Electrical behavior of the Ruddlesden–Popper phase, (Nd0.9La0.1)2 Ni0.75Cu0.25O4 (NLNC) and NLNC- x wt% Sm0.2Ce0.8O1.9 (SDC) (x = 10, 30 and 50), as intermediate‐temperature solid oxide fuel cells cathode

In the present work, a new Ruddlesden-Popper phase, (Nd_0.9La_0.1)₂ Ni_0.75Cu_0.25O₄ (NLNC) has been synthesized by solid state reaction for intermediate-temperature solid oxide fuel cells (IT-SOFCs) applications. The effect of sintering temperature on the microstructure and electrical properties of the NLNC cathode material is investigated. Likewise, composite cathode materials were also prepared by mixing the NLNC with 10, 30 and 50 wt% of Sm_0.2Ce_0.8O_1.9 (SDC) powders, and firing in the temperature range of 1000–1300 °C. The crystal structure and chemical compatibility of NLNC and SDC, and their microstructures were studied by XRD and SEM, respectively. Electrical conductivity and performance of monolithic and composite electrodes as a function of the electrode composition is investigated experimentally through four probe method and electrochemical impedance spectroscopy (EIS). The results proved that no reaction occur between NLNC and SDC compounds even at a temperature as high as 1300 °C. Maximum total electrical conductivity of 114.36 S cm^?1 at 500 °C is recorded for the pure NLNC material sintered at 1300 °C. The polarization resistance of pure NLNC cathode was 0.43 Ω cm² at 800 °C; the NLNC–SDC composite cathodes including 10, 30 and 50 wt% SDC displayed Rp value of 0.27 Ω cm², 0.11 Ω cm², and 0.19 Ω cm² at 800 °C, respectively.     

Synthesis and oxidation behavior of (Mo0.97Nb0.03)(Si0.97Al0.03)2 ceramic

(Mo_0.97Nb_0.03)(Si_0.97Al_0.03)₂ ceramic was synthesized by self-propagating high-temperature synthesis using commercial elemental powders, and then dense monolithic ceramic was prepared via vacuum hot pressing. The oxidation behavior of the bulk ceramic was investigated at 500 °C and 1200 °C. At 500 °C, due to the preferential oxidation of Nb-rich phase and the formation of uniform oxidation layer, the oxidation rate of (Mo_0.97Nb_0.03)(Si_0.97Al_0.03)₂ is lower than pure MoSi₂. At 1200 °C, (Mo_0.97Nb_0.03)(Si_0.97Al_0.03)₂ shows better oxidation resistance than MoSi₂, owing to the uniform complex oxide layer with SiO₂, Al₂O₃ and Nb₂O₅ formed on the surface of the prepared ceramic.     

Experiments and transient finite element simulation of γ-Y2Si2O7/B2O3-Al2O3-SiO2 glass coating on porous Si3N4 substrate under thermal shock

A dense γ-Y₂Si₂O₇/B₂O₃-Al₂O₃-SiO₂ glass coating was fabricated by slurry spraying method on porous Si₃N₄ ceramic for water resistance. Thermal shock failure was recognized as one of the key failure modes for porous Si₃N₄ radome materials. In this paper, thermal shock resistance of the coated porous Si₃N₄ ceramics were investigated through rapid quenching thermal shock experiments and transient finite element analysis. Thermal shock resistance of the coating was tested at 700 °C, 800 °C, 900 °C and 1000 °C. Results showed that the cracks initiated within the coating after thermal shock from 800 °C to room temperature, thus leading to the reduction of the water resistance. Based on the finite element simulation results, thermal shock failure tended to occur in the coating layer with increasing temperature gradient, and the critical thermal shock failure temperature was measured as 872.24 °C. The results obtained from finite element analysis agree well with that from the thermal shock tests, indicating accuracy and feasibility of this numerical simulation method. Effects of thermo-physical properties for the coating material on its thermal shock resistance were also discussed. Thermal expansion coefficient of the coating material played a more decisive role in decreasing the tangent tensile stress.     

Influence of small amounts of gallium oxide addition on ionic conductivity of La0.9Sr0.1Ga0.8Mg0.2O3-δ solid electrolyte

The effects of small amounts of gallium oxide on intragrain and intergrain conductivity of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ are investigated by impedance spectroscopy in the 280–420 °C range. Bulk specimens with 0.5, 1.0 and 1.5 mol% gallium oxide are prepared by solid state reaction at 1350 °C. All specimens achieved relative density values higher than 95%. The additive promotes grain growth indicating solid solution formation. A small fraction of the additive remains at grain boundaries and increases the fraction of the gallium-rich, LaSrGa₃O₇, impurity phase. The intragrain conductivity of gallium oxide containing specimens is higher than that of the parent solid electrolyte. Similar effect is found for the intergrain conductivity, which is maximum for 1 mol% gallium oxide addition.     

The effect of synthesis conditions on the physicochemical properties of magnesium aluminate materials

Magnesium aluminate-based materials were prepared by applying different methods: (i) mechanochemical milling of the initial mixture of magnesium and aluminium nitrate powders (in appropriate stoichiometric amounts) followed by heat treatment at temperatures of 650 °C and 850 °C and (ii) melting of the mixture of nitrate precursors at 240 °C followed by thermal treatment at 650 °C, 750 °C and 850 °C. The effect of synthesis method on the structure and morphology of the obtained solids was studied by using various techniques such as: nitrogen adsorption-desorption isotherms, powder XRD, IR spectroscopy and SEM. It was shown that the mechanochemical milling performed before calcination procedure leads to obtaining of nanocrystalline magnesium aluminate spinel phase at lower temperature of 650 °C in comparison with the method using thermal treatment only (at 750 °C). The obtained nanomaterials exhibit mesoporous structure.     

Microstructure and elastic modulus of electrospun Al2O3-SiO2-B2O3 composite nanofibers with mullite-type structure prepared at elevated temperatures

In the present work, Al₂O₃-SiO₂-B₂O₃ composite nanofibers with mullite-type structure were prepared using electrospinning technique. The microstructure and elastic modulus of the composite nanofibers obtained at elevated temperatures were studied. The results showed that Al₄B₂O₉ phase formed at 900 °C and then transformed to Al₁₈B₄O₃₃ at 1100 °C. Mullite was also detected in the nanofibers prepared at 1100 °C. Amorphous SiO₂ existed in all samples even the calcination temperature reached up to 1400 °C. The continuous and uniform structure of the composite nanofibers was kept after calcining at different temperatures, while rougher surface was evident due to the growth of the grain caused by the elevated temperature. An increase of elastic modulus of the samples from 9.47 ± 1.91 GPa to 27.30 ± 2.61 GPa was observed when calcination temperatures increased from 800 °C to 1400 °C.     

Liquid-phase synthesis of SrCo0.9Nb0.1O3-δ cathode material for proton-conducting solid oxide fuel cells

A novel liquid-phase synthesis strategy is demonstrated for the preparation of the Nb-containing ceramic oxide SrCo_0.9Nb_0.1O_3-δ (SCN). In comparison with the traditional solid-state reaction (SSR) method, the liquid-phase synthesis route offers a couple of advantages, including a lower phase formation temperature and a smaller particle size of the SCN materials that are beneficial for applications as proton-conducting fuel cell cathode. With BaCe_0.4Zr_0.4Y_0.2O_3-δ (BCZY442) as the electrolyte and the SCN synthesized in this work as the cathode, a proton-conducting solid oxide fuel cell (SOFC) shows a peak power density of 348 mW cm^?2 at 700 °C, significantly higher than that of a SOFC fabricated with SCN cathode prepared using the SSR method, which can only deliver 204 mW cm^?2 at the same temperature. Additionally, this new synthesis strategy allows impregnation of Sr²⁺, Co³⁺and Nb⁵⁺ on the solid backbone in aqueous solution, further improving cell performance to reach a peak power density of 488 mW cm^?2 at 700 °C.     

Preparation of Al2O3 and MgAl2O4-based samples with tailored macroporous structures

In this work we successfully obtained freeze-cast alumina (Al₂O₃) and magnesium aluminate spinel (MgAl₂O₄) samples. Camphene was used as the freezing vehicle in this study. The specimens prepared herein were examined by Archimedes tests, scanning electron microscopy, and X-ray powder diffraction. Cold crushing tests were also carried out at room temperature. It was observed that the pore structure of Al₂O₃ samples can be tailored by changing the solid loading and freezing rate; the higher the solid loading and freezing rate, the finer the pore structure of the freeze-cast sample. MgAl₂O₄-based specimens were fabricated by keeping the solid loading in the starting slurry at 30 vol% and using liquid nitrogen as the cooling agent. The material obtained from a 60 Al₂O₃?40 MgO slurry showed a spinel amount of about 90%, an expressive total porosity (63 ± 3%), and a significant cold crushing strength (58 ± 6 MPa). In addition, this material exhibited the finest pore structure among the composition studied herein, showing a mean pore size of about 4 µm.     

Structural characterization of YSZ/Al2O3 nanostructured composite coating fabricated by electrophoretic deposition and reaction bonding

Suspension of YSZ and Al particles in acetone in presence of 1.2 g/l iodine as dispersant was used for electrophoretic deposition of green form YSZ/Al coating. Results revealed that applied voltage of 6 V and deposition time of 3 min were appropriate for deposition of green composite form coating. After deposition, a nanostructured dense YSZ/Al₂O₃ composite coating was fabricated by oxidation of Al particles at 600 °C for 2 h and subsequently sintering heat treatment at 1000 °C for 2 h. Melting and oxidation of Al particles in the green form composite coating not only caused reaction bonding between the particles but also lowered the sintering temperature of the ceramic coating about 200 °C. The EDS maps confirmed that the composition of fabricated coating was uniform and Al₂O₃ particles were dispersed homogenously in YSZ matrix.     

Fabrication of porous Al2O3-based ceramics using ball-shaped powders by preceramic polymer process in N2 atmosphere

Porous Al₂O₃-based ceramics were successfully fabricated using ball-shaped powders by preceramic polymer process in N₂ atmosphere. These results showed that the amorphous Si-O-C ceramics were formed on the surface of ball-shaped Al₂O₃ particles by the pyrolysis of the silicone resin during sintering in N₂ atmosphere, which played a role in connecting the Al₂O₃ particles by forming the sintering necks. When the sintering temperatures increased from 1100 °C to 1600 °C, the formed Si-O-C ceramics still existed in the amorphous state and had no crystallization. Interestingly, the amorphous β-SiC formed at 1300 °C and its amount gradually increased with further increasing temperatures. The linear shrinkage rate of the samples varied from 0.49% to 0.73% and the weight loss rate increased from 2.01% to 10.77%. The apparent porosity remarkably varied with the range of 24.9% and 34.5%, as the bulk-density varied from 2.66 to 2.47 g/cm³. The bending strength gradually increased from 9.36 to 22.51 MPa with increasing temperatures from 1100 °C to 1500 °C, however, the bending strength remarkably decreased at 1600 °C, which was attributed to the comprehensive function of the high porosity, broken Al₂O₃ particles and weak connection between Al₂O₃ particles in the samples.     

Sintering behaviour,microstructural characterisation and thermal expansion properties of Sn substituted ZrMo2O8

The coefficient of thermal expansion (CTE) of ZrMo₂O₈ can be fine-tuned by controlling the amount of tin substitution in zirconium lattice sites. The sintering challenges associated with this material and the optimal sintering conditions were investigated in this study. Powders of tin substituted ZrMo₂O₈ were synthesised by co-precipitation technique. X-ray diffraction studies confirmed the formation of cubic SnMo₂O₈. Sintered pellets were produced from the powders and optimal sintering without decomposition of the phase was achieved at the expense of porosity. The material was found to be thermally stable up to 600 °C using thermogravimetric analysis. Dilatometric analysis of the sintered compacts shows that the CTE of the sample is in the order of 3.9 × 10^?6/K, between 25 °C and 600 °C.     

Highly efficient La0.8Sr0.2MnO3-δ - Ce0.9Gd0.1O1.95 nanocomposite cathodes for solid oxide fuel cells

La_0.8Sr_0.2MnO_3-δ-Ce_0.9Gd_0.1O_1.95 (LSM-CGO) nanostructured cathodes are successfully prepared in a single process by a chemical spray-pyrolysis deposition method. The cathode is composed of nanometric particles of approximately 15 nm of diameter, providing high triple-phase boundary sites for the oxygen reduction reactions. A low polarization resistance of 0.046 Ω cm² is obtained at 700 °C, which is comparable to the most efficient cobaltite-based perovskite cathodes. A NiO-YSZ anode supported fuel cell with the nanostructured cathode generates a power output of 1.4 W cm^?2 at 800 °C, significantly higher than 0.75 W cm^?2 for a cell with conventional LSM-CGO cathode. The results suggest that this is a promising strategy to achieve high efficiency electrodes for Solid Oxide Fuel Cells in a single preparation step, simplifying notably the fabrication process compared to traditional methods.     

Low-temperature sintering of Ti1−xCux/3Nb2x/3O2 (x = 0.23) microwave dielectric ceramics with CuO and B2O3 addition

The influence of CuO and B₂O₃ addition on the sintering behavior, microstructure and microwave dielectric properties of Ti_1?xCu_x/3Nb_2x/3O₂ (TCN, x = 0.23) ceramic have been investigated. It was found that the addition of CuO and B₂O₃ successfully reduced the sintering temperature of TCN ceramics from 950 to 875 °C. X-ray diffraction studies showed that addition of CuO-B₂O₃ has no effect on the phase composition. The TCN ceramics with 0.5 wt% CuO-B₂O₃ addition showed a high dielectric constant of 95.63, τ_f value of + 329 ppm/°C and a good Q × f value of 8700 GHz after sintered at 875 °C for 5 h, cofirable with silver electrode.     

Processing,microstructure and performance of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics prepared by laser floating zone melting with ultra-high temperature gradient

Directionally solidified Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary eutectic/off-eutectic composite ceramics with high density, homogeneous microstructures, well-oriented growth have been prepared by laser floating zone melting at different solidification rates from 4 to 400 µm/s. Uniform and stable melting zone is obtained by optimizing temperature field distribution to keep continuous and stable eutectic growth and prevent from cracks and defects. The as-solidified composite ceramic exhibits complexly irregular eutectic structure, in which the eutectic spacing is rapidly refined but dotted ZrO₂ number inside Al₂O₃ phase is decreased as increasing the solidification rate. The formation mechanism of ZrO₂ distributed inside Al₂O₃ matrix is revealed by examining the depression of solid/liquid interface. Furthermore, after heat exposure 1500 °C for 200 h, the eutectic microstructure only shows tiny coarsening, which indicates it has excellent microstructural stability. As increasing the ZrO₂ content, the fracture toughness can be improved up to 3.5 MPa m^1/2 at 20.6 mol% ZrO₂.     

Facile synthesis of LaAlO3 and Eu(II)-doped LaAlO3 powders by a solid-state reaction

Lanthanum aluminate (LaAlO₃) powder was prepared by a solid-state reaction between lanthanum(III) carbonate fluoride (LaFCO₃) and alumina (Al₂O₃) powders at elevated temperatures, and characterized by powder X-ray diffraction (XRD), ²⁷Al magic-angle spinning nuclear magnetic resonance spectroscopy, and scanning electron microscopy. The formation temperature (1000 °C) of LaAlO₃ was much lower than that in other solid-state reactions. The Eu(II)-doped LaAlO₃ powder was prepared by calcination of a mixture of Eu(III)-doped LaFCO₃ and Al₂O₃ in a nitrogen atmosphere and characterized by powder XRD, and photoluminescence and X-ray photoelectron spectroscopy. The intensity of blue emission due to Eu(II) ions in the LaAlO₃ powder increased with increasing calcination temperature up to 1200 °C but decreased with further increases in temperature. The origin of the Eu(II) ions was explained by the thermal decomposition of EuF₃.     

Facile modified synthesis and intermediate temperature electrical properties of Sn0.95Al0.05P2O7/KSn2(PO4)3 composite electrolyte

In this study, Sn_0.95Al_0.05P₂O₇ and a novel dense Sn_0.95Al_0.05P₂O₇/KSn₂(PO₄)₃ composite electrolytes were synthesized. The structural characterization of X–ray diffraction (XRD) and microstructual properties of scanning electron microscopy (SEM) were carried out. The XRD results indicated that an in-situ reaction between Sn_0.95Al_0.05P₂O₇ and inorganic melt salt take place to form the Sn_0.95Al_0.05P₂O₇/KSn₂(PO₄)₃ composite. The intermediate temperature electrical properties were determined by using impedance spectroscopy, oxygen concentration cell and hydrogen concentration discharge cell. Finally, the H₂/O₂ fuel cell using the Sn_0.95Al_0.05P₂O₇/KSn₂(PO₄)₃ as electrolyte membrane was constructed and the obtained maximum power output densities were 67.7 mW cm^?2 and 142.1 mW cm^?2 at 650 °C and 700 °C, respectively.     

The effect of slip casting and spark plasma sintering (SPS) temperature on the transparency of MgAl2O4 spinel

MgAl₂O₄ bulk samples were fabricated by two different approaches to investigate the effect of slip casting and sintering temperature on their transparency. Three MgAl₂O₄ samples containing 1 wt% LiF, as the sintering aid, were prepared by the spark plasma sintering process (SPS) at 1400 °C and 1500 °C, under 100 MPa, for 15 min. Also, another MgAl₂O₄ sample was prepared by slip casting followed by SPS under similar conditions. It was observed that utilizing slip casting led to more transparency (10% in the visible region and 20% in the IR region) due to the more homogeneous structure. It was also observed that by reducing the SPS temperature from 1500 °C to 1400 °C, the transparency increased (20% in the IR region) because of the lower grain growth rate at the lower temperature.     

The phase formation process of Bi0.5(Na0.8K0.2)0.5TiO3 thin films prepared using the sol-gel method

Lead-free Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ (abbreviated as BNKT) thin films were grown on Pt(111)/Ti/SiO₂/Si substrates using a sol-gel/spin coating technique and were then annealed at different temperatures (350 °C, 550 °C, 750 °C and 850 °C). Analysis of the XRD patterns and FT-IR spectra were used to determine the main reactions and the phase formation process of BNKT thin films during the sol-gel process. The results show that the dielectric constant of the thin films attains a maximum at a set temperature and then decreases at higher annealing temperatures, which can be attributed to phase formation and transformation. Moreover, the morphologies of the BNKT thin films improve with the increase in grain size and the formation of distinct grain boundaries. Furthermore, through increasing the pH of the precursor solutions, the size of the sol-gel colloidal particles increases slightly and the grains formed from the corresponding solutions tend to be small and uniform.     

Theoretical analysis and synthesis of Al4O4C and Al2CO phase in the resin bonded Al-Al2O3 refractory in N2-flowing

In flowing nitrogen, Al₄O₄C and Al₂CO bonded Al₂O₃-based composite was successfully prepared by a gaseous phase mass transfer pathway at 1600 °C for 3 h after an Al-AlN core-shell structure was formed in the resin bonded Al-Al₂O₃ refractory at 580 °C for 8 h. The formation mechanism of Al₄O₄C and Al₂CO phase is as follows. An Al-AlN core–shell structure is built at 580 °C for 8 h and broken at higher temperatures, and then, Al(g) reacts with C from the resin and N₂ to form Al₄C₃ and AlN, respectively. Owing to the exothermic reaction of the Al₄C₃ and AlN formation, the reaction temperature in the resin bonded Al-Al₂O₃ refractory is above the practical environmental temperature; for instance, the reaction temperature is above 1715 °C at 1600 °C in this work. Therefore, Al₄C₃ reacts with Al₂O₃ to generate Al₄O₄C and then Al₄O₄C is transformed into Al₂OC by the reaction ${\mathrm{Al}}_4{\mathrm{O}}_{\mathrm{4}}{C(s)+\text{Al}}_4{\mathrm{C}}_{\mathrm{3}}(s)\to {\mathrm{4Al}}_{\mathrm{2}}\text{OC}(s)$ at elevated temperatures. Al₂OC solid solution is finally formed through the dissolution of AlN into Al₂OC.     

Co2O3 substitution effects on the structure and microwave dielectric properties of low-firing (Zn0.9Mg0.1)TiO3 ceramics

Low-firing (Zn_0.9Mg_0.1)_1?xCo_xTiO₃ (x = 0.02–0.10) (ZMC_xT) microwave dielectric ceramics with high temperature stability were synthesized via conventional solid-state reaction. The influences of Co₂O₃ substitution on the phase composition, microstructure and microwave dielectric properties of ZMC_xT ceramics were discussed. Rietveld refinement results show the coexistence of ZnTiO₃ and ZnB₂O₄ phases at x = 0.02–0.10. (Zn_0.9Mg_0.1)_1?xCo_xTiO₃ ceramic with x = 0.06 (ZMC_0.06T) obtains the best combination microwave dielectric properties of: ε_r = 21.58, Q × f = 53,948 GHz, τ_f = ? 54.38 ppm/°C. For expanding its application in LTCC field, 3 wt% ZnO-B₂O₃-SiO₂ (ZBS) and 9 wt% TiO₂ was added into ZMC_0.06T ceramic, great microwave dielectric properties were achieved at 900 °C for 4 h: ε_r = 26.03, Q × f = 34,830 GHz, τ_f = ? 4 ppm/°C, making the composite ceramic a promising candidate for LTCC industry.