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.     

Tailored temperature window of CuOx/WOx–ZrO2 for NOx reduction via adjusting the calcination temperature of WOx–ZrO2

WO_x–ZrO₂ support was calcined at various temperatures for obtaining controllable activity of copper catalysts for NO_x reduction by ammonia. The temperature window of copper catalyst for over 80% NO_x conversion shifts from 180–300 to 230–350 °C by elevating the calcination temperature of WO_x–ZrO₂ support from 500 to 600 °C, due to the increased Brønsted acidity and reduced structure and electronic interactions between copper oxides and tungsten oxides arising from the polymerization of WO_x clusters on surface of support. Calcining WO_x–ZrO₂ support at 700 °C leads to the reduced redox property of copper oxides on the Cu–O–W interface and the formation of bulk-like CuO, results in a low activity of catalyst.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Metastable Pb1−xCdxS epitaxial films I. Growth and physical properties

The method of preparing (100)-oriented epitaxial layers of Pb_1?xCd_xS on (100) NaCl substrates is described. It is found that, by a careful control of substrate temperature and deposition rate, single-crystal films can be grown over the composition range 0?x?0.40, well in excess of the bulk phase diagram limits. Vegard's law is observed, and the lattice constant a₀ varies as

$\text{a}{}_0\text{= 5.938?0.375x}$

Some precipitation is observed as x approaches 0.40, together with a rejection of excess cadmium-rich material to the surface. It is concluded that metastable films can be readily prepared from 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.     

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.     

Fabrication and photocatalytic property of Pt-intercalated layered perovskite niobates H1−xLaNb2−xMoxO7 (x = 0–0.15)

H_1−xLaNb_2−xMo_xO₇ was prepared by solid-state reaction followed by an ion-exchange reaction. Pt was incorporated in the interlayer of H_1−xLaNb_2−xMo_xO₇ by the stepwise intercalation reaction. The H_1−xLaNb_2−xMo_xO₇ showed hydrogen production activity and the activities were greatly enhanced by Pt co-incorporating. The x value in H_1−xLaNb_2−xMo_xO₇ had an important effect on the photocatalytic activity of the catalyst. When the x = 0.05, the H_1−xLaNb_2−xMo_xO₇/Pt showed a photocatalytic activity of 80 cm³ h⁻¹ g⁻¹ hydrogen evolution rate in 10 vol.% methanol solution under irradiation from a 100 W mercury lamp at 333 K.     

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.     

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).     

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.     

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 of β-phase Ag1−xCuxI (x = 0-0.5) solid solutions nanocrystals

A series of single β-phase nano-Ag_1−xCu_xI (x = 0-0.5) solid solutions powders were synthesized by wet-chemical-chelating reaction processing and citric acid used as complexing agent. The Ag_1−xCu_xI powders were determined by X-ray diffraction and transmission electron microscopy. It was demonstrated that the crystalline size and lattice parameter of the Ag_1−xCu_xI powders decrease with an increase in the amount of CuI substitution. The copper in the lattice of the Ag_1−xCu_xI can effectively prevent the crystalline growth of the Ag_1−xCu_xI powders and citrate used in the Ag_1−xCu_xI powders synthesized process can accelerate single β-phase crystalline structure formation.