Characterization of NdSrCo1−xFexO4+δ (0 ≤ x ≤ 1.0) intergrowth oxide cathode materials for intermediate temperature solid oxide fuel cells

NdSrCo_1−xFe_xO_4+δ (0 ≤ x ≤ 1.0) intergrowth oxides have been investigated as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). All the cathodes prepared by a glycine nitrate process (GNP) indicated single phase intergrowth oxides. The introduction of Fe for Co leads to decrease TEC values and electrical conductivity, and increase polarization resistance and oxygen content. The polarization resistance of NdSrCoO_4+δ composition is 0.16 Ω cm² at 800 °C in air atmosphere, which is the best electrochemical performance compared with other compositions.     

Properties of sol–gel derived Al2O3–15 wt.% ZrO2 (3 mol% Y2O3) nanopowders using two different precursors

An alumina–15 wt.% zirconia (3 mol% yttria) nanopowder was synthesized by sol–gel method using salt and alkoxide as precursors. Al(NO₃)₃·9H₂O, ZrOCl₂·8H₂O and Y(NO₃)₃·6H₂O were used as salt precursors and Al(OC₄H₉)₃ and Zr(OC₄H₉)₄ were used as alkoxide precursors. The dried gels of two precursors were heat treated in the range of 450–1350 °C. The powders produced by alkoxide precursors calcined at 750 °C were in the range of 15–75 nm, while those prepared by salt precursors had the size in the range of 30–90 nm. The former powders had a higher surface area and smaller mean pore diameter compared with the later powder, i.e. 152 m²/g and 5.63 nm comparing with 121 m²/g and 9.79 nm, respectively. Therefore phase transformation in the former occurred in lower temperatures.     

High-temperature proton conductor Sr(Ce0.6Zr0.4)0.9Y0.1O3 − δ: Preparation,sintering and electrical properties

Sr(Ce_0.6Zr_0.4)_0.9Y_0.1O_3 − δ was prepared by a wet chemical route and the stages of its formation, as well as the characterization of the resulting compounds were carried out using TG–DTA, XRD, TEM, SEM and EPMA techniques. Experimental results indicate that a calcination temperature of 900–1100 °C, which is much lower than that for the conventional solid state reaction process, is sufficient to the formation of single perovskite phase. Sr(Ce_0.6Zr_0.4)_0.9Y_0.1O_3 − δ powders obtained are fine, narrowly distributed and well crystallized. This strongly improves the sinter properties and the formation of a dense Sr(Ce_0.6Zr_0.4)_0.9Y_0.1O_3 − δ. Sintered at T ≥ 1350 °C, samples with density ≥97.16% of the theoretical could be obtained. In addition, the proton conductivities of Sr(Ce_0.6Zr_0.4)_0.9Y_0.1O_3 − δ ceramic were measured by impedance spectroscopy in 5% H₂/Ar and the evolution of the spectra with increasing temperature was analyzed.     

Structural,dielectric and piezoelectric properties of xBiFeO3–(1−x)BaTi0.9Zr0.1O3 ceramics

Effect of BiFeO₃ (BFO) content on the microstructure and electrical properties of BaTi_0.9Zr_0.1O₃ (BTZ) ceramics prepared by the solid-state reaction technique was investigated. X-ray diffraction analyses show that BFO diffused into the lattice of BTZ to form a solid solution with perovskite structure. The relative density of the BTZ ceramics is increased by the introduction of BFO. The dielectric study reveals that the dielectric constant and the average dielectric loss of the solid solution decreased simultaneously with an increase in BFO content. The materials undergo a diffuse type ferroelectric phase transition. The diffusivity increases with increase in BFO contents in the studied composition range. On the other hand, the piezoelectric coefficient and electromechanical coupling coefficient decrease simultaneously with increasing the BFO content, whereas the mechanical quality factor increases gradually. The structure–property relationship and the mechanism associated with the change of the electrical properties are discussed intensively.     

Sintering behaviour of nano-crystalline γ-Al2O3 powder without additives at 2-7 GPa

Al₂O₃ ceramics were fabricated without additives under high pressure (2-7 GPa) at different temperatures (600-1200 °C) using nanocrystalline alumina powder with metastable γ-Al₂O₃ phase as the starting material.It was shown that high pressure increases the nucleation rate while reducing the growth rate of the transformed α phase so that its grain size decreases and nano-scale grains in the sintered structure can be achieved.On the other hand the sintered samples at 7 GPa and high temperature (1000 °C) have shown micron-scale large grain sizes compared to those sintered at lower pressures, for the same temperature and sintering time. This could be attributed to the higher input energy in the system at high pressure and high temperature conditions, thereby reaching the final stage in sintering more quickly.In this work, the best combination of grain size (∼200 nm) and density (98.0% TD) was obtained under the sintering condition of 1000 °C at 7 GPa with a holding time of 1 min.Thus for high pressure/high temperature conditions, the sintering time should be reduced to prevent grain growth.     

Phase equilibria and dielectric properties of Bi3+(5/2)xMg2−xNb3–(3/2)xO14−x cubic pyrochlores

Subsolidus pyrochlores with the proposed formula, Bi_3+(5/2)xMg_2−xNb_3−(3/2)xO_14−x (0.14≤x≤0.22) were successfully synthesised at the firing temperature of 1025 °C using conventional solid-state reaction. The excess Bi³⁺ charge was offset by removal of relative proportion of Mg²⁺ and Nb⁵⁺ together with creation of oxygen non-stoichiometry in order to preserve electroneutrality of the system. These samples were crystallised in cubic structure with space group of Fd3m, No. 227 and their refined lattice parameters were in the range of 10.5706 (3)–10.5797 (7) Å. The surface morphologies of the samples as confirmed by scanning electron microscopy analysis were of irregular shaped grains while their crystallite sizes of ~30–85 nm were calculated using the Scherrer equation and the Williamson–Hall method. No thermal event was discernable indicating these pyrochlores were thermally stable within a studied temperature range of ~30–1000 °C. The recorded dielectric constants of Bi_3+(5/2)xMg_2−xNb_3−(3/2)xO_14−x (0.14≤x≤0.22) subsolidus pyrochlores were generally above ~160 and their dielectric losses were in the order of 10⁻⁴–10⁻³ at the frequency of 1 MHz and temperature of ~30 °C. Meanwhile, these ceramic samples also exhibited negative temperature coefficient of relative permittivity between −528 and −742 ppm/°C in the temperature range of ~30–300 °C.     

Performance of Ba0.5Sr0.5Co0.8Fe0.2O3 + δ membrane after laser ablation for methane conversion

A cross-shaped pattern was formed on the surface of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ oxygen permeation membrane by laser ablation. A membrane reactor made from this membrane was operated for partial oxidation of methane to syngas in the presence of Ni/ZrO₂ catalyst. The CH₄ conversion and CO selectivity of the membrane reactor were 98.8% and 91.5%, respectively, and the oxygen permeation flux through the membrane was 11.0 ml/cm² min at 850 °C. The effects of space velocity (SV) on CH₄ conversion and CO selectivity in such reactor were discussed. The mechanism of POM in such membrane reactor may follow the combustion and reforming mechanism.     

Effect of sintering temperature on the microstructure and mechanical properties of ZrO2-3 mol%Y2O3 sol–gel films

The microstructural evolution and mechanical properties of ZrO₂-3 mol%Y₂O₃ films were investigated as a function of the sintering temperature in the range from 100 °C to 1500 °C, using a battery of characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and nanoindentation. It was found that the crystallization occurs at temperatures close to 300 °C. A gradual increase in the grain and crystallite sizes is observed as the sintering temperature increases up to 1000 °C, and above this sintering temperature the tendency changes abruptly with a rapid increase in these values. Although Young's modulus of the coatings did not change with sintering temperature, a slight decrease was observed in the hardness values above 1000 °C which is attributed to microstructure coarsening. Finally, a slight degradation of the films occurs above 1300 °C, which is due to the occurrence of a process of grain spheroidization.     

Electrochemical properties of Sr1−xCexMnO3 (0.1≤x≤0.4) – GDC composite cathodes for IT-SOFCs

The electrochemical properties of Sr_1−xCe_xMnO₃ (SCM, 0.1≤x≤0.4)–Gd_0.2Ce_0.8O_2−x (GDC) composite cathodes were determined by impedance spectroscopy. The study focused on the doping effect of Ce in the composite cathodes. Single-phase perovskite was obtained for 0.1≤x≤0.3 in SCM. No reaction occurred between the Sr_0.7Ce_0.3MnO₃ electrode and the GDC electrolyte at an operating temperature of 800 °C for 100 h. In the single phase perovskite region, lattice expansion occurred due to the reduction of Mn⁴⁺ to Mn³⁺ at B-sites, and this was attributed to an increase in Ce content. Ce doping enhanced the electrode performance of SCM–GDC composite cathodes, and best electrode performance was achieved for the Sr_0.7Ce_0.3MnO₃–GDC composite cathode (0.93 Ω cm² and 0.47 Ω cm² at 750 °C and 800 °C, respectively). The improvement in electrode performance was attributed to increases in charge carriers induced by a shift of some Mn from +4 to +3 and to the formation of surface oxygen vacancies caused by Mn⁴⁺ to Mn³⁺ conversion at high temperatures.     

Palladium(II) complexes with Pd2S2 rings. Synthesis and theoretical characterization of [Pd2(dppe)2(μ-S)2] and X-ray characterization of [Pd3(dppe)3(μ3-S)2]Cl2

The use of the dppe ligand and the fine tuning of the experimental conditions has allowed isolation of [Pd₂(dppe)₂)(μ-S)₂], whose structural characterization has been achieved by ab initio MP2 calculations, and of [Pd₃(dppe)₃(μ₃-S)₂]Cl₂, the X-ray structure of which is also reported.     

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.     

Phase transformation behavior of 3 mol% yttria partially-stabilized ZrO2 (3Y–PSZ) precursor powder by an isothermal method

The phase transformation behavior of freeze-dried 3 mol% yttria–partially-stabilized zirconia (3Y–PSZ) precursor powder has been studied. When the freeze-dried 3Y–PSZ precursor powder was calcined at 773–1073 K for 2 h, the crystalline structure was composed of tetragonal and monoclinic ZrO₂ as primary and secondary phases, respectively. The freeze-dried 3Y–PSZ precursor powder after calcination at 773 K, the monoclinic ZrO₂ content abruptly increased from 8.00% to 31.51% and the tetragonal ZrO₂ content suddenly decreased from 92.00% to 68.49%, with the duration increasing from 0.5 to 1 min. The activation energy of the isothermal transformation from tetragonal to monoclinic was 7.02 kJ/mol. The kinetics equation for the phase transformation from tetragonal to monoclinic in the freeze-dried 3Y–PSZ precursor powder between 773 K and 1273 K for various durations is described as ln(1/1−α)=1/2.61[t^2.61(1.50×10⁻³)^2.61exp(−7.02×10³/RT)]$\ln (1/1-\alpha )=1/2.61[t^{2.61}{(1.50\times {10}^{-3})}^{2.61}\exp (-7.02\times {10}^3/RT)]$; whereas, the HRTEM image shows a typical monoclinic ZrO₂ domain because of the stress-induced tetragonal to monoclinic ZrO₂ martensitic transformation that has occurred.     

Electrocaloric effect based on the depolarization transition in (1−x)Bi0.5Na0.5TiO3–xKNbO3 lead-free ceramics

Lead-free ferroelectric ceramics (1−x)Bi_0.5Na_0.5TiO₃−xKNbO₃ (BNT–xKN) with x=0.00, 0.04, 0.06 and 0.08 were synthesized by the conventional solid state reaction method. The effects of the KNbO₃ addition on the dielectric behavior, ferroelectric properties, as well as electrocaloric effect of the ferroelectric ceramic BNT–xKN were investigated. The results show that the depolarization temperature decreases with the increment of KN content. A high ECE of 1.73 °C is achieved at 76 °C in BNT–0.06KN. The relation between electrocaloric effect and depolarization transition was discussed. This investigation indicates that the depolarization transition below Curie transition in BNT-based ceramics is a promising approach in ECE technique.     

Depolarization temperature and piezoelectric properties of (Bi1/2Na1/2)TiO3–(Bi1/2Li1/2)TiO3–(Bi1/2K1/2)TiO3 lead-free piezoelectric ceramics

The phase transition temperature and piezoelectric properties of x(Bi_1/2Na_1/2)TiO₃–y(Bi_1/2Li_1/2)TiO₃–z(Bi_1/2K_1/2)TiO₃ [x + y + z = 1] (abbreviated as BNLKT100y–100z) ceramics were investigated. BNLKT100y–100z ceramics were prepared by conventional ceramic fabrication. The depolarization temperature T_d was determined by the temperature dependence of the dielectric and piezoelectric properties. This study focuses on the effect of Li¹⁺ and K¹⁺ ions on T_d and the piezoelectric properties of BNT ceramics. BNLKT100y–100z (y = 0–0.08) has a morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases at z = 0.18–0.20, and high piezoelectric properties were obtained at the MPB composition. The piezoelectric constant d₃₃ increased with increasing y; however, T_d decreased above y = 0.06. The d₃₃ and T_d values of BNLKT4-20 and BNLKT8-20 were 176 pC/N and 171 °C, and 190 pC/N and 115 °C, respectively.     

Microstructure evolution of Si3N4/Si3N4 joint brazed with Ag–Cu–Ti + SiCp composite filler

The Si₃N₄ ceramic was brazed by Ag–Cu–Ti + SiCp composite filler (p = particle) prepared by mechanical mixing. Effects of the content of Ti and SiC particles on microstructure of the joint were investigated. A reliable Si₃N₄/Si₃N₄ joint was achieved by using Ag–Cu–Ti + SiCp composite filler at 1173 K for 10 min. A continuous and compact reaction layer, with a suitable thickness, forms at the Si₃N₄/braze interface. The SiC particles react with Ti in the brazing layers, forming Ti₃SiC₂ thin layers around the SiC particles themselves and Ti₅Si₃ small particles in the Ag[Cu] and Cu[Ag] based solid solution. The higher content of SiC particles in the filler (≥10 vol%) depresses interfacial bonding strength between the Si₃N₄ substrate and composite brazing layer due to the thinner reaction layer and the bad fluidity of the filler. The Ti₃SiC₂ → TiC + Ti₅Si₃ reaction occurs when Ti concentration around SiC particles in the filler increases.     

Comparative study of different crystallographic structure of LiNi0.5Mn1.5O4−δ cathodes with wide operation voltage (2.0-5.0 V)

LiNi_0.5Mn_1.5O_4−δ cathode materials with two different structures ( and P4₃32) were synthesized by ultrasonic spray pyrolysis method. The X-ray diffraction (XRD) data was confirmed that face-centered spinel () transformed into primitive simple cubic (P4₃32) structure by annealing process at 700 °C. In spite of two electrons operated cut-off voltage range between 2.0 and 5.0 V, LiNi_0.5Mn_1.5O₄ with P4₃32 structure has better electrochemical behaviors than the cathode with simple cubic structure. Ex situ XRD study of the electrode revealed that LiNi_0.5Mn_1.5O₄ (P4₃32) has reversible crystal transformation between fully lithiated state (2.0 V) and delithiated state (5.0 V) whereas LiNi_0.5Mn_1.5O_4−δ () showed irreversible phase transformed at two voltage region. The LiNi_0.5Mn_1.5O_4−δ () has voltage drop was occurred after 20th cycled compared without any voltage drop of the P4₃32 structure.     

Mechanical properties up to 1100 °C of Al2O3–MgO-extruded graphite pellets castables reinforced with steel fibres

The influences of adding 19, 25 and 50 mm carbon steel and 19 mm stainless steel fibres in the range of 0–3 volume percentages on the mechanical properties of alumina–magnesia-extruded graphite pellet castables have been studied at intermediate temperatures between 800° and 1100 °C, in argon atmosphere, as well as at room temperature. Wedge splitting test results at room temperature have shown that both carbon and stainless steel fibres increases work of fracture. Same tests at 1100 °C have shown decrease in strength with all length and volume percentage of carbon steel fibre while improving work of fracture values as 50% over fibre-free castables. Strength degradation is caused by the defect generation in the castable structure.     

Microstructure and electrical properties of (Na0.5K0.5)1−2xMgxNbO3–Bi0.5Na0.5TiO3 lead-free piezoelectric ceramics

0.975[(Na_0.5K_0.5)_1−2xMg_xNbO₃]–0.025(Bi_0.5Na_0.5TiO₃) (KNMN–BNT, x=0, 0.01, 0.02, 0.03, 0.04 and 0.05) lead-free piezoelectric ceramics were fabricated by the conventional solid-state sintering method. The dependence of Mg content on the microstructure and electrical properties of the ceramics is investigated. The X-ray diffraction (XRD) analysis revealed that an appropriate amount of Mg diffused into the KNN–BNT lattice to form a stable solid solution, the ceramics possessed a pure perovskite structure, and a morphotropic phase boundary (MPB) between the orthorhombic and tetragonal phases was observed with the composition of 0.02≤x≤0.05. The orthorhombic–tetragonal transition temperature (T_O–T) is less than 95 °C and the Curie temperature (T_c) is almost unchanged (~360 °C) with the increase of MgO content. The ceramics with x=0.02 showed enhanced piezoelectric and ferroelectric properties because of close proximity to the MPB, i.e., d₃₃~210 pC/N, k_p~0.41, 2E_c~22.4 kV/cm and 2P_r~39.2 μC/cm². Moreover, the dielectric properties exhibited optimal effects with x=0.02, that is ε_r~637 and tan δ~0.09. These results indicate that the introduction of MgO is an effective method to improve the density as well as the electrical properties and the temperature stability of the KNN–BNT ceramics. As a result, the KNMN–BNT ceramic is a promising candidate for lead-free piezoelectric materials.     

Influence of Al2O3 whisker concentration on flexural strength of Al2O3(w)–ZrO2 (TZ-3Y) composite

On investigating the possibility of using alumina whisker reinforced 3 mol% Yttria stabilized Tetragonal Zirconia (TZ-3Y) composite for bioceramic applications, presented here is the influence of varying whisker concentration (2, 5, 10, 15 and 20 wt%) on flexural strength of the composite. Whiskers of hydrothermally synthesized Ammonium Aluminum Carbonate Hydroxide (AACH) were used for composite synthesis. These whiskers transformed in situ into alumina during sintering. It was found that with addition of alumina whiskers, strength was increased and reached a maximum value of 1212±60 MPa and 1325±65 MPa, in pure and 1 wt% CTAB added samples respectively, at a concentration of 10 wt% Al₂O₃ whiskers. The strength values of the synthesized composite can compete well with commercially available materials for dental applications.