Effect of Fe2O3 on properties and densification of Ce0.8Gd0.2O2−δ by PCAS

Dense Ce_0.8Gd_0.2O_2−δ was sintered by pulsed current activated sintering (PCAS) within 6 min from Ce_0.8Gd_0.2O_2−δ nanopowder prepared by co-precipitation method. Sintering was accomplished under the combined effects of a pulsed current and mechanical pressure. Highly dense Ce_0.8Gd_0.2O_2−δ with relative density of up to 96.3% was produced under simultaneous application of an 80-MPa pressure and the pulsed current. The effects of Fe₂O₃ additions on the sintering behavior, ionic conductivities, and mechanical properties of the Ce_0.8Gd_0.2O_2−δ were investigated.     

Electrochemical characteristics of electrospun La0.6Sr0.4Co0.2Fe0.8O3−δ-Gd0.1Ce0.9O1.95 cathode

The poor activity of cathode materials for electrochemical reduction of oxygen in intermediate and low temperature regime (<700 °C) is a key obstacle to reduced-temperature operation of solid oxide fuel cells (SOFCs). In our previous work, the direct methane fuel cell exhibits approximately 1 W cm⁻² at 650 °C in hydrogen atmosphere without any functional layers when the electrospun LSCF–GDC cathode was applied into the La₂Sn₂O₇–Ni–GDC anode-supported cell, which is approximately two times higher performance than 0.45 W cm⁻² of the cell with the conventional LSCF–GDC cathode. For detailed analysis of the fibrous cathode, the symmetrical cells with the electrospun and conventional LSCF–GDC cathode are fabricated, and then their electrochemical characteristics are measured by using electrochemical impedance spectroscopy (EIS). Each resistance contribution is determined by equivalent circuit consisting of a series resistance (R_s) and three arcs to describe the polarization resistance of the cathode. Total polarization resistance of the electrospun LSCF–GDC cathode is approximately two times lower than that of the conventional LSCF–GDC cathode at 650 °C, which is attributed to fibrous microstructures and large amount of pores in 100–200 nm. The results correspond to the difference in the cell performances obtained from our previous work.     

Comparison of the superconducting properties of Cu0.5Tl0.5Ba2Ca2(Cu3−yZny)O10−δ prepared at different synthesis temperatures

The superconducting properties of Zn-doped Cu_0.5Tl_0.5Ba₂Ca₂(Cu_3−yZn_y)O_10−δ {CuTlZn-1223} (y=0, 0.83, 1.66, 2.5) samples prepared at 820, 830, 850 and 860 °C have been compared. The samples were investigated by x-ray diffraction (XRD), dc-resistivity, ac-susceptibility and Fourier Transform Infrared (FTIR) absorption measurements. Almost all the superconducting properties have been increased to their maximum in all CuTlZn-1223 samples synthesized at 860 °C, which shows that 860 °C is the optimum temperature to achieve CuTlZn-1223 with enhanced superconducting properties.     

Creep behaviour of tubular Ba0.5Sr0.5Co0.8Fe0.2O3−δ gas separation membranes

The creep behaviour of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) membrane material was investigated in the temperature range 600-950 °C using tubular specimens. Oxygen partial pressure (4 and 2 × 10⁻⁶ mbar), as well as compressive stress (20-63 MPa) were varied. The steady state strain rates from compressive creep tests under axial loading were described by a standard creep equation considering as exponential fitting parameters the influence of temperature, stress, oxygen partial pressure and grain size. Differences in activation energy in the low and high temperature regime appeared to be associated with the disappearance of a second phase. After the creep tests, no significant changes in grain morphology were observed. In addition to the compressive tests, C-ring shaped specimens were machined from the tubes and deformed at 900 °C in a combined tensile-compressive creep mode that revealed the formation of pores along grain boundaries normal to the local tensile stresses that ultimately might lead to creep rupture.     

SrCo0.8Fe0.2O3−δ sorbent for high-temperature production of oxygen-enriched carbon dioxide stream

Perovskite-type SrCo_0.8Fe_0.2O_3−δ (SCF) has been prepared by a liquid citrate method and used to produce O₂-CO₂ gas mixture for oxyfuel combustion. Oxygen is desorbed and an oxygen-enriched carbon dioxide stream is obtained when SCF is exposed in a carbon dioxide stream at high temperature. Oxygen is adsorbed when SCF is regenerated in an air stream. A carbonation-reaction mechanism for O₂-desorption has been identified with the evidences of XRD and TGA analysis. It is found that the theoretical oxygen sorption capacity decreases with the increase of temperature. The sorption kinetics over a temperature range of 700-900 °C has been examined by TGA experiment. Both desorption and sorption processes exhibit a high reaction rate in an initial stage followed by a slower rate in a second stage. It is difficult to reach the theoretical oxygen sorption capacity during the whole temperature range due to the slow oxygen desorption rate. Optimal temperatures for oxygen sorption and desorption processes are determined to be 900 and 850 °C, respectively. Multiple sorption and desorption cycles indicate that SCF sorbent has high reactivity and cyclic stability. Comparison with the reference La_0.1Sr_0.9Co_0.5Fe_0.5O_2.6 (LSCF) and Sr_0.5Ca_0.5Co_0.5Fe_0.5O_2.47 (SCCF) sorbents shows that SCF has faster carbonation reaction at high temperature, i.e., 850 and 900 °C, and much higher theoretical oxygen sorption capacities.     

Suppression of 3D mobility of carrier and superconductivity by Y substitution in Cu0.5Tl0.5Ba2(Ca2−xYx)Cu3O10−δ samples

Bulk Cu_0.5Tl_0.5Ba₂(Ca_2−xY_x)Cu₃O_10−δ superconductor ceramic samples were synthesized by the conventional solid-state method and characterized by X-ray diffraction, dc-resistivity, ac-susceptibility and Fourier Transform Infrared spectroscopy. The main purpose of this study was to investigate the role of charge carriers and the effect of Y substitution at Ca sites in between the CuO₂ planes on superconductivity. The superconducting properties are suppressed by Y substitution at Ca sites in between the CuO₂ planes of Cu_0.5Tl_0.5Ba₂(Ca_2−xY_x)Cu₃O_10−δ samples. It is most likely that Y³⁺ may create correlated domains in between the CuO₂ planes and localizes the carriers, which lowers the diamagnetic screening and suppresses the superconductivity. Therefore, cationic substitution reduces the three dimensional (3D) mobility of carriers, resulting in the reduction of the Fermi vector and velocity of the carriers, which in turn suppresses the superconducting properties of the material.     

Further performance improvement of Ba0.5Sr0.5Co0.8Fe0.2O3−δ perovskite membranes for air separation

Perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) is a promising mixed conducting ceramic membrane material for air separation. In this work, BSCF powder was synthesized by a modified Pechini sol–gel technique at relatively lower temperature. The O₂ permeation through a series of BSCF membranes has been tested at different temperatures and various O₂ partial pressure gradients. Theoretical investigation indicated that bulk diffusion and the O₂ exchange reactions on membrane surfaces jointly controlled the O₂ permeation through BSCF membranes with thickness of between 1.1 and 0.75 mm. To further improve the O₂ fluxes, effective efforts are made on membrane thickness reduction and surface modification by spraying porous BSCF layers on both surfaces. When the membrane thickness was reduced from 0.75 to 0.40 mm, the O₂ fluxes were increased by 20–60% depending on the operating conditions. The surface modification further improved the O₂ flux by another 20–40%. The high O₂ fluxes achieved in this work are quite encouraging with a maximum value reaching 6.0 mL min⁻¹ cm⁻² at 900 °C.     

Electrochemical performance of silver-modified Ba0.5Sr0.5Co0.8Fe0.2O3−δ cathodes prepared via electroless deposition

Silver-modified Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) cathodes for intermediate-temperature solid-oxide fuel cells (IT-SOFCs) were prepared by an electroless deposition process using N₂H₄ as the reducing agent at room temperature. This fabrication technique together with tailored electrode porosity, modified the BSCF electrodes with silver content that varied from 0.3 to 30 wt.% without damaging the electrode microstructure. Both the Ag loading and firing temperatures were found to have a significant impact on the electrode performance, which could facilitate or block the electrochemical processes of the BSCF-based cathodes, processes that include charge-transfer, oxygen adsorption and oxygen electrochemical reduction. At an optimal Ag loading of 3.0 wt.% and firing temperature of 850 °C, an area specific resistance of only 0.042 Ω cm² at 600 °C was achieved for a modified BSCF cathode.     

Effects of heating rate on microstructures and microwave dielectric properties of (1 − x)ZnAl2O4–xTiO2 (x = 0.21) ceramics

(1 − x)ZnAl₂O₄–xTiO₂ (x = 0.21) ceramics were synthesized at 1500 °C for 3 h using the solid-state reaction at a heating rate from 1 to 7 °C/min. The effects of heating rate on the microstructure, phase composition and oxidation state of titanium in the ceramics were investigated. The XRD results show that this system is composed of two phases, i.e. ZnAl₂O₄ spinel and rutile. The “black core” phenomenon resulting from reduction of Ti⁴⁺ ion valence appears after the ceramics are sintered at the speed of 1 and 3 °C/min. As the heating rate increases, the density and quality factor (Q·f) increase initially and reach the maximum value when the heating rate is 5 °C/min, and then reduce quickly to the minimum, while the dielectric constant (?_r) and temperature coefficient of resonator frequency (τ_f) nearly do not change. The optimal microwave dielectric properties can be achieved in (1 − x)ZnAl₂O₄–xTiO₂ (x = 0.21) ceramics sintered at a heating rate of 5 °C/min with an ?_r value of 11.6, a Q·f value of 74,000 GHz (at about 6.5 GHz), and a τ_f value of −0.4 ppm/°C.     

Electrical conductivity of La1−xCaxFeO3−δ solid solutions

La_1−xCa_xFeO_3−δ solid solutions (x=0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) were investigated. The samples were prepared by the polymerizable complex route and characterized by X-ray diffraction and complex impedance spectroscopy techniques. Results reveal the formation of a single perovskite phase for the La_1−xCa_xFeO_3−δ (0≤x≤0.5) compositions. However, the La_0.4Ca_0.6FeO_3−δ sample is a mixture of many phases: perovskite, calcium ferrite and iron oxide. The unsubstituted lanthanum ferrite oxide, as well as the substituted samples, exhibits an orthorhombic symmetry. The direct current conductivity analyses reveal a typical negative temperature coefficient of the resistance behaviour for all the samples. The incorporation of calcium into the lanthanum ferrite lattice results in a significant improvement of the direct current conductivity. In fact, La_0.8Ca_0.2FeO_3−δ oxide shows the optimal conduction value. For all the studied compositions, a change in the activation energy is highlighted around 440 °C. This behaviour is attributed to the antiferromagnetic to paramagnetic transition of lanthanum ferrite. As for the alternating current conductivity, it obeys the Jonsher's power law. The correlated barrier hopping model is proposed to describe the transport mechanism in the studied matrix.     

Intrinsic kinetics for the synthesis of acetonitrile from ethanol and ammonia over Co–Ni/γ–Al2O3 catalyst

The intrinsic kinetics was studied for the synthesis of acetonitrile from amination–dehydrogenation of ethanol over the Co–Ni/γ–Al₂O₃ catalyst in the fixed-bed reactor. Experiments were carried out at reaction temperatures in the range 613–643 K, reactor pressure of 0.1 MPa, the ratios of volume feed velocity to catalyst volume (V₀/V_R) more than 12.99 min⁻¹ and large excesses of ammonia concentration over that of ethanol. The power-law model was used to fit the experimental data, and the model parameters were estimated using the Matlab software. Finally, a reaction kinetic model was proposed to describe the reaction, the calculated activation energy was 51.18 kJ mol⁻¹ and the reaction order to ethanol was 1.183.     

Enhanced microwave absorption of La0.7Sr0.3MnO3−δ based composites with added carbon black

Composites consisting of carbon black (CB) particles, La_0.7Sr_0.3MnO_3−δ (LSMO) powder, and epoxy resin were prepared for development of a high performance microwave absorber. This study investigated the influence of adding amounts of LSMO powder (60, 70, and 80 wt%) on complex permittivity, complex permeability, and reflection loss for CB (5 wt%)-epoxy composites. The variation of complex permittivity and complex permeability with frequency of the composites was measured by the cavity perturbation technique in the range of 7–14 GHz. It was found that the real part of the complex permittivity increased with increasing LSMO addition and the imaginary part of the complex permeability decreased with increasing frequency. The microwave absorption results indicated that the composite filled with 5 wt% CB particles and 80 wt% LSMO powder had the best absorption performance. The maximum reflection loss was −23.63 dB at 7.87 GHz and the absorbing bandwidth at −10 dB was 1.75 GHz with a matching thickness of 5 mm.     

Chemical expansion of nonstoichiometric Pr0.1Ce0.9O2−δ: Correlation with defect equilibrium model

Undoped and acceptor doped cerium dioxide is known to exhibit non-stoichiometry induced chemical expansion at elevated temperatures and reducing environments with impact on the mechanical integrity of solid oxide fuel cells and permeation membranes. In this paper, the chemical expansion of Pr_0.1Ce_0.9O_2−δ is measured and analyzed with respect to its defect equilibria and the chemical coefficient of expansion, analogous to thermal coefficient of expansion, is extracted. The addition of Pr to CeO₂ leads to major deviations from stoichiometry, and correspondingly to large chemical expansions, under readily accessible experimental conditions (e.g. in air at elevated temperatures). Pr_0.1Ce_0.9O_2−δ, therefore, serves as a model system for studying chemical expansion in ceria-based solid solutions in order to predict the conditions in which they exhibit suppressed chemical expansion.     

Synthesis of La0.9Sr0.1Ga0.8Mg0.2O3−δ electrolyte via ethylene glycol route and its characterizations for IT-SOFC

Strontium and magnesium doped lanthanum gallate (La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ), known as LSGM, was first prepared via ethylene glycol method. This route of preparation showed improved electrical conductivity, better surface area and high density. X-ray diffraction patterns of LSGM sintered at different temperatures indicated that pure LSGM phase was formed after sintering at 1400 °C. X-ray Rietveld refinement confirmed the formation of pure perovskite orthorhombic phase of the LSGM. The sintered sample showed 99% relative density. Scanning electron microscopic study of LSGM also depicted fairly densed grain morphology. X-ray photoelectron spectroscopy measurement confirmed the stability of the sintered sample in air and the existence of constituent elements in their characteristic valence states. A surface without porosity was observed in BET measurement. Average thermal expansion coefficient was found to be 9.78×10⁻⁶/°C in the measured temperature range (RT–1000 °C). The frequency dependent electrical conductivity of the sample was measured in the temperature range 400–800 °C. Total electrical conductivity of the LSGM pellet was found to be 0.056 S cm⁻¹ at 800 °C.     

Conductivity and electrochemical performance of (Ba0.5Sr0.5)0.8La0.2Fe1−xMnxO3−δ cathode prepared by the citrate-EDTA complexing method

This study reports the successful preparation of single-phase perovskite (Ba_0.5Sr_0.5)_0.8La_0.2Fe_1−xMn_xO_3−δ (x = 0-0.2) by the citrate-EDTA complexing method. The crystal structure, thermal gravity analysis, coefficient of thermal expansion, electrical conductivity, and electrochemical performance of (Ba_0.5Sr_0.5)_0.8La_0.2Fe_1−xMn_xO_3−δ were investigated to determine its suitability as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The lattice parameter a of (Ba_0.5Sr_0.5)_0.8La_0.2Fe_1−xMn_xO_3−δ decreases as the amount of Mn doping increases. The coefficients of thermal expansion of the samples are in the range of 21.6-25.9 × 10⁻⁶ K⁻¹ and show an abnormal expansion at around 400 °C associated with the loss of lattice oxygen. The electrical conductivity of the (Ba_0.5Sr_0.5)_0.8La_0.2Fe_1−xMn_xO_3−δ samples decreases as the amount of Mn-doping increases. The electrical conductivity of the samples reaches a maximum value at around 400 °C and then decreases as the temperature increases. The charge transfer resistance, diffusion resistance and total resistance of a (Ba_0.5Sr_0.5)_0.8La_0.2Fe_0.8Mn_0.15O_3-δ-Ce_0.8Sm_0.2O_1.9 composite cathode electrode at 800 °C are 0.11 Ω cm², 0.24 Ω cm² and 0.35 Ω cm², respectively.     

Investigation of the effect of ZrO2 and ZrO2/Al2O3 additions on the hot-pressing and properties of equimolecular mixtures of α- and β-Si3N4

In this work, hot-pressing of equimolecular mixtures of α- and β-Si₃N₄ was performed with addition of different amounts of sintering additives selected in the ZrO₂–Al₂O₃ system. Phase composition and microstructure of the hot-pressed samples was investigated. Densification behavior, mechanical and thermal properties were studied and explained based on the microstructure and phase composition. The optimum mixture from the ZrO₂–Al₂O₃ system for hot-pressing of silicon nitride to give high density materials was determined. Near fully dense silicon nitride materials were obtained only with the additions of zirconia and alumina. The liquid phase formed in the zirconia and alumina mixtures is important for effective hot-pressing. Based on these results, we conclude that pure zirconia is not an effective sintering additive. Selected mechanical and thermal properties of these materials are also presented. Hot-pressed Si₃N₄ ceramics, using mixtures from of ZrO₂/Al₂O₃ as additives, gave fracture toughness, K_IC, in the range of 3.7–6.2 MPa m^1/2 and Vicker hardness values in the range of 6–12 GPa. These properties compare well with currently available high performance silicon nitride ceramics. We also report on interesting thermal expansion behavior of these materials including negative thermal expansion coefficients for a few compositions.     

Electrochemical characterization of Co-doped Sr0.8Ce0.2MnO3−δ cathodes on Sm0.2Ce0.8O1.9-electrolyte for intermediate-temperature solid oxide fuel cells

Electrochemical properties of Co-doped Sr_0.8Ce_0.2MnO_3−δ cathode were investigated at the cathode/Sm_0.2Ce_0.8O_1.9 electrolyte interface. The electrochemical impedance spectroscopy was measured under applied cathodic voltages (E = −0.4 to 0 V). At E = 0 V, the area-specific resistance decreased from 2.20 Ω cm² to 0.19 Ω cm² at 700 °C with Co doping. Under the cathodic polarization, the rate determining step of oxygen reduction process was different for both cathodes: the charge transfer for Sr_0.8Ce_0.2MnO_3−δ and the diffusion process for Sr_0.8Ce_0.2Mn_0.8Co_0.2O_3−δ. Besides, the overpotential also decreased from 124 mV to 19 mV at the current density of 0.1 A cm⁻² at 800 °C with Co doping. The improved electrochemical properties of Co-doped Sr_0.8Ce_0.2MnO_3−δ can be ascribed to the formation of more oxygen vacancies and more active sites for oxygen reduction reaction.     

Electrical conduction behaviors of isovalent and acceptor dopants on B site of (La0.8Ca0.2)CrO3−δ perovskites

The electrical conduction behaviors of isovalent and acceptor dopants on B site of (La_0.8Ca_0.2)CrO_3−δ perovskites at high and low oxygen activities were investigated systematically. In this study, the concept of defect chemistry is used to explain the relationship between the concentration of electron hole with the electrical conductivity. The information of charge compensation mechanisms and defect formation may be valuable for a better understanding of the interconnect of (La_0.8Ca_0.2)CrO_3−δ-based ceramics used for solid oxide fuel cells (SOFCs). Since (La_0.8Ca_0.2)CrO_3−δ-based specimens belong to p-type conductors, their conductivities are proportional to the concentration of electron hole. In reducing atmosphere, the oxygen may be lost and ionic compensation may be take place through the formation of oxygen vacancies and the electrical compensation may arise by changing the valence of Cr from tri-valence to tetra-valence in reducing atmosphere. However the formation of oxygen vacancies has no contribution to electrical conductivity, the compensation mechanism is dominated by the electrical compensation, i.e. the take place a transition of Cr³⁺ → Cr⁴⁺ rather than that of ionic compensation, i.e. the formation of oxygen vacancies. Based on the defect chemical reactions and the results of electrical conductivity, the concentration of electron hole at high oxygen activity is larger than that at low oxygen activity. Therefore the electrical conductivity of (La_0.8Ca_0.2)CrO_3−δ-based ceramics at air is larger than that at 5% H₂–95% Ar forming gas. The compensation mechanisms contain ionic and electrical compensation and the ratios of electrical to ionic compensation varied with the kind of dopant which significantly effects the electrical conductivity. The results suggest that the (La_0.8Ca_0.2)Cr_0.9Co_0.1O_3−δ specimen shows high electrical conductivity in air (σ_{850 °C} = 59.59 S/cm) and 5% H₂–95% Ar forming gas (σ_{850 °C} = 47.98 S/cm) leading it a promising candidate as an interconnect material for SOFCs applications.