Electrical conductivity and nonstoichiometry in doped Sr3Ti2O7

A series of doped Ruddlesden–Popper phases, of general formula Sr₃Ti_2-xM_xO_7-δ (M=Al, Ga, Co), were synthesized and their electrical conductivity characterized as a function of temperature and oxygen partial pressure. For fixed-valent dopants, p-type conductivity predominates at p(O₂)> 10⁻⁵ atm, followed by a p(O₂)-independent electrolytic regime, and n-type electronic conductivity at very low p(O₂). The electrolytic regime exhibits activation energies in the range 1·7–1·8 eV. Doping with transition metals such as Co results in a very significant increase in total conductivity with a p-type conductivity at high p(O₂). Furthermore, an apparent ionic regime at intermediate p(O₂) is observed, characterized by high conductivity (> 10⁻² S/cm at 700°C) and low activation energy (0·6 eV). This interpretation is consistent with iodometric measurements as interpreted by a defect chemical model. Other measurements are in progress to confirm this conclusion.     

Ultrasound assisted synthesis of Gd and Nd doped ceria electrolyte for solid oxide fuel cells

In this study, the ceramic powders of Ce_1?xGd_xO_2?x/2 and Ce_1?xNd_xO_2?x/2 (x=0.05, 0.10, 0.15, 0.20 and 0.25) were synthesized by ultrasound assisted co-precipitation method. The ionic conductivity was studied as a function of dopant concentration over the temperature range of 300–800 °C in air, using the impedance spectroscopy. The maximum ionic conductivity, σ_800 °C=4.01×10^?2 Scm^?1 with the activation energy, E_a=0.828 kJmol^?1 and σ_800 °C=3.80×10^?2 Scm^?1 with the activation energy, E_a=0.838 kJmol^?1 were obtained for Ce_0.90Gd_0.10O_1.95 and Ce_0.85Nd_0.15O_1.925 electrolytes, respectively. The average grain size was found to be in the range of 0.3–0.6 μm for gadolinium doped ceria and 0.2–0.4 μm for neodymium doped ceria. The uniformly fine crystallite sizes (average 12–13 nm) of the ultrasound assisted prepared powders enabled sintering of the samples into highly dense (over 95%) ceramic pellets at 1200 °C (5 °C min^?1) for 6 h.     

Doping effect on secondary phases,microstructure and electrical conductivities of LaGaO3 based perovskites

The intermediate temperature electrolytes La_1?xSr_xGa_1?yMg_yO_3?δ (LSGM, where δ = (x + y)/2) with perovskite structure were prepared using a poly(vinyl alcohol) (PVA) solution polymerization method. Three secondary phases were identified by X-ray diffraction, LaSrGaO₄, LaSrGa₃O₇ and La₄Ga₂O₉. The relative amount of these secondary phases depended on the doping compositions. Sr doping produced more Sr rich secondary phases with increasing content, while enhanced solid solubility was observed with Mg addition. Sintered samples showed dense microstructures with well-developed equiaxed grains, and the secondary phases were mainly in the grain boundaries. LaSrGaO₄ could not be detected by SEM for the sintered pellets. The oxygen ionic conductivity was enhanced by doping with Sr and Mg. Mg doping showed the increased conductivity activation energy. La_0.8Sr_0.2Ga_0.9Mg_0.1O_2.85 had the highest ionic conductivity σ = 0.128 S/cm at 800 °C in this work.     

Influence of Yb3+ doping on phase stability and thermophysical properties of (Y1-xYbx)3Al5O12 under high temperature

In this work, Yb³⁺ was selected to replace the Y³⁺ in yttrium aluminum garnet (YAG) in order to reduce its thermal conductivity under high temperature. A series of (Y_1-xYb_x)₃Al₅O₁₂ (x=0, 0.1, 0.2, 0.3, 0.4) ceramics were prepared by solid-state reaction at 1600 °C for 10 h. The microstructure, thermophysical properties and phase stability under high temperature were investigated. The results showed that all the Yb doped (Y_1-xYb_x)₃Al₅O₁₂ ceramics were comprised of a single garnet-type Y₃Al₅O₁₂ phase. The thermal conductivities of (Y_1-xYb_x)₃Al₅O₁₂ ceramics firstly decreased and subsequently increased with Yb ions concentration rising from room temperature to 1200 °C. (Y_0.7Yb_0.3)₃Al₅O₁₂ had the lowest thermal conductivity among investigated specimens, which was about 1.62 W m⁻¹ K⁻¹ at 1000 °C, around 30% lower than that of pure YAG (2.3 W m⁻¹ K⁻¹, 1000 °C). Yb had almost no effect on the coefficients of thermal expansion (CTEs) of (Y_1-xYb_x)₃Al₅O₁₂ ceramics and the CTE was approximate 10.7×10⁻⁶ K⁻¹ at 1200 °C. In addition, (Y_0.7Yb_0.3)₃Al₅O₁₂ ceramic remained good phase stability when heating from room temperature to 1450 °C.     

Preparation and characterization of neodymium-doped ceria electrolyte materials for solid oxide fuel cells

The microstructure, thermal expansion, microhardness, indentation fracture toughness, and ionic conductivity of neodymium-doped ceria (NDC) prepared by coprecipitation were investigated. The results revealed that the average particle size (D_BET) ranged from 20.1 to 25.8 nm, crystallite dimension (D_XRD) varied from 17.5 to 20.7 nm, and the specific surface area distribution was from 31.25 to 40.27 m²/g for neodymium-doped ceria stacking powders. Dependence of lattice parameter, a, versus dopant concentration, x, of Nd³⁺ ion shows that these solid solutions obey Vegard's rule as a(x) = 5.4069 + 0.1642x for Ce_1?xNd_xO_2?(1/2)x for x = 0.05–0.25. For neodymium-doped ceria ceramics sintered at 1500 °C for 5 h, the bulk density was over 95% of the theoretical density. The maximum ionic conductivity, σ_800°C = 4.615 × 10^?2 S/cm, with the minimum activation energy, E_a = 0.794 eV was found for the Ce_0.75Nd_0.25O_1.875 ceramic. Trivalent, neodymium-doped ceria ceramics revealed high fracture toughness, the fracture toughness distribution was in the range of 6.236 ± 0.021 to 6.846 ± 0.017 MPa m^1/2. The high indentation fracture toughness of neodymium-doped ceria was attributed to crack deflection. Moreover, the porosity may influence the mechanical properties such as microhardness and fracture toughness. It was observed that as the porosity reduced, the microhardness and fracture toughness increased.     

Oxygen ionic transport in SrFe1−yAlyO3−δ and Sr1−xCaxFe0.5Al0.5O3−δ ceramics

The oxygen permeability of mixed-conducting Sr_1−xCa_xFe_1−yAl_yO_3−δ (x=0–1.0; y=0.3–0.5) ceramics at 850–1000 °C, with an apparent activation energy of 120–206 kJ/mol, is mainly limited by the bulk ionic conduction. When the membrane thickness is 1.0 mm, the oxygen permeation fluxes under pO₂ gradient of 0.21/0.021 atm vary from 3.7×10⁻¹⁰ mol s⁻¹ cm⁻² to 1.5×10⁻⁷ mol s⁻¹ cm⁻² at 950 °C. The maximum solubility of Al³⁺ cations in the perovskite lattice of SrFe_1−yAl_yO_3−δ is approximately 40%, whilst the brownmillerite-type solid solution formation range in Sr_1−xCa_xFe_0.5Al_0.5O_3−δ system corresponds to x>0.75. The oxygen ionic conductivity of SrFeO₃-based perovskites decreases moderately on Al doping, but is 100–300 times higher than that of brownmillerites derived from CaFe_0.5Al_0.5O_2.5+δ. Temperature-activated character and relatively low values of hole mobility in SrFe_0.7Al_0.3O_3−δ, estimated from the total conductivity and Seebeck coefficient data, suggest a small-polaron mechanism of p-type electronic conduction under oxidising conditions. Reducing oxygen partial pressure results in increasing ionic conductivity and in the transition from dominant p- to n-type electronic transport, followed by decomposition. The low-pO₂ stability limits of Sr_1−xCa_xFe_1−yAl_yO_3−δ seem essentially independent of composition, varying between that of LaFeO_3−δ and the Fe/Fe_1−γO boundary. Thermal expansion coefficients of Sr_1−xCa_xFe_1−yAl_yO_3−δ ceramics in air are 9×10⁻⁶ K⁻¹ to 16×10⁻⁶ K⁻¹ at 100–650 °C and 12×10⁻⁶ K⁻¹ to 24×10⁻⁶ K⁻¹ at 650–950 °C. Doping of SrFe_1−yAl_yO_3−δ with aluminum decreases thermal expansion due to decreasing oxygen nonstoichiometry variations.     

Electronic conductivity,oxygen permeability and thermal expansion of Sr0.7Ce0.3Mn1−xAlxO3−δ

The maximum solubility of aluminum cations in the perovskite lattice of Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ is approximately 15%. The incorporation of Al³⁺ increases oxygen ionic transport due to increasing oxygen nonstoichiometry, and decreases the tetragonal unit cell volume and thermal expansion at temperatures above 600 °C. The total conductivity of Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ (x = 0–0.2), predominantly electronic, decreases with aluminum additions and has an activation energy of 10.2–10.9 kJ/mol at 350–850 °C. Analysis of the electronic conduction and Seebeck coefficient of Sr_0.7Ce_0.3Mn_0.9Al_0.1O_3−δ, measured in the oxygen partial pressure range from 10⁻¹⁸ to 0.5 atm at 700–950 °C, revealed trends characteristic of broad-band semiconductors, such as temperature-independent mobility. The temperature dependence of the charge carrier concentration is weak, but exhibits a tendency to thermal excitation, whilst oxygen losses from the lattice have an opposite effect. The role of the latter factor becomes significant at temperatures above 800 °C and on reducing p(O₂) below 10⁻⁴ to 10⁻² atm. The oxygen permeability of dense Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ (x = 0–0.2) membranes, limited by both bulk ionic conduction and surface exchange, is substantially higher than that of (La, Sr)MnO₃-based materials used for solid oxide fuel cell cathodes. The average thermal expansion coefficients of Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ ceramics in air are (10.8–11.8) × 10⁻⁶ K⁻¹.     

Phase relationships and transport in Ti-, Ce- and Zr-substituted lanthanum silicate systems

The solubility of Ti⁴⁺ in the lattice of apatite-type La_9.83Si_6−xTi_xO_26.75 corresponds to approximately 28% of the Si-site density. The conductivity of La_9.83Si_6−xTi_xO_26.75 (x = 1–2) is predominantly oxygen-ionic and independent of the oxygen partial pressure in the p(O₂) range from 10⁻²⁰ to 0.3 atm. The electron transference numbers determined by the modified faradaic efficiency technique are lower than 0.006 at 900–950 °C in air. The open-circuit voltage of oxygen concentration cells with Ti-doped silicate electrolytes is close to the theoretical Nernst value both under oxygen/air and air/10%H₂–90%N₂ gradients at 700–950 °C, suggesting the stabilization of Ti⁴⁺ in the apatite structure. Titanium addition in La_9.83Si_6−xTi_xO_26.75 (x = 1–2) leads to decreasing ionic conductivity and increasing activation energies from 93 to 137 kJ/mol, and enhanced degradation in reducing atmospheres due to SiO volatilization. At p(O₂) = 10⁻²⁰ atm and 1223 K, the conductivity decrease after 100 h was about 5% for x = 1 and 17% for x = 2. The solubility of Zr⁴⁺ in the La_9.83Si_6−xZr_xO_26.75 system was found to be negligible, while the maximum concentration of Ce⁴⁺ in La_9.4−xCe_xSi₆O_27−δ is approximately 5% with respect to the number of lanthanum sites.     

Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method

Mn²⁺-doped Sn_1−xMn_xP₂O₇ (x = 0–0.2) are synthesized by a new co-precipitation method using tin(II)oxalate as tin(IV) precursor, which gives pure tin pyrophosphate at 300 °C, as all the reaction by-products are vaporizable at <150 °C. The dopant Mn²⁺ acts as a sintering aid and leads to dense Sn_1−xMn_xP₂O₇ samples on sintering at 1100 °C. Though conductivity of Sn_1−xMn_xP₂O₇ samples in the ambient atmosphere is 10⁻⁹–10⁻⁶ S cm⁻¹ in 300–550 °C range, it increases significantly in humidified (water vapor pressure, pH₂O = 0.12 atm) atmosphere and reaches >10⁻³ S cm⁻¹ in 100–200 °C range. The maximum conductivity is shown by Sn_0.88Mn_0.12P₂O₇ with 9.79 × 10⁻⁶ S cm⁻¹ at 550 °C in ambient air and 2.29 × 10⁻³ S cm⁻¹ at 190 °C in humidified air. It is observed that the humidification of Sn_1−xMn_xP₂O₇ samples is a slow process and its rate increases at higher temperature. The stability of Sn_1−xMn_xP₂O₇ samples is analyzed.     

Evaluation of the La0.75Sr0.25Mn0.8Co0.2O3−δ system as cathode material for ITSOFCs with La9Sr1Si6O26.5 apatite as electrolyte

In the past years, a major interest has been devoted to decrease the working temperature of solid oxide fuel cells (SOFCs) down to about 700 °C.Apatite materials (La_10?xSr_xSi₆O_27?x/2) are attractive candidates for solid electrolytes, with a high ionic conductivity at 700 °C, a chemical and a dimensional stability for a pO₂ ranging from 10^?25 to 0.2 atm. A perovskite oxide (La_0.75Sr_0.25Mn_0.8Co_0.2O_3?δ) has been used as a cathode material.Symmetrical cathode/electrolyte/cathode cells were fabricated by stacking layers obtained by tape casting of apatite and perovskite powders and co-sintering at 1400 °C for 2 h in air.Impedance spectroscopy measurements were performed on these cells in order to determine the electrode resistance. It has been shown that the latter decreases with the porosity content of the cathode and with the use of a composite material (apatite/perovskite) instead of a simple perovskite.     

Electrochemical and microstructural characteristics of nanoperovskite oxides Ba0.2Sr0.8Co0.8Fe0.2O3−δ (BSCF) for solid oxide fuel cells

Nanoperovskite oxides, Ba_0.2Sr_0.8Co_0.8Fe_0.2O_3?δ (BSCF), were synthesized via the co-precipitation method using Ba, Sr, Co, and Fe nitrates as precursors. Next, half cells were fabricated by painting BSCF thin film on Sm_0.2Ce_0.8O_x (samarium doped ceria, SDC) electrolyte pellets. X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrochemical impedance spectroscopy (EIS) measurements were carried out on the BSCF powders and pellets obtained after sintering at 900 °C. Investigations revealed that single-phase perovskites with cubic structure was obtained in this study. The impedance spectra for BSCF/SDC/BSCF cells were measured to obtain the interfacial area specific resistances (ASR) at several operating temperatures. The lowest values of ASR were found to be 0.19 Ω cm², 0.14 Ω cm² 0.10 cm², 0.09 Ω cm² and 0.07 Ω cm² at operating temperatures of 600 °C, 650 °C, 700 °C, 750 °C and 800 °C, respectively. The highest conductivity was found for cells sintered at 900 °C with an electrical conductivity of 153 S cm^?1 in air at operating temperature of 700 °C.     

Effect of CaO addition on the structure and electrical conductivity of the pyrochlore-type GdSmZr2O7

Polycrystalline GdSm_1?xCa_xZr₂O_7?x/2 (0 ≤ x ≤ 0.20) ceramics have been prepared by the solid-state reaction method. The effects of CaO addition on the microstructure and electrical properties of the pyrochlore-type GdSmZr₂O₇ ceramic were investigated. GdSm_1?xCa_xZr₂O_7?x/2 (x ≤ 0.05) ceramics exhibit a pyrochlore-type structure; however, GdSm_1?xCa_xZr₂O_7?x/2 (0.10 ≤ x ≤ 0.20) ceramics consist of the pyrochlore-type structure and a small amount of CaZrO₃. The total conductivity of GdSm_1?xCa_xZr₂O_7?x/2 ceramics follows the Arrhenius relation, and gradually increases with increasing temperature from 723 to 1173 K. GdSm_1?xCa_xZr₂O_7?x/2 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10^?4–1.0 atm at each test temperature. The highest total conductivity is about 1.20 × 10^?2 S cm^?1 at 1173 K for the GdSm_0.9Ca_0.1Zr₂O_6.95 ceramic.     

Study of Cr-SnO2 ceramic pigment and of Ti/Sn ratio on formation and coloration of these materials

In this study, Cr_xSn_1−xO₂ (0 ≤ x ≤ 0.06) and Cr_0.03Sn_0.97−yTi_yO₂ (0 < y ≤ 0.97) compositions were synthesized by the ceramic method and characterized by X-ray diffraction, UV–vis spectroscopy and CIE L*a*b* (Commission Internationale de l’Eclairage L*a*b*) parameters measurements. From Cr_xSn_1−xO₂ samples fired at 1600 °C/1 h, x = 0.03 was established as the composition limit of formation of solid solutions. When x ≤ 0.01, better coloration of glazed tiles were obtained from short thermal treatment (1400 °C/1 h or 1600 °C/1 h) than from long thermal treatment (1400 °C/24 h). When 0.01 < x < 0.06 small variations of color in glazed tiles were obtained from samples fired at 1400 °C/24 h and 1600 °C/1 h. From Cr_0.03Sn_0.97−yTi_yO₂ compositions, a better purple color was obtained when y = 0.02 (Ti/Sn ∼ 2.1 × 10⁻²) than when y = 0.     

Effect of molybdenum substitution on electrochemical performance of Li[Li0.2Mn0.54Co0.13Ni0.13]O2 cathode material

Molybdenum doping is introduced to improve the electrochemical performance of lithium-rich manganese-based cathode material. X-ray diffraction (XRD) results illustrate that the crystallographic parameters a, c and lattice volume V become larger with the increase of Mo content. The scanning electron microscope (SEM) shows that the molybdenum substitution increases the crystallinity of the primary particles. When evaluated as cathode material, the as-prepared Li[Li_0.2Mn_0.54-x/3Ni_0.13-x/3Co_0.13-x/3Mo_x]O₂ (x = 0.007) delivers a discharge capacity of 155.5 mA h g⁻¹ at 5 C (1 C = 250 mA g⁻¹) and exhibits the capacity retention of 81.8% at 1 C after 200 cycles. The results of cyclic voltammetry (CV) and electronic impedance spectroscopy (EIS) tests reflect that the molybdenum substitution is able to significantly reduce the electrode polarization and lower the charge-transfer resistance. Within appropriate amount of Mo doping, the lithium ion diffusion coefficient of the material can reach to 8.92 × 10^–15 cm² s⁻¹, which is ~ 30 times higher than that of pristine materials (2.65 × 10^–16 cm² s⁻¹).     

Blue-violet ceramic pigments based on Co and Mg Co2−xMgxP2O7 diphosphates

Solid solutions of Co and Mg diphosphates with compositions Co_2?xMg_xP₂O₇ (x = 0, 0.1, 0.2, 0.3, 0.5, 0.7, 1.0, 1.5 and 1.8) have been prepared and characterized for the first time as alternative low-toxicity blue ceramic pigments. The compositions were prepared through the conventional coprecipitation route and calcined up to 1000 °C/2 h. Samples were characterized by thermal analysis, XRD, SEM/EDX, UV–vis-NIR spectroscopy and colour measurements (CIE-L^*a^*b^*). Isostructural Co_2?xMg_xP₂O₇ diphosphate solid solutions (monoclinic system and P21/c spatial group) formed successfully within the studied range of compositions, accompanied only by a minor quantity of residual Co or Mg orthophosphates (M₃(PO₄)₂). Interestingly, the obtained solid solutions developed nice blue-violet colourations even with high Mg doping after enamelling within double-firing (x = 1.5–1.8) and single-firing (x = 1.0–1.5) ceramic glasses. These optimal compositions containing a minimized Co amount (measured values around 7–16 wt%) could be therefore less toxic alternatives to the conventional Co₃(PO₄)₂ blue ceramic pigment.     

Preparation and ionic conduction of CaZr1−xScxO3−α ceramics

A series of CaZr_1−xSc_xO_3−α (x=0, 0.05, 0.10, 0.15) perovskite oxide ceramics were successfully fabricated at 1400 °C for 10 h and then further sintered at 1650 °C for 10 h via a solid-state reaction sintering process. Conductivities of the ceramics were measured under the atmosphere that contains 1% H₂/Ar and 5.63 kPa H₂O/Ar by the electrochemical impedance spectra technique. It was found that the conductivities of CaZr_1−xSc_xO_3−α (x=0, 0.05, 0.10, 0.15) ceramics increased with the increase of the measuring temperature, and the conductivity achieved its maximum value of 2.03×10⁻⁵–6.5×10⁻³ S cm⁻¹ when the doping amount of Sc (x) was 0.10. Additionally, element doping can increase the conductivities and decrease the conductivity activation energies of CaZr_1−xSc_xO_3−α ceramics. The results of transport number measurement indicated that the CaZr_0.9Sc_0.1O_3−α is almost a pure protonic conductor at 500–750 °C, while it is a mixed protonic-oxygen ionic-electronic conductor at 750–1300 °C.     

Structural determination and microwave properties of (x)Re(Co1/2Ti1/2)O3–(1 − x)CaTiO3 (Re = La and Nd) solid solutions

Solid solutions of (x)Re(Co_1/2Ti_1/2)O₃–(1 − x)CaTiO₃ (Re = La and Nd, abbreviated to xLCT and xNCT, respectively) where x = 0, 0.25, 0.5, 0.75 and 1 have been fabricated using solid state synthesis. Samples have been examined using X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy (TEM) and their dielectric properties measured at microwave (MW) frequencies. Formation of single phase solid solutions were confirmed by XRD and the measured lattice parameters varied linearly from LCT (a = 5.66 Å, b = 7.867 Å and c = 5.494 Å) and NCT (a = 5.636 Å, b = 7.914 Å and c = 5.461 Å) to CT (a = 5.596 Å, b = 7.731 Å and c = 5.424 Å). XRD and TEM confirmed both in-phase and antiphase rotations of O-octahedra consistent with an a⁻a⁻c⁺ tilt system across the entire solid solution series. Electron diffraction revealed that LCT and NCT have reflections associated with B-site cation ordering which is absent for x ≤ 0.75. MW dielectric measurements showed that LCT and NCT were highly insulating with microwave quality factor (Qf₀) values of 39,000 and 34,000, respectively. Compositions anticipated to have a zero temperature coefficient of resonant frequency (τ_f) are 0.48LCT-CT and 0.52NCT-CT with ɛ_r = 45 and Qf₀ ∼ 5000 and ɛ_r = 43 and Qf₀ ∼ 4000, respectively.     

Structural,morphological, optical,cation distribution and Mössbauer analysis of Bi3+ substituted strontium hexaferrite

Single-phase M-type hexagonal ferrites, SrBi_xFe_12−xO₁₉ (0.0≤x≤1.0), were prepared by a co-precipitation assisted ceramic route. The influence of the Bi³⁺ substitution on the crystallization of ferrite phase has been examined using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and Mössbauer spectroscopy. The XRD data show that the nanoparticles crystallize in the single hexagonal magnetoplumbite phase with the crystallite size varying between 65 and 82 nm. A systematic change in the lattice constants, a=b and c, was observed because of the ionic radius of Bi³⁺ (1.17 Å) being larger than that of Fe³⁺ ion (0.64 Å). SEM analysis indicated the hexagonal shape morphology of products. From ⁵⁷Fe Mössbauer spectroscopy data, the variation in line width, isomer shift, quadrupole splitting and hyperfine magnetic field values on Bi substitutions have been determined.     

Improved dielectric properties and grain boundary response of SrTiO3 doped Y2/3Cu3Ti4O12 ceramics fabricated by Sol-gel process for high-energy-density storage applications

A chemical solution processing method based on sol-gel chemistry (SG) was used to synthesize (1-x)Y_2/3Cu₃Ti₄O₁₂-xSrTiO₃ (x = 0, 0.05, 0.1, 0.15, 0.2, 0.25) ceramics successfully. The 0.85Y_2/3Cu₃Ti₄O₁₂-0.15SrTiO₃ ceramics sintered at 1050 °C for 20 h showed fine-grained microstructure and high dielectric constant (ε′ ≈ 1.7 × 10⁵) at 1 kHz. Furthermore, the 0.85Y_2/3Cu₃Ti₄O₁₂-0.15SrTiO₃ ceramics appeared distinct pseudo-relaxor behavior. Two electrical responses were observed in the combined modulus and impedance plots, indicating the presence of Maxwell-Wagner relaxation. Sr vacancies and additional oxygen vacancies had substantial contribution to the sintering behavior, an increase in grain growth, and relaxation behaviors in grain boundaries. The contributions of semiconducting grains with the nanodomain and insulating grain boundaries (corresponding to high-frequency and low-frequency electrical response, respectively) played important roles in the dielectric properties of (1-x)Y_2/3Cu₃Ti₄O₁₂-xSrTiO₃ ceramics. The occurrence of the polarization mechanism transition from the grain boundary response to the electrode one with temperature change was clearly evidenced in the low frequency range.     

Crystal structure,thermal expansion and electrical conductivity of perovskite oxides BaxSr1−xCo0.8Fe0.2O3−δ (0.3 ≤ x ≤ 0.7)

Ba_xSr_1−xCo_0.8Fe_0.2O_3−δ (0.3 ≤ x ≤ 0.7) composite oxides were prepared and characterized. The crystal structure, thermal expansion and electrical conductivity were studied by X-ray diffraction, dilatometer and four-point DC, respectively. For x ≤ 0.6 compositions, cubic perovskite structure was obtained and the lattice constant increased with increasing Ba content. Large amount of lattice oxygen was lost below 550 °C, which had significant effects on thermal and electrical properties. All the dilatometric curves had an inflection at about 350–500 °C, and thermal expansion coefficients were very high between 50 and 1000 °C with the value larger than 20 × 10⁻⁶ °C⁻¹. The conductivity were larger than 30 S cm⁻¹ above 500 °C except for x > 0.5 compositions. Furthermore, conductivity relaxation behaviors were also investigated at temperature 400–550 °C. Generally, Ba_0.4Sr_0.6Co_0.8Fe_0‘2O_3−δ and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ are potential cathode materials.