Effect of mechanical milling on structural,dielectric and magnetic properties of BaTiO3–Ni0.5Co0.5Fe2O4 multiferroic nanocomposites

The coexistence of ferroelectricity and ferromagnetism has triggered great interest in multiferroic materials. Multiferroic with strong room temperature magnetoelectric (ME) coupling can provide a platform for future technologies. In this paper, we have investigated the effect of mechanical milling on the properties of multiferroic nanocomposites synthesized by mixing barium titanate (BaTiO₃) (BT) and nickel cobalt ferrite (Ni_0.5Co_0.5Fe₂O₄) (NCF). This process has resulted into reliable disposal of a given quantity of NCF nanoparticles in BT grid and composite samples of different particle sizes (<500 nm) have been obtained by varying the duration of ball-milling for 12, 24, and 48 h. The presence of NCF within BT powder has been confirmed by X-ray Diffraction (XRD) and magnetization measurements (MH). Structural analysis was performed by using Reitveld refinement method that shows that the tetragonality of BaTiO₃ structure get reduced in submicron range. Variations in ferroelectric and dielectric properties with reduction in particle size/milling duration have been studied by P-E loop tracer and Impedance analyzer. The dielectric constant value of 400 has been observed for BT-NCF0 that increases to 9.7 K for composite sample ball mill at 48 h whereas remnant polarization increases to 4.2 μC/cm². These composites with high dielectric constant that changes with temperature and particles size find application in energy storage devices, sensor and memory devices.     

Microstructural,magnetic and electric properties of sol-gel synthesized Na0.5Bi0.5TiO3–CoFe2O4 composites

In this work, we presented investigations on the microstructural, magnetic, dielectric and conductive properties of (1-x)Na_0.5Bi_0.5TiO₃-xCoFe₂O₄ (NBT-CFO) composite ceramics (x = 0.1, 0.2, 0.3 and 0.5). The composites were synthesized using the in-situ sol-gel method, and the resulting CFO/NBT embedded structure was observed in the 0.9NBT–0.1CFO sample. Rietveld refinement confirmed that the embedded structure had remarkable effects on the c/a ratios of NBT-CFO composites. Increasing normalization saturation magnetizations were observed with increasing of CFO contents, thus revealing the influence of phase interfaces on magnetism. Dielectric relaxation anomalies were observed on both temperature dependent permittivity and loss spectra. Two sets of anomalies in these spectra were caused by space charges on the phase interfaces of embedded CFO grains and the NBT matrix. The conduction behavior of ceramics with and without the embedded structure obeyed the double power law and Jonscher's fraction power law, respectively, which inferred a transformation from the AC to the DC conduction mechanism with the disappearance of the embedded structure.     

Effect of microstructure on oxygen permeation of Ba0.5Sr0.5Co0.8Fe0.2O3−δ and SrCo0.8Fe0.2O3−δ membranes

The effect of grain size on oxygen permeation properties of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) and SrCo_0.8Fe_0.2O_3?δ (SCF) membranes was investigated by variation of the dwell time. The membrane microstructure was examined by field-emission scanning microscopy (FE-SEM) and then evaluated using a statistical approach. With longer dwell times the grain growth was stimulated and leaded to grains with a narrower size distribution. The grains of SCF (average size from 11.3 to 19.9 μm) were found to be smaller than those of BSCF (average size from 13.9 to 41.3 μm). The oxygen permeation flux of BSCF membranes was found to be independent of grain size in the range from 24 to 42 μm. However, membranes with smaller grains (13.9 μm) show a decreased oxygen permeation flux. For the SCF membranes a decrease in permeation flux with larger grains was observed for average grain sizes between 11.3 and 19.9 μm. By transmission electron microscopy (TEM) formation of an oxygen ordered SrCo_0.8Fe_0.2O_2.5 brownmillerite by-phase could be observed at the oxygen-depleted sweep side of the membrane.     

Structure,dielectric and ferroelectric properties of 0.92Na0.5Bi0.5TiO3–0.06BaTiO3–0.02K0.5Na0.5NbO3 lead-free ceramics: Effect of Co2O3 additive

0.92Na_0.5Bi_0.5TiO₃–0.06BaTiO₃–0.02K_0.5Na_0.5NbO₃+x wt% Co₂O₃ (NBKT–xCo, x=0, 0.2, 0.4, 0.6, 0.8) lead-free ferroelectric ceramics were prepared via a conventional solid state reaction method. Effects of Co₂O₃ additive on crystallite structure, microstructure, dielectric and ferroelectric properties of the NBKT–xCo ceramics were studied. X-ray diffraction results showed that the rhombohedral–tetragonal morphotropic phase boundary existed in all the ceramics, with relative amount of tetragonal phase varying with the content of Co₂O₃. Average grain size, maximum value of dielectric constant, Curie temperature and ferroelectric properties of the ceramics were close related to the content of Co₂O₃. The dielectric anomaly caused by the phase transition between the ferroelectric phase and the so-called “intermediate phase” was observed in the ceramics with x≤0.2, while it disappeared with further increasing x. All the ceramics showed a diffuse phase transition between the “intermediate phase” and the paraelectric phase. The change in the ferroelectric properties with changing the content of Co₂O₃ was discussed by considering the competitive effects among grain size, relative amount of the tetragonal phase and oxygen vacancies.     

A case study of mechanical properties of perovskite-structured Ba0.5Sr0.5Co0.8Fe0.2O3−δ oxygen transport membrane

This study has investigated mechanical properties of perovskite-structured Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) oxygen transport membrane. The Young’s modulus and fracture toughness are determined by both macroscopic-scale and microscopic-scale methods. Both three-point and ring-on-ring bending tests as macroscopic-scale methods produce broadly similar results with a Young’s modulus, which is lower than that measured from micro-indentation method under a 10 N load. Young’s modulus and fracture toughness of BSCF show strongly dependent of the porosity. However, the fracture toughness of BSCF is independent of grain size. The fracture toughness determined by macroscopic-scale method is similar with that measured by microscopic-scale method. The crack shape of BSCF under a 10 N load is determined to be a median-radial mode. The intrinsic Young’s modulus and fracture toughness are determined to be 105.6 GPa and 1.49 MPa m^0.5, respectively, according the Minimum Solid Area (MSA) model. Annealing decreases the fracture toughness of BSCF between RT and 800 °C.     

Ammonia oxidation in Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane reactor

Selective oxidation of ammonia to NO was studied in a dense mixed ion electron conducting Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ membrane reactor, which integrates the separation and catalytic reaction process in a single reactive separation unit. The influence of the temperature and feed concentration on the membrane reaction performance were investigated in detail. Under reaction conditions, the oxygen permeation flux through the dense membrane increases with increasing temperature and ammonia flow rate. The lower temperature and ammonia concentration can favor the formation of NO, in which higher catalytic performance is obtained, suggesting that the membrane reactor operation is much beneficial for selective oxidation of ammonia.     

Enhancing the dielectric and energy storage properties of lead-free Na0.5Bi0.5TiO3–BaTiO3 ceramics by adding K0.5Na0.5NbO3 ferroelectric

A novel strategy of enhancing the dielectric and energy storage properties of Na_0.5Bi_0.5TiO₃–BaTiO₃ (NBT–BT) ceramics by introducing a K_0.5Na_0.5NbO₃ (KNN) ferroelectric phase is proposed herein, and its underlying mechanism is elucidated. The lead-free KNN ceramic decreases the residual polarisation and increases the electric breakdown strength of the NBT–BT matrix through the simultaneous modification of its A-sites and B-sites. The obtained NBT?BT?x?KNN ceramics have a perovskite structure with unifying grains. A bulk 0.9NBT–BT–0.1KNN ceramic sample with a thickness of 0.2 mm possesses a high energy storage density of 2.81 J/cm³ at an applied electric field of 180 kV/cm. Moreover, it exhibits good insulation properties and undergoes rapid charge and discharge processes. Therefore, the obtained 0.9NBT–BT–0.1KNN ceramic can be potentially used in high-power applications because of its high energy density, good insulation properties, and large discharge rate.     

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.     

ZrxCo0.8−xNi0.2−xFe2O4-graphene nanocomposite for enhanced structural,dielectric and visible light photocatalytic applications

Zirconium doped nickel cobalt ferrite (Zr_xCo_0.8−xNi_0.2−xFe₂O₄) nanoparticles and Zr_xCo_0.8−xNi_0.2−xFe₂O₄-graphene nanocomposites were synthesized by a cheap and facile co-precipitation method. Annealing was done at 750 °C for 6.5 h. Spinel cubic structure of prepared nanoparticles was confirmed by X-ray powder diffraction (XRD) technique. Crystalline size of nanoparticles was observed in the range of 18–27 nm. Graphene was synthesized by Hummer's method. Formation of rGO was confirmed by UV-visible spectroscopy (UV-vis) and XRD. Zr_xCo_0.8−xNi_0.2−xFe₂O₄-graphene nanocomposites were prepared by ultra-sonication route. Grain size of nanoparticles and dispersion of nanoparticles between rGO layers was determined by Scanning electron microscopy (SEM). In application studies of nanoparticles and their nanocomposites, photocatalytic efficiency of nanoparticles under visible light irradiation was observed by degradation of methylene blue. Charge transfer resistance was measured by electrochemical impedance spectroscopy (EIS) and the variation in dc electrical resistivity was analyzed by room temperature current voltage characteristics (I-V). Dielectric constant was also evaluated in frequency range from 1 MHz to 3 GHz. All these investigations confirmed the possible utilization of these materials for a variety of applications such as visible light photocatalysis, high frequency devices fabrication etc.     

Enhancement of magnetic and dielectric properties of KNbO3–CoFe2O4 multiferroic composites via thermal treatment

In this work, multiferroic composites were produced from CoFe₂O₄ and KNbO₃ mixtures via control of the heat treatment temperature. For this, CoFe₂O₄ nanoparticles were produced by sol-gel method, while KNbO₃ was synthesized by microwave-assisted hydrothermal synthesis. The powders were homogenized and subjected to heat treatment at 300, 400 and 500 °C for 5 h. The structural, electrical and magnetic properties were characterized. The results of X-ray diffraction indicated that there was no formation of secondary phases with heat treatment. Raman vibrational modes confirmed the presence of KNbO₃ and CoFe₂O₄ in the prepared composites. SEM analysis showed that the composite microstructure consists of smaller ferrite particles arranged on the surface of largest cubic KNbO₃ particles. The improvement of coercivity (H_C = 382.1Oe) and dielectric constant (?’~7860) was observed for the composite thermally treated at 300 °C. The obtained results show the potential application of KN:CFO composites for multifunctional devices.     

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 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.     

Structural,dielectric and ferroelectric properties of Ba1−x(Bi0.5Na0.5)xTiO3 ceramics

Lead free Ba_1?x(Bi_0.5Na_0.5)_xTiO₃ (x=0, 0.02, 0.04, 0.06, 0.08, 0.1) ferroelectric ceramics were synthesized by conventional solid state reaction technique. Sintering was done at 1200 °C for 2 h in air atmosphere. The final products have tetragonal symmetry with decreasing c/a ratio confirmed by X-ray diffraction analysis. The grain size varies between 300 nm to 1000 nm for x=0 to 0.1. With increase in Bi_0.5Na_0.5TiO₃ [BNT] content, the room temperature permittivity decreases whereas the Curie temperature (T_c) increases and its highest value was found to be 155 °C for 10 mol% of BNT addition. The ceramics show stable and low dielectric loss characteristics. The remnant polarization (P_r) and the coercive field (E_c) increases monotonously with increase in BNT content. The highest value of 2P_r (=17 μC/cm²) and 2E_c (=22 Kv/cm) was obtained for x=10 mol% BNT addition.     

Polyaniline (PANI)–Co0.5Mn0.5Fe2O4 nanocomposite: Synthesis,characterization and magnetic properties evaluation

Polyaniline (PANI)/Cobalt-manganese ferrite, (PANI)/Co_0.5Mn_0.5Fe₂O₄, nanocomposite was synthesized by oxidative chemical polymerization of aniline in the presence of ammonium peroxydisulfate (APS). Microwave assisted synthesis method was used for the fabrication of core CoFe₂O₄ nanoparticles. The structural, morphological, thermal and magnetic properties of the nanocomposite were investigated in detail by X-ray diffraction (XRD), fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). The average crystallite size of (PANI)/Co_0.5Mn_0.5Fe₂O₄ nanocomposite by the line profile method was 20±9 nm. The magnetization measurements revealed that (PANI)/Co_0.5Mn_0.5Fe₂O₄ nanocomposite has superparamagnetic behavior with blocking temperature higher than 300 K. The saturation magnetization of the composite is considerably low compared to that of CoFe₂O₄ nanoparticles due to the partial replacement of Co²⁺ ions and surface spin disorder. As temperature decreases, both coercivity and strength of antiferromagnetic interactions increase which results in unsaturated magnetization of the nanocomposite.     

Structure,dielectric, and ferroelectric properties of (1-x)Bi0.5Na0.5TiO3 – X K0.5Na0.5NbO3 (0 ≤ x ≤ 0.1) solid solution

The correlation of the phase structure, dielectric, and ferroelectric properties of lead-free (1-x)(Na_0.5Bi_0.5)TiO₃–xK_0.5Na_0.5NbO₃ (NBTKNx) (0 = x ≤ 0.1) polycrystalline ceramics, fabricated via a solid state reaction technique, were investigated. The Rietveld refinement allowed identifying the crystallographic transformation from a rhombohedral to a coexisting rhombohedral-tetragonal or tetragonal long range-ordered ferroelectric (FE) phase. The dielectric investigations showed an increase of the dielectric diffuseness (1.53 = γ ≤ 1.73) and a clear shift of the depolarization temperature (T_d) to a lower temperature while increasing substitution. More importantly, the lattice disorder also generated a plateau-like dielectric anomaly, leading to a thermally stable ϵ_r ∼2859 ± 20% (120–500 °C) and ∼3112 ± 10% (120–420 °C) for x = 0.075 and 0.1 samples, respectively. At room temperature (RT), Raman spectroscopy investigations revealed a downshift of the frequencies as a function of the composition with an inhomogeneous broadening of the Raman lines. On heating, Raman spectra showed changes in the region where the dielectric transitions are observed. Moreover, the composition dependence of the current peaks in the I-E loops confirmed the occurrence of a phase transition from a non-ergodic polar phase to an ergodic weakly polar after the applying of an electric field of 60 kV/cm⁻¹.     

A novel Na0.5K0.5NbO3–La(Zn0.5Ti0.5)O3 ceramics with excellent dielectric properties at wide temperature range

Ceramic-based dielectrics are considered as the best candidates for high temperature capacitors because of their outstanding mechanical and electrical properties. Nevertheless, conventional barium titanate-based capacitors show narrow operating temperature ranges owing to the low tetragonal-cubic phase transition temperature. In order to increase the working temperature and relative permittivity, a novel (1-x)Na_0.5K_0.5NbO₃- xLa(Zn_0.5Ti_0.5)O₃ (NKN-xLZT) ceramics were chosen to meet the targets in this work. The NKN-xLZT ceramics with sub-micrometer grains (0.2–0.4 μm) were synthesized via a conventional solid-state sintering route. A relative permittivity (ε’ = 1560 ± 15%) with low loss tangent over wide temperature range from 96 °C to 350 °C was obtained in the x = 0.02 ceramics. Additionally, the crystal structure distortion and conduction behaviors of the NKN-xLZT ceramics were systematically studied. The decrease of oxygen octahedron distortion induced a weak polarization, and the high resistance (9 × 10⁶ Ωcm at 400 °C) greatly suppressed the long-term migration of defective ions in the ceramics. Therefore, the low loss tangent and high permittivity were still stabilized at the high temperature. It believes that the NKN-xLZT ceramic system in this work will become one of the most promising candidates for high-temperature capacitor devices.     

A case study of the effect of Ni substitution on the sintering behaviours of Ba0.5Sr0.5Co0.8Fe0.2O3−δ oxygen transport membranes

ABSTRACT

The sintering behaviours of undoped and 8 mol% B-site Ni-doped Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) under various conditions are systematically investigated. Ni doping increases the lattice parameter of BSCF. The grain size of the undoped BSCF increases from 1.52 to 64.8 μm with increasing sintering temperature from 900 at 1150 °C, while the grain size of the Ni-doped counterpart increases from 3.06 to 34.1 μm as the sintering temperature increases from 1000 to 1200 °C. The grain growth kinetics is analysed using the expression Dⁿ = tK ₀exp(-Q/RT). The grain growth exponent n is found to be 3 for the undoped BSCF and 4.7 for the Ni-doped BSCF. The grain growth activation energy (Q) is 650.7±30 kJ/mol for the undoped BSCF and 803.4±37 kJ/mol for the Ni-doped BSCF. This indicates that the Ni dopant has a negative influence on the densification and grain growth of BSCF.     

Thermochemical expansion of mixed-conducting (Ba,Sr)Co0.8Fe0.2O3−δ ceramics

The thermal and chemical expansion of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and SrCo_0.8Fe_0.2O_3−δ (SCF) mixed ionic-electronic conductors were studied in combination with oxygen nonstoichiometry (δ) at 298–1223 K and p(O₂) = 10⁻⁴ to 1.00 atm. In order to minimize the effects of phase separation or oxygen-vacancy ordering processes, the data were collected in dynamic cooling mode using dense ceramic samples. The procedure was justified by a very fast equilibration at given p(O₂) in high-temperature range demonstrated for ceramics samples with different specific surface area. The difference in nonstoichiometry of BSCF and SCF at temperatures ≥973 K was found to be ≤0.03 oxygen atoms per formula unit. BSCF demonstrates favorably smaller chemical expansion compared to SCF and many other mixed conductors, originating from smaller δ variations and larger unit cell less sensitive to temperature and nonstoichiometry changes. Excessive thermochemical expansion impedes however the use of BSCF in single-phase fuel cell cathodes and planar mixed-conducting membranes.     

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.