Synthesis, structure and oxygen-storage capacity of Pr1−xZrxO2−δ and Pr1−x−yPdyZrxO2−δ

Nanocrystalline Pr_1−xZr_xO_2−δ (0 ≤ x ≤ 1) and Pr_1−x−yPd_yZr_xO_2−δ (x = 0.50, y = 0.02) solid solutions have been synthesized by a single step solution combustion method. The whole range of solid solution compositions crystallize in cubic fluorite structure. The lattice parameter ‘a’ linearly varied up to x = 1.0. Oxygen-storage capacity (OSC) and redox properties of Pr_1−xZr_xO_2−δ (0.0 ≤ x ≤ 0.8) solid solutions have been investigated by temperature-programmed reduction (TPR) and are compared with those of Ce_1−xZr_xO₂. Pr_1−xZr_xO_2−δ exhibited H₂ uptake and CO oxidation at a lower temperature than Ce_1−xZr_xO₂. Small amount of Pd ion (y = 0.02) substitution was found to bring down the temperature of oxygen release-storage significantly.     

Y0.08Sr0.92FexTi1−xO3−δ perovskite for solid oxide fuel cell anodes

A single phase perovskite Y_0.08Sr_0.92Fe_xTi_1−xO_3−δ (x = 0.05, 0.1,0 0.20, 0.25, 0.40, and 0.50) was fabricated at 1400 °C in air by a solid state reaction method and its electrical conductivity and electrochemical properties as an anode were investigated as a function of the Fe content. Doping with Y for Sr allowed the SrFe_xTi_1−xO_3−δ perovskite to be stable at 800 °C in a reducing atmosphere. At 900 °C, metallic Fe precipitated and the stability of the perovskite phase under a reducing atmosphere decreased as the Fe content increased. The conductivity of Y_0.08Sr_0.92Fe_xTi_1−xO_3−δ (x = 0.40) was greater than that of the x = 0.20 sample. The conductivity of Y_0.08Sr_0.92Fe_xTi_1−xO_3−δ was found to be 2 × 10⁻¹ Scm⁻¹ at 800 °C in H₂. Sintering the Y_0.08Sr_0.92Fe_xTi_1−xO_3−δ anode at 1200 °C was found to be optimum to obtain not only good interfacial adhesion, but also a fine grain structure. The Y_0.08Sr_0.92Fe_0.25Ti_0.75O_3−δ anode exhibited the lowest polarization resistance (0.7 and 1.8 Ωcm² at 800 and 700 °C).     

Phase transitions in Sr1 + xCo0.8Fe0.2O3 − δ oxides

The phase transitions that take place in Sr_1 + xCo_0.8Fe_0.2O_3 − δ (− 0.2 ≤ x ≤ 0.1) oxides are reported here. Thermogravimetric analysis (TGA) showed that the oxides with − 0.2 ≤ x ≤ 0 were prone to undergo oxygen-vacancy disorder-order phase transitions, while others with x = 0.05, 0.1 had more stable crystal structures during oxygen-desorption processes in nitrogen. These results were further confirmed by high-temperature in-situ X-ray techniques. The changes in activation energies of three typical oxides, Sr_1 + xCo_0.8Fe_0.2O_3 − δ (x = − 0.2, 0, 0.1), used as oxygen-permeable membranes were investigated. The phase transitions in Sr_1 + xCo_0.8Fe_0.2O_3 − δ (x = − 0.2, 0) have also been detected in differential scanning calorimetry (DSC) profiles.     

Properties of superconducting, polycrystalline dysprosium-doped Bi1.6Pb0.5Sr2−xDyxCa1.1Cu2.1O8+δ (0 ≤ x ≤ 0.5)

The structural and superconducting properties of dysprosium (Dy) doped (Bi,Pb)-2212 superconductor have been studied. Dy concentration is varied from x = 0.0 to 0.5 in a general stoichiometry of Bi_1.6Pb_0.5Sr_2−xDy_xCa_1.1Cu_2.1O_8+δ. It is found that the Dy atoms enter into the crystal structure by replacing Sr atoms and induce significant change in lattice parameter, microstructure, hole-concentration and normal state conductivity of the system. The critical temperature (T_C) and critical current density (J_C) at self-field of the Dy-doped samples enhance considerably at optimum doping levels. Maximum T_C of 92.3 K (for x = 0.4) and J_C of 1390 A/cm² at 64 K (for x = 0.2) are observed for doped samples as against 79.4 K and 127 A/cm², respectively, for the pure sample. The results are discussed on the basis of the change in hole-concentration due to Dy-doping at Sr-site of (Bi,Pb)-2212 superconductor.     

The chemical stability and conductivity of BaCe0.9−xYxNb0.1O3−σ proton-conductive electrolyte for SOFC

BaCe_0.8Y_0.2O_3−δ and BaCe_0.9−xY_xNb_0.1O_3−δ (x = 0.1, 0.15, 0.2, 0.25, 0.3, 0.35) were prepared by a solid-state reactions. It was found that the BaCe_0.8Y_0.2O_3−δ samples decomposed into CeO₂ and BaCO₃ after being exposed in the atmosphere (3% CO₂ + 3% H₂O + 94% N₂) at 700 °C for 10 h. However, samples containing Nb remains unchanged in the same conditions, demonstrating a better stability in the presence of CO₂ and H₂O. The conductivity of BaCe_0.9−xY_xNb_0.1O_3−δ increased with the increase of Y content (x ≤ 0.30), and the highest value was observed at x = 0.30 where a significant decrease in conductivity took place at x = 0.35. The conductivity of BaCe_0.6Y_0.3Nb_0.1O_3−δ reaches 0.01 S/cm in humid hydrogen at 700 °C, slight lower than BaCe_0.8Y_0.2O_3−δ, 0.012 S/cm in the same conditions. Fuel cell with BaCe_0.6Y_0.3Nb_0.1O_3−δ as-prepared was successfully prepared and humidified hydrogen was supplied as fuels in evaluating the fuel cell performance. The open circuit voltage, peak power density and interfacial resistance at 700 °C were 1.02 V, 345 mW/cm² and 0.27 Ω cm², respectively.     

Structural and electrical properties of Ce0.75(Gd0.95−xSrxCa0.05)0.25O2−δ thick film electrolyte

The effects of Sr doping on the electrical properties of Ce_0.75(Gd_0.95−xSr_xCa_0.05)_0.25O_2−δ (x = 0, 0.01, 0.02, 0.03, 0.04, 0.05 mol%) electrolytes thick films were investigated. The samples sintered at 1400 °C for 8 h. X-ray diffraction (XRD) showed typical XRD patterns of a cubic fluorite structure, and the ionic conductivity was examined by AC impedance spectroscopy. From the experimental results, it was observed that Ce_0.75(Gd_0.95−xSr_xCa_0.05)_0.25O_2−δ (x = 0.04 mol%) electrolytes thick film have higher conductivity and minimum activation energy at 600 °C. This is explained in terms of the increased in the oxygen vacancy concentration at the grain boundary.     

Reducibility study of oxide-ion conductors La2−xBaxMo2−yAyO9−δ (A = W, Al, Ga) assessed by impedance spectroscopy

The reducibility of oxide-ion conductors La_2−xBa_xMo_2−yW_yO_9−δ (x = 0, 0.06; y = 0, 0.1, 0.5, 1.0) and La₂Mo_1.9A_0.1O_9−δ (A = Al, Ga) were studied in the reducing atmosphere of 5%H₂ + Ar by means of impedance spectroscopy measurement. The introduction of Ba at La site in La₂Mo_2−yW_yO₉ can lower the sintering temperature by about 100 K in comparison with the La₂Mo_2−yW_yO₉ samples. All substitutions can enhance the conductivity and improve the reducibility in the temperature range from 548 to 923 K. The double substitution of Ba and W as well as substitution of Al or Ga at Mo sites has a better stabilizing effect than the single tungsten substitution. Among these substitutions the introduction of Al has the best stabilizing effect.     

Dielectric properties of Ba3−xKxCaNb2O9−δ (0.5 < x < 1.25) (KBCN) double perovskites

We report synthesis, structure and dielectric properties of double perovskite-type Ba_3−xK_xCaNb₂O_9−δ (x = 0.5, 0.75, 1, 1.25) (KBCN). Powder X-ray diffraction (XRD) confirmed the formation of double perovskite-type structure and lattice constant decreases with increasing K in KBCN. AC impedance study showed a single semicircle over the investigated temperatures and frequencies in dry H₂, H₂ + 3% H₂O, 3% H₂O + N₂, while two semicircles were observed at low temperatures in air, which could be attributed to bulk and grain-boundary contributions. Unlike un-doped BCN, KBCN exhibits negligible grain-boundary and electrode effects to the total electrical properties and is consistent with perovskite-type K-doped BaZrO₃. The bulk dielectric constant and dielectric loss were found to increase with increasing K content in KBCN and also found to change with sintering temperature. Among the samples investigated, Ba_1.75K_1.25CaNb₂O_9−δ sintered at 1100 °C showed the highest dielectric constant of 65 at 10⁶ Hz and dielectric loss of 0.14 at 400 °C in air. Isothermal dielectric constant and electrical conductivity at 1 MHz were found to be independent at elevated temperatures, while vary at low-frequency and low temperatures. Below 700 °C, dielectric constant and dielectric loss decreases with increasing frequency, whereas an opposite trend was observed for the electrical conductivity.     

Preparation and properties of La1 − xAgyMnO3 + δ thin epitaxial films

We report here the preparation and properties of La_1 − xAg_yMnO_3 + δ thin epitaxial films. The original two-step preparation procedure was developed. At first, La_1 − xMnO_3 + δ were grown epitaxially by metal-organic chemical vapor deposition on the single-crystal substrates (001) and (110) SrTiO₃, (001) LaAlO₃, (111) and (001) ZrO₂(Y₂O₃). Treatment by the vapor of the metallic silver in the oxygen atmosphere (at 1 bar and 20 bar) was the second step resulting in the selective absorption of silver by La_1 − xMnO_3 + δ phase. The value of y depended on the process conditions and revealed different kinetics of the silver absorption for (001) and (110) orientation of La_1 − xMnO_3 + δ films. The films prepared were characterized by X-ray diffraction, scanning electron microscopy with energy-dispersion X-ray analysis, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, electrical resistivity and magnetoresistance measurements in a four-probe configuration. We have found that metal-insulator transition temperature (T_p) in the series La_1 − xAg_xMnO_3 + δ possessed a maximum of 380 K at x = 0.15. Thus, T_p of La_1 − xAg_xMnO_3 + δ films was significantly higher than ever reported in the literature for the La_1 − xAg_xMnO_3 + δ ceramics. La_1 − xAg_xMnO_3 + δ films demonstrated the important role of the ferromagnetic fluctuations above Curie temperature T_c resulting in the sign change of the resistivity curve temperature slope dR / dT and a significant shift of T_p well above T_c. The maximum of the magnetoresistance on the temperature scale was close to dR / dT maximum. The intrinsic magnetoresistance values as high as 22% at 310 K and 50% at 280 K were measured in the magnetic field of 1 T in the series of La_1 − xAg_yMnO_3 + δ epitaxial films.     

Fabrication of LiNi0.5 + δMn0.5 − δO2 nanofibers by electrospinning

Polycrystalline tetranary LiNi_0.5 + δMn_0.5 − δO₂ nanofibers have been successfully fabricated by a sol-gel assisted electrospinning technique. The structures and properties of fabricated nanofibers were characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and thermal gravimetric analysis (TGA). After heat treatment of the electrospun fibers at a temperature of 800 °C, the LiNi_0.5 + δMn_0.5 − δO₂ phase was found without other trace phases. Multilayered nanoparticles with a grain size of 50 nm or less within a single fiber are notable from TEM. In this study, it was shown that the sol-gel assisted electrospun LiNi_0.5 + δMn_0.5 − δO₂ fibers could be formed with the α-NaFeO₂ type crystal structure at a temperature lower than that in a typical solid-state or sole sol-gel process and possess good thermal stability as high as 800 °C.     

Proton conduction and chemical stability of (La0.5Sr0.5)(Mg0.5+yNb0.5−y)O3−δ

Electrical conduction properties of complex perovskite-type oxides in the (La_0.5Sr_0.5)(Mg_0.5+yNb_0.5−y)O_3−δ (y = 0.02-0.06) series at intermediate-high temperatures were investigated; introduction of protons by hydration of oxide-ion vacancies was expected by increasing the Mg/Nb ratio from unity. The conductivity depended on y and a maximum conductivity was obtained at y = 0.04: σ = 4.9 × 10⁻⁶ S cm⁻¹ at 400 °C in wet H₂ atmospheres. From electromotive force measurements of hydrogen and water vapor concentration cells, electrical conduction in wet H₂ atmospheres can be attributed to ionic conduction, and proton conduction is dominant below 700 °C. Unlike other perovskite-type proton conductors, (La_0.5Sr_0.5)(Mg_0.54Nb_0.46)O_3−δ was stable in CO₂ atmospheres even in the low-intermediate temperature region due to dilution of reactive strontium by lanthanum.     

Low temperature magnetoresistance properties of Ag2+δSexTe1−x nanocrystalline thin films

Ag_2+δSe_xTe_1−x (x = 0-1) nanocrystals (NCs) with different Se and Te content are prepared by a simple hydrothermal process using a Se_xTe_1−x NC template. Both rod- and dot-shaped NCs are obtained, a variation from the rod-shaped Se_xTe_1−x template. The Ag_2+δSe_xTe_1−x NC thin films are dense with an atomic ratio δ between Ag and Se_xTe_1−x that can be controlled in the range of δ = 0.1-0.3. The MR effect of Ag_2+δSe_xTe_1−x NCs is found to be related to the composition as well as annealing temperature. MR of the Ag_2.2Se_0.2Te_0.8 NC thin films shows a rapid increase to 68% at 239 K and 8 T, an observation providing very useful fundamental information necessary for future applications in the fabrication of high-quality MR sensors and other electronic devices.     

Electrical conductivity and oxygen sensing behavior of SrFe0.2−xTi0.8CoxO3−δ (x = 0.05-0.2)

SrFe_0.2−xTi_0.8Co_xO_3−δ (x = 0.05-0.2) were prepared by solid-state reaction method. Phase characterization and lattice parameter evaluation were done by X-ray diffraction studies. Relative concentrations of iron in various oxidation states in these compositions were estimated using Mossbauer spectroscopy. Electrical conductivities of these bulk samples were measured in various ambient and temperatures using AC impedance spectroscopy. SrFe_0.15Ti_0.8Co_0.05O_3−δ and SrTi_0.8Co_0.2O_3−δ have been found to exhibit good change in electrical conductivity between 21% O₂ and 10 ppm O₂ in argon.     

The Fe K-edge X-ray absorption characteristics of La1−xSrxFeO3−δ prepared by solid state reaction

Perovskites of the composition La_1−xSr_xFeO_3−δ (x = 0.0, 0.1, 0.5, 0.9, 1.0) were prepared by the conventional solid state reaction route. The single phase behaviour was assessed by XRD analysis, the electronic properties were investigated by Fe K-edge X-ray absorption spectroscopy. The work is focused on the valence state of iron and the oxygen vacancies of the perovskites investigated. The XRD measurements revealed that the solid state reaction yields cubic perovskites for x = 1, 0.9, rhombohedral perovskites for x = 0.5, and orthorhombic perovskites for x = 0, 0.1. The X-ray absorption data are discussed in detail with respect to Fe K-edge shift, white-line intensity, pre-edge features, and the EXAFS data. The first peak in the Fourier transform of the Chi × k³ and Chi × k² functions was simulated for a detailed analysis of scattering contributions from the first oxygen shell to evaluate the Fe-O bond length. The substitution of lanthanum by strontium leads to a corresponding increase of the iron valence state and thus to the formation of the Jahn-Teller Fe⁴⁺ ion. This is causing disorder in the first coordination shell and thus an increase of the Debye-Waller factor with increasing x. The Fe-O bond length obtained from XRD and especially from X-ray absorption data are consistent with δ-values close to zero.     

Ce1−xNdxO2−δ/Si thin films obtained by pulsed laser deposition: Microstructure and conduction properties

Substituted Ce_1−xNd_xO_2−δ cerium dioxide thin films are obtained by pulsed laser deposition technique. The films are deposited for various deposition times and at.% Nd, on [100] Si substrates, covered by a thin native SiO₂ layer. The evolution of the cell parameters with Nd content shows that a solid solution is formed, up to x = 0.27. The thin films are homogenous in composition at a nanometer scale. The morphology of the grains does not change significantly with Nd content. The microstructure is columnar, with a preferential [100] growth direction. The width of the grains varies from 20 to 30 nm. The conductivities of the thin films are determined from impedance spectroscopy analyses, in the temperature range 200 °C to 600 °C. The experimental data are explained in the frame of the space charge layer model.     

Synthesis and electrical properties of scheelite Ca1−xSmxMoO4+δ solid electrolyte ceramics

Scheelite-type Ca_1−xSm_xMoO_4+δ electrolyte powders were prepared by the sol-gel auto-combustion process. The crystal structure of the samples was determined by employing the techniques of X-ray diffraction (XRD). According to the XRD analysis, the formed continuous series of Ca_1−xSm_xMoO_4+δ (0 ≤ x ≤ 0.3) solid solutions had the structure of tetragonal scheelite, and the lattice parameters increased with increasing x in the Sm-substituted system. Results of sinterability and electrochemical testing revealed that the performances of Sm-doped calcium molybdate were superior to that of pure CaMoO₄. Ca_1−xSm_xMoO_4+δ ceramics show higher sinterability, and the Ca_0.8Sm_0.2MoO_4+δ sample with 98.7% of the theoretical density were obtained after being sintered at 1250 °C for 4 h. The conductivity increased with increasing samarium content, and a total conductivity 9.54 × 10⁻³ S cm⁻¹ at 800 °C could be obtained in Ca_0.8Sm_0.2MoO_4+δ sintered at 1250 °C for 4 h.     

The effect of A-site deficiency on the performance of La1−sFe0.4Ni0.6O3−δ cathodes

The effect of lowering the A-site stoichiometry of La-Fe-Ni based perovskite solid oxide fuel cell cathodes was investigated with electrochemical impedance spectroscopy on cone-shaped electrodes using a Ce_0.9Gd_0.1O_1.95 electrolyte. It was shown that a lowering of the A-site stoichiometry lowers the amount of Ni in the perovskite phase, as powder XRD revealed that NiO was expelled from the perovskite lattice when the A-site stoichiometry was decreased. NiO inhibit the reduction of oxygen as the activity of a nominally A-site deficient La_1−sFe_0.4Ni_0.6O_3−δ perovskite was worse than the activity of the corresponding LaFe_0.4+sNi_0.6−sO_3−δ perovskite without NiO. NiO is therefore poison for the reduction of oxygen at the cathode in a solid oxide fuel cell.     

Structural transformation and magnetic properties of CaMn1 − xFexO3 − δ (0.0 ≤ x ≤ 0.5)

We have investigated the structural and magnetic properties of CaMn_1 − xFe_xO_3 − δ (0.0 ≤ x ≤ 0.5). Solid state method is used for the synthesis of these samples. Sintering of these compositions at 1300 °C stabilizes higher ionic radii Fe^+ 3 (0.645 Å) at Mn^+ 4 (0.53 Å) site in CaMn_1 − xFe_xO_3 − δ. Structural transformation from orthorhombic to tetragonal to pseudo cubic crystal system and the increase in lattice parameters have been observed with the substitution of Fe at Mn site in CaMn_1 − xFe_xO_3 − δ (0.0 ≤ x ≤ 0.5). The magnetization data show the transformation of G type of antiferromagnetic arrangement of Mn^+ 4 electrons spins in CaMnO₃ into paramagnetic spin type arrangement with the substitution of Fe at Mn site. The compositions x = 0.05, x = 0.1and x = 0.2 show a small kink ~ 100 K in the magnetization data, which resulted due to the competition between antiferromagnetic and paramagnetic states with the Fe substitution.     

Structural characteristics, sinterability and electrical conductivity of Ba-site non-stoichiometric BaxCe0.50Zr0.40Y0.10O3−δ

The influence of barium content on the structural characteristics, sinterability and electrical conductivity of proton conductor Ba_xCe_0.50Zr_0.40Y_0.10O_3−δ (x = 0.95, 0.97, 1.00, 1.03, 1.05) is investigated. Compositions with barium deficiency show the presence of fluorite precipitate detected by powder X-ray diffraction, whilst pure perovskite phase is present for other samples. Barium deficiency promotes the densification process of the samples. The electrical conductivity of Ba_xCe_0.50Zr_0.40Y_0.10O_3−δ increases with barium content, which is mainly ascribed to the decreased activation energy due to the increasing lattice volume, especially for the case in wet 5% H₂/Ar. The present results suggest that it is very important to control the stoichiometry of cations to obtain desirable perovskite type high temperature proton conductors.     

Microstructural characterization of Ce1−xTbxO2−δ (0.60 ≤ x ≤ 0.90) sintered samples

To understand the secondary phase formation in ceria based oxide, the microstructure need to be studied in a wide compositional range. However, in most previous studies, the doping concentration is lower than 50 at.%. In this work, the microstructure of Ce_1−xTb_xO_2−δ sintered samples with Tb concentrations of 0.60 ≤ x ≤ 0.90 were investigated by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Besides the fluorite-structured matrix, a secondary phase with a superstructure formed from a structural modulation of the fluorite structure along [1 1 0] and [0 0 1] directions was observed, whose amount and size reached a maximum at x = 0.90. It has a cubic structure with a lattice constant twice as large as that of the fluorite-structured matrix.