Thermoelectric response of oxygen nonstoichiometric YBaCo2O5+δ cobaltites synthesized via non-ion selective EDTA-citrate-metal complexing

The physical transport properties of layered 112-type YBaCo₂O_5+δ cobaltites, synthesized via non-ion selective EDTA-citrate-metal complexing, were systematically investigated by means of electrical and thermal conductivity as well as through thermopower measurements. The transport properties were shown to be markedly dependent on the oxygen content δ. The temperature dependence of the thermopower, S(T), exhibits p-type conductivity in the temperature range 60–320 K. In the low-temperature antiferromagnetic region, S(T) exhibits an unusual behavior (broad peak), which can be explained by the electron magnon scattering mechanism. The temperature dependence of the thermal conductivity exhibits a glass-like behavior, and no thermal conductivity peak can be seen at low temperatures for any of the samples. The absence of the phonon peak at low temperatures is associated with the oxygen nonstoichiometry, which brings about strong lattice disorder. The calculated figure of merit (ZT) shows values (∼10⁻⁵ at room temperature) too small for applications, as expected from 112-type cobaltite systems.     

Thermal and structural characterization of the Bi2-xSmxSr2Ca2Cu3O10+δ glass-ceramic system

In this work, investigations on the crystallization and oxidization kinetics of the Sm substituted BiSrCaCuO glass-ceramic system were performed. It has been shown that the Sm ions changed the glassification properties of the BiSrCaCuO system. While fully glass samples were obtained for low Sm-substitution levels (x=0.2 and x=0.4), the Sm containing particles on the surface have grown especially at high Sm-substitution levels (x>0.6) and glassification problem increased. Nonisothermal crystallization kinetics including activation energy for crystallization, E_a, and Avrami parameter, n, of the samples prepared was investigated using differential thermal analysis (DTA) at four different uniform heating rates. The values of glass transition temperature and crystallization temperature exhibited compositional and heating rate dependence. E_a value of the samples showed an increase with increasing the Sm concentration. The Avrami parameter, n, was found approximately 4.5, suggesting the growth of small particles with an increasing nucleation rate. The calculated values of the oxidization rates and the activation energy for oxygen out-diffusion process, E, indicated that the oxygen deficiencies by the Sm substitution in the unit cell of BiSrCaCuO were formed and more oxygen atoms were absorbed to fill the oxygen deficiencies in the system.     

Electrical impedance spectroscopy and structural characterization of liquid-phase sintered ZnO–V2O5–Nb2O5 varistor ceramics doped with MnO

The influence of the MnO content on the microstructure and the electrical response of liquid-phase sintered ZnO–V₂O₅ varistor ceramics wereanalysed using A.C. impedance spectroscopy on samples prepared via the conventional solid state route.The impedance spectra were analysed with the help of one model equivalent circuit at high frequencies and another at low frequencies, involving both resistor and non-Debye constant phase elements (CPEs). The results indicate a significant contribution of grain boundary resistance to the total resistance and non-ohmic characteristic of the studied materials. The Arrhenius plots show two slopes with a turnover at 150 °C/200 °C for both the higher- and lower-frequency time constants. These behaviours can be related to the decrease of the minor charge carrier density. These activation energies were associated with the adsorption and reaction of O₂ (as well as V) species at the grain boundary interface. Consequently, better varistor performance is achieved for 96.9 mol% ZnO+0.5 mol% V₂O₅+0.10 mol% Nb₂O₅+2.5 mol% MnCO₃ with nonlinear coefficient α=21.6, breakdown field E_1mA=191.47 V/mm, leakage current density J_L=36.46 µA/cm² and activation energies of 0.639 eV and 0.644 eV. X-ray diffraction shows that in addition to the major ZnO phases, Zn₃(VO₄)₂ and ZnV₂O₄, were detected as minor secondary phases. SEM analysis of the morphology shows that the grain growth increases with increases in the MnO doping level.     

Compaction pressure effect on microstructure and electrochemical performance of GdBaCo2O5+δ cathode for IT-SOFCs

In order to optimize the morphology of starting powder, raw GBCO powder synthesized via solid state reaction was repeatedly compacted by uniaxial die pressing at two apparent compaction pressures of 500 and 1000 MPa. The particle size distribution curves and SEM images indicated that, with increasing compaction pressure and number of compaction times, the larger particles in the powder were gradually broken apart and the particle size became small and uniform. Then the effect of pressing treatment for the starting GBCO particles on the microstructure and performance of sintered cathode was studied. The results demonstrated that, after being sintered under the same conditions, the cathode prepared from the treated GBCO particles showed a finer microstructure compared with that prepared from the raw GBCO particles. In addition, optimizing the morphology of the starting GBCO powder by pressing treatment could improved the cathode performance and made the polarization resistance of final cathode reduce from 1.33 Ω cm² to 0.40 Ω cm² at 600 °C.     

Catalytic activity and characterization of V2O5/γ-Al2O3 for ammoxidation of m-xylene system

An ammoxidation of m-xylene was evaluated in a fixed-bed reactor using V₂O₅ on various oxides. Catalysts were prepared by wet impregnation method. At first, the loading of V₂O₅ was varied from 5 wt% to 20 wt% on γ-Al₂O₃ support to estimate the most effective amount of V₂O₅. Second, the effect of catalyst supports was examined at 10 wt% loading of V₂O₅. V₂O₅/TiO₂ and V₂O₅/SiO₂ catalysts were employed to compare the ammoxidation reaction with V₂O₅/γ-Al₂O₃. Most catalytic activity was observed when γ-Al₂O₃ was used as a support. Careful characterization was followed by physicochemical techniques, such as BET measurement, X-ray diffraction (XRD), Raman spectroscopy and temperature-programmed reduction (TPR). The results provided the clue that monolayer V₂O₅ was favorably dispersed on the surface of γ-Al₂O₃ up to 10 wt%, which led to the highest yield of isophthalonitrile (IPN).     

中温固体氧化物燃料电池YBaCo2O5+δ阴极材料的改性研究

为了提高中温固体氧化物燃料电池层状钙钛矿氧化物YBaCo2O5+δ(YBC)阴极材料的电化学性,通过掺杂和包覆对其进行了改性研究.采用乙二胺四乙酸-柠檬酸络合法合成了YBC和YBa0.5Sr0.5Co2O5+δ(YBSC)氧化物粉体;采用溶液浸渍法制备了La2NiO4+δ(LN)包覆YBSC的复合阴极La2NiO4+δ...     

Electrical and mechanical characterization by instrumented indentation technique of La0.85Sr0.15Ga0.8Mg0.2O3−δ electrolyte for SOFCs

La_0.85Sr_0.15Ga_0.8Mg_0.2O_3?δ pellets obtained by the polymeric organic complex solution method, isostatic pressing and sintering at 1350 °C have been electrical and mechanically studied. Electrical measurements evidenced reasonable ionic conductivities (0.01 S cm^?1 at 800 °C), which were comparable to those reported for the La_1?xSr_xGa_1?yMg_yO_3?δ prepared by other synthesis methods. On the other hand, the mechanical properties (elastic modulus, E and hardness, H) have been determined at micro/nanometric scale using the instrumented indentation technique. While E did not vary significantly with the increasing indentation depth (h), H values strongly decreased with the indentation depth up to 500 nm. For h > 500 nm, both mechanical properties remained almost constant, thus obtaining E = 271 ± 6 GPa and H = 13.2 ± 0.4 GPa. Finally, the residual imprints and fracture mechanisms have been observed by atomic force microscopy (AFM).     

Electrical conduction behavior in nonstoichiometric BaBixNb5O15±δ tungsten bronze ceramics

The BaBi_xNb₅O_15±δ (BB_xN, 0.98 ≤ x ≤ 1.02) ceramics were synthesized via solid-state reaction to investigate the effect of Bi³⁺ nonstoichiometry on their microstructure and electrical conduction behavior. The study of the relationship between structure and conductive behavior revealed two main conclusions: (1) as the concentration of Bi³⁺ content increased, from deficiency to excess, the oxygen vacancies decreased, and the lattice unit volume V gradually increased; (2) there was a low-frequency Warburg electrode response besides the medium-frequency grain boundary response and high-frequency grain response, and the Bi³⁺ introduction could reduce conductivity. In addition, the dielectric anomalies indicated by the T₁ peak and the T₂ peak at 300 °C and 500 °C are related to the Warburg electrode response and to the Bi vacancy and oxygen vacancy defects, respectively.     

Oxygen transport,thermal and electrochemical properties of NdBa0.5Sr0.5Co2O5+δ cathode for SOFCs

In this study, the crystal structure, thermal, oxygen transport, electrical conductivity and electrochemical properties of the perovskite NdBa_0.5Sr_0.5Co₂O_5+δ (NBSC55) are investigated. In the temperature range of 250 °C–350 °C, the weight loss upon heating was due to a partial loss of lattice oxygen and along with a reduction of Co⁴⁺ to Co³⁺. The tend of weight-loss slows down as temperature increased above 350 °C indicating a reduction of Co³⁺ to Co²⁺ during this stage. The oxygen migration is dominated by surface exchange process at high temperature range (650-800 °C); however, the bulk diffusion process prevails at low temperature range (500–600 °C). For long-term testing, the polarization resistance of NBSC55 increases gradually form 3.13 Ω cm² for 2 h to 3.34 Ω cm² for 96 h at 600 °C and an increasing-rate for polarization resistance is around 0.22% h^?1. The power density of the single cell with NBSC55 cathode reached 341 mW cm^?2 at 800 °C.     

Combustion Synthesis of PrBa2Cu3O7−δ and YBa2Cu3O7−δ cuprate materials

PrBaCuO and YBaCuO cuprate materials were prepared from cooper, barium peroxide, and yttrium/praseodymium oxide by SHS and standard solid-state synthesis. SHS reactions were carried out using relatively large cooper particles (< 63 im) to obtain small product samples (13 mm in diameter). High ambient temperature was used to stabilize a combustion front in the ignited pellets. Explored was the effect of cooper particles size, starting density, and ambient temperature on phase evolutions in synthesized materials.     

Preparation and characterization of mesoporous γ-Al2O3 membranes

以粒径为0.3～0.4 μm的α-Al₂O₃为原料,通过悬浮液真空抽吸法,制备出平均孔径约为70 nm的完整无缺陷的片状α-Al₂O₃支撑体;以仲丁醇铝为前驱体,采用颗粒溶胶路线制备出稳定的Boehmite溶胶,以此溶胶采用浸浆法,在制备的α-Al₂O₃支撑体上制备出完整无缺陷的γ-Al₂O₃中孔膜,并考察了烧成温度对γ-Al₂O₃中孔膜性能的影响。结果表明,本文制备出的γ-Al₂ O₃膜的孔径约为3 nm,对PEG的截留分子量为2800～5300,纯水渗透通量为11.5～25.9 L.m^-2.h^-1［7.6×10⁵ Pa,（14±1）℃］。说明在孔径为70 nm左右的载体上直接制备孔径为3 nm的完整的中孔膜是可行的。     

DC and AC susceptibility study of sol–gel synthesized Bi2Sr2CaCu2O8+δ superconductor

We study DC susceptibility along with amplitude and frequency dependence of AC susceptibility of sol–gel synthesized polycrystalline samples of Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) sintered at different temperatures. In particular, it is demonstrated that susceptibility techniques are an effective tool to characterize granular characteristics of high temperature superconductors. The results are discussed in the framework of Bean's critical state model whence the field and temperature dependence of critical current density is determined. Flux pinning force density is calculated and possibility of the pinning mechanisms prevalent in type II superconductors are investigated. Flux creep activation energy is determined in the light of vortex dynamics exhibited by frequency dependence of AC susceptibility. Since polycrystalline samples are granular in nature, we calculated grain volume fraction and separated the contribution of grain and matrix susceptibility from total measured AC susceptibility. We establish that increase in the sintering temperature not only changes the grain morphology but affects the superconducting properties significantly, validating the impact of grain boundaries on superconducting performance of studied Bi-2212 superconductor.     

Effect of Fluorine Ion on the Electrical Properties of Glasses in the Na2O–P2O5 System

The temperature–concentration dependence of the electrical conductivity of glasses in the NaPO₃–NaF system has been investigated. The regularities revealed are interpreted from the standpoint of the structural microinhomogeneity of glasses, which is due to the formation of polar structural units of the Na⁺[O^–POO_2/2], Na₂ ⁺[O^– ₂POO_1/2], Na⁺[F^–POO_2/2], and Na⁺F^– types. It is shown that the concentration dependence of the electrical conductivity is governed by the ratio between the concentrations of these structural units.     

Investigation of oxygen ion transport and surface exchange properties of PrBaFe2O5+δ

The oxygen transport properties and chemical stability of PrBaFe₂O_5+δ (PBF) double-perovskite oxide were systematically investigated as a chemically stable, highly oxygen-permeable membrane and solid oxide fuel cell (SOFC) electrode under a CO₂-containing or reducing atmospheres. The oxygen permeation flux of 0.7 mm-thick samples and the oxygen ion conductivity were 4.7 × 10^?1 mL?cm^?2 min^?1 and 0.12 S?cm^?1 at 900 °C, respectively, which are comparable to those of PrBaCo₂O_5+δ, exhibiting the most superior performance among oxides with a double-perovskite structure. Moreover, the bulk diffusion and surface exchange coefficients estimated from the electrical conductivity relaxation analysis were generally comparable to those of PrBaCo₂O_5+δ. The characteristic thickness estimated from the membrane and conductivity relaxation tests was ～0.6 mm at 900 °C. The results indicate the significant influence of the surface exchange reaction on the permeability within a thickness of 0.5–1.7 mm. The PBF double-perovskite oxide exhibited superior chemical stability, compared to typical oxides such as PrBaCo₂O_5+δ under a CO₂-containing atmosphere. All results suggest that PrBaFe₂O_5+δ exhibits high oxygen diffusivity with high chemical stability under CO₂-containing or reducing atmospheres.     

Studies on electrical and dielectric properties of co-precipitated Aurivillius phase Ca1−xBaxBi2Nb2O9 ceramics

Polycrystalline samples of lead free Ca_1−xBa_xBi₂Nb₂O₉ ceramics has been synthesized by simple chemical co-precipitation method. Single phase layered perovskite structure was confirmed by X-ray diffraction with average crystallite size in the range of 46–57 nm. Randomly oriented grains with inhomogeneous morphology was observed by scanning electron microscopy. Study of dielectric constant and corresponding tangent loss was carried out in the frequency range of 1 Hz to 5 MHz at room temperature. Ca_1−xBa_xBi₂Nb₂O₉ shows maximum value of dielectric constant of 535 at 1 Hz. The highest polarization of 0.026 μC/cm² was obtained at an applied electric field of 10 kV/cm using PE loop measurement.     

Structural and thermal characterization of CaO–MgO–SiO2–P2O5–CaF2 glasses

The influence of varying the CaO/MgO ratio on the structure and thermal properties of CaO–MgO–SiO₂–P₂O₅–CaF₂ glasses was studied in a series of eight glass compositions in the glass forming region of diopside (CaMgSi₂O₆)–fluorapatite [Ca₅(PO₄)₃F]–wollastonite (CaSiO₃) ternary system. The melt-quenched glasses were characterized for their structure by infrared spectroscopy (FTIR) and magic angle spinning (MAS)-nuclear magnetic resonance (NMR) spectroscopy. Silicon is predominantly present as Q² (Si) species, while phosphorus tends to coordinate in orthophosphate environment. The sintering and crystallization parameters of the glasses were obtained from differential thermal analysis (DTA) while crystalline phase fractions in the sintered glass–ceramics were analyzed by X-ray diffraction adjoined with Rietveld refinement. Diopside, fluorapatite, wollastonite and pseudowollastonite crystallized as the main crystalline phases in all the glass–ceramics with their content varying with respect to variation in CaO/MgO ratio in glasses. The implications of structure and sintering behaviour of glasses on their bioactivity were discussed.     

PrBaFe2O5+δ promising electrode for redox-stable symmetrical proton-conducting solid oxide fuel cells

The proton conduction behavior and electrochemical performance of PrBaFe₂O_5+δ (PBF) are firstly studied for use as promising anode materials in symmetrical proton-conducting solid oxide fuel cells (PCFCs). This study focuses on the investigation of protonic defect formation in PBF and its properties as an electrode, including its chemical stability, electrochemical performance and redox stability. PBF is found to have proton conductivity at intermediate temperatures, which contributes to improved hydrogen oxidation reaction and water formation reaction at the anode and cathode, respectively. Hence, the symmetrical PCFC exhibits a peak power density of 301 mW?cm^?2 and total resistance of 0.77 Ω?cm² at 700 °C. Further, it shows excellent redox-cycle stability upon cycling between fuel and air under a current load. The electrochemical analysis and redox cycling tests of the PBF anode demonstrate that PBF is an attractive candidate for alternative material for symmetrical PCFCs.