2% Ca2+-doped Y3GaO6 was found to be more effective in increasing the ionic conductivity as ionic radii mismatch is minimum for Y3+/Ca2+. The total conductivity of 2% Ca-doped Y3GaO6 composition calculated using the complex impedance plot was found to be ∼0.14 × 10–3 S cm–1 at 700 °C, which is comparable to many other reported solid electrolytes at the same temperature, making it a potential candidate for future electrolyte material for solid oxide fuel cells (SOFCs). Total electrical conductivity measurement as a function of oxygen partial pressure suggests dominating oxide-ion conduction in a wide range of oxygen partial pressure (ca. 10–20–10–4 atm). The oxygen-ion transport is attributed to the presence of oxygen vacancies that arise from doping and conducting oxide-ion layers of one, two-, or three-dimensional channels within the crystal structure. The oxide-ion migration pathways were analyzed by the bond valence site energy (BVSE)-based approach. Photoluminescence analysis, dilatometry, Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy studies were also performed to verify the experimental findings.

Iron activity measurements and spinel-slag equilibria in alumina-bearing iron silicate slags

Alumina is a common substance deporting in copper smelting slags when various secondary copper fractions, e.g. e-scrap or WEEE, are used as feedstock as such or along with primary sulphide concentrates. Properties of iron-silicate slags at high alumina concentrations, in the iron-alumina spinel saturation, have been studied at 1300 °C by a high temperature equilibration-quenching method combined with EPMA (electron probe microanalysis) phase composition data from the polished sections. The equilibrations were performed in fixed oxygen activity with platinum or palladium powder, which dissolved iron from the slag and generated a heterogeneous equilibrium system, characterised by the general equilibrium criterium in isothermal and isobaric conditions, as.μalloy(Fe) = μslag(Fe) = μspinel(Fe).This criterium was used for measuring experimentally iron activities of molten silicate slags. The locations of the spinel-liquid slag tie-lines were also determined in the oxygen partial pressure range of 10−6–10−10 atm. A comparison with the recent critical thermodynamic assessments of the Fe–O–Al2O3 system indicates that the iron-alumina spinel-corundum phase boundary in silica-containing systems as a function of oxygen partial pressure is too steep and thus the assessed databases do not match with the experimental data of this study. The liquid slag domain from silica to iron oxide saturation is also smaller than expected earlier, as the spinel primary phase boundary locates at higher silica concentrations than e.g. obtained in the assessments of the Mtox database.

Defect Chemistry and Oxygen Ionic Conducting Properties of Gd2O3 at Elevated Temperatures

Gadolinium sesquioxide (Gd2O3), an oxide exhibiting outstanding thermal stability, wide bandgap (>5eV) and relatively high dielectric constant (>14), is attractive as a high-k dielectric for CMOS, particularly when high temperature processing is required. However, in contrast to its conventional application as an electrical insulator, recent research on magneto-ionic devices suggests that Gd2O3 can serve as a solid electrolyte for field driven devices operating near room temperature. [1][2] According to previous research, proton transport in Gd2O3 is relatively swift among the binary oxides. [3] However, the defect chemistry and the oxygen ion conductivity of Gd2O3 have only been researched to a limited extent. In this work, we investigate the defect chemistry and electrical properties of doped and phase controlled Gd2O3 by impedance spectroscopy, as a function of oxygen partial pressure and temperature.Nominally undoped, as well as intentionally donor and acceptor doped cubic and monoclinic phase polycrystalline specimens of Gd2O3 were prepared. Studies on the two different phases were possible since the monoclinic phase can stabilize below ~1420 K as a metastable phase. Ca2+& Sr2+and Ce4+& Zr4+ were used as acceptor and donor dopants respectively. 2-point impedance measurements were conducted on samples over a wide range of pO2 (10-12 atm – 1 atm) and for 5 isotherms ranging from 695 – 895 ˚C.By analyzing the measured conductivity in terms of the developed defect model, relevant thermodynamic and kinetic parameters were extracted. In bixbyite (defective fluorite) phase Gd2O3, the formation enthalpy of anion Frenkel pairs was found to be 1.8 eV, lower than those of related fluorite oxides. The monoclinic phase Gd2O3 showed a somewhat higher, but still a rather low anion Frenkel pair formation enthalpy (2.1eV). Surprisingly, the more close-packed monoclinic structured Gd2O3 exhibited higher oxygen ion conductivity than more open cubic structured Gd2O3. The most highly ionic conducting samples of each phase were Ce doped cubic and Sr doped monoclinic Gd2O3, although the donor doped cubic had a significantly lower ionic conductivity and larger migration energy (1.7 eV and 0.8 eV respectively). All the Gd2O3 specimens showed mixed ionic and p-type conduction in synthetic air within the measured temperature range, implying excess oxygen nonstoichiometry.

Oxygen surface exchange kinetics and electronic conductivity of the third-order Ruddlesden-Popper phase Pr4Ni2.7Co0.3O10-δ

The third-order Ruddlesden-Popper phase Pr4Ni2.7Co0.3O10-δ (PNCO43) was synthesized by a freeze drying process. Phase purity and crystal structure were determined by X-ray diffraction and Rietveld analysis. The electronic conductivity of a bulk sample obtained by a two-step sintering process was measured by the four-point dc van der Pauw method as a function of temperature (50 ≤ T/°C ≤ 800) and oxygen partial pressure (1 × 10−3 ≤ pO2/bar ≤1). Dense thin-film PNCO43 microelectrodes were prepared by pulsed laser deposition and photolithography on yttria-stabilised zirconia substrates. The thin-films were characterized by X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy, and inductively coupled plasma optical emission spectroscopy. Individual resistive and capacitive processes were investigated with electrochemical impedance spectroscopy as a function of the oxygen partial pressure (1 × 10−3 ≤ pO2/bar ≤1) and temperature (600 ≤ T/°C ≤ 850). Oxygen surface exchange coefficients kq, calculated from the resistance of the electrode, show relatively high values (e.g. kq = 1.5 × 10−6 cm s−1 at 800 °C and 2 × 10−1 bar pO2). Chemical surface exchange coefficients kchem of oxygen were obtained from the peak frequency or the chemical capacitance as determined by impedance spectroscopy.

Structural, Optical and Electrical Properties of Electron Beam Evaporated Tioxny Films as Selective Solar Absorber Coatings

Titanium oxinitride (TiOx Ny ) solar absorber coatings were deposited at different oxygen partial pressures onto Cu, Si and glass substrates using electron beam evaporation technique. XRD diffraction patterns evidenced (111), (200) and (220) orientation of TiNx phase. The preferred orientation of the films changed with oxygen partial pressure. XPS revealed the intensity of both Ti 2P3/2 and Ti 2P1/2 increases as a function of oxygen flow, and also shifted towards higher binding energy, indicating more oxidized state of Ti species than that of TiO2 due to incorporation of nitrogen atoms. Formation of uniformly distributed spherical like particles and an increase in surface roughness of the TiOx Ny films were observed as a function of oxygen partial pressure as depicted from SEM and AFM, respectively. Ellipsometric and resistivity measurements showed a shift from metallic to semiconductor behaviour of the TiOx Ny films as oxygen flow changed. A solar absorptance value of 0.94 in the solar spectrum region and a low thermal emittance value of 0.05 were achieved for the TiOx Ny solar absorber coatings prepared at the oxygen partial pressure of 7.5x10-5 Torr due to both interference and intrinsic absorption. This study confirmed that a single layer of TiOx Ny film can be a good candidate as selective solar absorber.

Performance of GDC with alkali metal carbonates as sintering aids

Gd-doped ceria electrolytes (GDC, 10 at.% Gd) were prepared using a eutectic mixture of Li and Na carbonates as sintering aid (NLC - 0, 1, 3 and 5 mol%), using two processing routes (ceramic and chemical). Structural and microstructural characterization provided no evidence for significant solubility of alkali metal cations in GDC. Materials sintered at 1100 °C/4 h were similar in densification (>95%) and microstructure to those obtained from a commercial powder sintered at 1500 °C/4 h. Impedance spectroscopy in air (200–600 °C) and as a function of oxygen partial pressure (from 0.21 to about 10−25 atm, 600–750 °C) showed that the low temperature conductivity of materials prepared using chemical synthesis surpassed the conductivity of a commercial GDC due to higher grain boundary conductivity. The estimated n-type conductivity of materials with NLC was also slightly higher than for pure GDC. The role of sintering aid and processing routes is discussed taking into consideration decomposition and volatilization of alkali salts.

Control of Platinum Loss in WEEE Smelting

In spite of significant economic value, the solubilities of the platinum group and precious metals in metallurgical copper smelting slags are not well known. Recent experimental information on iron-free and low-iron silicate melts indicates that the chemical solubility of platinum is very low, < 1 ppmw (part per million weight). In this study, the concentration of platinum in alumina spinel-saturated iron silicate slags in equilibrium with a solid iron-platinum alloy was measured as a function of oxygen partial pressure at 1300°C. The results were converted to unit activity of platinum by the thermodynamic properties of the iron-platinum alloy formed. This allowed the mechanism of dissolution of platinum in the slag and the forms of platinum species in alumina-rich iron silicate slags in copper scrap smelting and refining conditions to be obtained. Our findings explain some inconsistent results in the geochemical literature by proposing an anionic dissolution mechanism at low oxygen partial pressures in iron-containing silicate slags.

Effect of Structure on Sensor Properties of Oxygen-Deficient Perovskites, A2BB′O5 (A = Ca, Sr; B = Fe; B′ = Fe, Mn) for Oxygen, Carbon Dioxide and Carbon Monoxide Sensing

The remarkable sensor properties of a series of oxygen-deficient perovskites, Ca2Fe2O5, Sr2Fe2O5, Ca2FeMnO5 and Sr2FeMnO5, and the impact of structural order on gas sensing response have been investigated. The sensor behavior of defect-ordered oxygen-deficient perovskites have not been studied before. Among the four materials investigated in this work, the ordered compound, Ca2Fe2O5, shows remarkably versatile sensing properties for all three gases, oxygen, carbon dioxide and carbon monoxide. This material is a semiconductor, and its electrical charge-transport increases systematically as a function of oxygen partial pressure, and decreases as a function of carbon dioxide and carbon monoxide concentrations. The recovery of charge-transport upon completion of gas sensing cycles for all gases is nearly perfect, and its crystal structure remains intact, as confirmed by Rietveld refinements with powder x-ray diffraction data. The variation of gas sensing properties as a function of structure and composition is interesting. Most remarkable is the versatility of Ca2Fe2O5, and its sensitivity to O2, CO2 and CO.

Impact of MgO and K2O on Slag-Nickel Matte Equilibria

Slag chemistry of the direct nickel matte smelting was studied in typical industrial high-grade nickel matte smelting conditions at 1400 °C and 0.1 atm pSO2. The experimental technique used involved equilibration, quenching and direct elemental phase composition analysis by Electron Probe X-ray Microanalysis. Magnesia and potassia, a typical gangue constituent of sulfidic nickel concentrates and a common impurity of industrial grade silica flux (sand), respectively, were adopted as slag modifiers in concentrations typical to industrial operations. Their effects on oxidation degree of the nickel-copper-iron matte and equilibrium concentrations of Ni and Cu in the slag were studied as a function of oxygen partial pressure. Solubility of silica in the slag increased significantly with additions of MgO and K2O in the constrained case studied, at silica saturation. Equilibrium concentrations of Ni and Cu in the slag containing MgO and K2O were about a quarter lower compared to the pure iron silicate slag, in the entire oxygen partial pressure range studied.

Role of surface exchange kinetics in coated zirconia dual-phase membrane with high oxygen permeability

In order to comprehend the enhancement mechanism of oxygen permeability by surface coating in the zirconia-based dual-phase membrane, it is necessary to comprehend surface exchange reactions in coated-membrane. Surface exchange reactions significantly contribute to oxygen permeability in a thin membrane with a thickness lower than characteristic thickness; thus, it is extremely important to define the correlation of surface exchange kinetics between the membrane and coating material. The permeation fluxes of zirconia-rich composite membrane [70 vol% Zr0.79Sc0.2Ce0.01O2-δ (ScSZ): 30 vol% La0.7Sr0.3MnO3±δ (LSM), LSM30] in the thickness range of 40–340 μm have been investigated as a function of oxygen partial pressure on the feed and permeate sides to elucidate the enhancement in the permeation flux by a Nd2NiO4+δ (NNO)-coating layer. The oxygen permeation resistances were estimated from oxygen fluxes as a function of PO2 through the oxygen permeation model. The surface exchange (k' and k'') and diffusion (D) coefficients are obtained from the resistances. The diffusion coefficient of the coated membrane is similar to that of ScSZ, which corresponds to the main element of the membrane as the ionic conductor. Additionally, the surface exchange coefficient of the coated membrane is similar to that of NNO as the coating material. The surface exchange reaction on the feed side is faster than that on the permeate side under the operating condition. In order to ensure a more detailed elucidation, the k values obtained from the permeation model are compared with those obtained from the electrical conductivity relaxation method and characteristic length (LC). Despite differences in the experimental conditions such as morphologies, the surface exchange coefficients exhibited similar values and trend in the temperature range of 750–900 °C, irrespective of the experimental method.

High-temperature transport in perovskite-type Ca0.25Sr0.75Fe0.75Mo0.25O3 − δ

The comparative study of transport characteristics of SrFe0.75Mo0.25O3 − δ and Ca0.25Sr0.75Fe0.75Mo0.25O3 − δ was carried out in order to evaluate calcium substitution usability to improve functional properties of electrode materials based on SrFe1 − xMoxO3 − δ. The electrical conductivity in CaxSr1 − xFe0.75Mo0.25O3 − δ was measured as a function of oxygen partial pressure varying in the range of 10−21–0.5 atm at the temperature range of 750–950 °C. The thermogravimetric measurements have revealed that calcium substitution results in a decrease of oxygen content in SrFe0.75Mo0.25O3 − δ. The conductivity and oxygen content analysis has shown that calcium introduction in SrFe0.75Mo0.25O3 − δ provides about 30% conductivity increase under reducing conditions attributed to an increase in both concentration and mobility of n-type charge carriers. In contrast, hole conduction has been found unaffected by calcium content in CaxSr1 − xFe0.75Mo0.25O3 − δ. The improved electrical conductivity in Ca0.25Sr0.75Fe0.75Mo0.25O3 − δ under reducing conditions calls for testing this oxide as an anode material.

Phase stability and magnetic properties of SrFe18O27 W‐type hexagonal ferrite

AbstractW‐type ferrite is a member of the hexagonal ferrite family and a potential permanent magnet material. However, its synthesis conditions are not fully understood yet. Samples were sintered either at 1400°C in air and quenched, or at 1300°C at reduced oxygen partial pressure. The precise stability conditions of this W‐type ferrite were investigated in the temperature range of 1200°C‐1400°C using thermogravimetry, XRD, and electron microscopy. At 1300°C, the ferrite is stable at oxygen partial pressures of . At more oxidizing conditions, the ferrite decomposes into M‐type ferrite and hematite, while at more reducing atmospheres Sr4Fe6O13 and magnetite are formed. The nonstoichiometry δ of SrFe18−δO27 was derived from thermal analysis data at 1300°C as function of oxygen partial pressure and was found to be mainly due to cation vacancies. Magnetization measurements show that this W‐type ferrite exhibits Ms = 103 emu/g at T = 4 K, which agrees well with a ferrimagnetic spin arrangement according to Gorter's model. As alternative, Zn‐substituted W‐ferrite was found to be stable in air at 1200°C with a large Ms = 123 emu/g at 4 K.

Tailoring Nonstoichiometry and Mixed Ionic Electronic Conductivity in Pr0.1Ce0.9O2-δ/SrTiO3 Heterostructures.

The oxygen deficiency or excess, as reflected in the nonstoichiometry of oxide films, plays a crucial role in their functional properties for applications such as micro solid oxide fuel cells, catalysis, sensors, ferroelectrics, and memristors. High concentrations of oxygen vacancies may be beneficial or detrimental according to the application, and hence there is interest in controlling the oxygen content of films without resorting to compositional changes. Here, we demonstrate that substantial changes in the nonstoichiometry of Pr0.1Ce0.9O2-δ (PCO), a model mixed ionic electronic conductor, can be achieved by fabricating multilayers with an inert material, SrTiO3 (STO). We fabricated heterostructures using pulsed laser deposition, keeping the total thickness of PCO and STO constant while varying the number of layers and thickness of each individual layer, to probe the effects of the PCO/STO interfaces. Conductivity measurements as a function of oxygen partial pressure (PO2) and temperature showed a significant weakening of the PO2 dependence compared to bulk PCO, which scaled with the density of interfaces. We confirmed that this change was due to variations in nonstoichiometry, by optical transmission measurements, and show that the lower oxygen content is consistent with a decrease in the effective oxygen reduction enthalpy of PCO. These results exemplify the dramatic differences in properties between films and their bulk counterparts, achievable by interface engineering, and provide generalized insight into tailoring the properties of mixed ionic electronic conductors at the nanoscale.

Electrical conduction mechanisms and effect of atmosphere annealing on the electrical properties of BiFeO3-BaTiO3 ceramics

BiFeO3-BaTiO3 (BFBT) is a promising high temperature piezoelectric ceramic system, as it possesses both high electromechanical properties and high Curie temperature. However, pure BFBT commonly exhibits large electrical conductivity at high temperatures, and there is considerable disagreement on its electrical conduction mechanism. Here we investigated the electrical conduction mechanisms of 0.70BiFeO3-0.30BaTiO3 and 0.67BiFeO3-0.33BaTiO3 ceramics by systematic impedance spectroscopy measurements as a function of oxygen partial pressure and temperature. Both ceramics exhibit higher electrical conductivity with an increase of oxygen partial pressure, consistent with p-type semiconductor behaviors. Particularly, reduced conductivity and improved ferroelectric remanent polarization can be obtained in BFBT when annealed in inert atmosphere. This study provides fundamental basis for preparing insulating and high performance BFBT ceramics.