Influence of microstructure and architecture on oxygen permeation of La(1−X)SrXFe(1−Y)(Ga,Ni)YO3−δ perovskite catalytic membrane reactor

Catalytic membrane reactors (CMR) have been an economically attractive process for natural gas reforming to syngas (H₂ + CO) since more than twenty years.The CMR studied in this paper consists of a mixed ionic and electronic conductor dense layer (La_(1?X)Sr_XFe_(1?Y)Ga_YO_3?δ). High temperature X-ray diffraction analysis, from room temperature to 900 °C under air and nitrogen atmosphere, show a reversible monoclinic to rhombohedral phase transition around 300 °C, and good chemical and dimensional stabilities of La_0.8Sr_0.2Fe_0.7Ga_0.3O_3?δ material.The La_0.8Sr_0.2Fe_0.7Ga_0.3O_3?δ dense layer elaborated by tape casting has been respectively coated with La_0.8Sr_0.2Fe_0.7Ga_0.3O_3?δ on the air side and La_0.8Sr_0.2Fe_0.7Ni_0.3O_3?δ on the inert side using screen printing. The influences of the dense membrane microstructure and of the surface exchange kinetics on the oxygen semi-permeation performances are evaluated. Small grain size, mainly below 1 μm in the dense membrane significantly increases the oxygen flux. A porous layer of La_0.8Sr_0.2Fe_0.7Ni_0.3O_3?δ or La_0.8Sr_0.2Fe_0.7Ga_0.3O_3?δ on the air or inert side of the membrane increased strongly the specific oxygen semi-permeation. The impact of the porous layer is much more important than the reduction of the grain size. In this case, surface exchange kinetics are the limiting steps of oxygen permeation, and Ni-containing formulation leads to the highest flux.     

Investigation of structural,optical, electrical,and magnetic properties of Fe-doped La0.7Sr0.3MnO3 manganites

In this work, the physical properties of nanocrystalline samples of La_0.7Sr_0.3Mn_1−xFe_xO₃ (0.0 ≤ x ≤ 0.20) perovskite manganites synthesized by the reverse micelle (RM) technique were explored in detail. The phase purity, crystal structure, and crystallite size of the samples were determined using X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. All the samples had rhombohedral crystal structure and crystallite size increased with increase in Fe content in La_0.7Sr_0.3MnO₃. The scanning electron micrographs (SEMs) exhibited smooth surface morphology and nonuniform shape of the particles. The optical properties studied using UV-visible absorption spectroscopy revealed a decrease in the absorbance and optical band gap with an increase in Fe content in La_0.7Sr_0.3MnO₃ compound. The temperature-dependent resistivity measurements revealed semiconducting nature of x = 0 and 0.1 samples up to the studied temperature range, while a metal-to-insulator transition was observed at higher Fe doping. Magnetic studies revealed weak ferromagnetism in all the samples and a reduction in the maximum magnetization with an increase in Fe content. A close correlation between electrical transport and magnetic properties was observed with the doping of Fe ion in La_0.7Sr_0.3MnO₃ at Mn site. These results advocate strong interactions associated with the double exchange mechanism among Fe³⁺ and Mn³⁺ ions.     

Surface Energetics of Nanoscale LaMnO3+δ Perovskite

Perovskite type LaMnO₃ and related materials are important compounds with many useful and unique physical and chemical properties. There is a lack of experimental thermochemical data on the energetics of LaMnO₃ nanoparticles. In this work, a series of LaMnO_3+δ samples were synthesized via the citrate method and calcined at 700°C–1050°C. All samples displayed rhombohedral structure (X‐ray diffraction) with similar oxygen stoichiometry 3+δ = 3.16–3.18 (iodometric titration coupled with gravimetric analysis). The BET surface area varied from null for bulk sample to 6.88 ± 0.08 × 10³ m²/mol for the sample calcined at 700°C. The water content varied linearly with the surface area with the highest value being 2.34 wt%. The chemisorbed water adsorption enthalpy was ?63.0 ± 4.1 kJ/mol with the chemisorbed water coverage of 8 H₂O/nm². High‐temperature oxide melt drop solution calorimetry, performed in sodium molybdate at 702°C, yielded enthalpy of formation from La₂O₃, Mn₂O₃, and O₂ of bulk LaMnO_3.16 of ?77.85 ± 1.94 kJ/mol. After correction of drop solution enthalpies of nanometric samples for water content, the calorimetric data were used to calculate the surface energy of LaMnO_3+δ. The energy of the anhydrous surface was 2.27 ± 0.29 J/m², and that of the hydrous surface was 2.02 ± 0.27 J/m². These values are higher than the surface energies of LaMnO_3.00 predicted elsewhere by theoretical methods, probably due to the different oxygen content and possibly more complex surface structure and exposed surface planes. The measured surface energy of LaMnO_3+δ lies between the values reported recently for BaTiO₃ and PbTiO₃ and close to the reported values for MnO₂. This suggests that LaMnO_3+δ surface is predominantly MnO₂‐terminated, in line with the trends predicted by theoretical calculations.     

Structure and properties of (1-x)La0.67Sr0.33MnO3/xMnOδ multicomponent composite

(1-x)La_0.67Sr_0.33MnO₃/xMnO_δ [(1-x)LSMO/xMnO_δ] multicomponent composites were prepared. Their structure and properties were investigated as function of composition. X-ray diffraction and x-ray photoelectron spectroscopy confirmed the coexistence of MnO, Mn₂O₃ and MnO₂. It was found that MnO_δ introduction led to decreased average grain size and metal-insulator transition temperature. But it can increase maximum resistivity and magnetoresistance. The corresponding values were 2.0 μm, 370 K, 0.017 Ω cm, −24.7% (10 K, 2 T) for x = 0 and 0.7 μm, 225 K, 1.899 Ω cm and −25.6% (10 K, 2 T) for x = 0.3. However, the ferromagnetic Curie temperature was almost composition-independent with the value of 305 K. These results indicate that forming multicomponent composite by introducing ferromagnetic second phases can suppress the drawbacks of conventional ferromagnetic materials.     

Chemical Bulk Diffusion Coefficient of a La0.5Sr0.5CoO3−δ Cathode for Intermediate‐Temperature Solid Oxide Fuel Cells

In the first part of this study, the characteristics of a La_0.5Sr_0.5CoO_3?δ cathode are described, including its chemical bulk diffusion coefficient (D_chem), and electrical conductivity relaxation experiments are performed to obtain experimental D_chem measurements of this cathode. The second part of this study describes two methods to improve the single‐cell performance of solid oxide fuel cells. One method uses a composite cathode, i.e., a mix of 30 wt% electrolyte and 70 wt% cathode; the other method uses an electrolyte‐infiltrated cathode, i.e., an active ionic‐conductive electrolyte with nano‐sized particles is deposited onto a porous cathode surface using the infiltration method. In this work, 0.2M Ce_0.8Sm_0.2O_1.9 (SDC)‐infiltrated La_0.5Sr_0.5CoO_3?δ exhibits a maximum peak power density of 1221 mW/cm² at an operating temperature of 700°C with a thick‐film SDC electrolyte (30 μm), a NiO + SDC anode (1 mm) and a La_0.5Sr_0.5CoO_3?δ cathode (10 μm). The enhancement in electrochemical performances using the electrolyte‐infiltrated cathode is attributed to the creation of electrolyte/cathode phase boundaries, which considerably increases the number of electrochemical sites available for the oxygen reduction reaction.     

Large increase of Curie temperature in (110)-oriented La0.7Sr0.3MnO3 films

From the perspectives of scientific researches and practical applications, it is desirable to explore high operating temperature ferromagnetic films. The effect of biaxial strain on magnetic properties of (110)-oriented La_0.7Sr_0.3MnO₃ films was studied. High quality La_0.7Sr_0.3MnO₃ films were grown on (110)-oriented perovskite single crystal substrates using pulsed laser deposition, varying substrate-induced misfit strains from ??2.27–0.75%. A remarkable enhancement of Curie temperature has been achieved for (110)-oriented La_0.7Sr_0.3MnO₃ films clamped with small misfit strains (i.e., grown on LAST (110)). The enhanced Curie temperature of (110)-oriented La_0.7Sr_0.3MnO₃ films could be attributed to the misfit strain between the films and the underlying substrates and may have technological implication for applications at high temperature environments.     

Structural and Mössbauer Investigation of Nanocrystalline SrFe1−xTixO3−δ

Crystal structure and cation distribution of nanocrystalline SrFe_1?xTi_xO_3?δ (0 ≤ x ≤ 0.3) synthesized by combined high‐energy ball milling and solid‐state reactions are investigated using Neutron powder diffraction and Mössbauer spectroscopy. Ti doping stabilizes the single phase tetragonal structure with I4/mmm space group up to x = 0.3. The neutron and Mössbauer data confirm that Fe exists in three different sites both crystallographically as well as magnetically in all the four compositions. The cation distribution at various sites is established through Rietveld refinement.     

Effect of synthesis process on the densification,microstructure, and electrical properties of Ca0.9Yb0.1MnO3 ceramics

Ca_0.9Yb_0.1MnO₃ thermoelectric materials have been prepared, through a classical solid‐state sintering method, from attrition‐ and ball‐milled precursors. After calcination step, microstructural observations have shown that attrition‐milled precursors possess much smaller particle sizes than the obtained by ball milling. Smaller precursors sizes lead to higher reactivity, producing higher density, hardness, and thermoelectric phase content in the sintered materials. The thermoelectric properties reflect the microstructural features, decreasing electrical resistivity in the attrition milling prepared samples without a drastic decrease in the Seebeck coefficient. As a consequence, power factor values are higher than the obtained in the classical solid‐state method samples. Moreover, the highest power factor values at 800°C are much higher than the best results obtained in this CaMnO₃ family. As a result, it has been found that it is possible to tailor the thermoelectric properties of Ca_0.9Yb_0.1MnO₃ ceramics by designing the appropriate preparation procedure while keeping in mind its industrial scalability.     

A New Composite Material Ca3Co4O9+δ+La0.7Sr0.3CoO3 Developed for Intermediate‐Temperature SOFC Cathode

In this study, Ca₃Co₄O_9+δ (CCO) and La_0.7Sr_0.3CoO₃ (LSC) have been mixed as mass fraction by 1:1, to prepare novel two‐phase composites with high electrical conductivity and low thermal expansion coefficient (TEC), for potential application in intermediate‐temperature solid oxide fuel cells. The conductivity of the composite, Ca₃Co₄O_9+δ (50 wt.%) + La_0.7Sr_0.3CoO₃ (50 wt.%) (CCO‐LSC50), is improved to be three times that of single phase CCO material. And, the TEC of CCO‐LSC50 has been effectively improved to be 15.3 × 10^–6 °C^–1, about 20% lower than single phase LSC cathode, which ensures better chemical compatibility with adjacent electrolyte. As a result, compared with pure LSC and CCO cathodes, CCO‐LSC50 composite cathode improves the electrochemical performance, a percentage of 16 and 84%, respectively, according to the impedance spectra experiments. In addition, cathodic overpotential and oxygen reduction kinetics have also been researched to reveal what is driving the results. The microstructures and phases of cathodes were also compared and analyzed.     

Improved electrochemical performance of La0.7Sr0.3MnO3 and carbon co-coated LiFePO4 synthesized by freeze-drying process

Carbon coated LiFePO₄ particles were first synthesized by sol-gel and freeze-drying method. These particles were then coated with La_0.7Sr_0.3MnO₃ nanolayer by a suspension mixing process. The La_0.7Sr_0.3MnO₃ and carbon co-coated LiFePO₄ particles were calcined at 400 °C for 2 h in a reducing atmosphere (5% of hydrogen in nitrogen). Nanolayer structured La_0.7Sr_0.3MnO₃ together with the amorphous carbon layer forms an integrate network arranged on the bare surface of LiFePO₄ as corroborated by high-resolution transmission electron microscopy. X-ray diffraction results proved that the co-coated composite still retained the structure of the LiFePO₄ substrate. The twin coatings can remarkably improve the electrochemical performance at high charge/discharge rates. This improvement may be attributed to the lower charge transfer resistance and higher electronic conductivity resulted from the twin nanolayer coatings compared with the carbon coated LiFePO₄.     

Tailoring La0.8Sr0.2MnO3/La/Sr nanocomposite using a novel complementary method as well as dissecting its microwave,shielding, optical,and magnetic characteristics

In this work, a novel approach was introduced to reduce the oxide nanoparticles and extract the pure metal from them. Accordingly, La_0.8Sr_0.2MnO₃ nanoparticles were prepared through the conventional citrate gel method, and then they were reduced using a solvothermal method by ethylene glycol as a reductive agent. Chemical species, magnetic parameters, crystal structures, and morphological properties of the fabricated structures were deeply studied by Fourier transform infrared (FT-IR), vibrating sample magnetometer (VSM), X-ray powder diffraction (XRD), and field emission scanning electron microscopy (FE-SEM) analyses, respectively. Noticeably, the curves of the diffuse reflection spectroscopy (DRS) suggested a lower energy gap for the La_0.8Sr_0.2MnO₃/La/Sr nanocomposite. Finally, the microwave absorbing characteristics of the specimens were scrupulously investigated using the polystyrene (PS) and polyvinylidene fluoride (PVDF) media. It was found that La_0.8Sr_0.2MnO₃/La/Sr blended in PVDF gained a remarkable reflection loss of 94.68 dB at 15.31 GHz with an only thickness of 1.75 mm, meanwhile displaying an efficient bandwidth as wide as 6.74 GHz (reflection loss (RL) > 10 dB). Noteworthy, La_0.8Sr_0.2MnO₃/PS illustrated a considerable efficient bandwidth of 2.36 GHz (RL > 20 dB). Moreover, La_0.8Sr_0.2MnO₃ composites demonstrated more than 88% electromagnetic interference shielding efficiency (SE) along the X and Ku-band frequency.     

Validation of BaIn0.3Ti0.7O2.85 as SOFC Electrolyte with Nd2NiO4, LSM and LSCF as Cathodes

Due to its high ionic conductivity level, BaIn_0.3Ti_0.7O_2.85 (BIT07) is a potential electrolyte material for SOFC. In order to validate the use of this material, its chemical, mechanical and electromechanical compatibilities with three classical cathode materials (La_0.7Sr_0.3MnO_{3 – δ} (LSM), La_0.58Sr_0.4Co_0.2Fe_0.8O_{3 – δ} (LSCF) and Nd₂NiO_{4 + δ}) have been tested. The chemical compatibility has been demonstrated by XRD experiments, which show that BIT07 does not react with the cathode materials below 1,000 °C. The mechanical compatibility has been investigated by combining measurements of the thermal expansion coefficients of the cathode materials from X‐ray thermo diffraction and SEM observations. Symmetrical cells cathode/electrolyte/cathode have been studied by electrochemical impedance spectroscopy in order to quantify the quality of the cathode/electrolyte interface and to study the long‐term stability of the cell. The main conclusion of the study is that LSCF presents the best compromise between an acceptable mechanical compatibility and very good electrical properties with BIT07.     

In-situ exsolution of PrO2−x nanoparticles boost the performance of traditional Pr0.5Sr0.5MnO3-δ cathode for proton-conducting solid oxide fuel cells

By synthesizing the nominal Pr_xSr_0.5MnO_3-δ materials (x = 0.5, 0.6, 0.7, 0.8), new Pr_0.5Sr_0.5MnO_3-δ (PSM50)+PrO_2−x composite cathodes for proton-conducting solid oxide fuel cells (SOFCs) were developed. The structure analysis and morphology observations verified the exsolution of PrO_2−x particles, and the amount of exsolved PrO_2−x increased with the amount of Pr in Pr_xSr_0.5MnO_3-δ. An H-SOFC with a Pr_0.7Sr_0.5MnO_3-δ (PSM70) cathode enabled the highest reported fuel cell output for H-SOFCs with manganate cathodes. The construction of a PSM50/PrO₂ heterostructure interface can reduce the formation energy of oxygen vacancies, hence accelerating the cathode oxygen reduction reaction (ORR) kinetics, as confirmed by oxygen diffusion and surface exchange experiments. The excellent electrochemical performance was combined with its good chemical stability against CO₂ and H₂O, allowing a stable operation of the cell for over 100 h, indicating that PSM70, which was in fact PSM50 +PrO_2−x, was a highly efficient and durable cathode material for H-SOFCs.     

Preparation of Fe3Si‐Al2O3 Nanocomposite Powders by Mechanochemical Reaction of Fe3O4‐Si‐Al Powder Mixtures

Phase evolution and morphology of Fe₃O₄‐Si‐Al powder mixtures during ball milling from 30 min to 20 h were investigated. A 3‐h critical milling was necessary for the occurrence of mechanically activated combustion reaction. The reaction results in the formation of Fe (Si), Fe₃Si, and α‐Al₂O₃. During ball milling from 3 to 20 h, Fe (Si) and Fe₃Si were combined into disordered Fe₃Si intermetallic and Fe₃Si‐Al₂O₃ composite powder was formed. The presence of in situ formed alumina leads to a decrease in crystallite and particle sizes. The Fe₃Si‐Al₂O₃ particles after milling for 20 h had a crystalline size of 10~12 nm.     

Physical properties of microwave and solid state synthesized La0.7Na0.3MnO3

We report the structural, magnetic, electrical and broadband microwave absorption in La_0.7Na_0.3MnO₃ sample synthesized by microwave (MW) irradiation (Na_0.3LMO_MW) and compare them to the sample synthesized by solid-state (SS) reaction method (Na_0.3LMO_SS). Single phase Na_0.3LMO_MW was synthesized at 800 °C in 30 min, whereas, Na_0.3LMO_SS sample was obtained by sintering at 1200 °C for 48 h. Although both these samples show ferromagnetic transition at T_C ～324.8 K, the MW-synthesized sample shows distinct physical properties: broad ferromagnetic transition, smaller saturation magnetization, a large difference between the magnetic ordering and metal-insulator transition temperatures, a large high-field magnetoresistance, a table top-like magnetocaloric effect, and a large low-field microwave absorption compared to the solid state synthesized sample. These differences are suggested to arise from magnetic heterogeneity induced by smaller grain size and surface spin disorder in the MW synthesized La_0.7Na_0.3MnO₃.     

Synthesis of CoFe2O4 Nanoparticles by a Low Temperature Microwave‐Assisted Ball‐Milling Technique

Well‐crystallized Cobalt ferrite nanoparticles with mean size of 20 nm and high saturation magnetization (82.9 emu/g) were synthesized at a low temperature (≤100°C) by microwave‐assisted solid–liquid reaction ball‐milling technique without subsequent calcination. CoC₂O₄·4H₂O and Fe powder were used as raw materials and stainless steel or pure iron milling balls with diameter of 1.5 mm were used. As a contrast, solid–liquid reaction ball milling without microwave assistance was also investigated. The results showed that this is a simple, environmentally friendly, and energy‐saving technique for ferrite nanocrystal synthesis.     

Effect of High‐Energy Ball Milling and Silica Fume Addition in BaCO3–Al2O3. Part I: Formation of Cementing Phases

The effects of high‐energy ball milling and subsequent calcination on the formation of barium aluminate cementing phases using mixtures of Al₂O₃ and BaCO₃ were investigated. Silica fume was further added in the raw mixtures to observe its role on the cementing phase formation. Results indicated that the decomposition temperature of BaCO₃ lowered remarkably with the increase in milling time. Barium aluminate cements with grain size in nanometer range were obtained from high‐energy ball‐milled raw mixtures. X‐ray diffraction (XRD) results confirmed several crystalline barium‐silicate and barium aluminate phases present. Formation of crystalline BaO·Al₂O₃ phase was observed between 1000°C and 1100°C in the raw mixtures, which were obtained in amorphous state after milling for 5 h. This temperature is at least 300°C lower than that used in the traditional solid‐state method. Fume SiO₂ additions resulted in BaO·Al₂O₃·2SiO₂ (celsian) formation which acted as a retarder, provides more workability and mechanical strength.     

Phase interaction and oxygen transport in oxide composite materials

Abstract

The oxygen permeability of oxide composite membranes containing similar volume fractions of the components, including (La_0.9 Sr_0.1)_0.98 Ga_0.8 Mg_0.2 O_3-δ(LSGM)–La_0.8 Sr_0.2Fe_0.8Co_0.2O_3-δ (LSFC), LSGM–La₂Ni_0.8Cu_0.2O_4+δ (LNC), SrCoO_3-δ–Sr₂Fe₃O_{6.5 ±δ}, Ce_0.8Gd_0.2O_2-δ (CGO)–LSFC and CGO–La_0.7Sr_0.3MnO_3-δ (LSM), was studied at 973–1223 K. In most cases, oxygen transport is substantially affected by component interaction, decreasing ionic conductivity due to cation interdiffusion, and formation of intermediate phases and/or blocking layers at grain boundaries. This interaction is maximised in systems where the phase components have similar structure and thus may form continuous solid solutions, for example LSGM–LSFC, or intermediate compounds such as Roddlesden–Popper phases in LSGM–LNC composites. The results show that, in addition to knowledge of the transport properties and volume fractions of percolating phases, analysis of ionic conduction in oxide composite materials requires assessment of phase interaction and grain boundary processes.     

A-site Ca/Sr co-doping to optimize room-temperature TCR of La0.7Ca0.3-xSrxMnO3 films

Based on the analysis of crystal structure, Mn³⁺/Mn⁴⁺ pairs, distortion of MnO₆ octahedron, and electrical transport properties of La_1-xCa_xMnO₃ and La_1-xSr_xMnO₃ materials, room-temperature coefficient of resistivity (TCR) of La_0.7Ca_0.3-xSr_xMnO₃ (LCSMO) films was optimized by Ca/Sr co-doping at the A-site. LCSMO films are successfully fabricated on LaAlO₃ (100) substrates via facile spin coating technology. The microstructure of LCSMO films is characterized by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, atomic force microscopy and X-ray photoemission spectroscopy. Results reveal that A-site Ca/Sr co-doping significantly influenced crystal structure, formation of Mn³⁺/Mn⁴⁺ pairs, and distortion of MnO₆ octahedron. The correlation between microstructure and electrical transport properties was explained through the phenomenological percolation model, double-exchange mechanism and Jahn-Teller effect. Furthermore, the TCR reached 10.2% K^-1 at 296.1 K in La_0.7Ca_0.18Sr_0.12MnO₃ films.     

Nitric Oxide Reduction Using Hydrogen Over Perovskite Catalysts with Promotional Effect of Platinum on Catalytic Activity

The catalytic activities of strontium substituted La_0.7Sr_0.3MnO₃ type perovskite catalysts for NO reduction using H₂ as reducing agent has been studied, which is further improved by incorporation of Pt outside (0.1 wt.%Pt/La_0.7Sr_0.3MnO₃) and inside (La_0.7Sr_0.3Mn_0.97Pt_0.03O₃) the perovskite lattice structure. Pt shows excellent enhancement in catalytic selectivity towards N₂ when supported on the perovskite. The catalysts were characterized using thermo-gravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and specific surface area. Catalysts evaluations were carried out using thermo-gravimetric analysis coupled with mass spectra (TG-MS).