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.     

Oxygen nonstoichiometry, thermal expansion and high-temperature electrical properties of layered NdBaCo2O5+δ and SmBaCo2O5+δ

Layered LnBaCo₂O_5+δ (Ln = Nd, Sm) with the cation-ordered double perovskite structure were synthesized by the solid-state reaction route and characterized by X-ray diffraction, thermogravimetric analysis and dilatometry. For NdBaCo₂O_5.73 and SmBaCo₂O_5.61 equilibrated with atmospheric oxygen at low temperatures, tetragonal and orthorhombic polymorphs were found to form, respectively. The oxygen content at 300-1300 K decreases with decreasing rare-earth cation size, whilst δ variations and chemical contribution to the apparent thermal expansion in air are substantially lower compared to the disordered (Ln, A)CoO_3−δ (A = Ca, Sr) analogues. The average thermal expansion coefficients are 23.1 × 10⁻⁶ K⁻¹ for NdBaCo₂O_5+δ and 20.8 × 10⁻⁶ K⁻¹ for SmBaCo₂O_5+δ at 300-1370 K and atmospheric oxygen pressure. These values are comparable to those of Bi₂O₃-based ionic conductors, but are incompatible with common electrolytes such as stabilized zirconia or doped ceria. The oxygen partial pressure dependencies of the total conductivity and Seebeck coefficient, studied in the P(O₂) range from 10⁻¹⁰ to 1 atm, confirm predominant p-type electronic conductivity.     

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.     

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.     

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).     

Thermodynamic properties of LaFeO3−δ and LaFe12O19

The standard Gibbs energies of formation of lanthanum orthoferrite (LaFeO_3−δ) and hexaferrite (LaFe₁₂O₁₉) were determined using solid-state electrochemical cells incorporating yttria-stabilized zirconia as the electrolyte and pure oxygen gas at ambient pressure as the reference electrode. From emf of the solid-state cell, the Gibbs energy of formation of nonstoichiometric orthoferrite (LaFeO_3−δ) is obtained. To derive values for the stoichiometric phase, variation of the oxygen nonstoichiometric parameter δ with oxygen partial pressure was measured using thermogravimetry under controlled gas mixtures. The results obtained for LaFeO₃ and LaFe₁₂O₁₉ can be summarized by the following equations, which represent the formation of ternary oxides from their component binary oxides: ½La₂O₃ + ½Fe₂O₃ → LaFeO₃; ΔG° (LaFeO₃) (±450) (J mol⁻¹) = −62920 − 2.12T (K), and ½La₂O₃ + 9/2Fe₂O₃ + Fe₃O₄ → LaFe₁₂O₁₉; ΔG° (LaFe₁₂O₁₉) (±200) (J mol⁻¹) = −103900 + 21.25T (K). These data are discussed critically in comparison with thermodynamic values reported in the literature from a variety of measurements. The values obtained in this study are consistent with calorimetric entropy and enthalpy of formation of the perovskite phase and with some of the Gibbs energy measurements reported in the literature. For the lanthanum hexaferrite (LaFe₁₂O₁₉) there are no prior thermodynamic measurements for comparison.     

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.     

Electrical conductivity,Seebeck coefficient,and defect structure of oxygen nonstoichiometric Nd2−xSrxNiO4+δ

To elucidate the electronic state and the conduction mechanism of Nd₂NiO_4+δ series oxides at high temperatures, the electrical conductivity, Seebeck coefficient, and nonstoichiometric oxygen content of Nd_2−xSr_xNiO_4+δ (x = 0, 0.2, 0.4) were measured as a function of the Sr content, temperature, and oxygen partial pressure. The hole mobility is estimated from the electrical conductivity and the hole concentration which is defect chemically determined. The mobility slightly decreases as temperature increases as in metals at high temperatures. The relationships between the Seebeck coefficient, electrical conductivity, and hole concentration can be explained by Mott's equation, which expresses the Seebeck coefficient for metals. Semi-quantitative analyses strongly indicate that the electron or hole is itinerant in Nd_2−xSr_xNiO_4+δ, and the conduction mechanism is metal-like band conduction at high temperatures. Based on the experimental results, schematics for energy level and band structure are proposed. At high temperatures, free holes in the σ_x2−y2 band composed of d_x2−y2 orbitals contribute to metallic conduction.     

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.     

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.     

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.     

Preparation and characterisation of La10−xGe5.5Al0.5O26±δ apatites by freeze-drying precursor method

Nanocrystalline powders of La_10−xGe_5.5Al_0.5O_26±δ (x = 0-0.5) with an average crystallite size of 50 nm were prepared by a freeze-drying precursor method. These powders were used to obtain dense ceramic materials at rather low temperature as 1100-1200 °C for 1 h and to study the transport properties by impedance spectroscopy. The composition with the highest La-content (x = 0) exhibits a second-order phase transition from triclinic () to hexagonal (P6₃/m) space groups around 750 °C, whereas for x ≥ 0.2 the materials presents hexagonal structure in the whole temperature range studied. The thermal properties of these materials were investigated by high temperature X-ray diffraction (XRD), thermal analysis (TG/DTA) and impedance spectroscopy. These results confirmed the incorporation of water in the germanate-apatite structure. However, the conductivity resulted to be independent on the gas atmosphere used, which seems to indicate that the proton contribution to the overall conductivity is negligible in these materials.     

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.     

Synthesis, thermal stability, and oxygen intake/release characteristics of YBa(Co1−xAlx)4O7+δ

The YBaCo₄O_7+d (Y-114) phase has recently attracted interests as a potential oxygen storage material due to its oxygen intake/release capability at 200-400 °C. Nevertheless, thermal instability of Y-114 has been an obstacle for future applications, since this compound immediately starts to decompose when the sample is heated at 700-800 °C in oxygen-rich atmosphere. Here we demonstrate that Al-for-Co substitution in Y-114 drastically enhances the thermal stability. Substituting only 10 at.% of aluminum for cobalt in Y-114 essentially suppresses the decomposition reaction seen at 700-800 °C, while well retaining its remarkable oxygen intake/release capability at 200-400 °C. It is also revealed that the addition of aluminum effectively reduces the particle size. The Al-substituted Y-114 products exhibit superior oxygen intake/release response to the Al-free products upon switching the atmosphere between O₂ and N₂.     

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.     

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.     

Investigation of SmBaCuCoO5+δ double-perovskite as cathode for proton-conducting solid oxide fuel cells

SmBaCuCoO_5+δ, a double-perovskite oxide, was synthesized by the modified Pechini method and developed as cathode material for proton-conducting solid oxide fuel cells. The SmBaCuCoO_5+δ powders calcined at 800 °C, show the double-perovskite structure in powder XRD pattern. SmBaCuCoO_5+δ has a more suitable thermal expansion coefficient than SmBaCo₂O_5+δ for BaCe_0.7Zr_0.1Y_0.2O_3−δ electrolyte-based solid oxide fuel cells. The single cell was tested with humidified hydrogen (∼3% H₂O) as the fuel and static air as the oxidant. The performance of the cell was characterized by DC Electronic Load and AC impedance spectroscopy. The peak power densities reached 355-86 mW cm⁻² in the range of 700-550 °C and the interfacial polarization resistance decreased with increasing operation temperature, from 3.1 Ω cm² at 550 °C to 0.22 Ω cm² at 700 °C. The high power density and low polarization demonstrate that SmBaCuCoO_5+δ is a potential candidate for proton-conducting solid oxide fuel cells.     

Chemical stability and electrical properties of BaCe0.85Y0.15O3−δ–Ce0.85Y0.15O2−δ composite bulk samples for use as central membrane materials in dual PCFC–SOFC fuel cells

The goal of the presented research was to determine the physicochemical properties of composite samples obtained by mixing BaCe_0.85Y_0.15O_3−δ (BCY15) and Ce_0.85Y_0.15O_2−δ (YDC15) in different ratios, and to achieve a better understanding of how these ratios affect the electrical conductivity, chemical stability and morphology of BCY15–YDC15 composite materials. It was determined that the samples are chemically stable in H₂O-containing atmospheres at 600 °C. Furthermore, the porosity of the samples increases with the addition of YDC15 to BCY15. Both the porosity and the BCY15/YDC15 ratio affect the stability of the studied samples. The total activation energy (E_t) values of the composite samples, determined via resistance measurements conducted in air at temperatures between 200 and 800 °C, are in the range of 0.590 ± 0.017 eV (E_t of BCY15) to 1.132 ± 0.008 eV (E_t of YDC15). This indicates that the properties of activation energy for composite materials are additive; the presence of both BCY15 and YDC15 affects the activation energy values. The different morphologies of the samples also influence the conductivity within the respective samples. The electrical conductivity values of the composite samples obtained at temperatures from 200 to 500 °C are in the order of magnitude of 10⁻⁷–10⁻³ S/cm. These values are between those determined for pure BCY15 and YDC15 at the respective measuring temperatures. Consequently, the materials show promise for application as porous central membranes (CM) in dual PCFC–SOFC fuel cells operating in the temperature range 600–700 °C.     

Study on the chemical stability of Y-doped BaCeO3 − δ and BaZrO3 − δ films deposited by aerosol deposition

Barium cerate (BaCeO₃) has high proton conductivity but rather poor chemical stability in CO₂-containing atmospheres. Barium zirconate (BaZrO₃), in contrast, is a rather stable material, but exhibits poor sinterability. In the present work, powders of Y-doped BaCeO₃ and BaZrO₃ were synthesized via the solid solution reaction method, and dense ceramic membranes with BaCe_0.9Y_0.1O₃ and BaZr_0.85Y_0.15O₃ were prepared by the aerosol deposition method at room temperature. Aerosol deposition method is a technique that enables the fabrication of ceramic films at room temperature with a high deposition rate as well as strong adhesion to the substrate. The powders and aerosol-deposited membranes were characterized by X-ray diffraction, particle size analysis, scanning electron microscopy, and X-ray elemental mapping. The chemical stability of powders and aerosol-deposited membranes with BaCe_0.9Y_0.1O₃ and BaZr_0.85Y_0.15O₃ against water and carbon dioxide has been investigated, and it was found that BaZr_0.85Y_0.15O₃ materials showed a better chemical compatibility.     

Influence of Cr deficiency on sintering character and properties of SOFC interconnect material La0.7Ca0.3Cr1−xO3−δ

The SOFC interconnect materials La_0.7Ca_0.3Cr_1−xO_3−δ (x = 0-0.09) were prepared using an auto-ignition process and characterized. XRD analysis indicated that all the samples displayed a pure perovskite phase after sintered at 1400 °C for 4 h. The relative density increased from 67% (x = 0) to 95.8% (x = 0.02) and reached to about 97% (x > 0.02), as sintered at 1400 °C for 4 h. The electrical conductivity in air dramatically increased and then lowered slowly with x values. The sample with 0.03 Cr deficiency got a maximum conductivity of 61.7 S cm⁻¹ at 850 °C in air, which is about three times as high as that of the sample with no Cr deficiency (20.6 S cm⁻¹). The sample with 0.06 Cr deficiency exhibited the highest electrical conductivity of 3.9 S cm⁻¹ at 850 °C in pure H₂. The thermal expansion coefficient (TEC) were below 11.8 × 10⁻⁶ K⁻¹ for samples of x = 0.02-0.09, that was of well compatibility with other components in SOFCs. Results indicate that the materials with 0.02-0.06 Cr deficiency have high properties and are much suitable for SOFC interconnect.