Bi0.5K0.5TiO3–CaTiO3 ceramics: Appearance of the pseudocubic structure and ferroelectric-relaxor transition characters

In this study, solid solution ceramics of (1−x)Bi_0.5K_0.5TiO₃–xCaTiO₃ (BKT-CT, x = 0, 0.12, 0.15, 0.18, 0.21, and 0.25) were prepared. A phase transition from the tetragonal symmetry to the pseudocubic symmetry is discovered near x = 0.25. The reasons for the appearance of the pseudocubic phase were discussed. The compositions of x ≤ 0.21 show the ferroelectric ordering at room temperature. The remnant polarization (P_r) is 22.4 μC/cm² for the x = 0.15 composition. The temperature dependence of the relative permittivity suggests two dielectric anomalies for x ≤ 0.18. The dielectric anomaly in the low-temperature range is related to a spontaneous transition between the ferroelectric and relaxor states. The temperature (T_F-R) for the transition decreases with the CT addition, falling from 211°C for x = 0.12-134°C for x = 0.18. Only one relaxor-like dielectric anomaly was observed for x ≥ 0.21. The thin double ferroelectric hysteresis loops have been observed during the ferroelectric-relaxor transition process for x ≥ 0.21. The maximum electrostrain (S_m) reaches 0.155% at 100°C for x = 0.21. The low-temperature Raman measurement suggests the intrinsic tetragonal distortions for x = 0.25.     

Structural properties of multiferroic 0.5BiFeO3–0.5Pb(Fe0.5Nb0.5)O3 solid solution

Magnetoelectric multiferroics are very promising materials because of their practical applications and fundamental interests. The most widely studied magnetoelectric oxides are ABO₃ perovskites. In the paper structural properties of BiFeO₃ and Pb(Fe_0.5Nb_0.5)O₃ solid solution are described. The material crystallizes in rhombohedral R3c crystal structure which parameters are presented. Mössbauer spectroscopy was used to study local changes in an iron environment due to Fe/Nb substitution and hyperfine interaction parameters of different local surroundings of iron atoms are presented. The random distribution of B-site sublattice cations was confirmed. Ab initio calculations of the studied solid solution were conducted and theoretical crystal structure parameters were compared with the experimental data. The theoretical magnetic and electric properties are discussed. The local iron magnetic moments were estimated and their dependence on the local surrounding changes is shown. The calculated electrons densities and Bader's topological analysis were used to describe chemical bonding properties.     

Tailoring the electrochemical reduction kinetics of dual-phase BaCe0.5Fe0.5O3-δ cathode via incorporating Mo for IT-SOFCs

The BaCe_0.5Fe_0.5O_3-δ (BCF) cathode consists of the ion-electron mixed conducting phase BaCe_0.15Fe_0.85O_3-δ (BCF1585) and the proton-conducting phase BaCe_0.85Fe_0.15O_3-δ (BCF8515). In this paper, the electrochemical performance is improved by incorporating the high valence element Mo into the BCF and applied to intermediate-temperature solid oxide fuel cells (IT-SOFCs). High-temperature X-ray diffraction (HT-XRD) and O₂-temperature programmed desorption (O₂-TPD) results show that Mo doping enhances the structural stability of BCF. The X-ray photoelectron spectroscopy (XPS) results suggest that the introduction of Mo increases the amount of adsorbed oxygen and thus the oxygen reduction reaction (ORR) catalytic activity. Compared to BCF, the polarization impedance of BaCe_0.5Fe_0.45Mo_0.05O_3-δ (BCFM) at 800 °C is 0.154 Ω·cm², a reduction of 22 %. Meanwhile, the BCFM output power at 800 °C is 778.01 mW·cm⁻², an improvement of 32.17 %, and maintains a stable current density after 250 h at 0.7 V. The results demonstrate that Mo doping is an effective strategy to enhance the electrochemical performance of BCF.     

Destabilization of the ferroelectric order in Na0.5Bi0.5TiO3–6 wt% BaTiO3 ceramics through doping

One of the most promising candidates to replace lead-based compounds in actuator applications are Na_0.5Bi_0.5TiO₃ (NBT)-based materials. K_0.5Na_0.5NbO₃ (KNN)-modified NBT-BaTiO₃ (NBT-BT) solid solutions exhibit giant large-signal strain–electric-field coefficients (S_max/E_max) exceeding 500 pm V^?1. However, despite the promising properties of the ceramics reported in the literature, the synthesis of these materials remains challenging, leaving gaps in the understanding of the synthesis-property relationship. In this contribution, we investigate the microstructure and the electrical properties while changing the composition to destabilize the ferroelectric order in the material, which is the key to achieve large strain response. Measurements of dielectric and ferroelectric properties reveal that Na- or Ti-deficiency or excess of Bi decrease the ferroelectric-to-relaxor transition temperature and remnant polarization, indicating a destabilization of the ferroelectric order. Additionally, the use of KNO₃ instead of K₂CO₃ as the potassium source in KNN results in an additional destabilizing effect on the ferroelectric order, which can be attributed to better incorporation of K⁺ into the perovskite structure. The results identify the key aspects of the synthesis of NBT-BT-KNN ceramics to obtain high S_max/E_max values.     

Influence of calcination temperature on the piezoelectric properties of Ag2O doped 0.94(K0.5Na0.5)NbO3–0.06LiNbO3 ceramics

The effects of calcination temperature on the bulk density, piezoelectric, and ferroelectric properties were investigated for the Ag₂O doped 0.94(K_0.5Na_0.5)NbO₃–0.06LiNbO₃ ceramics. The calcination temperatures were varied from 750 to 950 °C by 50 °C differences. An tetragonal XRD pattern, consistent with single-phase 0.94(K_0.5Na_0.5)NbO₃–0.06LiNbO₃ was obtained after calcination at 850 °C for 2 h. And the experimental results showed that Ag₂O doped 0.94(K_0.5Na_0.5)NbO₃–0.06LiNbO₃ ceramics calcined at 850 °C had a remnant polarization P_r=24.5 μC/cm², bulk density=4.32 g/cm³, piezoelectric constant d₃₃=282 pC/N and electromechanical coefficient k_p=37.8%.     

Relationship between the Bond Valence and the Temperature Coefficient of the Resonant Frequency in the Complex Perovskite (Pb1−xCax)[Fe0.5(Nb1−yTay)0.5]O3

The temperature coefficient of the resonant frequency (TCF) of complex perovskite (Pb_1−xCa_x)[Fe_0.5(Nb_1−yTa_y)_0.5]O₃ ceramics (x= 0.5, 0.55; 0.0 ≤y≤ 1.0) was investigated, relative to the bond valence of the A- and B-site ions in the ABO₃ perovskite structure (such as the barium-, strontium-, and calcium-based complex perovskites). The TCF of these complex perovskite compounds varied with the bond valence of the A- and B-sites and the tolerance factor (t) in the perovskite structure. In the tilted region (t < 1.0), the tilting of the oxygen octahedra increased and the TCF decreased, because of the increased bond valence of the B-site. Also, the dependence of TCF on the bond valence of the A-site was similar to its dependence on t.     

Features of Polymerization of Methyl Methacrylate using a Photocatalyst—the Complex Oxide RbTe1.5W0.5O6

Journal of Inorganic and Organometallic Polymers and Materials - Radical polymerization of methyl methacrylate in an aqueous emulsion was carried out using the complex oxide RbTe1.5W0.5O6 as a...     

Enhanced energy storage properties of BiAlO3 modified Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3 lead-free antiferroelectric ceramics

Lead-free (1−x)(0.75Bi_0.5Na_0.5TiO₃–0.25Bi_0.5K_0.5TiO₃)–xBiAlO₃ (BNT–BKT–100xBA, x=0–0.10) ceramics were prepared by two-step sintering method and their phase structure, micro morphology and electrical properties were systematically investigated. X-ray diffraction analysis indicates a pure perovskite phase for x≤0.06 as well as a structural evolution from a tetragonal toward a pseudocubic phase. Transmission electron microscopy study of the x=0.04 composition reveals the existence of antiferroelectric phase with a⁰a⁰c⁺ oxygen octahedron tilting which is in the form of nano-domains. Polarization-electric field and current-electric field hysteresis loops demonstrate that the increase of BA concentration destroys the ferroelectric order and strengthens antiferroelectric order. A much enhanced energy storage density of 1.15 J/cm³ and efficiency of 73.2% is achieved under 105 kV/cm at x=0.06. In addition, its energy storage property is found to depend weakly on temperature within the measurement range of 25–150 °C.     

Effect of Fe doping on the performance of suspension plasma-sprayed PrBa0.5Sr0.5Co2−xFexO5+δ cathodes for intermediate-temperature solid oxide fuel cells

PrBa_0.5Sr_0.5Co_2–xFe_xO_5+δ (PBSCF) (X = 0, 0.3, 0.4, and 0.5) is investigated as cathode material for intermediate-temperature solid oxide fuel cells. Suspension plasma spraying is used as a low cost and large-scale manufacturing process to prepare PBSCF cathodes. Fe substitution effects on the crystal structure and electrochemical performance are characterized. All plasma-sprayed PBSCF cathodes exhibit a pure and stable cubic structure. The suspension plasma-sprayed PBSCF deposits show a porous and fine structure, and the microstructures are insensitive to Fe substitution. Subsequent to Fe doping, the polarization resistance of PBSCF cathodes rapidly decreases for increasing Fe substitution concentration from 0 to 0.4. Further increase of the Fe doping concentration increases the cathode polarization instead. At 600 and 700 °C, a 20% Fe-doped (x = 0.4) PBSCF cathode exhibits remarkably low area-specific polarization resistances (R_p) of 0.074 Ω cm² and 0.012 Ω cm², respectively. Moreover, the R_p of all cathodes remains almost identical after isothermal annealing at 600 °C for 300 h. Furthermore, the thermally-sprayed porous metal-supported cell assembled with the optimal PBSCF cathode shows excellent performance with peak power densities of 0.37, 0.8, and 1.35 W cm⁻² at 500, 600, and 700 °C, respectively.     

Effect of annealing temperature on the multiferroic properties of 0.7BiFeO3–0.3Bi0.5Na0.5TiO3 solid solution prepared by sol–gel method

Room-temperature multiferroic 0.7BiFeO₃–0.3Bi_0.5Na_0.5TiO₃ solid solution ceramics have been prepared by the sol–gel method. We have discussed the annealing temperature dependence of the multiferroic properties. The samples are annealed at 1023, 1123, 1223 and 1323 K for 3 h, respectively. X-ray diffraction patterns identify that all samples are pure. Scanning electron micrographs present the increasing grain size with higher annealing temperature. Magnetic, ferroelectric and dielectric properties are enhanced obviously with the increase in annealing temperature. The coexistence of ferroelectric and ferromagnetic properties is also proved at room temperature. In addition, it reveals that the optimal annealing temperature accompanied with favorable multiferroic properties of 0.7BiFeO₃–0.3Bi_0.5Na_0.5TiO₃ solid solution ceramics is near 1223 K.     

Understanding the microwave dielectric properties of (Pb0.45Ca0.55)[Fe0.5(Nb1−xTax)0.5]O3 ceramics via the bond valence

Microwave dielectric properties of the (Pb_0.45Ca_0.55)[Fe_0.5(Nb_1−xTa_x)_0.5]O₃ ceramics were investigated in the range of 0.0⩽x(mol)⩽1.0. With increasing Ta₂O₅, the dielectric constant and the temperature coefficient of the resonant frequency (TCF) decreased, while the Q·f value of the specimens increased. The change of dielectric constant was evaluated by the correlation between ionic polarizability and bond valence. TCF of the specimens decreased with the tilting of oxygen octahedra due to an increase of bond valence of the B-site. The Q·f values were studied by infrared reflectivity spectra from 50 to 4000 cm⁻¹, and calculated by the Kramers-Kronig analysis and classical oscillator model. The specimens with 0.4 mol of Ta₂O₅ sintered at 1150 °C for 3 h showed ε_r of 82, Q·f of 7650, and TCF of −5 ppm/°C.     

The impact of the discovery of the polymerization of the α-olefins on the development of the stereospecific polymerization of vinyl monomers

After a short survey of the first indications on the existence of stereoregularity in the main chains of the vinyl polymers, the first syntheses of stereoregular poly(vinylethers) and polystyrene are briefly summarized. The difficulties encountered in the early investigations of the propylene polymerization by the CoO/MoO₃/Al₂O₃ and CrO₃/SiO₂/Al₂O₃ catalysts are considered and the isolation and identification of a crystalline component in the mixture of diastereomeric macromolecules thus obtained, are discussed. The results obtained by different research groups, independently investigating the stereospecific polymerization of vinyl monomers, are compared and finally an attempt is made to explore the origin of the very rapid progress made in this field by Natta and coworkers in connection with their discovery of the stereospecific polymerization of α-olefins with the ‘Ziegler-Natta catalysts’.     

Flocculation of heterodisperse suspensions of fine coal (−0.5 mm)

Coal particles (−0.5 mm) were flocculated with fine magnetite by polyacrylamide-based polymers. The magnetic flocs obtained were retained in a magnetic field and their stability studied under different flow rates of water. Flocs formed by coarser particles were more easily broken. It is postulated that this is due to breakage of polymer bridges between particles.     

The role of the symmetry and the flexibility of the anion on the characteristics of the nanostructures and the viscosities of ionic liquids☆

The transport properties of ionic liquids (ILs) are crucial properties in view of their applications in electrochem-ical devices. One of the most important advantages of ILs is that their chemical–physical properties and conse-quently their bulk performances can be well tuned by optimizing the chemical structures of their ions. This will require elucidating the structural features of the ions that fundamentally determine the characteristics of the nanostructures and the viscosities of ILs. Here we showed for the first time that the“rigidity”, the order, and the compactness of the three-dimensional ionic networks generated by the anions and the cation head groups determine the formation and the sizes of the nanostructures in the apolar domains of ILs. We also found that the properties of ionic networks are governed by the conformational flexibility and the symmetry of the anion and/or the cation head group. The thermal stability of the nanostructures of ILs was shown to be con-trolled by the sensitivity of the conformational equilibrium of the anion to the change of temperature. We showed that the viscosity of ILs is strongly related to the symmetry and the flexibility of the constitute ions rather than to the size of the nanostructures of ILs. Therefore, the characteristics of the nanostructures and the viscosities of ILs, especially the thermal stability of the nanostructures, can be fine-tuned by tailoring the symmetry and the conformational flexibility of the anion.     

Crystal structure,microstructure and electrical properties of (1−x−y)Bi0.5Na0.5TiO3–xBi0.5K0.5TiO3–yBiFeO3 ceramics near MPB prepared via the combustion technique

(1?x?y)Bi_0.5Na_0.5TiO₃–xBi_0.5K_0.5TiO₃–yBiFeO₃ (BNKFT-x/y with 0.12≤x≤0.24, 0≤y≤0.07) lead-free piezoelectric ceramics have been prepared by the combustion technique. The effects of amounts of x and y on structures and electrical properties were examined. Powders and ceramics can be well calcined and sintered at 750 °C for 2 h and 1025–1050 °C, respectively. The results indicated that the crystalline structure and microstructure changed with the increase of x and y concentrations. XRD results of BNKFT-x/0.03 and BNKFT-0.18/y ceramics with 0.12≤x≤0.24 and 0≤y≤0.07 showed the rhombohedral–tetragonal morphotropic phase boundary (MPB). The addition of y caused a promoted grain growth while the addition of x suppressed the grain growth. The highest density (ρ=5.85 g/cm³), superior dielectric properties at T_c (ε_r=7846 and tan δ=0.02), remnant polarization measured at 40 kV/cm (P_r = 20.1 μC/cm²) and piezoelectric coefficient (d₃₃=213 pC/N) were obtained for x=0.18 and y=0.03.     

Dynamical behavior of step-like transition of La0.5Sr0.5Mn1−xTixO3 in a widened field sweep rate

The magnetization behavior of Ti⁴⁺ doped perovskite manganites La_0.5Sr_0.5Mn_1−xTi_xO₃ (x=0.15, 0.175 and 0.2) was investigated. Experimental results show that Ti⁴⁺ dopant suppresses the antiferromagnetic charge ordering and leads to a step-like magnetization behavior below 3 K. The step-like transitions and the critical magnetic fields are strongly dependent on the Ti doping level, magnetic field sweep rate and temperature. Above 3 K, the step-like transitions transform to broad ones. In addition, under pulsed high magnetic fields with an ultrafast field sweep rate of ~10³ T/s, the sharp step-like transitions observed in the static magnetization measurements become smooth metamagnetic transition at low temperatures. This feature is correlated with the collapse of the balance between the magnetic energy and elastic energy in the phase separation system within a martensitic-like scenario.     

Analysis of the rhombohedral–tetragonal symmetries coexistence in lead-free 0.94(Bi0.5Na0.5)TiO3–0.06BaTiO3 ceramics from nanopowders

Ceramics with perovskite-type structure and 0.94(Bi_0.5Na_0.5)TiO₃–0.06BaTiO₃ (BNBT) composition have been studied by conventional powder X-ray diffraction in Bragg–Brentano geometry. Ceramics were obtained from sol–gel autocombustion nanopowders and processed either by hot pressing and subsequent recrystallisation or pressureless sintering in two steps. These methods provided single-phase, sub-micron grain size (<700?nm), dense ceramics with good piezoelectric performance (96–94% of theoretical density and d₃₃?=?143–124?pC?N^–1, respectively). For the considered ceramics, the splitting of the peaks of the cubic perovskite-type structure with 111 and 200 Miller indices has been repeatedly used as a symmetry identification criterion. In this work a simple, yet powerful, procedure to validate the consistency of the mentioned splitting interpretation is presented. Based on peaks fitting and well-known crystallographic expressions, the rhombohedral and tetragonal symmetries' coexistence is verified. The suggested procedure can be applied to the study of peak splitting in ceramics at Morphotropic Phase Boundaries in a general way. In a given series of BNBT ceramics, inconsistencies for interplanar distances, intensities' ratios and the evolution of these from not-poled to poled samples have been found. In poled ceramics, special care has been taken when carrying out this analysis, due to the anisotropic strains arising from ferroelectric (FE) domain orientation. Poling gives rise to a displacement of the peaks angular positions and modification of the intensity ratios. However, the interplanar distance changes associated with the angular deviations here observed are one order of magnitude higher than those expected from anisotropic strains. These results set up a doubt on the sufficiency of the [rhombohedral?+?tetragonal] model to characterise the considered ceramics. A model of a mesoscopic FE phase with rhombohedral symmetry, a mesoscopic and globally weakly polar phase, with cubic symmetry, and a nanosised phase, also cubic, is presented as a plausible alternative.     

The effect of morphotropic phase boundary on dielectric properties of Bi0.5Na0.5Ti1−xZrxO3 solid solutions

Lead-free bismuth sodium titanate zirconate (Bi_0.5Na_0.5Ti_1?xZr_xO₃ or BNTZ) solid solutions with varied composition of x=0.50, 0.55, 0.58, 0.60, 0.63, 0.65, 0.68, 0.70, 0.73, 0.75 and 0.78 mol fraction were obtained using a conventional mixed-oxide method. XRD analysis indicated that the increase in concentration of Zr led to compositions across morphotropic phase boundary region. A quantitative structural investigation was carried out using the X-ray powder diffraction data. The rhombohedral phase was found to dominate for x<0.68 with space group R3c. In the morphotropic phase boundary (MPB) region i.e. 0.68≤x≤0.75, it was demonstrated that coexistence of rhombohedral and orthorhombic phase was observed. For x=0.78, the phase was completely orthorhombic with space group Pmna. Furthermore, the dielectric properties showed some enhanced activity of dipole movement at MPB boundaries which supported the presence of MPB region in this material system.     

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.