Impedance spectroscopy and piezoresponse force microscopy analysis of lead-free (1 − x) K0.5Na0.5NbO3 − xLiNbO3 ceramics

(1 ? x) K_0.5Na_0.5NbO₃ ? xLiNbO₃ (where x = 0.0, 5.0, 5.5, 6.0, and 6.5 wt.%) (KNLN) perovskite structured ferroelectric ceramics were prepared by the solid-state reaction method. X-ray diffraction patterns indicate that single phase was formed for pure KNN while a small amount of second phase (K₆Li₄Nb₁₀O₃₀, ～3%) was present in LN doped KNN ceramics. Phase analysis indicated the change in the crystal structure from orthorhombic to tetragonal with increase in LN content. The electrical behavior of the ceramics was studied by impedance spectroscopy technique in the high temperature range. Impedance analysis was performed using an equivalent circuit model. The impedance response in pure KNN and KNLN ceramics could be deconvoluted into two contributions, associated with the bulk (grains) and the grain boundaries. Activation energies for conductivity were found to be strongly frequency dependent. The activation energy obtained from dielectric relaxation data was attributed to oxygen vacancies. From PFM we found that the composition with 6.5 wt.% LN displays stronger piezocontrast as compared to pure KNN implying an evidence of a pronounced piezoelectric coefficient.     

Phase transitions and dielectric properties of (1 − x)KNbO3 − xK0.5Bi0.5TiO3 ceramics synthesized by a stirred hydrothermal process

Dielectric ceramic materials (1 − x) KNbO₃ − xK_0.5Bi_0.5TiO₃ (0 ≤ x ≤ 0.3) have been successfully synthesized via a stirred (dynamic) hydrothermal method. The microstructure, relative density and dielectric properties were studied as a function of KBT doping. The structure of the solid solutions changed from orthorhombic (x = 0; 0.05) to tetragonal (x = 0.1; x = 0.3) at room temperature. The morphotropic phase limit was obtained at x = 0.075 where we have noted the coexistence of the orthorhombic and tetragonal structures. The mean value of the measured dielectric permittivity ε_r was 700 and dielectric loss tanδ was about 0.06 at room temperature. The dielectric properties of the studied ceramics, from 80 to 450 K, depend not only on their microstructure but also on their relative density. A relaxation behavior was observed for the tanδ curves at temperature below 150 K. The activation energy (E_a) of this phenomenon increases from 0.15 to 0.34 eV with the increase of KBT amount. The conductivity σ_ac remains constant at about 10⁻⁶ S m⁻¹.     

Improving La0.6Sr0.4Co0.2Fe0.8O3 − δ cathode performance by infiltration of a Sm0.5Sr0.5CoO3 − δ coating

La_xSr_1 ? xCo_yFe_1 ? yO_3 ? δ (LSCF) represents one of the state-of-the-art cathode materials for solid oxide fuel cells (SOFCs) due primarily to its high ionic and electronic conductivity. In this study, a one-step infiltration process has been developed to deposit, on the surface of a porous LSCF cathode, a thin film (50–100 nm) of Sm_0.5Sr_0.5CoO_3 ? δ (SSC), which is catalytically more active for oxygen reduction. Electrochemical impedance spectroscopy reveals that the SSC coating has dramatically reduced the polarization resistance of the cathode, achieving area-specific resistances of 0.036 Ω cm² and 0.688 Ω cm² at 750 °C and 550 °C, respectively. It has also maintained the stability of LSCF cathodes. In particular, the peak power densities are increased by ~ 22% upon the infiltration of SSC onto the porous LSCF cathodes of our best performing cells. These results demonstrate that a conductive backbone (e.g., LSCF) coated with a catalytic film (e.g., SSC) is an attractive approach to achieving an active and stable SOFC cathode for low-temperature solid oxide fuel cells.     

Synthesis and characterization of (Pr0.75Sr0.25)1 − xCr0.5Mn0.5O3 − δ as anode for SOFCs

The complex perovskite (Pr_0.75Sr_0.25)_1 − xCr_0.5Mn_0.5O_3 − δ (PSCM) has been prepared and studied as possible anode material for high-temperature solid oxide fuel cells (SOFCs). PSCM exhibits GdFeO₃-type structure and is both physically and chemically compatible with the conventional YSZ electrolyte. The reduction of PSCM resulted in structural change from orthorhombic Pbnm to cubic Pm-3m. Selected area electron diffraction (SAED) analysis on the reduced phases indicated the presence of a √2 × √2 × 2 superlattice. The total conductivity values of ∼ 75% dense Pr_0.75Sr_0.25Cr_0.5Mn_0.5O_3 − δ at 900 °C in air and 5% H₂/Ar are 9.6 and 0.14 S cm^− 1 respectively. The conductivity of PSCM drops with decreasing Po₂ and is a p-type conductor at all studied Po₂. The average TEC of Pr_0.75Sr_0.25Cr_0.5Mn_0.5O_3 − δ is 9.3 × 10^− 6 K^− 1, in the temperature range of 100–900 °C and is close to that of YSZ electrolyte. The anode polarization resistance of PSCM in wet 5%H₂ is 1.31 Ω cm² at 910 °C and in wet CH₄ at 930 °C; the polarization resistance is 1.29 Ω cm². PSCM was unstable at 900 °C in unhumidified hydrogen. Cell performance measurements carried out using graded PSCM and La_0.8Sr_0.2MnO₃ as anode and cathode respectively yielded a maximum power density of 0.18 W cm^− 2 in wet 5%H₂/Ar at 910 °C and the corresponding current density was 0.44 A cm^− 2 at 0.4 V. The activation energy for the electrochemical cell operating in wet (3% H₂O) 5%H₂/Ar fuel is 85 kJ mol^− 1.     

High-temperature compressive creep behaviour of the perovskite-type oxide Ba0.5Sr0.5Co0.8Fe0.2O3 − δ

Compressive creep tests have been performed on perovskite-type Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 ? δ ceramics. The activation energy, stress exponent and inverse grain size exponent of the steady-state creep rates are evaluated at p(O₂) = 0.21 ? 10⁵ Pa and 0.01 ? 10⁵ Pa in the stress, temperature and grain size ranges 5–20 MPa, 1078–1208 K and 2.5–17.4 µm, respectively. The results indicate that the creep rate of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 ? δ is controlled by diffusion of cations via both the oxide lattice (bulk diffusion) and along grain boundaries. The creep rate of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 ? δ increases profoundly by more than one order of magnitude at 1153–1178 K, which is tentatively linked with the onset of the hexagonal-to-cubic phase transition in this compound.     

Characterization of doped La0.7A0.3Cr0.5Mn0.5O3 − δ (A = Ca,Sr, Ba) electrodes for solid oxide fuel cells

Doped lanthanum manganese chromite based perovskite, La_0.7A_0.3Cr_0.5Mn_0.5O_3 ? δ (LACM, A = Ca, Sr, Ba), on yttria-stabilized zirconia (YSZ) electrolyte is investigated as potential electrode materials for solid oxide fuel cells (SOFCs). The electrical conductivity and electrochemical activity of LACM depend on the A-site dopant. The best electrochemical activity is obtained on the La_0.7Ca_0.3Cr_0.5Mn_0.5O_3 ? δ/YSZ (LCCM/YSZ) composite electrodes. The conductivity of LCCM is 29.9 S cm^? 1 at 800 °C in air, and the electrode polarization resistance (R_E) of the LCCM/YSZ composite cathode for the O₂ reduction reaction is 0.5 Ω cm² at 900 °C. The effect of Gd-doped ceria (GDC) impregnation on the LCCM cathode polarization resistances is also studied. GDC impregnation significantly enhances the electrochemical activity of the LCCM cathode. In the case of the 6.02 mg cm^? 2 GDC-impregnated LCCM cathode, R_E is 0.4 Ω cm² at 800 °C, ~ 60 times smaller than 24.4 Ω cm² measured on a LCCM cathode without the GDC impregnation. Finally the electrochemical activities of the doped lanthanum manganese chromites for the H₂ oxidation reaction are also investigated.     

Thermal,Raman and dielectric study of 0.5K0.5Bi0.5TiO3–0.5PbTiO3 ceramics

The 0.5K_0.5Bi_0.5TiO₃–0.5PbTiO₃ ceramics were prepared by following a standard solid-state method. The Raman, thermal and dielectric properties of these ceramics were investigated. The X-ray measurements showed that samples have single perovskite-type structure with tetragonal symmetry. Dielectric study revealed that the dielectric behaviour of the investigated ceramics is rather of normal ferroelectrics with large thermal hysteresis. The transition temperature observed by means of differential scanning calorimetry measurements is in good agreement with that obtained from dielectric study.     

0.5BiFeO3–0.5PbFe0.5Nb0.5O3 Multiferroic Ceramic: Structure,Dielectric and Magnetodielectric Properties

Physics of the Solid State - The structure, dielectric characteristics, and magnetoelectric effect of a 0.5BiFeO3–0.5PbFe0.5Nb0.5O3 multiferroic ceramics are studied. We found that the...     

Microstructure and electrical properties of Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3–LiNbO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics of (1?x?y)Bi_0.5Na_0.5TiO₃–xBi_0.5K_0.5TiO₃–yLiNbO₃ (BNT–BKT–LN-x/y) have been fabricated by a conventional solid-state reaction method, and their microstructure and electrical properties have been investigated. The results of X-ray diffraction (XRD) measurement show that K⁺, Li⁺ and Nb⁵⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a solid solution with a pure perovskite structure. The BKT and LN addition has no remarkable effect on the crystal structure. However, a significant change in grain size took place. Simultaneously, with increasing amount of LN, the temperature for a ferroelectric–antiferroelectric phase transition is clearly reduced. The temperature dependence of dielectric properties suggests that the ceramics have diffuse-type phase transition characteristics. The piezoelectric constant d₃₃ and the electromechanical coupling factor k_p of the ceramics attain maximum values of 195 pC/N and 0.336 at x=0.18 and y=0.01.     

Intercomparison of Fluctuation Induced Conductivity of Cu0.5Tl0.5Ba2Can−1CunO2n+4−y (n = 2, 3, 4) superconductor thin films

An intercomparison of Fluctuation Induced Conductivity (FIC) of Cu_0.5Tl_0.5Ba₂Ca_n?1Cu_nO_2n+4?y (n = 2, 3, 4) [CuTl-12(n ? 1)n] superconductor thin films is given. We tried to find any correlation between the critical temperature and the parameters extracted from the excess conductivity data i.e. cross-over temperature, pseudogap temperature and fluctuation amplitudes. We found that the critical temperature seems to depend on the fluctuation amplitude; greater the fluctuation amplitude higher is the critical temperature.     

Magnetic properties of Nd0.5Pb0.5-xSrxMnO3 materials

The structure and magnetic properties of Nd0.5Pb0.5-xSrxMnO3 (0≤x≤0.4) manganites were systematically investigated. Significant changes in Curie temperature and metal-insulator (MI) transition temperature of the samples were observed. All samples exhibited a transition from paramagnetic semiconducting to ferromagnetic metallic state.Curie temperature Tc and the MI transition temperature Tp increased with increasing Sr content. We attributed these behaviours to the enhancing of both the double exchange mechanism and the Jahn-Teller electron-phonon coupling.     

Dielectric and piezoelectric properties of Li-substituted lead-free (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–BaTiO3 ceramics

Lead-free 0.79(Bi_0.5Na_0.5)TiO₃–0.14[Bi_0.5(K_0.5−xLi_x)]TiO₃–0.07BaTiO₃ (BNBK79 + xLi, x = 0.0, 0.1, 0.2, 0.25, 0.3, and 0.4) ceramics were prepared by conventional solid state reaction process. The crystalline structures and surface morphologies are investigated by X-ray diffraction method and scanning electron microscopy. Dielectric and piezoelectric properties were measured. With increasing of lithium substitution, the Curie temperatures of BNBK79 + xLi ceramics increase, but the maximum value of the dielectric constant decreases. And a relatively large remnant polarization of 17.6 μC/cm² and 157 pC/N of d₃₃ has been obtained when x = 0.3.     

Phase-transformation-induced microstructure in lead-free ferroelectric ceramics based on (Bi0.5Na0.5)TiO3–BaTiO3–(Bi0.5K0.5)TiO3

Lead-free ferroelectric ceramics with a morphotropic phase boundary (MPB) composition 85.4% (Bi_0.5Na_0.5)TiO₃–2.6%BaTiO₃–12.0% (Bi_0.5K_0.5)TiO₃ (BNT-BT-BKT at a molar ratio of 85.4: 2.6: 12.0) doped with 0.8?mol% Nb₂O₅ were studied for their crystalline phases and microstructure. The crystalline phases were identified using X-ray diffractometry (XRD) with the contents determined using the Rietveld refinement technique. The phase-transformation-induced microstructure was analyzed using transmission electron microscopy (TEM) and the crystal symmetries were determined using the convergent-beam electron diffraction (CBED) technique. Samples sintered at 1200°C contain a mixture of cubic (C-), tetragonal (T-) and rhombohedral (R-) phases at a ratio of C/T/R?=?56.6: 28.4: 15.0?wt%. Two types of grains are produced: one characterized by a featureless contrast consisting of nano-scale T-domains dispersed in a C-phase matrix; the other a core-shell structure with a shell containing twin and anti-phase-boundary (APB) domains coexisting with a (C?+?T)-phase mixture core. The T- and R-twin boundaries are determined to {111}_T and {110}_R, respectively, and the fault vector for T-APB to R?=?1/2?110]_T. The characteristic microstructure is discussed in terms of the reduction in the point group symmetry and changes in the unit cell volume or the Bravais lattice upon phase transformation among the C-, T- and R-phases. The twin and the APB domains are induced and explained.     

Magnetic properties of Nd0.5Pb0.5—xSrxMnO3 materials

The structure and magnetic properties of Nd_{0.5}Pb_{0.5-x}Sr_xMnO_3 (0≤x≤0.4) manganites were systematically investigated. Significant changes in Curie temperature and metal-insulator (MI) transition temperature of the samples were observed. All samples exhibited a transition from paramagnetic semiconducting to ferromagnetic metallic state. Curie temperature T_C and the MI transition temperature T_p increased with increasing Sr content. We attributed these behaviours to the enhancing of both the double exchange mechanism and the Jahn－Teller electron－phonon coupling.     

SrTiO3(100) − √5 × √5 − R26.6 surface observed by high-resolution scanning tunneling microscopy

SrTiO₃(100) – (√5 × √5) – R26.6 surfaces were studied by means of high-resolution scanning tunneling microscopy (STM) under ultrahigh vacuum conditions. By varying the bias voltage in the occupied state, it was possible to observe the arrangement of titanium and oxygen atoms in the unit cells of a (√5 × √5) surface superstructure, which revealed that the TiO₂ layer is the terminating plane of the (√5 × √5) surface. In the STM images, peculiar protrusions were seen at the oxygen fourfold hollow site responsible for √5 × √5 periodicity. The protrusions are asymmetrical in contrast, which would be an important consideration in proposing accurate structural models for (√5 × √5) surface superstructures.     

Structural and electrochemical properties of LiNi0.5Mn0.5 − xAlxO2 (x = 0, 0.02, 0.05, 0.08, and 0.1) cathode materials for lithium-ion batteries

Layered LiNi_0.5Mn_0.5 ? xAl_xO₂ (x = 0, 0.02, 0.05, 0.08, and 0.1) series cathode materials for lithium-ion batteries were synthesized by a combination technique of co-precipitation and solid-state reaction, and the structural, morphological, and electrochemical properties were examined by XRD, FT-IR, XPS, SEM, CV, EIS, and charge–discharge tests. It is proven that the aliovalent substitution of Al for Mn promoted the formation of LiNi_0.5Mn_0.5 ? xAl_xO₂ structures and induced an increase in the average oxidation number of Ni, thereby leading to the shrinkage of the lattice volume. Among the LiNi_0.5Mn_0.5 ? xAl_xO₂ materials, the material with x = 0.05 shows the best cyclability and rate ability, with discharge capacities of 219, 169, 155, and 129 mAh g^? 1 at 10, 100, 200, and 400 mA g^? 1 current density respectively. Cycled under 40 mA g^? 1 in 2.8–4.6 V, LiNi_{0. 5}Mn_0.45Al_0.05O₂ shows the highest discharge capacity of about 199 mAh g^? 1 for the first cycle, and 179 mAh g^? 1 after 40 cycles, with a capacity retention of 90%. EIS analyses of the electrode materials at pristine state and state after first charge to 4.6 V indicate that the observed higher current rate capability of LiNi_{0. 5}Mn_0.45Al_0.05O₂ can be understood due to the better charge transfer kinetics.     

Effect of Eu dopant concentration in SrCe1 − xEuxO3 − δ on ambipolar conductivity

The europium dopant concentration in strontium cerate was studied to achieve maximum hydrogen permeation. In order to determine high ambipolar conductivity, total conductivity and open circuit potential measurements were performed. Among the three different compositions of Eu-doped SrCe_1 ? xEu_xO_3 ? δ (x = 0.1, 0.15 and 0.2) studied, SrCe_0.9Eu_0.1O_3 ? δ showed highest total conductivity between 600 °C and 900 °C. However, transference number measurements showed increasing electronic conductivity with increasing dopant concentration and a stronger temperature dependence for electronic conduction. Therefore, the highest ambipolar conductivity was obtained over the compositional range from SrCe_0.85Eu_0.15O_3 ? δ to SrCe_0.8Eu_0.2O_3 ? δ depending on temperature. Finally, the hydrogen permeation flux was calculated based on the ambipolar conductivity and compared with experimental results.     

On the superconductivity and structural properties of YBa2Cu3 − xCdxO7 − y

A series of superconducting cuprates with the nominal composition YBa₂Cu_3 − xCd_xO_7 − yand the effect of Cd substitution on Cu sites in this compound is presented. X-ray powder diffraction patterns for these cadmium cuprates with reduced diamagnetism indicate an orthorhombic unit cell like-perovskite structure for (0 ≤ x ≤ 0.15), while for higher Cd concentration, i.e.x = 1.0 the material is polyphasic. The observed superconducting transition temperature of the samples is nearly the same ([formula] K), except for (x = 1.0) whereT_cdrops to 72 K and a transition from metallic to semiconducting behavior of the normal state of the resistivity is observed. Such a decrease inT_cfor higher Cd concentration could be attributed to the presence of the green phase in this composition.     

La0.5Ca0.5-xBaxMnO3体系中的异常磁电阻行为

实验研究了La0.5Ca0.5MnO3系统Ca位上Ba掺杂对其磁电阻效应的影响.结果表明,掺杂量低于0.2的样品,磁电阻效应随温度降低而增大,在转变温度附近达到最大值,然后随温度降低而变小,随着温度进一步降低磁电阻效应再次增大.而对于掺杂量高于0.2的样品,磁电阻效应在整个温区随温度降低而单调增加.文中通过考虑铁磁性和反铁磁性两种效应的竞争而对实验观察给出解释.