O3 Perovskite Research Articles

Electrophoretically Deposited LaNi0.6Fe0.4O3 Perovskite Coatings on Metallic Interconnects for Solid Oxide Fuel Cells

Interconnects present major technical challenges in the commercialization of solid oxide fuel cells (SOFCs) with high performance and durability. To mitigate surface oxidation (chromia growth) and chromium evaporation under high-temperature oxidizing conditions, metallic interconnects should be coated with conductive metal oxides. In this study, we report a protective coating based on perovskite-type LaNi0.6Fe0.4O3 (LNF) fabricated by a facile electrophoretic deposition (EPD) process. LNF powders with a narrow size distribution are synthesized by using the Pechini method. EPD parameters, including solid content in a suspension, deposition time, and applied voltage, are adjusted to control the microstructure, morphology, and thickness of the LNF coating. An LNF coating of ∼7.7 μm thickness is fabricated on an SOFC interconnect via EPD under optimized conditions. The electrophoretically deposited LNF layer serves as an effective protective coating for SOFC interconnects, showing a low resistance (∼4.2 mΩ cm2) as well as high stability during long-term operation at 800°C.

Relationship between negative thermal expansion and lattice dynamics in a tetragonal PbTiO3–Bi(Mg1/2Ti1/2)O3 perovskite single crystal

The negative thermal expansion of a tetragonal PbTiO3–Bi(Mg1/2Ti1/2)O3 perovskite single crystal is correlated to its lattice dynamics and spontaneous polarization.

Local structural distortion and electrical transport properties of Bi(Ni1/2Ti1/2)O3 perovskite under high pressure

Perovskite-structure materials generally exhibit local structural distortions that are distinct from long-range, average crystal structure. The characterization of such distortion is critical to understanding the structural and physical properties of materials. In this work, we combined Pair Distribution Function (PDF) technique with Raman spectroscopy and electrical resistivity measurement to study Bi(Ni1/2Ti1/2)O3 perovskite under high pressure. PDF analysis reveals strong local structural distortion at ambient conditions. As pressure increases, the local structure distortions are substantially suppressed and eventually vanish around 4 GPa, leading to concurrent changes in the electronic band structure and anomalies in the electrical resistivity. Consistent with PDF analysis, Raman spectroscopy data suggest that the local structure changes to a higher ordered state at pressures above 4 GPa.

Magnetic fluctuations driven insulator-to-metal transition in Ca(Ir(1-x)Rux)O3.

Magnetic fluctuations in transition metal oxides are a subject of intensive research because of the key role they are expected to play in the transition from the Mott insulator to the unconventional metallic phase of these materials, and also as drivers of superconductivity. Despite much effort, a clear link between magnetic fluctuations and the insulator-to-metal transition has not yet been established. Here we report the discovery of a compelling link between magnetic fluctuations and the insulator-to-metal transition in Ca(Ir1−xRux)O3 perovskites as a function of the substitution coefficient x. We show that when the material turns from insulator to metal, at a critical value of x ~ 0.3, magnetic fluctuations tend to change their character from antiferromagnetic, a Mott insulator phase, to ferromagnetic, an itinerant electron state with Hund’s orbital coupling. These results are expected to have wide-ranging implications for our understanding of the unconventional properties of strongly correlated electrons systems.

Nickel nanocatalyst exsolution from (La,Sr) (Cr,M,Ni)O3 (M[dbnd]Mn,Fe) perovskites for the fuel oxidation layer of Oxygen Transport Membranes

La0.75Sr0.25Cr0.5Mn0.5O3 (LSCM) and La0.75Sr0.25Cr0.5Fe0.5O3 (LSCF) perovskites with partial substitution of the B-site by 5mol% of Ni were synthesised and characterised for the fuel oxidation layer of Oxygen Transport Membranes (OTMs). X-ray diffraction showed that single phase compounds were obtained at 1300°C. LSCM exhibits a rhombohedral structure and LSCF an orthorhombic structure. Both materials retain their structures, after doping the B site with Ni, with small changes on the unit cell parameters. Thermal analysis under reducing conditions was used to investigate the reducibility of these perovskites up to 900°C, which appears to be enhanced by doping with Ni. The formation of Ni nanoparticles (30–50nm) on the LSCMNi and LSCFNi grains was observed by scanning electron microscopy, after reducing the samples in 5% H2/Ar at 900°C, indicating possible enhanced catalytic activity and potential benefits for the development of the OTMs. DC conductivity measurements conducted under a redox cycle, indicate that LSCFNi presents a better redox stability than LSCMNi.

Engineered spinel–perovskite composite sensor for selective carbon monoxide gas sensing

In the present work, a novel strategy has been adopted to develop highly sensitive and selective carbon monoxide (CO) gas sensor using (x)NiFe2O4 (spinel)–(1−x)La0.8Pb0.2Fe0.8Co0.2O3 (perovskite) (0≤x≤1) composite nano-powders. These powders were prepared through a citric acid mediated wet chemical pechini route. We have investigated the effect of the inclusion of perovskite phase in the spinel matrix to achieve high sensitivity, selectivity, lowering of the optimum temperature, rapid response and recovery time toward CO sensing. Improvement of these gas sensing parameters are essential to yield smart CO sensing system. We have demonstrated that optimized 25%NiFe2O4–75%La0.8Pb0.2Fe0.8Co0.2O3 composite exhibits excellent sensitivity and selectivity toward CO sensing in operating temperatures ranging between 125 and 175°C. The plausible reasons for the improvement in gas sensing parameters are discussed.

Magnetic Interaction and Electronic Transport in La<sub>0.4</sub>Bi<sub>0.6</sub>Mn<sub>0.5</sub>Ti<sub>0.5</sub>O<sub>3</sub> Manganite

We report magnetic interactions and electronic transport properties in La0.4Bi0.6Mn0.5Ti0.5O3 perovskite manganite synthesized using solid-state route. After characterizing the samples structurally, the systematic investigations of magnetic and electrical transport behaviors have been undertaken. It has been observed that at low temperatures near TC , the sample is magnetically frustrated leading to the second-order magnetic transition. A justification for the observed magnetic behavior has been explained based on Arrott’s plot study. The resistivity as a function of temperature in the absence and the presence of applied magnetic field suggests semiconducting nature of the sample. The conduction of the charge carriers is explained using Shklovskii-Efros-type variable-range-hopping mechanism.

Electrical conductivity and complex impedance analysis of La0.7−xNdxSr0.3Mn0.7Ti0.3O3 (x≤0.30) perovskite

Polycrystalline samples La0.7−xNdxSr0.3Mn0.7Ti0.3O3 (x=0.10; 0.20 and 0.30) were prepared by a high-temperature solid-state reaction technique. The X-ray diffraction shows that all the samples crystallize in the orthorhombic structure, Pbnm space group, with presence of a minor unreacted Nd2O3. The electrical response was studied by impedance complex spectroscopy over a broad frequency range (40–100MHz) at room temperature. The values of ac conductivity for all samples were fitted by the Jonscher law σ(ω)=σdc+Aωs. For x=0.10 and 0.20, hopping occurs between neighboring sites, whereas for x=0.30 the hopping process occurs through longer distance. Complex impedance plots exhibit semicircular arcs described by an electrical equivalent circuit, which indicates that the Nd-doped compounds obey a non-Debye relaxation process.

Magneto-transport and magnetic entropy change correlation in Nd0.67Ba0.33Mn0.98Fe0.02O3 perovskite

We have investigated both temperature and magnetic field dependences of the resistivity of Nd0.67Ba0.33Mn0.98Fe0.02O3 perovskite. Electrical-resistivity measurement in the temperature range 5–230 K shows a metal – semiconductor transition at peak temperature, TMS, which shifts to higher temperature when the magnetic field increases. At a field of 7 T the sample presents a large magnetoresistance (MR) effect (∼97%). In 5–40 K temperature range the sample shows a minimum of resistivity, presumably due to the combined effect of weak localization, electron–electron and electron–phonon scattering. In 40–230 K temperature range the resistivity data were well fitted with the phenomenological percolation model, which is based on the phase segregation of ferromagnetic-metallic clusters and paramagnetic-semiconductor regions. The temperature and field dependences of resistivity data, ρ(T, H), allowed us to determine the magnetic entropy change, ΔSM, using the following equation ΔSM=−α∫0H(∂Ln(ρ)/∂T)HdH, which relates the magnetic order to the transport behavior of our sample. Results show that the as obtained magnetic entropy change values are similar to those determined using data obtained from the investigation of temperature and magnetic field dependences of magnetization.

Development of structural ordering in (Ba0.8Pb0.2)(Mg1/3Ta2/3)O3 perovskite

Developments of a perovskite superlattice and Ba0.5TaO3 were investigated as a function of sintering time for (Ba0.8Pb0.2)(Mg1/3Ta2/3)O3. During the sintering step of powder compacts, fundamental reflections of the perovskite structure developed along with secondary phases of perovskite superlattice as well as Ba0.5TaO3. Effects of sintering time on phase developments of the two structures were investigated.

Electro-caloric effect in lead-free ferroelectric Ba1−xCax(Zr0.1Ti0.9)0.925 Sn0.075O3 ceramics

The influence of the incorporation of Calcium in lead-free ferroelectric Ba(Zr0.1Ti0.9)0.925Sn0.075O3 perovskite on its structural phase transition, dielectric properties, ferroelectric behavior, pyroelectric current, and electrocaloric effect was investigated. X-ray diffraction study allowed the identification of a continuous solid solution in the composition range 0≤x≤20mol%. Dielectric and pyroelectric measurements permitted to determine the exits phase transition. The adiabatic temperature change ∆T was calculated by the indirect method from the thermal variation of P–E hysteresis loops using the Maxwell relation. The incorporation of Calcium was found to increase the electrocaloric responsivity within a broad range of temperature with an optimum of ΔT/ΔE=0.27Kmm/kV observed for x=15mol%.

(K0.5Na0.5)NbO3–Bi(Zn0.5Zr0.5)O3 perovskite ceramics: High relative permittivity, low dielectric loss and good thermal stability

A novel lead-free perovskite ceramic system with the composition of (1−x)(K0.5Na0.5)NbO3–xBi(Zn0.5Zr0.5)O3 [(1−x)KNN–xBZZ, x=0–0.02] was developed. The effects of BZZ addition on the phase transition, microstructure and dielectric performances of KNN ceramic were systematically investigated. The BZZ addition can significantly enhance the relative permittivity and thermal stability of permittivity of ceramics below the Curie temperature. When x=0.015, the ceramic possessed the optimized dielectric performance with small Δε/ε115°C value (≤±15%) and relatively high permittivity (~1500) over the temperature range from 115 to 343°C. Moreover, the dielectric loss was dramatically reduced below 4.5% from room temperature to ~544°C, which suggests that this ceramic is a candidate for high-temperature capacitor application.

Electrochemical Impedance Spectroscopy Investigation of the Anodic Functionalities and Processes in LSCM-CGO-Ni Systems

Full ceramic anodes have been developed over the last decade as an alternative to the state-of-the-art Ni/YSZ cermet. The objective is to overcome the technical limitations of the cermet, especially in high fuel utilization operational mode, when hydrocarbon fuels are used, or in systems where rapid heat up is required. Numerous oxide compositions were proposed and investigated. (La,Sr)Cr0.5Mn0.5O3(LSCM) perovskite has been reported as a very promising anode material because of its high tolerance to carbon and sulfur, and its redox and thermal cycling stability. However, similarly to other oxide anode materials, its overall performance still needed further improvement to match that of Ni/YSZ. The relatively low performance of LSCM compared to Ni/YSZ, used to be attributed to its relatively low electronic conductivity. Recently, we have reported that the pre-coating LSCM with Ni nitrate results in a considerable decrease in the anodic polarization resistance and activation energy. Ni dissolves in the perovskite phase at high temperature under oxidizing atmosphere, and then exsolves in the form of nanoparticles under reducing atmospheres. This type of anode composition with split functionalities (ionic conduction, electronic conduction, and catalytic activity) gives a unique opportunity to investigate the specific influence of each of the above functions on the anodic mechanisms and performance. In this work, electrochemical impedance spectroscopy was used to characterize anode compositions containing LSCM and different levels (15, 40, and 60 wt%) of gadolinia doped ceria (CGO), with and without additional submicron Ni, as well as Ni nanoparticles exsoluted from pre-coated larger LSCM particles. Impedance measurements were performed in a three electrode – half cell configuration from 700 to 900°C under two flow rates of 3% wet H2(50 and 150 ml/min). At 900°C and under 150 mL/min of 3% wet H2, lower values of the polarization resistance (Rp) were measured for the samples containing CGO compared to pure LSCM ones. The Rp showed a slight decrease when the content of CGO was increased (15 to 40 and 60 wt%). This was characterized by a decrease of the high frequency part of the impedance diagrams, likely due to the higher ionic conductivity brought by the addition of CGO. The addition of 5 wt% of submicron Ni to LSCM-CGO led to a remarkable decrease in Rp. This effect was even more spectacular for the samples containing 5 wt% of exsoluted Ni nanoparticles, characterized by a decrease of the whole impedance diagram, and especially the high and the middle frequency parts, which suggests an enhancement of the electrochemical processes related to anode charge transfer. Further analysis of the impedance diagrams, measured for the above anodes at different temperatures and gas flow rates, is in progress in order to define the effect of different anode functionalities on the anodic processes involved, and thus its overall performance.

Synthesis and crystal structure determination of YCo1−x Fe x O3 (x = 0, 0.33, 0.5, 0.67 and 1) perovskites

The results on synthesis, crystal structure determination and calculation of crystallochemical parameters of YCo1−x Fe x O3 (x = 0, 0.33, 0.5, 0.67 and 1) perovskites are presented in this work. The compounds within this series were synthesized by solution combustion method using two different fuels: urea and citric acid. It was found that iron-containing perovskites, obtained by citric acid as a fuel are of better quality and crystallinity. All the compounds crystallize in Pnma space group with Z= 4. According to the structure and the calculated crystallochemical parameters, the coordination number of Y3+ in these perovskites is 8. The unit cell parameter relationship is of O−type suggesting that the main reason for distortion of ideal perovskite structure is the octahedral tilting. The deformation of the octahedrons, as well as the tilting angles, are increasing with the increasing content of Fe3+ but the calculated global instability indices (GII) show that the stability of the perovskite structure is increasing with increasing of the Fe3+ content. The perovskites YCo1−x Fe x O3 (0 ≤ x ≤ 1) obtained by solution combustion method are isomorphous and crystallize in Pnma space group. The main reason for deviation from the ideal perovskite structure is tilting of the octahedra. The substitution of Co3+ with Fe3+ leads to a more distorted and tilted octahedron.

Neutron diffraction study of the low-temperature transitions in the SrMo0.9Co0.1O3 oxide

SrMo0.9Co0.1O3 perovskite has recently been described as a performing anode material in solid-oxide fuel cells (SOFC). In this work, we describe the structural phase transitions that this oxide undergoes below room-temperature, studied by “in-situ” neutron powder diffraction (NPD) data and differential scanning calorimetry (DSC) measurements. The oxide is defined in a cubic unit cell with space group Pm3¯m at room temperature. Upon cooling the sample, two structural phase transitions appear, the first one from the cubic structure to a tetragonal I4/mcm structure close to 240K, and the second one to an orthorhombic Imma phase below 150K. By DSC measurements the first phase transition was identified to happen at 243K.

Structural, dielectric and vibrational properties of Ba(1−x)Bix[Ti0.95(Yb0.5Nb0.5)0.05](1−x)FexO3 ceramics (0≤x≤0.2)

Lead-free Ba(1−x)Bix[Ti0.95(Yb0.5Nb0.5)0.05](1−x)FexO3 (0≤x≤0.2) ceramics were prepared using the solid state reaction method. The effect of cationic substitution of Bismuth and iron in A and B sites, respectively, in BaTi0.95(Yb0.5Nb0.5)0.05O3 perovskite on structural and physical properties have been studied using X-ray diffraction, dielectric measurements and Raman spectroscopy techniques. X-ray diffraction analysis revealed that all the samples crystallized at room temperature in a pseudo-cubic Pm3m perovskite structure. Dielectric measurements indicated that the prepared compositions showed a transformation from a classical ferroelectric to relaxor behavior as the rate of substituted x increased. The substitution of Baruim by Bismuth in the A site and Ytterbium/Niobium/Titane by iron in the B site introduced an amelioration in dielectric properties, observed principally in the increasing of the dielectric permittivity maximum and in the decreasing of dielectric loss. Relaxor behavior was also noted to improve, with ΔTm=21K for Ba0.95Bi0.05[Ti0.95(Yb0.5Nb0.5)0.05]0.95Fe0.05O3 at Tm=299K near room temperature. Raman spectra were studied as a function of compositions and temperatures. The results correlated well with dielectric measurements and allowed for a deeper understanding of microstructural properties.

Effect of ZnO Addition on Dielectric and Piezoelectric Properties of (Ba0.85Ca0.15)(Ti0.915Zr0.085)O3 Ceramics

Lead-free (Ba0.85Ca0.15)(Ti0.915Zr0.085)O3 + x wt%ZnO piezoelectric ceramics were synthesized using a conventional solid state sintering method and the effect of ZnO addition on their piezoelectric and dielectric properties was investigated. The X-ray diffraction date exhibited that ZnO diffuses into the lattice of (Ba0.85Ca0.15)(Ti0.915Zr0.085)O3 (abbreviated as BCTZ) and pure perovskite structure phase was formed at all the specimens, And no secondary phases are observed in the measurement range of XRD. In addition, all the specimens showed tetragonal phase structure. The Curie temperature of BCTZ ceramics decreases with increasing the ZnO content. At the 0.2 wt% ZnO added (Ba0.85Ca0.15)(Ti0.915Zr0.085)O3 ceramics sintered at 1360°C, the optimized piezoelectric properties were obtained; piezoelectric constant(d33) = 348 [pC/N], dielectric constant(ϵr) = 3263, mechanical quality factor(Qm) = 267, electromechanical coupling factor(kp) = 0.41, respectively.

Effect of A-site cation size on the structural, magnetic, and electrical properties of La1−xNdxMn0.5Cr0.5O3 perovskites

A systematic investigation of rare earth chromium manganites with general formula La1−xNdxMn0.5Cr0.5O3 (x=0.0, 0.3, 0.6 and 0.9) has been undertaken mainly to understand their structural, magnetic as well as electrical behavior. Upon Nd doping on La site, unit cell volume and the B–O–B bond angle are reduced. The magnetic interactions in the materials are predominantly ferromagnetic and we believe that double exchange interactions between Mn3+ and Mn4+, and Mn3+ and Cr3+ should be the origin of ferromagnetism. The Curie temperature decreases with increasing Nd content. All the samples are semiconducting throughout the temperature range studied. The electric transport behavior can be described by the adiabatic small polaron hoping model, with an apparent increase of polaron activation energy with Nd content.

Remarkable magnetic relaxation and metamagnetic transition in phase-separated La0.7Ca0.3Mn0.9Cu0.1O3 perovskite

The magnetic and electrical properties in phase-separated La0.7Ca0.3Mn0.9Cu0.1O3 perovskite were studied in detail. Experimental results show that the substitution of Cu at Mn-site suppresses the ferromagnetic (FM) phase of La0.7Ca0.3MnO3 matrix and induces a charge ordering transition at ∼146 K. At ∼256 K, a Griffiths-like behaviour is observed. At low temperatures, the occurrence of an irreversible metamagnetic transition associated with a large relaxation rate is analyzed, clearly indicating the dynamic phase coexistence and evolution of FM islands. A magnetic phase diagram is finally constructed based on the magnetic measurements, summarizing the dynamic evolution process of phase separation.

Low-cost (0.1BiYbO3-0.9PbTiO3)-PbZrO3-xMn high Curie temperature piezoelectric ceramics with improved high-temperature performance

Here, we report the structure and performance of a low-cost high Curie temperature (TC) Mn-doped ternary BiYbO3-Pb(Zr,Ti)O3 (BY-PZT-xMn) perovskite piezoelectric ceramic system. The partial substitution of Mn for the A-site Pb, B-site Ti and Zr of the ternary system enhanced both the piezoelectric coefficient and the mechanical quality factor, indicating that the variable valence element induced combinatory soft and hard doped characteristics. The improved electrical performance was found to be related to the improved density and homogeneity, and a correlation between the piezoelectricity and the grain size was also observed. More importantly, a Mn doping level of less than 0.8 mol. % also obviously enhanced the thermal stability of the samples, as reflected by the higher TC and depolarization temperature (Td) measured via both in situ Berlincourt method and annealing experiments, and a correlation between TC and tetragonality of BY-PZT-xMn system was observed. The sample with 0.8 mol. % Mn showed a remarkable overall performance with a d33 of 246 pC/N and a TC around 400 °C, which is expected to be promising for high temperature piezoelectric device applications.