Kinetic demixing/decomposition of Ba0.5Sr0.5CoxFe1−xO3−δ (x = 0.2 and 0.8)

Microstructural changes due to kinetic demixing within sintered BSCF ceramics (Ba_0.5Sr_0.5Co_xFe_1?xO_3?δ, x = 0.2 and 0.8: BSCF5528 and BSCF5582, respectively) have been investigated. When the specimens were subjected to 2 A/cm² at 1000 °C and pO₂ = 10^?5 atm, there was a significant enhancement of grain growth as well as 2nd phase formation observed in BSCF5528. At the anode, cobalt deficient aggregates within the grains; and, at the cathode, cobalt rich 2nd phase particles were observed on the grain surfaces of the microstructure. Such phenomena were not observed in BSCF5582, even under higher current density (7 A/cm²) and longer delay time. These results were explained by the kinetic demixing/decomposition.     

Structural properties and hyperfine interactions in Co–Zn Y-type hexaferrites prepared by sol–gel method

Y-type barium hexaferrites Ba₂Co_2−xZn_xFe₁₂O₂₂ (0.0≤x≤2.0) were prepared using sol–gel method and then sintering at temperatures between 900 and 1100 °C. The properties of the prepared samples were investigated using X-ray diffraction, scanning electron microscopy, and Mössbauer spectroscopy. XRD patterns revealed the presence of a single Y-type hexaferrite phase in the samples sintered at temperatures above 1000 °C. Mössbauer data indicated that Co²⁺ ions occupied octahedral sites in the T blocks, while Zn²⁺ ions were distributed between the two tetrahedral sites. This trend for cationic distribution resulted in weakening the superexchange interactions between spin-up and spin-down sublattices with increasing Zn content, and a consequent reduction in the hyperfine fields in Zn rich compounds.     

Frequency and temperature dependent electrical properties of Ni0.7Zn0.3CrxFe2−xO4 (0 ≤ x ≤ 0.5)

Series of the ferrite samples with a chemical formula Ni_0.7Zn_0.3Cr_xFe_2?xO₄ (x = 0.0–0.5) were prepared by a sol–gel auto-combustion method and annealed at 600 °C for 4 h. The prepared samples have the cubic spinel structure with no impurity phase. As the Cr³⁺ content x increases, the unit cell dimensions decrease with an increase in Cr³⁺ content x. The crystallite size is decreases from 37 nm to 21 nm as the Cr³⁺ content increases from x = 0.0 to 0.5. Resistivity increases whereas dielectric constant decreases with an increase in Cr³⁺ content x. Maxima in the dielectric loss tangent versus frequency appear when the frequency of the hopping charge carriers coincides with the frequency of the applied alternating field. Dielectric constant and dielectric loss tangent increases with increase in temperature. Saturation magnetization of sintered samples showed higher values as compared to as-prepared sample. Curie temperature deduced from AC susceptibility data decreases with increasing x.     

Structural,magnetic and Mössbauer study of BaLaxFe12−xO19 nanohexaferrites synthesized via sol–gel auto-combustion technique

BaLa_xFe_12−xO₁₉ (0.05≤x≤0.25) nanohexaferrites were synthesized by sol–gel auto combustion method. X-ray diffraction study revealed the hexagonal structure of the synthesized nanoferrites without any secondary phase and Rietveld analysis confirmed the P63/mmc space group. The crystallite size was observed to increase (49–63 nm) with the increasing substitution of La³⁺ ions. The particle size was observed to be in the range 49–63 nm. The remarkable increase in saturation magnetization upto 78.5 emu/g and an increase in magneto-crystalline anisotropic was observed with the increase in La³⁺ substitution. In addition, for the first time we have reported the Mösssbauer study of BaLa_xF_12−xO₁₉ nanohexaferrites in the present paper.     

Structural,vibrational, electrical and magnetic properties of Bi1−xPrxFeO3

Crystalline solid solution of Bi_1−xPr_xFeO₃ (x=0.05, 0.1, and 0.15) ceramics has been successfully synthesized by a low temperature assisted co-precipitation method. Rietveld-refinement of the X-ray diffraction data reveals rhombohedral structure for Bi_1−xPr_xFeO₃ (x=0.05, 0.10) and triclinic for Bi_1−xPr_xFeO₃ (x=0.15). The crystallite sizes of the Bi_1−xPr_xFeO₃ (x=0.05, 0.1 and 0.15) are found to be approximately 33, 27 and 22 nm respectively calculated using Debye–Scherrer equation. The SEM images of Bi_1−xPr_xFeO₃ (x=0.05, 0.10 and 0.15) ceramics show grains with almost spherical morphology. 4A1 and 7E Raman modes have been observed in the range 100–650 cm⁻¹ and two phonon modes centered around 1150–1450 cm⁻¹ have also been observed corresponding to 2A₄ (LO), 2E₈ (TO) and 2E₉ (TO) modes of Bi_1−xPr_xFeO₃ (x=0.05, 0.1 and 0.15). The changes in Raman modes such as prominent frequency shift, line broadening and intensity have been noticed with the increase of Pr concentration in BiFeO₃ (BFO) suggesting a structural transformation as revealed by the Rietveld refinement. An anomaly in the temperature dependent dielectric studies has been noticed in all the samples at the vicinity of Neel temperature (T_N) indicating a magnetic ordering and an increase in magnetization with increase of Pr concentration is noticed from the room temperature magnetic studies. Further, the leakage current density is found to be reduced with increasing Pr concentration.     

Structural and optical properties of Mg1−xMnxP2O6 (x = 0–1.0) magnesium metaphosphate

Structural and optical properties of Mg_1−xMn_xP₂O₆ (x = 0–1.0) magnesium metaphosphate were investigated in detail. The complete solid solution of MgP₂O₆–MnP₂O₆ is confirmed as monoclinic space group C2/c. The dynamic luminescence was studied by changing the Mn²⁺ content (0–100 mol%) and temperature (10–300 K). There is a good chemical homogeneity in Mg_1−xMn_xP₂O₆ (x = 0–1.0), which can be supported by the linearly varying cell size and the gradually changing vibration spectrum. However, the optical properties of the solid solution do not show a continuous change trend, that is, an obvious inflection point appeared when x = 0.5. Mg_1−xMn_xP₂O₆ (x = 0.1–0.5) shows a dominant O²⁻ → Mn²⁺ charge transfer (CT) absorption in the near UV region and feeble d–d transitions of Mn²⁺ in visible wavelength region. However, Mg_1−xMn_xP₂O₆ (x = 0.6–1.0) presents a strong d–d absorption transition and nearly disappeared CT band. The changing trend of optical absorption is also maintained in the excitation and emission of the solid solutions. In Mg_1−xMn_xP₂O₆ (x = 0.1–0.5), (Mn, Mg)O₆ octahedron has slight distortion, and the effective luminescence only occurs when CT band excitation is used. In contrast, in Mg_1−xMn_xP₂O₆ (x = 0.6–1.0), (Mn, Mg)O₆ octahedron is highly distorted, and only excitation at d–d transition produces effective luminescence. This research highlights the critical role of MnO₆ octahedral distortions in the luminescence properties of Mn²⁺ activators. The research provides a reference for developing optical materials.     

Physical–chemical property and activity evaluation of LaB0.5Co0.5O3 (B = Cr,Mn, Cu) and LaMnxCo1−xO3 (x = 0.1, 0.25, 0.5) nano perovskites in VOC combustion

The correlation between physical–chemical properties and activities of LaB_0.5Co_0.5O₃ (B = Cr, Mn, Cu) nano perovskites was studied in combustion of toluene. LaMn_0.5Co_0.5O₃ showed the highest activity among LaB_0.5Co_0.5O₃ catalysts and further optimization study was focused on LaMn_xCo_1?xO₃ (x = 0.1, 0.25, 0.5). The activity and reducibility of catalysts improved due to partial substitution of Co³⁺ by B cation. No direct relationship was between surface area and catalyst activity. T_50% of 2-propanol over LaMn_0.25Co_0.75O₃, LaMn_0.5Co_0.5O₃, LaMn_0.1Co_0.9O₃ and LaCoO₃ was 168, 200, 220 and 229 °C, respectively. LaMn_0.25Co_0.75O₃ was the optimum catalyst and showed robust stability in combustion of toluene and 2-propanol.     

Structural and electrical properties of magnetic ceramics of Co1−xCuxFe2O4 spinel nanoferrites

Abstract

Magnetic ceramics of the type of spinel nanoferrites of Co_1?xCu_xFe₂O₄ with Cu concentrations of x?=?0·00, 0·25, 0·50, 0·75 and 1·00 were prepared by chemical co-precipitation method. X-ray diffraction results confirmed the formation of a single spinel ferrite structure with crystallite size in the range of 20–63 nm. Scanning electron microscopy and EDS were used to study the morphological and compositional changes taking place with varying Cu concentration. DC electrical resistivity, activation energy and drift mobility are strongly influenced by both Cu concentration and temperature. Resistivities of the prepared magnetic ceramics were found to decrease with the increase in Cu concentration to follow Verwey mechanism. The semiconductor behaviour of the prepared nanoparticles was confirmed from the standard Arrhenius relation of resistivity versus temperature. The dielectric constants were measured in the frequency range of 100 Hz–3·0 MHz and are explained by the Maxwell–Wagner interfacial type of polarisation.     

Study of structural,dielectric, electric,magnetic and magnetoelectric properties of K0.5Na0.5NbO3 − Ni0.2Co0.8Fe2O4 composites

Magnetoelectric composites with general formula (x) Ni_0.2Co_0.8Fe₂O₄- (1- x) (K_0.5Na_0.5)NbO₃ (x = 0, 0.10, 0.20, 0.30, 0.40, 0.50 and 1.0) have been synthesized by solid state reaction method. X-ray diffraction pattern asserts the existence of both the constituent phases in the synthesized composites. FESEM micrographs are used to investigate the microstructure and for calculation of average grain size of the composites. Temperature dependent dielectric properties are investigated as a function of temperature and found to enhance with addition of ferrite in the composites. P-E hysteresis loops obtained for individual (K_0.5Na_0.5)NbO₃ (KNN) phase and composites indicate the ferroelectric ordering in the composites. Saturation and remnant magnetization show increasing trend with increase in NCFO concentration. FC-ZFC magnetization curves indicate charge ordering, metal-insulator transition (Verwey transition) in NCFO and composites. Impedance spectroscopy shows that bulk resistance reduces with increase in temperature, thereby indicating negative temperature coefficient of resistance (NTCR) behaviour of the composites. The magnetoelectric effect is confirmed by measuring magnetoelectric voltage coefficient, α_ME and the maximum value of α_ME is 5.389 mV/cm-Oe for 20% NCFO-80% KNN composite.     

Structural,optical and magnetic properties of Zn1−xCoxO prepared by the sol–gel route

Effects of Co doping on the structural, optical and magnetic properties of ZnO samples prepared by the sol–gel method are reported. The X-ray diffraction, X-ray photoelectron spectroscopy and UV–visible spectroscopy confirmed the substitution of Co ions on Zn sites without changing the wurtzite structure. No segregated secondary phases or Co rich clusters were detected. Optical absorption spectra of the samples exhibit a blue shift in the absorption band edge with increasing dopant concentration. The photoluminescence measurements show a blue shift in UV emission peak with the increase in Co concentration and a slight shift in the green emission band at around 509 nm which gets suppressed for higher sintering temperature. The field dependence of magnetization observed at room temperature exhibits clear ferromagnetic behavior. Efforts have been made to fit the experimental M–H data using the magnetic polarons model (BMP) which involves localized carriers and magnetic cations. The calculated concentration of the BMPs is found to be below the typical percolation threshold in ZnO. Thus BMP model alone is not sufficient to explain the room temperature ferromagnetic behavior in ZnO. To know the exact magnetic ordering in the system, we also attempted to fit temperature dependent magnetization curves with the Curie–Weiss Law which shows antiferromagnetic ordering in all samples.     

Structural,interfacial, magnetic and dielectric properties of (1−x)(Mg0.95Zn0.05)2(Ti0.8Sn0.2)O4@xNi0.4Zn0.6Fe2O4 composite at high frequency

(Mg_0.95Zn_0.05)₂(Ti_0.8Sn_0.2)O₄ powder was synthesized by a solid state reaction. Then, Ni_0.4Zn_0.6Fe₂O₄ was grown on the (Mg_0.95Zn_0.05)(Ti_0.8Sn_0.2)O₄ particles in a hydrothermal environment to form a core-shell structure. (1-x)(Mg_0.95Zn_0.05)₂(Ti_0.8Sn_0.2)O₄@xNi_0.4Zn_0.6Fe₂O₄ composite ceramics were sintered at 1200 °C with these powders. XRD, SEM, TEM analyses indicated that high dense core-shell ceramics without any foreign phase were obtained. Different types of sharp interfaces were self-assembled owing to the minimization of direct elastic energy in the hydrothermal environment. The composites enjoy good magnetic and dielectric properties, especially, good microwave dielectric properties with high saturation magnetization when the ferrite content is 0.3–0.5. The results provided a powerful experimental basis for the sensor and transducer.     

Microwave dielectric properties of SrLa[Ga1−x(Mg0.5Ti0.5)x]O4 and SrLa[Ga1−x(Zn0.5Ti0.5)x]O4 (x = 0.2-0.8) ceramics

SrLa[Ga_1−x(R_0.5Ti_0.5)_x]O₄ (R = Mg, Zn) ceramics were prepared by a standard solid state sintering method. The single-phase ceramics with K₂NiF₄-type layered perovskite structure and I4/mmm space group were obtained, indicating that SrLa(R_0.5Ti_0.5) and SrLaGaO₄ can form the unlimited solid solutions. With increasing x for R = Mg and Zn, ε_r increases monotonously, the Qf value first increases and then decreases, while τ_f increases from a negative to a positive value. The optimized microwave dielectric properties were obtained as following: ε_r = 23.3, Qf = 89 400 GHz, τ_f = −0.8 ppm/°C for SrLa[Ga_0.6(Mg_0.5Ti_0.5)_0.4]O₄ and ε_r = 23.3, Qf = 76 200 GHz, τ_f = 0.2 ppm/°C for SrLa[Ga_0.7(Zn_0.5Ti_0.5)_0.3]O₄, indicating that the present solid solution ceramics are the promising candidates as microwave resonator materials for the telecommunication applications.     

Mechanical behavior of ferroelastic porous La0.6Sr0.4Co0.2Fe0.8O3−δ prepared with different pore formers

The present work investigated the mechanical behavior of porous La_0.6Sr_0.4Co_0.2 Fe_0.8O_3−δ LSCF under uniaxial compression. The porous (LSCF) samples with the same grain size but different porous structures with 1.5–41% of porosity were prepared using three different pore formers. All the samples had ferroelastic domains and exhibited ferroelastic mechanical behaviors under uniaxial compression. Initial and loading moduli as well as critical stress monotonically decreased and remnant strain increased with increasing the porosity. The initial modulus can be determined by the actual porosity regardless of porous structure or grain size, whereas the other properties were more sensitive to experimental condition such as loading rate and maximum applied stress. Compressive fracture strength could be significantly influenced by porous structure.     

Synthesis,CO2-tolerance and rate-determining step of Nb-doped Ce0.8Gd0.2O2−δ–Pr0.6Sr0.4Co0.5Fe0.5O3−δ ceramic membranes

In this work, CO₂-tolerant Ce_0.8Gd_0.2O_2δ–Pr_0.6Sr_0.4Co_0.5Fe_0.5−xNb_xO_3−δ (CG–PSCF_0.5−xN_x; x=0–0.125) dual-phase dense oxygen permeation membranes were successfully developed. The crystal structure, microstructure, oxygen permeability, rate-determining step and CO₂ tolerance were systematically investigated. The experimental results showed that the increase in CG content improved oxygen permeability and CO₂ tolerance. Thermogravimetry–differential-scanning-calorimetry analysis, X-ray photoelectron spectra and oxygen permeation tests indicated that the increase in Nb content caused a slight decrease in oxygen permeability, while the long-term CO₂ resistance can be improved significantly. According to the adopted permeation model, the weight ratio and thickness affect the oxygen permeability and permeation resistance distribution. By examining the distribution of three permeation resistances, we identified the rate-determining step and then optimized the weight ratio of the two phases, as well exploring the effects of thickness on oxygen permeability. All these experiments confirm that CG–PSCF_0.5−xN_x dual-phase membranes have great CO₂ tolerance and potential application in oxy-fuel combustion.     

Microstructures and mechanical properties of Ce1−xCaxO2−y (x = 0.05, 0.1, 0.2) with different sintering temperatures

In this study, the microstructures and mechanical properties of Ca-doped CeO₂ affected by different sintering temperatures and various dopant concentrations have been systematically investigated. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) revealed that a seemingly individual Ca-doped CeO₂ system may consist of multiple types of doped CeO₂ phases. X-ray photoelectron spectroscopy (XPS) analysis demonstrated that the chemical state of the Ce element is less dependent on the sintering temperature, while increasing the sintering temperature may cause segregation of the Ca elements to form CaO phase. Nanoindentation tests showed that Ca-doped CeO₂ has better fracture toughness compared to rare-earth (RE) doped CeO₂; the relationship between the sintering temperature and the mechanical property has been discussed.     

Electrical characterization of room temperature humidity sensors in La0.8Sr0.2Fe1−xCuxO3 (x = 0, 0.05, 0.10)

Semiconducting oxide gas sensors based on La_0.8Sr_0.2Fe_1?xCu_xO₃ (x = 0, 0.05, 0.10) (LSF, LSFC05, and LSFC10, respectively) were prepared by screen-printing for humidity detection at room temperature.The thick-films were heat-treated at 800, 900 and 1000 °C for 1 h and all the compositions proved to be effective in humidity sensing and presented a good reproducibility between several measurements. However, the best results were obtained with LSFC10 fired at 800 °C which showed a detection limit of 15% relative humidity and a maximum sensor response of about 87%, higher than the previous results. Copper addition to lanthanum strontium ferrites proved to be effective in lowering the sensors’ detection limit.     

Structural,magnetic, electrical and dielectric properties of MnxNi1−xFe2O4 spinel nanoferrites prepared by PEG assisted hydrothermal method

Mn_xNi_1?xFe₂O₄ (x=0.2, 0.4, 0.6) nanoparticles were synthesized by a polyethylene glycol (PEG)-assisted hydrothermal route. We present a systematic investigation on the structural, magnetic, electrical and dielectric properties of the products by using XRD, FT-IR, SEM, TGA, VSM and dielectric spectroscopy, respectively. Single phased cubic spinel structure was confirmed for all samples and the average crystallite size of the products was estimated using Line profile fitting and ranges between 6.5 and 11 nm. The nanoparticles have ferromagnetic nature with small coercivity. The samples showed semiconducting behavior which is revealed from temperature dependent conductivity measurements. Temperature and frequency dependent dielectric property; dielectric permittivity (ε) and ac conductivity (σ_AC) studies for the samples indicated that the dielectric dispersion curve for all samples showed usual dielectric dispersion confirming the thermally activated relaxation typical for Debye-like relaxation referring to it as the Maxwell–Wagner relaxation for the interfacial polarization of homogeneous double structure. The particle size, saturation magnetization, coercive field, conductivity and dielectric constant of the samples are strictly temperature dependent and increased with Mn concentration.     

Synthesis and electrical properties of Ce1−xGdxO2−x/2 (x = 0.05–0.3) solid solutions prepared by a citrate–nitrate combustion method

Ce_1?xGd_xO_2?x/2 (GDC) powders with different Gd³⁺ contents (x = 0.05–0.3) were prepared by a simple citrate–nitrate combustion method. The influence of the Gd³⁺ doping content on the crystal structure and the electrical properties of GDC were examined. Many analysis techniques such as thermal analysis, X-ray diffraction, nitrogen adsorption analysis, scanning electron microscopy and AC impedance analysis were employed to characterize the GDC powders. The crystallization of the GDC solid solution occurred below 350 °C. The GDC powders calcined at 800 °C showed a typical cubic fluorite structure. The lattice parameter of GDC exhibited a linear relationship with the Gd³⁺ content. As compared with that sintered at other temperatures, the GDC pellet that sintered at 1300 °C had a high relative density of 97%, and showed finer microstructure. The conductivity of GDC was firstly increased and then decreased with the increase of the Gd³⁺ content. The sintered GDC sample with the Gd³⁺ content of 0.25 exhibited the highest conductivity of 1.27 × 10^?2 S cm^?1 at 600 °C.     

Formation of sol–gel derived (Cu,Mn,Co)3O4 spinel and its electrical properties

An (Mn,Co)₃O₄ spinel layer effectively suppresses Cr outward diffusion, but the occurrence of spallation between coating and metal interfaces and the increasing oxidation rate over time significantly deteriorate cell performance. A transition metal cation, particularly Cu, is added into the (Mn,Co)₃O₄ to improve the long-term stability of the interconnect materials for low-to-intermediate-temperature solid oxide fuel cells (SOFCs). In this work, fine-crystalline (Cu,Mn,Co)₃O₄ spinel powders with an average crystallite size of 21 nm were successfully synthesized via the citric acid–nitrate method. The potential for use as a coating material for low-to-intermediate-temperature SOFC interconnects was explored. According to the TG curves, IR spectra, and XRD patterns, 800 °C is recommended as the minimum calcination temperature for (Cu,Mn,Co)₃O₄ spinel materials. Compared with (Mn,Co)₃O₄ spinel materials, the addition of Cu does not induce significant changes in the crystal structure but markedly improves the electrical conductivity (116 Scm⁻¹) and the activation energy (0.394 eV) of the (Cu,Mn,Co)₃O₄ spinel. Overall, the sol–gel derived (Cu,Mn,Co)₃O₄ spinel can be used as a coating material for low-to-intermediate-temperature SOFC interconnects and is superior to (Mn,Co)₃O₄ spinel because high electrical conductivity is preferred to minimize ohmic losses between the electrodes of adjacent cells.