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.     

Influence of Al-substitution on structures,resistivity and magnetic properties of LaxSr(2−x)Fe(1+y)Mo(1−y)O6 (x=3y,y=0.05, 0.1, 0.15, 0.2)

Two series of single-phased La_xSr_(2−x)Fe_(1+y)Mo_(1−y)O₆ and La_xSr_(2−x)Fe_(1+0.5y)Al_0.5yMo_(1−y)O₆ (x=3y, y=0.05, 0.1, 0.15 and 0.2) double perovskites were prepared by solid-state reaction. The effects of Al-substitution on the structures, resistivity and magnetic properties of La_xSr_(2−x)Fe_(1+y)Mo_(1−y)O₆ were investigated. Although Al-replacement exhibits a negligible influence of on the B-site ordering degree, it results in the suppression of magnetisation caused by non-magnetic Al³⁺ ions. Reduction of grain sizes leads to increased resistivity, thus an optimised magnetoresistance (MR) behaviour is observed. The greatest MR extent improvement can be obtained when y is 0.15 and the MR% of the Al-doped ceramics reaches −10.5% (10 K, 1 T), which is 2 times greater than that of the undoped ceramics (−4.6%, 10 K, 1 T). Interestingly, the Curie temperature (Tc) of both Al-doped and undoped samples maintained relatively constant values of approximately 420 K and 405 K, respectively, which were different results from the data obtained for similar electron-doping systems in the literature.     

Evaluation of GdBaCuCo0.5Fe0.5O5+δ as cathode material for intermediate temperature solid oxide fuel cells

The GdBaCuCo_0.5Fe_0.5O_5+δ (GBCCF) layered perovskite oxide was evaluated as novel cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). Its electrical conductivity was 9–13 S cm^?1 at 650–800 °C in air. The average thermal expansion coefficient (TEC) of GBCCF was 14.4 × 10^?6 K^?1, which was close to that of the typical electrolyte material. The cathode polarization resistance of GBCCF was 0.650 Ω cm² at 750 °C and it decreases to 0.118 Ω cm² when Ce_0.9Gd_0.1O_1.95 (GDC) was added to form a GBCCF–GDC composite cathode. Preliminary results indicated that layered perovskite GBCCF was a promising alternative cathode material for IT-SOFCs.     

Synthesis,structure, optical and magnetic properties of Nd1−xAxMn0.5Co0.5O3−δ (A = Ba,Sr and Ca; x = 0 and 0.25)

The structural, optical, and magnetic properties of polycrystalline Nd_1-xA_xMn_0.5Co_0.5O_3−δ (A = Ba, Sr and Ca; x = 0 and 0.25) perovskite oxides were investigated. The powder XRD pattern demonstrates that the unit cell volume decreases with the changing A-site dopant type. The estimated bandgap energy (Eg) from UV–vis spectroscopic for NdMn_0.5Co_0.5O_3−δ, Nd_0.75Ba_0.25Mn_0.5Co_0.5O_3−δ, Nd_0.75Sr_0.25Mn_0.5Co_0.5O_3−δ and Nd_0.75Ca_0.25Mn_0.5Co_0.5O_3−δ are 3.27, 3.82, 3.79 and 3.53 eV respectively. The substitution of divalent element alters the absorption spectrum, while the redshift optical transition was observed with an increasing ionic radius of dopant. Temperature-dependent magnetization exposes that the Curie temperature (TC) gradually decreases with the decreasing size of alkaline earth metals, and glassy nature was observed at a lower applied magnetic field. The observation of TC can be well explained by the considering of the cationic size disorder parameter in A-site than the random distribution of B-site ions.     

Structure and thermal properties of SrCe1-xSnxO3 (x=0.1 … 0.5) as a promising thermal barrier coating

Gas turbines efficiency growth is primarily associated with an increase in the operating temperature of the combustion chamber, which places new stringent requirements on the materials of thermal barrier coatings. Strontium cerate doped with tin SrCe_{1- x}Sn_xO₃ where x = 0.1 … 0.5, was proposed as a promising material. The research has shown that the lightly doped solid solution SrCe_1-xSn_xO₃ has an orthorhombic Pnma structure at x < 0.3, whereas at a high content of Sn⁴⁺ the monoclinic structure P2₁/m becomes more favorable. Thermogravimetric analysis (TGA) in reducing atmosphere (5%H₂ in Ar) shows no mass lost as a result of unchangeable charge of Ce⁴⁺ and Sn⁴⁺. An increase in the distortion of the crystal lattice, due to the large difference in the ionic radii of Ce⁴⁺ and Sn⁴⁺, leads to a deterioration in the symmetry of the crystal lattice, a reduction of thermal conductivity (from 1.9 to 1.4 W m⁻¹ K⁻¹ at 1000 °C) and at the same time, growth of hardness and porosity. The increase in porosity, along with an increase in the required temperature of solid-state synthesis, indicates an enhancement in the melting point of the obtained materials. For the compounds with an orthorhombic structure, the thermal expansion coefficient increases with a growth in the Sn content, achieving a highest point 12.47·10⁻⁶ K⁻¹ at 1100 °C for x = 0.3. The combination of the revealed properties and their comparison with advanced refractories makes the solid solution, primarily SrCe_0.5Sn_0.5O₃, a promising material for application as thermal barrier coatings.     

Structural,morphological, and electrochemical behavior of titanium-doped SrFe1-xTixO3-δ (x = 0.1–0.5) perovskite as a cobalt-free solid oxide fuel cell cathode

Titanium, Ti-doped SrFe_1-xTi_xO_3-δ (x = 0.1–0.5) perovskite-structured ceramics were synthesized via solution combustion. The structural, morphological, and electrochemical behaviors of the as-synthesized materials were investigated to determine the applicability of SrFe_1-xTi_xO_3-δ as a cobalt-free cathode material for intermediate-temperature solid oxide fuel cells. X-ray diffraction analysis confirmed the formation of a single-phase cubic perovskite structure. The unit volume of this perovskite structure increased as the amount of Ti dopant increased. Morphological analysis revealed that the porosity of the SrFe_1-xTi_xO_3-δ perovskite cathode film was inversely proportional to the amount of Ti dopant. The cathode SrFe_0·9Ti_0·1O_3-δ film exhibited a high porosity of 24.74 ± 0.52%, a low but acceptable hardness value of 0.70 ± 0.01 GPa and an area specific resistance of 0.57 Ω cm². These results suggested that cobalt-free SrFe_1-xTi_xO_3-δ cathode was still not good enough to be compared with the existing cobalt-containing cathode such as lanthanum strontium cobalt ferrite. But, the results obtained from this work can be considered as a major turning point as the literature works on SrFe_1-xTi_xO_3-δ cathode showed excellence electrochemical performance. The contradict result between the present and past studies proved that the use of SrFe_1-xTi_xO_3-δ cathode is worthy of being studied into details to confirm its capability.     

Colossal dielectric permittivity and electrical properties of the grain boundary of Ca1−3x/2YbxCu3−yMgyTi4O12 (x=0.05, y=0.05 and 0.30)

Dielectric properties of Ca_1−3x/2Yb_xCu_3−yMg_yTi₄O₁₂ (x=0.05, y=0.05 and 0.30) prepared using a modified sol–gel method and sintered at 1070 °C for 4 h were investigated. The mean grain sizes of the CaCu₃Ti₄O₁₂ and co-doped Ca_0.925Yb_0.05Cu_3−yMg_yTi₄O₁₂ (y=0.05 and 0.30) ceramics were ≈15.86, ≈3.37, and ≈2.32 μm, respectively. Interestingly, the dielectric properties can be effectively improved by co-doping with Yb³⁺ and Mg²⁺ ions to simultaneously control the microstructure and properties of grain boundaries, respectively. These properties were improved over those of single-doped and un-doped CaCu₃Ti₄O₁₂ ceramics. A highly frequency−independent colossal dielectric permittivity (≈10⁴) in the range of 10²–10⁶ Hz with very low loss tangent values of 0.018–0.028 at 1 kHz were successfully achieved in the co-doped Ca_0.925Yb_0.05Cu_3−yMg_yTi₄O₁₂ ceramics. Furthermore, the temperature stability of the colossal dielectric response of Ca_1−3x/2Yb_xCu_3−yMg_yTi₄O₁₂ was also improved to values of less than ±15% in the temperature range from −70 to 100 °C.     

Thermal expansion and thermal conductivity of SmxZr1−xO2−x/2 (0.1 ≤ x ≤ 0.5) ceramics

A study was conducted of the effect of additions of samarium oxide on the thermal expansion and thermal conductivity of zirconium oxide for thermal barrier coatings. Sm_xZr_1?xO_2?x/2 (0.1 ≤ x ≤ 0.5) ceramic powders synthesized with a chemical-coprecipitation and calcination method were sintered at 1873 K for 15 h. Structures of the synthesized powders and sintered ceramics were identified by X-ray diffractometer. The morphologies of ceramic powders were observed by transmission electron microscope. The thermal expansion coefficients and thermal diffusion coefficients of Sm_xZr_1?xO_2?x/2 ceramics were studied with a high-temperature dilatometer and a laser flash diffusivity technique from room temperature to 1673 K. The thermal conductivity was calculated from thermal diffusivity, density and specific heat of bulk ceramics. Sm_0.1Zr_0.9O_1.95 ceramics consists of both monoclinic and tetragonal structures. However, Sm_0.2Zr_0.8O_1.9 and Sm_0.3Zr_0.7O_1.85 ceramics only exhibit a defect fluorite structure. Sm_0.4Zr_0.6O_1.8 and Sm_0.5Zr_0.5O_1.75 ceramics have a pyrochlore-type lattice. With the increase of Sm₂O₃ content, the linear thermal expansion of Sm_xZr_1?xO_2?x/2 ceramics increases except for Sm_0.1Zr_0.9O_1.95. The thermal conductivities of Sm_xZr_1?xO_2?x/2 ceramics ranged from 1.41 at 873 K to 1.86 W m^?1 K^?1 at room temperature in a test temperature range of room temperature to 1673 K, and the results can be explained by phonon scattering mechanism.     

Characterization of Ba0.5Sr0.5M1−xFexO3−δ (M = Co and Cu) perovskite oxide cathode materials for intermediate temperature solid oxide fuel cells

Ba_0.5Sr_0.5Co_1?xFe_xO_3?δ (x = 0.2, 0.6, and 0.8) and Ba_0.5Sr_0.5Cu_1?xFe_xO_3?δ (x = 0.6 and 0.8) perovskite oxides have been investigated as cathode materials for intermediate temperature solid oxide fuel cells. All the samples synthesized by a citrate–EDTA complexing method were single-phase cubic perovskite solid solutions. Then, the thermal expansion coefficient, electrical conductivities, the oxygen vacancy concentrations, the polarization resistances (R_p), and the power densities were measured. An increase in the Co content resulted in a decrease in the polarization resistance, the electrical conductivities at low temperatures, and the inflection point of the thermal expansion coefficient, but it led to an increase in the electrical conductivities at high temperatures, the oxygen vacancy concentrations, and the maximum power densities. The Cu-based system has similar behavior to the Co-based system; yet, in terms of the electrical conductivities, high Cu content gave a better result than low content for the entire range of temperatures.     

Evaluation of A-site deficient Sr1−xSc0.175Nb0.025Co0.8O3−δ (x=0, 0.02, 0.05 and 0.1) perovskite cathodes for intermediate-temperature solid oxide fuel cells

This work strives to improve the performance of SrSc_0.175Nb_0.025Co_0.8O_3−δ (SSNC) cathode by introducing A-site cation deficiency up to 10 mol%. Three different Sr-deficient compositions, i.e., Sr_1−xSc_0.175Nb_0.025Co_0.8O_3−δ (S_1−xSNC, x=0.02, 0.05 and 0.1) including the non-deficient analogue, SSNC are prepared. Powder X-ray diffraction patterns indicate that the original cubic perovskite structure is retained. The thermal expansion coefficient between 50 °C and 900 °C increases progressively with increasing Sr deficiency, consistent with the “β-oxygen” release profiles trend. The electrical conductivities for SSNC, S_0.98SNC, S_0.95SNC and S_0.9SNC show a maximum-type profile against increasing temperature, i.e., semiconducting behavior followed by metallic behavior. Despite the consistent increase in the oxygen non-stoichiometry with increasing Sr deficiency, the oxygen reduction reaction performance increases in the order of SSNC, S_0.9SNC, S_0.98SNC and S_0.95SNC. That the highest oxygen reduction reaction (ORR) performance is demonstrated by S_0.95SNC indicates the trade-off between the increase in the concentration of oxygen vacancies and the formation of the phase impurities. At 650 °C, S_0.95SNC shows an area specific resistance of 0.017 Ω cm² (from symmetric cell test) and a peak power density of 1263 mW cm⁻² (from single fuel cell test on a Ni-Sm_0.2Ce_0.8O_1.9 (SDC) anode supported SDC electrolyte).     

Preparation and dielectric properties of (Sr1−xCax)Fe0.5Nb0.5O3; (x=0.0, 0.1, 0.2) ceramics

In the present work, strontium calcium iron niobate ((Sr_1?xCa_x)Fe_0.5Nb_0.5O₃; SCFN) (x=0, 0.1, and 0.2) powders were synthesized for the first time using a molten salt technique. The pure phase perovskite obtained at a relative low calcination temperature of 800 °C was characterized using the X-ray diffraction technique (XRD). SCFN ceramics were fabricated and their properties were investigated. The XRD data of the SCFN ceramics was consistent with an orthorhombic symmetry. However, the solubility of Ca in the SCFN ceramics had an upper limit at x=0.1. All ceramics showed a large dielectric constants. The Ca doping inhibited grain growth, but produced an improvement in dielectric–temperature stability. Furthermore, the doping reduced loss tangent, especially for the x=0.1 sample. These results suggest that the SCFN ceramics prepared from molten salt synthesis exhibit a good dielectric performances, compared to many high dielectric materials that have been prepared using the conventional method.     

Crystal structure,microstructure and electrical properties of (1−x−y)Bi0.5Na0.5TiO3–xBi0.5K0.5TiO3–yBiFeO3 ceramics near MPB prepared via the combustion technique

(1?x?y)Bi_0.5Na_0.5TiO₃–xBi_0.5K_0.5TiO₃–yBiFeO₃ (BNKFT-x/y with 0.12≤x≤0.24, 0≤y≤0.07) lead-free piezoelectric ceramics have been prepared by the combustion technique. The effects of amounts of x and y on structures and electrical properties were examined. Powders and ceramics can be well calcined and sintered at 750 °C for 2 h and 1025–1050 °C, respectively. The results indicated that the crystalline structure and microstructure changed with the increase of x and y concentrations. XRD results of BNKFT-x/0.03 and BNKFT-0.18/y ceramics with 0.12≤x≤0.24 and 0≤y≤0.07 showed the rhombohedral–tetragonal morphotropic phase boundary (MPB). The addition of y caused a promoted grain growth while the addition of x suppressed the grain growth. The highest density (ρ=5.85 g/cm³), superior dielectric properties at T_c (ε_r=7846 and tan δ=0.02), remnant polarization measured at 40 kV/cm (P_r = 20.1 μC/cm²) and piezoelectric coefficient (d₃₃=213 pC/N) were obtained for x=0.18 and y=0.03.     

Stability and electrical conductivity of BaCe0.85Tb0.05M0.1O3−δ (M=Co,Fe, Y,Zr, Mn) high temperature proton conductors

High temperature proton conductors (HTPCs) must possess both high electrical conductivity and good chemical and structural stability for practical applications. In this work, BaCe_0.85Tb_0.05M_0.1O_3−δ (BCTM) high temperature proton conductors doped with M=Co, Fe, Y, Zr, Mn were prepared by the sol-gel combustion method using EDTA and citric acid as the chelating agents. The structural and chemical stability, and electrical conductivity of the BCTM perovskites were measured in different atmospheres by the powder X-ray diffraction (XRD) and four-probe techniques. The results indicate that the perovskite phase formed in the BCTM oxides after calcination at 1000 °C can be preserved well in H₂ and water vapor-containing atmospheres, whereas the perovskites may be segregated into metal oxides and react with CO₂ into carbonates in the CO₂ environment due to the strong alkalinity of the BaCeO₃ composites. The electrical conductivity of the BCTM oxides in hydrogen atmosphere is noticeably higher than in the air atmosphere, confirming the dominant protonic conductivity in the BCTM perovskites. Doping with Mn or Co ions in the BCTb perovskite favors improving the electrical conductivity with lower activation energy. The BCTMn perovskite demonstrates an N-shaped variation trend in electrical conductivity with temperature due to the multi-valence states for Mn ions presented at different temperatures.     

Synthesis and characterisation of MnGaxCr2−xO4 (0.1≤x≤1) spinels as potential electrode support materials for intermediate-temperature solid oxide fuel cells

MnGa_xCr_2−xO₄ (MGCO, x=0.1, 0.2, 0.4, 0.8, 1) oxides are synthesised using a citric acid nitrate combustion method. The influence of Ga substitution on the structure, electrical conductivity and electrochemical performance are systematically investigated. The chemical and thermal compatibility of MGCO materials with yttrium-stabilised zirconia (YSZ) are also studied. All the samples exhibit a single phase spinel structure. Thermal expansion coefficients (TECs) of the MGCO oxides are in the range of 9–12×10⁻⁶ K⁻¹, indicating a good thermal match with the YSZ electrolyte. No chemical reactions are detected between MGCO materials and YSZ, indicating their good chemical compatibility with YSZ. The magnitude of electrical conductivity of all the obtained samples is in the order of about 10⁻³ S cm⁻¹at 800 °C measured in air. The polarisation resistance reaches a value as low as 5.2 Ω cm² for x=0.4 at 800 °C. The preliminary results demonstrate that MGCO materials could be used as electrode support materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs).     

Na2TiGeO5—A self-light-emitting phosphor with the stable structure and tunable emission resulted from Cr3+-doped for FEDs

For matching the high energy excitation by the low voltage electron beams, the phosphors with stable crystal structure and high energy level excitation band is necessary. In this work, it is interesting to find a suitable phosphor of Na₂TiGeO₅ with self-light emitting property. The tight crystal structure of Na₂TiGeO₅ formed by edge-shared NaO₆ and TO₅ polyhedrons corresponds to its good thermal stability and degradation resistance property. The results indicate that the PL and CL intensity can still remain beyond 80% and 90% at 150°C and after 1 hour electron beams bombardment, respectively. Under the 254 nm excitation, the bright blue light peaked at 460 nm can be detected and the light control has been realized with the Cr³⁺ doped, which changes blue light to cyan light. The high energy excitation is attributed to the charge transfer band (CTB) of Ti⁴⁺–O²⁻, which matches the electron beams. The bright CL emission light centered at 450 nm with good degradation resistance property has been detected. The results indicate its potential application in field emission display (FED)s.     

Lithium ion conductivity in the solid electrolytes (Li0.25La0.25)1−xM0.5xNbO3 (M=Sr,Ba, Ca,x=0.125) with perovskite-type structure

Perovskite-type structured solid electrolytes with the general formula (Li_0.25La_0.25)_1−xM_0.5xNbO₃ (M=Sr, Ba, Ca, x=0.125) have been prepared by solid-state reaction. Their crystal structure and ionic conductivity were examined by means of X-ray diffraction analysis (XRD), scanning electron microscope (SEM), and alternating current (AC) impedance technique. All sintered compounds are isostructural with the parent compound Li_0.5La_0.5Nb₂O₆. Some impurity phase is detected at the grain boundary in the Ba- and Ca-substituted compounds. The substitution of partial Li⁺ by alkaline earth metal ions has responsibility for the cell volume expansion as determined by the XRD data. The densification is accelerated, with the overall porosity and grain boundary minimized as Sr²⁺ ions are doped. Among the investigated compounds, the perovskite (Li_0.25La_0.25)_0.875Sr_0.0625NbO₃ shows a remarkable ionic conductivity of 1.02×10⁻⁵ S/cm at room temperature (20 °C) and the lowest activation energy of 0.34 eV in comparison with 0.38 eV and 0.44 eV for the corresponding Ba- and Ca-doped samples, respectively. It is identified that the enhancement of ionic conductivity is attributed to a reduction in activation energy for ionic conduction which is related to an increase in the cell volume.     

Electro-spinning Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3−δ nanofibers infiltrated with Gd0.2Ce0.8O1.9 nanoparticles as cathode for intermediate temperature solid oxide fuel cell

Pr_0.4Sr_0.6Co_0.2Fe_0.7Nb_0.1O_3−δ (PSCFN) nanofibers and their corresponding Pr_0.4Sr_0.6Co_0.2Fe_0.7Nb_0.1O_3−δ–Gd_0.2Ce_0.8O_1.9 (PSCFN–GDC) composites have been synthesized and applied as cathodes for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this paper, PSCFN nanofibers were obtained through electro-spinning and the following pyrolysis process. The resultant PSCFN nanofibers were infiltrated with GDC precursor to prepare nanofiber-structured PSCFN–GDC composite cathodes. The optimal PSCFN: GDC mass ratio of 1: 0.10 was identified to possess the lowest interfacial polarization resistances of 0.264, 0.155, 0.039 and 0.018 Ω cm² at 650, 700, 750 and 800 °C, respectively, lower than those of the PSCFN–GDC nanoparticle-structured composite cathode. The PSCFN–GDC (1: 0.10) shows an excellent stability of electrochemical activity under a current density of 200 mA cm⁻² for 100 h at 800 °C. All results proved that the nanofiber-structured PSCFN–GDC composite could act as a highly efficient cathode candidate for the IT-SOFCs.     

Crystal phases and electric properties of (Na0.5K0.5)1−xNb1+x/5O3:yCuO,zLiSbO3 piezoceramics

In this study the electric property and the formation of crystal phases are characterized along with the increase of the A-site alkali deficiency(x) in the non-stoichiometric (Na_0.5K_0.5)_1?xNb_1+x/5O₃:yCuO + zLiSbO₃ (x = ?0.01 to 0.1; y = 0, 0.01; z = 0, 0.05) ceramics. Quantitative crystal phase analysis has been carried out using Rietveld method. The crystal structure of tetragonal tungstenbronze phase is discussed in relation with the P–E hysteresis and dielectric properties. The stoichiometric and the slightly alkali deficient samples show very leaky P–E loop. With increasing the alkali deficiency the electrical leakage decreases and the P–E loop shows the saturation. CuO and LiSbO₃ doping in the alkali deficient sample (x = 0.05, y = 0.01, z = 0) leads to the slim and pinched P–E loop shape. By CuO doping the P_r and P_s decreases to 13.9 and 20.87 μC/cm² from 25.6 and 27.2 μC/cm², respectively.     

Vapor-phase methylation of pyridine with methanol to 3-picoline over Zn1−xCoxFe2O4 (x=0, 0.2, 0.5, 0.8 and 1.0)-type ternary spinels prepared via a low temperature method

The reaction of pyridine with methanol was carried out over Zn_1−xCo_xFe₂O₄ (x=0, 0.2, 0.5, 0.8 and 1.0)-type systems in a fixed-bed down-flow reactor. The influences of surface acidity, cation distribution in the spinel lattice and various reaction parameters are discussed. The activity and selectivity were shown to be strongly dependent on the surface acidity of the systems. Over all compositions of the systems, 3-picoline was formed as the major product, even though the activity and selectivity show a strong dependence on composition and reaction conditions. Generally, the systems possessing more acidic sites (x≥0.5) favor the production of 3-picolines and 3,5-lutidine. Pyridine conversion increased with the progressive substitution of Zn²⁺ ions by Co²⁺ ions. Cation distribution in the spinel lattice influences their acidic properties. These factors have been adequately considered as helpful to evaluate the activity of the systems.