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.     

Synthesis and microwave absorption properties of Ni–Zn–Mn spinel ferrites

A series of Ni_0·5?xZn_0·3?xMn_0·2+2xFe₂O₄ ferrites was successfully prepared by the sol–gel autocombustion method. The structure and electromagnetic properties of the powders were characterised by X-ray diffraction, SEM and vector network analysis. The pure powders were formed by heating at 1200°C for 3 h in air, and grain sizes increased as the amount of substitution ranged from x?=?0·0 to x?=?0·25. For samples with x?=?0·1, a minimum reflection loss of ?27·57 dB was observed at 11·0 GHz with the less than ?10 dB absorption bandwidth at 8·0 GHz with 3·8 mm thickness. The results indicate that substitution with Mn and Zn ions can greatly improve the microwave absorption properties of NiFe₂O₄ ferrites.     

Investigation of structural,optical, electrical,and magnetic properties of Fe-doped La0.7Sr0.3MnO3 manganites

In this work, the physical properties of nanocrystalline samples of La_0.7Sr_0.3Mn_1−xFe_xO₃ (0.0 ≤ x ≤ 0.20) perovskite manganites synthesized by the reverse micelle (RM) technique were explored in detail. The phase purity, crystal structure, and crystallite size of the samples were determined using X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. All the samples had rhombohedral crystal structure and crystallite size increased with increase in Fe content in La_0.7Sr_0.3MnO₃. The scanning electron micrographs (SEMs) exhibited smooth surface morphology and nonuniform shape of the particles. The optical properties studied using UV-visible absorption spectroscopy revealed a decrease in the absorbance and optical band gap with an increase in Fe content in La_0.7Sr_0.3MnO₃ compound. The temperature-dependent resistivity measurements revealed semiconducting nature of x = 0 and 0.1 samples up to the studied temperature range, while a metal-to-insulator transition was observed at higher Fe doping. Magnetic studies revealed weak ferromagnetism in all the samples and a reduction in the maximum magnetization with an increase in Fe content. A close correlation between electrical transport and magnetic properties was observed with the doping of Fe ion in La_0.7Sr_0.3MnO₃ at Mn site. These results advocate strong interactions associated with the double exchange mechanism among Fe³⁺ and Mn³⁺ ions.     

Theoretical Study of Cubic and Orthorhombic Nd1–xSrxMnO3 as a Potential Solid Oxide Fuel Cell Cathode

In this paper, the atomic and electronic structures of cubic and orthorhombic Nd_1–xSr_xMnO₃ are investigated using the projector augmented‐wave (PAW) methods within the spin‐polarization generalized gradient approximation (GGA+U), where U is on‐site Coulomb interaction correction. The optimized structure parameters of both cubic and orthorhombic bulk phases are obtained. The difference between the AFM and FM structures for NdMnO₃ is very small, indicating a small magneto‐elastic effect. In Nd_1–xSr_xMnO₃, the pseudo‐cubic lattice constant decreases on increasing Sr doping due to the oxidation of Mn³⁺ cations to the smaller Mn⁴⁺ cations. The result of the total density of states shows the majority spin without gap and the minority spin with 4.8 × 10⁻¹⁹ J gap, indicating a half‐metallic ground state for NdMnO₃ in GGA+U treatment. The Bader effective charges in both cubic and orthorhombic phases are analyzed. The oxygen vacancy formation energy of Nd_1–xSr_xMnO₃ becomes smaller as the Sr doping concentration increases. The oxygen vacancies can be formed more easily on Nd_0.5Sr_0.5MnO₃ than other systems. These results suggest that Nd_0.5Sr_0.5MnO₃ could be a promising candidate for application in SOFC.     

Tuning Ba0.5Sr0.5Co0.8Fe0.2O3-δ cathode to high stability and activity via Ce-doping for ceramic fuel cells

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) fuel-cell cathode stands out because of its ultrahigh ionic conductivity and excellent electrocatalytic activity, but it is still very subject to instability. Here, a new strategy of Ce doping is proposed to boost the stability and activity of the BSCF cathode. A one-pot combustion method is employed to synthesize (Ba_0.5Sr_0.5)_1–xCe_xCo_0.8Fe_0.2O_3-δ (x=0–0.2) cathodes. Both BSCF and (Ba_0.5Sr_0.5)_0.9Ce_0.1Co_0.8Fe_0.2O_3-δ have a cubic perovskite structure. (Ba_0.5Sr_0.5)_0.8Ce_0.2Co_0.8Fe_0.2O_3-δ shows two phases of cubic perovskite and fluorite ceria. Proper Ce doping can boost the electrical conductivity of BSCF, and can dramatically reduce the polarization resistance of BSCF cathode. Ce doping significantly improved BSCF cathode long-term stability by 160 h. Moreover, ten-percent Ce doping in BSCF highly improves single-cell output performance from 516.33 mW cm^?2 to 629.75 mW cm^?2 at 750 °C. The results reveal that Ce doping as a potential strategy for enhancing the stability and activity of BSCF cathode is promising.     

Effect of Ce3+ doping on phase and microwave dielectric properties of 0.94MgTiO3−0.06(Ca0.8Sr0.2)TiO3 ceramics

In this study, 0.94Mg_(1-3x/2)Ce_xTiO₃−0.06(Ca_0.8Sr_0.2)TiO₃ (MCe_xT−CST, 0≤x≤0.01) composite ceramics were prepared at a low temperature of 1175°C by using the 50-nm-sized powders. The effects of Ce³⁺ doping on crystalline phase, microstructure, and microwave dielectric properties of MCe_xT−CST were studied. A main ilmenite (Mg,Ce)TiO₃ phase and a minor perovskite (Ca_0.8Sr_0.2)TiO₃ phase coexist well with the appearance of impurity MgTi₂O₅ phase in MCe_xT−CST. The dielectric properties of MCe_xT−CST are affected by the molecular polarizability, the impurity phase, and the Ce³⁺ doping. The replacement of Mg²⁺ by high valence Ce³⁺ could effectively inhibit the formation of oxygen vacancy, resulting in the enhancement of Q×f. When x = 0.005, MCe_xT−CST exhibits microwave dielectric properties with a moderate ε_r of 21.5, a high Q×f of 67 000 GHz, and a near-zero τ_f of −0.74 ppm/°C. The results reveal that the Ce³⁺ substitution is a prospective approach to optimize the microwave dielectric properties of MgTiO₃-based ceramics.     

Enhanced high temperature microwave absorption of La0.9Sr0.1MnO3/MgAl2O4 composite ceramics based on controllable electrical conductivity

The high temperature microwave absorbing efficiency (HTMAE) of xLa_0.9Sr_0.1MnO₃/(1 − x)MgAl₂O₄ composite ceramics was investigated by studying the crystal structure, electrical conductivity, and permittivity. The crystal structure of La_0.9Sr_0.1MnO₃ and MgAl₂O₄ were maintained, but the Mn³⁺ and Al³⁺ ions were exchanged with each other through doping. The conductivity and permittivity of the composite ceramics increased with the increase of La_0.9Sr_0.1MnO₃ content and test temperature. When x = 0.36, the electrical conductivity in La_0.9Sr_0.1MnO₃ significantly enhanced the microwave polarization of the composite ceramics at high temperature. According to transmission/reflection modelling, the composite ceramics with x = 0.24 showed excellent HTMAE near the optimal thickness of 1.8 mm. Although the optimal thickness of the composite with x = 0.36 was reduced to 1.1 mm, the HTMAE was seriously lessened due to an impedance mismatch. xLa_0.9Sr_0.1MnO₃/(1 − x)MgAl₂O₄ are promising as thin and efficient microwave absorbing materials at high temperatures and the microwave permittivity can be further enhanced by adjusting the conductivity of La_0.9Sr_0.1MnO₃.     

Properties of Ca1−xHoxMnO3 perovskite-type electrodes

In this work, Ca_1−xHo_xMnO₃ (x = 0, 0.1 and 0.2) perovskite oxide pelleted electrodes were prepared from the respective powders obtained by the citrate route method at 1173 K.The electrodes exhibit particle size that decreases with the holmium content in the oxide. All the samples reveal semiconductor behaviour and the presence of holmium induces a marked decrease in the electrical resistivity. The results can be well attributed to the changes in the Mn⁴⁺/Mn³⁺ ratio. Electrodes were characterized by cyclic voltammetry and chronopotentiometry. Cyclic voltammetric studies indicate a similar behaviour of the electrodes, irrespective of their composition. Two pairs of peaks were identified and associated, one to the Mn⁴⁺/Mn³⁺ redox couple and the other to the Mn⁷⁺/Mn⁴⁺ and Mn⁶⁺/Mn⁴⁺ redox couples. The voltammetric data provide evidence that the electrodes roughness factor increases with the introduction of Ho-ions in the oxide structure, what is consistent with the crystallite size obtained by X-ray diffraction (XRD) and the morphology observed by scanning electron microscopy (SEM). The Ho-substituted electrodes present higher current density when compared with CaMnO₃ electrodes what can be attributed both to higher electrical conductivity and smaller particle size. The chronopotentiometric studies have shown that the discharge occurs by different mechanisms for the oxide electrodes with and without Ho.     

Influence of magnesia doping on structure and electrical conductivity of pyrochlore type GdSmZr2O7

Abstract

Polycrystalline ceramic samples of magnesia doped GdSm_1–xMg_xZr₂O_7–x/2 have been prepared by conventional solid state reaction method using high purity oxides. The influence of magnesia dopant content on densification, microstructure and electrical properties of GdSm_1–xMg_xZr₂O_7–x/2 ceramics are investigated. Magnesia doping promotes the sintering densification behaviour of GdSm_1–xMg_xZr₂O_7–x/2 ceramics. GdSm_1–xMg_xZr₂O_7–x/2 (x?=?0, 0·05, 0·10) ceramics have a single phase of the pyrochlore type structure, while GdSm_1–xMg_xZr₂O_7–x/2 (x?=?0·15, 0·20) ceramics consist of the pyrochlore type structure and a small amount of magnesia as the second phase. The total conductivity of GdSm_1–xMg_xZr₂O_7–x/2 ceramics obeys the Arrhenius relation, and gradually increases with increasing temperature from 723 to 1173 K. GdSm_1–xMg_xZr₂O_7–x/2 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1·0×10^–4 to 1·0 atm at each test temperature. The maximum value of the total conductivity is 1·29×10^–2 S cm^–1 at 1173 K for the GdSm_0·85Mg_0·15Zr₂O_6·925 ceramic.     

Structure and electrical properties evolution of B-site complex ions (Li1/4Nb3/4) modification BNT–BT ceramics

Abstract

B-site complex ions (Li_1/4Nb_3/4)⁴⁺ modification (Bi_1/2Na_1/2)_0·94Ba_0·06TiO₃ ceramics with compositions of (Bi_1/2Na_1/2)_0·94Ba_0·06Ti_1?x(Li_1/4Nb_3/4)_xO₃ (x?=?0, 0·01, 0·03 and 0·06) have been synthesised via the conventional solid state reaction. The effect of (Li_1/4Nb_3/4)⁴⁺ content and sintering temperature on structures and electrical properties were investigated. It was found that both compositions and sintering temperatures have no significant effect on the crystal structure, and trace (Li_1/4Nb_3/4)⁴⁺ addition and sintering temperatures have a great influence on the microstructure. Two obvious dielectric anomaly peaks (T_d and T_m) were observed and dielectric constant for all poled specimens displayed significant frequency dispersion at T_d and diffusion phase transition at T_m. The piezoelectric properties of the ceramics are insensitive to the sintering temperatures, and the composition with x?=?0·03 sintering at 1150°C exhibits favourable piezoelectric properties of d₃₃?=?155 pC N^?1 and k_p?=?0·312.     

A-site Ca/Sr co-doping to optimize room-temperature TCR of La0.7Ca0.3-xSrxMnO3 films

Based on the analysis of crystal structure, Mn³⁺/Mn⁴⁺ pairs, distortion of MnO₆ octahedron, and electrical transport properties of La_1-xCa_xMnO₃ and La_1-xSr_xMnO₃ materials, room-temperature coefficient of resistivity (TCR) of La_0.7Ca_0.3-xSr_xMnO₃ (LCSMO) films was optimized by Ca/Sr co-doping at the A-site. LCSMO films are successfully fabricated on LaAlO₃ (100) substrates via facile spin coating technology. The microstructure of LCSMO films is characterized by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, atomic force microscopy and X-ray photoemission spectroscopy. Results reveal that A-site Ca/Sr co-doping significantly influenced crystal structure, formation of Mn³⁺/Mn⁴⁺ pairs, and distortion of MnO₆ octahedron. The correlation between microstructure and electrical transport properties was explained through the phenomenological percolation model, double-exchange mechanism and Jahn-Teller effect. Furthermore, the TCR reached 10.2% K^-1 at 296.1 K in La_0.7Ca_0.18Sr_0.12MnO₃ films.     

Synthesis of La1−xSrxMnO3 powders by polymerizable complex method: Evaluation of structural, morphological and electrical properties

Lanthanum strontium manganite (La_1−xSr_xMnO₃, LSM) powders were synthesized by polymerizable complex method, based on complexation of metal ions (MI) with citric acid (CA) and polyesterification between CA and ethylene glycol (EG). Firstly, the effect of the molar ratio of CA:MI (=1–3) was investigated on the synthesis of La_0.7Sr_0.3MnO₃ powders, which were characterized by thermal analysis (TGA/DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The results indicated that the molar ratio CA:MI = 3 is adequate for a good crystallization of pure perovskite phase after calcination, with nanometric crystallite sizes and porous microstructure. For the La_0.7Sr_0.3MnO₃ sample synthesized with CA:MI ratio of 3, it was investigated the effect of calcination temperature, showing that the perovskite structure is better crystallized at 900 °C, without secondary phase formation. Using this same CA:MI ratio and calcination temperature, powders with different Sr content (x = 0.2–0.4) were synthesized, with surface areas of 4–10 m² g⁻¹. These powders were sintered at 1100 °C to produce porous pellets. The porosity of the sintered pellets and the electrical conductivity, measured by two-probe technique, increased with increasing Sr content.     

Structural and electronic properties of Ce-doped Ti4O7 based on experiments and first-principles calculations

A series of Ti₄O₇ powders with different Ce-cation doping concentrations were successfully prepared through a new strategy combining the wet chemical method and hydrogen thermal reduction. It is shown that Ce, entering the lattice of Ti₄O₇ during the reduction process, only occurs in the form of Ce³⁺, and the solubility of Ce is equal to 8.93 at.%. Furthermore, the electrical conductivity of Ce-doped Ti₄O₇ powders was tested for the first time and theoretically explained by the first-principles calculations. It was revealed that Ce provides a negative effect on the electrical conductivity of Ti₄O₇ powders, which is due to the decrease in the number of electronic states near the Fermi energy level caused by the introduction of Ce.     

Tuning stoichiometry of high-entropy oxides for tailorable thermal expansion coefficients and low thermal conductivity

Thermal barrier coating materials with proper thermal expansion coefficient (TEC), low thermal conductivity, and good high-temperature stability are of great significance for their applications in next-generation turbine engines. Herein, we report a new class of high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x with different Ce⁴⁺ contents synthesized by a solid-state reaction method. They exhibit different crystal structures at different Ce⁴⁺ content, including a bixbyite single phase without Ce⁴⁺ doping (x = 0), bixbyite-fluorite dual-phase in the RE₂O₃-rich region (0 < x < 2), and fluorite single phase in the stoichiometric (x = 2) and CeO₂-rich region (x > 2). The high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x exhibit tailorable TECs at a large range of 9.04 × 10^–6–13.12 × 10^–6 °C^–1 and engineered low thermal conductivity of 1.79–2.63 W·m^–1·K^–1. They also possess good sintering resistance and high-temperature phase stability. These results reveal that the high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x are promising candidates for thermal barrier coating materials as well as thermally insulating materials and refractories.     

Characterization of Au/MnO x /TiO2 for Photocatalytic Oxidation of Carbon Monoxide

The Au/MnO _x /TiO₂ catalyst was used for the photocatalytic oxidation of carbon monoxide. The catalytic activity of Au/MnO _x /TiO₂ with low concentration of manganese (3–7 mol%) was much higher than that of Au/TiO₂. The surface of Au/MnO _x /TiO₂ was characterized by XPS and Raman spectroscopy. While the main state of manganese in 13.8 mol% MnO _x /TiO₂ was Mn⁴⁺ species, Mn³⁺ was the dominant species in the samples with below 6.5 mol% manganese. Raman spectroscopy revealed that the interaction between the MnO _x and TiO₂ form Mn–O–Ti species in which the state of manganese was Mn³⁺. The Au particles also interacted with both MnO _x and TiO₂ to modify the surface of them. In the case of the Au species, low loading of manganese produced the metallic Au⁰ and perimeter interfacial Au^δ+, whereas high loading showed the coexistence of three components which were metallic Au⁰, perimeter interfacial Au^δ+, and oxidic Au³⁺. The catalytic active component was the metallic Au⁰ and perimeter interfacial Au^δ+ species, which were dispersed on TiO₂ and Mn³⁺/TiO₂.     

Ferroelectric and dielectric properties of Nd2−xCexTi2O7 ceramics

The solid solution system Nd_2?xCe_xTi₂O₇ has been investigated. The solubility limit of Ce in Nd_2?xCe_xTi₂O₇ was found to be 0·5–0·75 according to X-ray diffraction and X-ray photoelectron spectroscopy results. Ce substitution increases the b and c axes and the volume of the unit cell due to its larger ionic radius. Nd_2?xCe_xTi₂O₇ (x?=?0·05, 0·25, 0·5, 0·75) textured ceramics were fabricated using spark plasma sintering. The ferroelectric and dielectric properties of the ceramics were studied. Ce substitution decreases the Curie point T_c of Nd_2?xCe_xTi₂O₇ compounds. The results suggest that the T_c of Ce₂Ti₂O₇ is <1445°C.     

Preparation and Application of Strong Near‐Infrared Emission Phosphor Sr3SiO5:Ce3+,Al3+,Nd3+

This work investigated the near‐infrared (NIR) emission properties of mCe³⁺, xNd³⁺ codoped Sr_3?m?x(Si_1?m?xAl_m+x)O₅ phosphors. Samples with various doping concentrations were synthesized by the high‐temperature solid‐state reaction. Al³⁺ ions have the ability to promote Ce³⁺ ions to enter into the Sr²⁺ sites and to improve the visible emission of Ce³⁺. Thus the NIR emission of Nd³⁺ is enhanced by the energy‐transfer process, which occurred from Ce³⁺ to Nd³⁺. The device based on these NIR emission phosphors is fabricated and combined with a commercial c‐Si solar cell for performance testing. Short‐circuit current density of the solar cell is increased by 7.7%. Results of this work suggest that the Sr_2.95Si_0.95Al_0.05O₅:0.025Ce³⁺, 0.025Nd³⁺ phosphors can be used as spectral convertors to improve the efficiency of c‐Si solar cell.     

Effect of Mn-doping on the structure and electrical properties of (Pb0.325Sr0.675)TiO3 ceramics

Pb_0.325Sr_0.675Ti_1-xMn_xO₃ ceramics (x?=?0, 0.001, 0.005, 0.01, and 0.05) were successfully prepared by traditional solid-state reaction method. It was found that the lattice constant calculated through Rietveld refinement initially increased and then decreased with increasing Mn content, which was attributed to the variation in valence state of Mn and Ti ions. The microstructure gradually varied from the coexistence of large grains and fine grains for x?=?0 to the uniform grain for x?=?0.05 by increasing the doping Mn ions. With increasing Mn content from x?=?0 to x?=?0.05, the Curie temperature (Tc) dramatically decreased from 25?°C to ??40?°C and dielectric maximum decreased from 27,100 to 13,200. Pb_0.325Sr_0.675Ti_1-xMn_xO₃ ceramics with x?=?0.001 showed the lowest dielectric loss of 0.006 with a relatively high dielectric peak value of ~ 21,000. The grain boundaries resistance obtained from the complex impedance decreased with the increase of Mn content. The decrease in resistance was ascribed to oxygen vacancies and electronics produced by the change of ionic valence state. X-ray photoemission spectroscopy revealed that Ti ions were Ti⁴⁺ and the valences of Mn ions were deduced to be mainly in the form of Mn²⁺ and/or Mn³⁺ for ceramics with low content of Mn, while the Ti ions were in the form of Ti³⁺ and Ti⁴⁺ and Mn ions were diverse valence states with the coexistence of Mn²⁺, Mn³⁺, and Mn⁴⁺ for ceramics with x?=?0.01 and 0.05.     

Effect of Ce-doping on microstructure and electrical properties of LaAlO3 ceramics

A series of novel negative temperature coefficient (NTC) thermistor materials based on La_1-xCe_xAlO₃ (0 ≤ x ≤ 0.2) ceramics were synthesized via the solid-state route. X-ray diffraction results confirmed the successful doping of Ce in the La_1-xCe_xAlO₃ crystal and the formation of a good solid solution. Scanning electron microscopy results indicated that Ce doping is beneficial for grain growth and reduces the porosity of the samples. With the increase in the Ce doping amount, the average grain size increased from 2.1793 to 10.7344 μm, and densities of the ceramics increased from 93.15% to 99.26%. The temperature vs resistance curve indicated that Ce doping reduces the resistivity of LaAlO₃ materials, while reducing the B_200/1400 value of the LaAlO₃ ceramic. For a doping amount of 0.2, the B_200/1400 value of the LaAlO₃ ceramic decreased from 18175.1 to 4897.7K, and the resistivity at 1000 °C decreased from 68971.87 to 1105.15 Ω cm. In addition, the La_1-xCe_xAlO₃ (0 ≤ x ≤ 0.2) series materials exhibited good linear NTC characteristics. X-ray photoelectron spectroscopy results revealed that the resistivity of the LaAlO₃ materials decreased after Ce doping owing to the transformation between the Ce⁴⁺ and Ce³⁺ valence states，and the concentration of Ce³⁺ increased with the increase in the Ce doping amount. Ce³⁺ increases the concentration of oxygen vacancies, decreasing the resistance. Impedance analysis findings suggested that the resistance of the La_1-xCe_xAlO₃ (0 ＜ x ≤ 0.2) material mainly originates from the grain. These results indicate that Ce doping is an effective method to reduce the resistivity of LaAlO₃. Consequently, La_1-xCe_xAlO₃ (0 ≤ x ≤ 0.2) is a promising material for NTC applications.