Preparation and properties of thick LaMnO3-based films

Thick (50 ±10 μm) La_1-xSr_xMnO₃ (x = 0.2, 0.4) and La_0.7Ca_0.3Mn0₃ films were prepared by screen printing, and their electrical properties were studied. The films were found to be close in magnetoresistance and resistivity to bulk materials     

Symmetry breaking and electrical conductivity of La0.7Sr0.3Cr0.4Mn0.6O3−δ perovskite as SOFC anode material

This work is focused on nanocrystalline solid oxide fuel cell synthesis and characterization (SOFC) anodes of La_0.7Sr_0.3Cr_0.4Mn_0.6O_3−δ (perovskite-type) with Nickel. Perovskite-type oxide chemical reactivity, nucleation kinetics and phase composition related with La_0.7Sr_0.3Cr_0.4Mn_0.6O_3−δ–NiO to La_0.7Sr_0.3Cr_0.4Mn_0.6O_3−δ–Ni transformation have been analyzed. SOFC anode powders were obtained by sol–gel synthesis, using polyvinyl alcohol as an organic precursor to get a porous cermet electrode after sintering at 1365 °C and oxide reduction by hydrogen at 800 °C/1050 °C for 8 h in a horizontal tubular reactor furnace under 10% H₂/N₂ atmosphere. Composite powders were compressed into 10-mm diameter discs with 25–75 wt% Ni.     

The chemical control of colossal magnetoresistance (CMR) in the new two-dimensional La1.2(Sr1.8−xCax)Mn2O7 system

The magnetic and electrical properties of the colossal magnetoresistance (CMR) La_1.2(Sr_1.8−xCa_x)Mn₂O₇ materials have been investigated. Based on the chemical titration method, the valence of Mn was determined to be 3.39±0.04 for all measured La_1.2(Sr_1.8−xCa_x)Mn₂O₇. The magnetoresistance ratio [ρ(0)/ρ(H)] was efficiently increased from ∼192% (135 K, 1.5 T) for x=0 to 208% (102 K, 1.5 T) for x=0.4 in La_1.2(Sr_1.8−xCa_x)Mn₂O₇. Moreover, the Curie temperature (T_c) decreased with increasing x and was strongly dependent on the apical bond lengths of Mn–O(2).     

Structure,chemical stability and electrical conductivity of perovskite La0.6Sr0.4M0.3Fe0.7O3−δ (M = Co,Ti) oxides

The perovskite La_0.6Sr_0.4M_0.3Fe_0.7O_3?δ (M = Co, Ti) powders have been synthesized by the citrate gel method. The structural and chemical stability of the La_0.6Sr_0.4M_0.3Fe_0.7O_3?δ (M = Co, Ti) oxides were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) techniques. The electrical conductivities of the sintered La_0.6Sr_0.4M_0.3Fe_0.7O_3?δ (M = Co, Ti) ceramics were measured. The results demonstrate the chemical stability in H₂/helium (He) atmosphere of the La_0.6Sr_0.4Ti_0.3Fe_0.7O_3?δ oxide is improved significantly compared to that of the La_0.6Sr_0.4Co_0.3Fe_0.7O_3?δ oxide. The incorporation of Ti^3+/4+ ions in the perovskite structure can significantly stabilize the neighboring oxygen octahedral due to the stronger bonding strength, leading to the enhanced structural and chemical stability of the La_0.6Sr_0.4Ti_0.3Fe_0.7O_3?δ. In addition, the perovskite La_0.6Sr_0.4M_0.3Fe_0.7O_3?δ (M = Co, Ti) oxides possess much higher chemical stability in CO₂/He atmosphere than that of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ oxide, in which the perovskite structure is destroyed completely in a flowing CO₂-containing atmosphere.     

Preparation of La2-x Sr x NiO4 (x = 0-1.3) conducting oxides via a citrate route

La_2-x Sr_xNiO₄ (x = 0-1.3) oxide materials were prepared by the citrate and ceramic routes. The citrate precursors and final products were characterized by thermal analysis (DTA + TG), x-ray diffraction, scanning electron microscopy, IR spectroscopy, and specific surface and conductivity measurements. The use of citrate mixtures leads to the formation of amorphous precursors to La₂O₃, NiO, and La_2-xSr_xNiO₄ (x <-0.4); reduces the temperature required for obtaining solid solutions by 150–200‡C as compared to the ceramic route; and enables the preparation of fine powders (≤5 Μm) and dense ceramics. Deceased.     

Griffiths Phase and Disorder in Perovskite Manganite Oxides La1?xCaxMnO3 and La0.7Sr0.3MnO3

Polycrystalline samples La_1−x Ca _x MnO₃ (x=0.17, 0.15, 0.10) and La_0.7Sr_0.3MnO₃ were prepared in order to investigate the Griffiths-like features induced by disorder compared with their counterpart single crystals. The magnetization data exhibit the traditional transition from ferromagnetic phase to paramagnetic phase. From the temperature dependence of inverse susceptibility, it can be testified that the Griffiths-like features still exist in as-prepared Ca doped samples, while non-Griffiths-like features exist in La_0.7Sr_0.3MnO₃. All these samples, however, exhibit the large effective spins resulting from formation of the short-order ferromagnetic clusters. The O K-edge X-ray absorption spectra indicate the Jahn–Teller (J-T) distortions are definitely present due to the J-T ion Mn³⁺, which indicate that static J-T distortion is not a sufficient condition for the existence of Griffiths phase in Sr-doped system. And, the size of J-T distortion is a little larger in polycrystalline La_0.7Sr_0.3MnO₃ than that in polycrystalline samples La_1−x Ca _x MnO₃ (x=0.17, 0.15, 0.10), revealed by X-ray diffraction parameters and extended X-ray fine structure absorption data of Mn K-edge. It also testifies that the disorder in La_0.7Sr_0.3MnO₃ caused by both chemical doping and J-T distortions is lower than that in polycrystalline samples La_1−x Ca _x MnO₃, which may be the reason of non-Griffiths-like phase existing in La_0.7Sr_0.3MnO₃ samples.     

Synthesis of Sr- and Fe-doped LaGaO3 perovskites by the modified citrate method

Fine particle strontium and iron substituted lanthanum gallates La_1–x Sr _x Ga_1–y Fe _y O_3–, where x = 0.2, 0.4, and 0.6; y = 0.2, 0.4, 0.6, and 0.8, have been synthesized by a modified citrate method. The formation of these powders was confirmed by the X-ray powder diffraction (XRD) and the fine particle of La_0.6Sr_0.4Ga_0.2Fe_0.8O_3– was investigated by scanning electron microscopy (SEM), and particle size analysis. The single phase of La_0.8Sr_0.2Ga_0.4Fe_0.6O_3–, La_0.6Sr_0.4Ga_0.2Fe_0.8O_3–, and La_0.4Sr_0.6Ga_0.2Fe_0.8O_3– powders could be obtained both with and without calcination. The amount of the secondary phase increased when the amount of Sr in La_1–x Sr _x Fe_0.6Ga_0.4O_3– was more than 0.2 (x > 0.2) and the amount of Fe in La_0.6Sr_0.4Ga_1–y Fe _y O_3– and La_0.4Sr_0.6Ga_0.2Fe_0.8O_3– was less than 0.8 (y < 0.8). The results indicated that in the pH range of 1.36–9.27, the single phase of La_0.6Sr_0.4Ga_0.2Fe_0.8O_3– was formed without calcination and the pH had negligible effects on the structure and lattice parameter. The fine particle of these calcined powders (<4 m) was obtained with the average particle size 1.70 m at pH = 1.36 and with the average particle size between 0.56–0.60 m at pH range between 3.39–9.27, and with a lattice parameter about 3.9 Å.     

Microstructure and H<Subscript>2</Subscript>S gas sensing properties of nanocrystalline perovskite-type La<Subscript>0.6</Subscript>Co<Subscript>0.4</Subscript>Mn<Subscript>0.5</Subscript>Ni<Subscript>0.5</Subscript>O<Subscript>3</Subscript>

Nanocrystalline La_1−x Co _x Mn_1−y Ni _y O₃ (x = 0.2 and 0.4; y = 0.1, 0.3, and 0.5) thick films sensors prepared by sol–gel method were studied for their H₂S gas sensitivity. The structural and morphological properties have been carried out by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Average particle size estimated from XRD and TEM analyses was observed to be 30–35 nm. The gas response characteristics were found to depend on the dopants concentration and operating temperature. The maximum H₂S gas response of pure LaMnO₃ was found to be at 300 °C. In order to improve the gas response, material doped with transition metals Co and Ni on A- and B-site, respectively. The La_0.6Co_0.4Mn_0.5Ni_0.5O₃ shows high response towards H₂S gas at an operating temperature 250 °C. The Pd-doped La_0.6Co_0.4Mn_0.5Ni_0.5O₃ sensor was found to be highly sensitive to H₂S at an operating temperature 200 °C. The gas response, selectivity, response time and recovery time were studied and discussed.     

Enhancing Electrochemical CO2 Reduction on Perovskite Oxide for Solid Oxide Electrolysis Cells through In Situ A-Site Deficiencies and Surface Carbonate Deposition Induced by Lithium Cation Doping and Exsolution

Solid oxide electrolysis cells (SOECs) hold enormous potential for efficient conversion of CO₂ to CO at low cost and high reaction kinetics. The identification of active cathodes is highly desirable to promote the SOEC's performance. This study explores a lithium-doped perovskite La_0.6-xLi_xSr_0.4Co_0.7Mn_0.3O_3-δ (x = 0, 0.025 0.05, and 0.10) material with in situ generated A-site deficiency and surface carbonate as SOEC cathodes  for CO₂ reduction. The experimental results indicate that the SOEC with the La_0.55Li_0.05Sr_0.4Co_0.7Mn_0.3O_3-δ cathode exhibits a current density of 0.991 A cm⁻² at 1.5 V/800 °C, which is an improvement of ≈30% over the pristine sample. Furthermore, SOECs based on the proposed cathode demonstrate excellent stability over 300 h for pure CO₂ electrolysis. The addition of lithium with high basicity, low valance, and small radius, coupled with A-site deficiency, promotes the formation of oxygen vacancy and modifies the electronic structure of active sites, thus enhancing CO₂ adsorption, dissociation process, and CO desorption steps as corroborated by the experimental analysis and the density functional theory calculation. It is further confirmed that Li-ion migration to the cathode surface forms carbonate and consequently provides the perovskite cathode with an impressive anti-carbon deposition capability, as well as electrolysis activity.     

Synthesis of Ni-doped LaSrMnO3 nanopowders by hydrothermal method for SOFC interconnect applications

Ni-doped lanthanum strontium manganite (LSMN) nanopowders, La_0.7Sr_0.3Mn_1?xNi_xO₃ (0.05?≤?x?≤?0.3) were synthesized at 150?°C for 8?h by hydrothermal reaction as a function of Ni doping concentration. The SEM analyses suggested that the calcination treatment influenced the morphology of the nanopowders. The calcined nanopowders at 1300?°C had agglomerated spherical structure of 44–77?nm. Meanwhile, the XRD studies revealed that the nanopowders have single crystalline phase over the range x?=?0.05–0.2. In addition, the LSMN nanopowders were sintered at elevated temperatures, 1250–1350?°C to examine their electrical conductivity for solid oxide fuel cell (SOFC) interconnect applications under actual SOFC working condition. Their electrical conductivity gradually increased to 90.05?S/cm with Ni doping concentration x?=?0.2, which were sintered at 1300?°C. These results suggest La_0.7Sr_0.3Mn_0.8Ni_0.2O₃ displays a good performance as an optimal composition of the LSMN.     

Effect of Co substitution for Mn on Y1−x Sr x MnO3 properties for SOFC cathode material

Y_0.6Sr_0.4Mn_1–y Co _y O₃ (0 y 0.4) perovskite oxides were prepared by the coprecipitation method. The effect of Co substitution for Mn on the crystal structure, electrical conductivity and thermal expansion properties were investigated. By X-ray powder diffraction, the crystal structure was found to change from hexagonal symmetry of Y_0.8Sr_0.2MnO₃ to orthorhombic of Y_0.6Sr_0.4Mn_1–y Co _y O₃. The differences in the structure of the unsubstituted Y_1–x Sr _x MnO₃ (0.2 x 0.4) are attributed to the average ionic radii of the cations and the amounts of Mn⁴⁺ present. The results of electrical conductivity analysis can be described by the small polaron hopping conductivity model. With Co substitution, the activation energy increases, possibly due to an increase of Jahn–Teller distortion, at an extent higher than the increase of the concentration of charge carriers; thus, the electrical conductivity decreases. In addition, the relative densities of the materials reached 94% with sintering at 1350°C for 12 h and had higher concentration of the available lattice sites, thus showing higher conductivity, than that with sintering at 1300°C for 6 h, which achieved 70% relative density. It is also found that the thermal expansion coefficient (TEC) increases as the Sr and Co content of Y_1–x Sr _x Mn_1–y Co _y O₃ increases and those with Co content of y = 0.2 exhibit TEC compatibility with YSZ.     

Electrical conductivity of LaxSr2−xFe1−yRuyO4±δ

Materials of the K₂NiF₄ structure type have been prepared and the electrical conductivity in air determined for a number of compositions in the La_xSr_2−xFe_1−yRu_yO_4±δ solid solution series including three with Ru substituted for Fe on the B site: La_0.2Sr_1.8Fe_0.6Ru_0.4O_4±δ, La_0.4Sr_1.6Fe_0.7Ru_0.3O_4±δ, and La_0.6Sr_1.4Fe_0.8Ru_0.2O_4±δ. Overall the total conductivity values measured were lower than expected for the Ru-doped materials, with a peak conductivity of ≈2 S cm⁻¹ at 700 °C. In the undoped La_xSr_2−xFeO_4±δ materials, a significant jump in conductivity was observed between the x = 0.7 and 0.8 compositions and was related to the bonding in the materials and the Fe³⁺/Fe⁴⁺ redox behaviour. In all materials, the conduction behaviour was found to follow a semi-conducting trend.     

Ni-doped MnCo2O4 nanoparticles as electrode material for supercapacitors

The Mn_1-xNi_xCo₂O₄ (0.00?≤?x?≤?0.20) nanoparticles were synthesized by a simple PAN-solution route. XRD, TEM, SAED, FESEM, XANES, XPS, and BET techniques were used to investigate the structural and morphology of Ni-doped MnCo₂O₄ nanoparticles. The effect of Ni ions substitution in MnCo₂O₄ nanoparticles on the electrochemical properties was examined on three-electrode systems. The results show that the substitution of Mn with Ni ions can improve the electrochemical properties of MnCo₂O₄ nanoparticles. The Mn_1-xNi_xCo₂O₄ electrode with x?=?0.15 exhibits the highest specific capacitance of 378 F g^?1 at the current density of 1 A g^?1. After 1000 charge-discharges times, this electrode has good capacity retention of 85%. The good capacitance characteristics and cyclic stability indicate that these Mn_1-0.85Ni_0.15Co₂O₄ nanoparticles could be applied as active materials for energy storage devices.

     

Effect of Sr and Sr-Ca substitution on the superconductivity of (Er0.76Ca0.24)Ba2(Cu2.76Co0.24)Oz

The structural and superconducting properties of (Er_1-xCa_x)Ba_2-ySr_y(Cu_2.76Co_0.24)O_z are investigated using X-ray diffraction, resistivity, ac susceptibility, and oxygen content measurements. Increasing the Sr content in (Er_0.76Ca_0.24)Ba_2-ySr_y(Cu_2.76Co_0.24)O_z lowers the oxygen content and decreases theT _c, which is attributed to the rearrangement of oxygen atoms in the basal plane. This suppression ofT _c at_{y =0.6} cannot be improved by appropriate hole doping with Ca in (Er_1-xCa_x)Ba_1.4Sr_0.6(Cu_2.76Co_0.24)O_z forx = 0.24-0.48.     

La0.6Sr0.4Co0.2Fe0.79M0.01O3−δ (M = Ni,Pd) perovskites synthesized by Citrate-EDTA method: Oxygen vacancies effect on electrochemical properties

La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF08), La_0.6Sr_0.4Co_0.2Fe_0.79Ni_0.01O_3?δ (LSCF08-Ni) and La_0.6Sr_0.4Co_0.2Fe_0.79Pd_0.01O_3?δ (LSCF08-Pd) perovskites were synthesized by Citrate-EDTA method, by using NiCl₂ or PdCl₂ as metal precursors, and their physicochemical properties were characterized by XRD, TGA, TPD and TPR. XRD data evidenced an expansion of the lattice parameters of LSCF08-Pd, while a contraction of the lattice occurred for LSCF08-Ni, with respect to the undoped LSCF, suggesting different oxygen vacancies content in the perovskite (confirmed by TGA) likely due to a different oxidation state of Ni and Pd species stabilized in the structure.TEM analyses performed over LSCF08-Pd revealed the presence of metallic Pd nanoparticles well dispersed in the matrix that accounts for the increased reducibility of the Co and Fe species with respect to LSCF08-Ni and undoped perovskite. AC impedance measurements that were carried out on symmetric cells consisting of LSCF-based materials deposited onto Ce_0.8Gd_0.2O_2?δ (GDC) electrolyte proved the enhanced electrochemical performances of Ni/Pd doped LSCF.The electrochemical characterization of LSCF08, LSCF08-Ni and LSCF08-Pd electrodes was completed by performing cyclic voltammetry experiments in the range of temperature 600–800?°C, varying the potential (U) between 0.3?V and ?1?V, at scan rates in the range 1–50?mV?s^?1 and working under flow of 0.7?vol% O₂ in He (30?ml/min).     

CuO as sintering additive to Sr0.4Ca0.6La4Ti5O17 ceramics

The effects of CuO addition on the sintering behavior and microwave dielectric properties of Sr_0.4Ca_0.6La₄Ti₅O₁₇ ceramics were investigated. CuO was selected as a liquid phase sintering aid to lower the sintering temperature of Sr_0.4Ca_0.6La₄Ti₅O₁₇ ceramics. With CuO addition, the sintering temperature of Sr_0.4Ca_0.6La₄Ti₅O₁₇ ceramics was effectively reduced from 1550 °C to 1475 °C. The crystalline phase exhibited no phase difference and no second phase was detected at all addition levels. The electric permittivity was not significantly affected by various amounts of CuO addition and ranged from 52 to 54. Small values (<+7 ppm/K) of the temperature coefficient of resonant frequency were obtained for Sr_0.4Ca_0.6La₄Ti₅O₁₇ ceramics. However, the unloaded quality factor Q _u was strongly dependent upon the CuO concentration. Q _u f _o ～ 10500 GHz was obtained for Sr_0.4Ca_0.6La₄Ti₅O₁₇ ceramics with 0.5 wt.% of CuO addition, sintered at 1475 °C.     

Magnetic properties and magnetocaloric effect in La1.4?xCexCa1.6Mn2O7 perovskites synthesized by sol–gel method

The magnetic properties and magnetocaloric effect near the critical phase transition temperatures of La_1.4−x Ce _x Ca_1.6Mn₂O₇ (x = 0, 0.2) double layered manganites are analyzed. Polycrystalline nanopowders of La_1.4Ca_1.6Mn₂O₇ and La_1.2Ce_0.2Ca_1.6Mn2O₇ were successfully synthesized by a sol–gel method. The magnetic measurements suggest an antiferromagnetic alignment of the Ce and Mn moments in La_1.2Ce_0.2Ca_1.6Mn₂O₇. Both Ce³⁺ and Ce⁴⁺ ions are present in the doped sample, as indicated by the XPS spectra. A moderate magnetocaloric effect was found for both samples, with the maximum entropy change located at temperatures near the magnetic transition ones. The high RCP(S) values together with the broadened magnetic entropy curves suggest the possibility to use these materials for magnetic refrigeration devices.     

Combined platinum/perovskite catalyst for an efficient nanostructured air electrode

This work shows the stepwise improvement of air electrodes by the right combination of catalysts. In all electrodes carbon nanotubes serve as carbon support. The electrodes are produced by ultrasonic mixing of the carbon nanotubes and the catalysts. Their catalytic activity towards oxygen reduction in alkaline solution is evaluated by polarisation curves and electrochemical impedance spectroscopy. In a first step La_1?xSr_xMnO₃ perovskites are investigated, as well as La_0.65Sr_0.35MnO₃ and La_0.6Sr_0.4CoO₃ are compared. It is found that La_0.65Sr_0.35MnO₃ and La_0.6Sr_0.4CoO₃ have a positive impact on different parts of the current–potential curve. In a second step the influence of small amounts of platinum as an additional catalyst besides the perovskite is analyzed with the result that platinum lowers significantly the activation polarisation. Finally, the optimum composition of the electrode is found by using the synergetic effect of platinum, La_0.65Sr_0.35MnO₃ and La_0.6Sr_0.4CoO₃.     

Electrochemical performance of La2NiO4+δ-La0.6Sr0.4Co0.2Fe0.8O3−δ composite cathodes for intermediate temperature solid oxide fuel cells

The composite cathodes La₂NiO_4+δ-La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ were prepared for intermediate temperature solid oxide fuel cells. La₂NiO_4+δ and La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ powders were synthesized successfully by glycine-nitrate process. The effect of composition on the electrochemical performance of the composite electrodes was studied by AC impedance spectroscopy and the optimal calcination temperature was determined when the electrode showed the minimum area specific resistance. The addition of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ to La₂NiO_4+δ electrode decreased the area specific resistance remarkably. The composite electrode with 30 wt% La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ calcined at 1150 °C exhibited the lowest area specific resistance of 0.125 Ω cm², about 60% of the area specific resistance of La₂NiO_4+δ electrode at 700 °C in air. The composite electrode with 30 wt% La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ can be a promising cathode material through the evaluation of electrical conductivity and thermal expansion behavior.     

Electromagnetic and microwave absorption performance of some transition metal doped La0.7Sr0.3Mn1−xTMxO3±δ (TM = Fe,Co or Ni)

In this study the transition metal doped La_0.7Sr_0.3Mn_1?xTM_xO_3±δ (TM = Fe, Co or Ni, x = 0, 0.2) powders were fabricated by the conventional solid state reaction method. The compositions, morphologies and crystal structures were characterized using different method. The influences of the incorporation of TM into La_0.7Sr_0.3MnO_3±δ on the complex permittivity, complex permeability and microwave absorption performance were investigated in the range of 5.85–18 GHz. It is found that the electromagnetic loss has been enhanced after TM doping. And the microwave absorption properties have been significantly improved. In present study La_0.7Sr_0.3Mn_0.8Fe_0.2O_3±δ had the best microwave absorption properties. The maximum reflection loss was 27.67 dB at 10.97 GHz, and the absorbing bandwidth above 6 dB was 6.80 GHz with 2 mm thickness.