Dielectric and magnetic properties of multiferroic BiFeO3 ceramics sintered with the powders prepared by hydrothermal method

BiFeO₃ ceramics were sintered in the temperature range of 700–900 °C by using the pure BiFeO₃ powders hydrothermally synthesized at 250 °C. The low reaction temperature and low sintering temperature prevent the element volatilization and phase decomposition. The ceramics sintered at 800 and 850 °C exhibit much dense microstructure with clear grains and grain boundaries. They also show high dielectric constant, dielectric dispersion and low loss tangent. At room temperature, the dielectric behaviors of BiFeO₃ ceramics are mainly attributed to the transition of localized charge carriers and the microstructure of grains and grain boundaries. The temperature dependence of dielectric constant and loss tangent confirms that the localized charge carriers are a main contribution to the dielectric permittivity. Activation energy E_α of relaxation process for the BiFeO₃ ceramic sintered at 850 °C is 0.397 eV. The obtained BiFeO₃ ceramics show magnetic responses, which are relative to the grain size.     

Size-controlled synthesis of BiFeO<Subscript>3</Subscript> nanoparticles by a soft-chemistry route

In this work we report the size-controlled synthesis of BiFeO₃ nanoparticles via a soft-chemistry route. In this route, the aqueous solution of inorganic Bi and Fe salt is gelled by using acrylamide and bisacrylamide. It is demonstrated that the grain size of resulted BiFeO₃ powders can be tailored by varying the ratio of acrylamide to bisacrylamide. With increase in the bisacrylamide content, the grain size decreases monotonously. By using this method, a series of BiFeO₃ samples with average grain size ranging from 110 to 52 nm have been prepared. The thermal decomposition process of precursor xerogels and the formation of BiFeO₃ phase are investigated by means of X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry analysis, and fourier transform infrared spectroscopy (FTIR). SEM observations reveal that the prepared BiFeO₃ nanoparticles are nearly spherical in shape with a narrow diameter distribution. Magnetic hysteresis loop measurement shows that the BiFeO₃ nanoparticles exhibit weak ferromagnetic behavior at room temperature, and a saturation magnetization of ~1.56 emu/g is achieved for the 52 nm sample.     

Preparation,dielectric and ferroelectric properties of (Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>)<Subscript>0.94</Subscript>Ba<Subscript>0.06</Subscript>TiO<Subscript>3</Subscript> thin films by a sol–gel process

(BiFeO₃)_1−x –(BaTiO₃) _x solid solution thin films are grown on Pt/Ti/SiO₂/Si substrates by chemical solution deposition method. Films with x = 0.00, 0.05 and 0.10 were prepared by annealing at 500°C. X-ray diffraction patterns indicate that the pure BiFeO₃ film adopts random orientation while the solid solution films are highly (100) preferentially oriented. Improved electric property at room temperature was observed in the BaTiO₃ incorporated BiFeO₃ films. The remanent polarization of the film with x = 0.0, 0.05 and 0.10 are 76.6, 51.9 and 19.7 μC/cm² respectively under a measuring electric field of 0.94 MV/cm. The BaTiO₃ incorporated BiFeO₃ films show improved fatigue endurance. By the solid solution with BaTiO₃, the leakage current density is reduced effectively.     

Preparation of nanocrystalline BiFeO<Subscript>3</Subscript> via a simple and novel method and its kinetics of crystallization

The precursor of nanocrystalline BiFeO₃ was obtained by solid-state reaction at low heat using Bi(NO₃)₃·5H₂O, FeSO₄·7H₂O, and Na₂CO₃·10H₂O as raw materials. The nanocrystalline BiFeO₃ was obtained by calcining the precursor. The precursor and its calcined products were characterized by differential scanning calorimetry (DSC), Fourier transform-infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). The data showed that highly crystallization BiFeO₃ with rhombohedral structure (space group R3c (161)) was obtained when the precursor was calcined at 873 K for 2 h. The thermal process of the precursor experienced three steps, which involve the dehydration of adsorption water, hydroxide, and decomposition of carbonates at first, and then crystallization of BiFeO₃, and at last decomposition of BiFeO₃ and formation of orthorhombic Bi₂Fe₄O₉. The mechanism and kinetics of the crystallization process of BiFeO₃ were studied using DSC and XRD techniques, the results show that activation energy of the crystallization process of BiFeO₃ is 126.49 kJ mol⁻¹, and the mechanism of crystallization process of BiFeO₃ is the random nucleation and growth of nuclei reaction.     

Preparation and functional characterization of BiFeO3 ceramics: A comparative study of the dielectric properties

In the present study, the electrical properties of BiFeO₃ ceramic specimens prepared by solid-state sintering method by using two thermal treatment strategies are comparatively investigated. The room temperature XRD pattern shows perovskite single-phase, in the limit of XRD accuracy, for BiFeO₃ ceramic prepared by single-step method. For two-step sintering method sample small amounts of secondary Bi₂Fe₄O₉ phases were identified. The ceramics show a non-homogeneous microstructure, consisting of ceramic grains with irregular morphology and interconnected porosity mainly in the grain boundary regions in the case of two-step sintering sample. The most interesting feature is the conduction anomaly observed on the conductivity in the low-frequency range close to dc-conductivity. The Arrhenius plot of the dc-conductivity determined at the lowest frequency vs. 1/T shows two distinct linear regions separated by the mentioned temperature range of (189–244) K, for which the dc conductivity could not be determined from the present impedance spectroscopy data only. It is clear that in the mentioned temperature range (for both samples), a conduction anomaly takes place.     

Preparation and characterization of pure single-phase BiFeO3 nanoparticles through thermal decomposition of the heteronuclear Bi[Fe(CN)6]·5H2O complex

The heteronuclear Bi[Fe(CN)₆]·5H₂O complex was synthesized and single-phase perovskite-type BiFeO₃ nanoparticles with an average size of 30 nm were obtained by its decomposition at 600 °C. The complex and its decomposition products were analyzed using elemental analysis, thermal analysis (TGA/DTA/DSC), X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FT-IR), UV–Vis spectroscopy, BET specific surface area measurement, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and magnetic measurements. The magnetic measurement confirms that the product shows a ferromagnetic order at room temperature, which may be ascribed to the size confinement effect. The DTA and DSC results confirm the multiferroic nature of the BiFeO₃ nanoparticles with Neel and Curie points at 372 and 825 °C, respectively. The BiFeO₃ prepared by this method could be an appropriate visible-light photocatalytic material due to its strong absorption band in the visible region. This method is simple, low-cost, safe and also suitable for industrial production of high purity perovskite-type BiFeO₃ nanoparticles for electromagnetic applications.     

Structure and magnetic properties of potassium doped bismuth ferrite

The influence of the potassium (K⁺) doping on the structure of multiferroic BiFeO₃ and its relation with ferroelectric and magnetic properties was investigated for perovskites with composition Bi_1−xK_xFeO₃ in the range 0?x?0.07. All the studied samples are described in R3c space group (No. 161). Typical cell parameters (BiFeO₃) in hexagonal setting are a_hex=5.5769(2) Å and c_hex=13.8531(2) Å with Z=6 formula units. The structure determination shows that as the K⁺ content increases, the average cations displacements decrease reducing the polar character of doped samples with respect to pure BiFeO₃ and leading to a change from rhombohedral to a pseudo-cubic symmetry. A structural disorder is related to the substitution of K⁺, which results in strong diffuse scattering (DS) located at the bottom of the Bragg peaks. Magnetic measurements reveal that all the compounds remain antiferromagnetic at room temperature (RT) with almost no change in the transition temperature (Néel temperature T_N).     

Phase transitions, electrical conductivity and chemical stability of BiFeO3 at high temperatures

The multiferroic perovskite BiFeO₃ is reported to display two first order structural phase transitions. The structural phase transition at 925±5 °C is demonstrated to be first order by calorimetry and dilatometry. Electrical conductivity measurements revealed that the high temperature phase above 925±5 °C is semiconducting, in disagreement with recent reports. The sign and magnitude of the volumes of transition reflect the sign and magnitude of the discontinuities in electrical conductivity across the two first order phase transitions. A high partial pressure of oxygen was demonstrated to stabilise BiFeO₃ towards peritectic decomposition. Finally, the origins of the commonly observed decomposition of BiFeO₃ at high temperatures are discussed.     

Synthesis of nanosized bismuth ferrite (BiFeO3) by a combustion method starting from Fe(NO3)3·9H2O-Bi(NO3)3·9H2O-glycine or urea systems

Two bismuth ferrite potential precursors systems, namely Fe(NO₃)₃·9H₂O-Bi(NO₃)₃·9H₂O-glycine/urea with different metal nitrate/organic compound molar ratios have been investigated in order to evaluate their suitability as BiFeO₃ precursors. The presence into the precursor of both reducing (glycine and urea) and oxidizing (NO₃⁻) components, modifies dramatically their thermal behaviour comparative with the raw materials, both from the decomposition stoichiometries and temperature occurrence intervals points of view. Also, the thermal behaviour is dependent on the fuel nature but practically independent with the fuel content. The fuel nature influences also some characteristics of the resulted oxides (phase composition, morphologies). In the case of the oxides prepared using urea as fuel, a faster evolution toward a single phase composition with the temperature rise is evidenced, the formation of the BiFeO₃ perovskite phase being completed in the temperature range of 500–550°C.     

Magnetoelectric perovskite (Bi0.5Pb0.5)(Fe0.5Zr0.5)O3: Preparation,structural and magnetic properties

The perovskite (Bi_0.5Pb_0.5)(Fe_0.5Zr_0.5)O₃ was synthesized by solid-state reaction in an attempt to find magnetoelectric materials, in which ferroelectricity and ferromagnetism coexist. This complex perovskite has been studied by X-ray and neutron powder diffraction in combination with magnetic measurements. The compound crystallizes in the orthorhombic space group Pbam with a ～ √2a_p, b ～ 2√2a_p and c ～ 2a_p (with a_p ～ 4.057 Å). The field and temperature dependence of the magnetization combined with neutron diffraction data showed antiferromagnetic behavior with the Neel temperature, T_N ～ 450 K. Rietveld refinements of neutron powder diffraction data collected at different temperatures, between 10 and 700 K, have been carried out in order to extract information about the thermal evolution of the nuclear and magnetic structures. A distorted orthorhombic perovskite structure was found within the whole temperature interval. The Bi/Pb and Fe/Zr ions were found to be partially ordered over the perovskite A-site and disordered over the B-site. The neutron diffraction patterns of the (Bi_0.5Pb_0.5)(Fe_0.5Zr_0.5)O₃ sample showed evidence of a long-range magnetic ordering below T_N with a propagation vector k = (0,0,0) and an antiferromagnetic arrangement of the magnetic moments of the Fe³⁺ cations in the B-site. This is consistent with an A_y-type magnetic structure. The factors governing the structural and magnetic properties of (1 ? x)BiFeO₃–xPbZrO₃ solid solutions are discussed and compared with those of pure BiFeO₃ and PbZrO₃. A solid solution strategy for developing magnetoelectric properties in BiFeO₃-based compounds is described, with the aim of realizing both a spontaneous polarization and magnetization at room temperature.     

BiFeO<Subscript>3</Subscript>-coated spinel LiNi<Subscript>0.5</Subscript>Mn<Subscript>1.5</Subscript>O<Subscript>4</Subscript> with improved electrochemical performance as cathode materials for lithium-ion batteries

Spinel LiNi_0.5Mn_1.5O₄ cathode material is a promising candidate for next-generation rechargeable lithium-ion batteries. In this work, BiFeO₃-coated LiNi_0.5Mn_1.5O₄ materials were prepared via a wet chemical method and the structure, morphology, and electrochemical performance of the materials were studied. The coating of BiFeO₃ has no significant impact on the crystal structure of LiNi_0.5Mn_1.5O₄. All BiFeO₃-coated LiNi_0.5Mn_1.5O₄ materials exhibit cubic spinel structure with space group of Fd3m. Thin BiFeO₃ layers were successfully coated on the surface of LiNi_0.5Mn_1.5O₄ particles. The coating of 1.0 wt% BiFeO₃ on the surface of LiNi_0.5Mn_1.5O₄ exhibits a considerable enhancement in specific capacity, cyclic stability, and rate performance. The initial discharge capacity of 118.5 mAh g^?1 is obtained for 1.0 wt% BiFeO₃-coated LiNi_0.5Mn_1.5O₄ with very high capacity retention of 89.11% at 0.1 C after 100 cycles. Meanwhile, 1.0 wt% BiFeO₃-coated LiNi_0.5Mn_1.5O₄ electrode shows excellent rate performance with discharge capacities of 117.5, 110.2, 85.8, and 74.8 mAh g^?1 at 1, 2, 5, and 10 C, respectively, which is higher than that of LiNi_0.5Mn_1.5O₄ (97.3, 90, 77.5, and 60.9 mAh g^?1, respectively). The surface coating of BiFeO₃ effectively decreases charge transfer resistance and inhibits side reactions between active materials and electrolyte and thus induces the improved electrochemical performance of LiNi_0.5Mn_1.5O₄ materials.     

Perovskite-type ferromagnetic BiFeO3 nanopowder: a new magnetically recoverable heterogeneous nanocatalyst for efficient and selective transfer hydrogenation of aromatic nitro compounds into aromatic amines under microwave heating

Perovskite-type ferromagnetic BiFeO₃ nanopowder was readily synthesized via thermal decomposition of Bi[Fe(CN)₆]·5H₂O complex and characterized using thermal analysis (TGA/DSC), X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FT–IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), magnetic measurement and Brunauer–Emmett–Teller (BET) specific surface area measurements. The magnetic measurements show a ferromagnetic behavior for the BiFeO₃ nanoparticles at room temperature. This nanosized ferromagnetic oxide with an average particle size of approximately 20 nm and a specific surface area of 48.5 m²/g was used as a new magnetically recoverable heterogeneous nanocatalyst for the highly efficient and selective reduction of aromatic nitro compounds into their corresponding amines by using propan-2-ol as the hydrogen donor under microwave irradiation. This method is regio- and chemoselective, clean, inexpensive and compatible with the substrates having hydrogenlyzable or reducible functional groups. As compared with conventional heating, this method is very fast and suitable for the large-scale preparation of different substituted anilines as well as other arylamines. The catalyst can also be reused without loss of activity.     

沉淀法制备不同形貌和结构的纳米BiVO4

采用液相沉淀法，通过选择不同起始原料并控制反应温度和pH值，制备得到不同形貌和结构的纳米BiVO₄。采用X-射线衍射(XRD)、透射电子显微镜(TEM)和紫外-可见吸收光谱(UV-Vis)技术对产物进行分析表征。结果表明，采用NH₄VO₃作V源，室温下可直接制备得到结晶好的四方晶系硅酸锆型BiVO₄球形纳米颗粒，提高反应液pH值或升高温度，可得到单斜晶系白钨矿型BiVO₄，采用NaVO₃作为V源，室温下可直接得到单斜晶系白钨矿型片状BiVO₄。加入不同类型表面活性剂则得到不同形貌的BiVO₄。     

Beneficial modification of the properties of multiferroic BiFeO3 by cation substitution

The effects of modifying the well-known multiferroic BiFeO₃ by substituting Fe by Mn and Bi by La have been investigated. It is shown that both the substitutions have a favourable effect on the multiferroic properties of BiFeO₃. Thus, both BiFe_1−xMn_xO₃ and Bi_1−xLa_xFeO₃ (x = 0.0–0.3) show increased magnetization accompanied by hysteresis loops as well as improved dielectric properties. The ferroelectric transition temperature is lower than that of BiFeO₃, but is in a more accessible range. In Bi_1−xLa_xFeO₃, there is a change in structure at x = 0.2.     

Synthesis of BiFeO3 nanoparticles with small size

Uniform bismuth ferrite BiFeO₃ nanoparticles (below 60?nm) were successfully synthesized via a simple hydrothermal route with the assistance of KOH as mineralizer and EDTA as the chelating agent. The effects of mineralizer and chelating agent on the phase formation, and morphology of these particles were investigated. Size dependent magnetic properties of as-prepared particles at room temperature were investigated.     

Sol–gel route to prepare well-ordered nanowires with anodic aluminum oxide template

In this work, we report a sol?Cgel route to fabricate oxide nanowires of BiFeO₃ using two-step anodic aluminum oxide as template. We prepared oxide nanowires with uniform dimensions that can vary from 35 to 100?nm in diameter and with 4???m in length, as confirmed by scanning electron microscopy and transmission electron microscopy measurements. Magnetization measurements, performed in a vibrating sample magnetometer, show that this nanostructures present ferromagnetism at room temperature.     

A new method of synthesis of BiFeO<Subscript>3</Subscript> prepared by thermal decomposition of Bi[Fe(CN)<Subscript>6</Subscript>]·4H<Subscript>2</Subscript>O

In order to investigate the formation of the multiferroic BiFeO₃, the thermal decomposition of the inorganic complex Bismuth hexacyanoferrate (III) tetrahydrate, Bi[Fe(CN)₆]·4H₂O has been studied. The starting material and the decomposition products were characterized by IR spectroscopy, thermal analysis, laboratory powder X-ray diffraction, and microscopic electron scanning. The crystal structures of these compounds were refined by Rietveld analysis. BiFeO₃ were synthesized by the decomposition thermal method at temperature as low as 600 °C. There is a clear dependence of the type and amount of impurities that are present in the samples with the time and temperature of preparation.     

Formation of nanocrystalline BiFeO<Subscript>3</Subscript> under hydrothermal conditions

The formation of bismuth orthoferrite under hydrothermal conditions at temperature 160, 180, or 200°С and pressure 100 MPa in aqueous solution of potassium hydroxide has been studied. The determined composition and structure of polycrystalline phase with sillenite structure have evidenced its formation at the interface of the crystallites of amorphous iron oxide. It has been shown that the formation of polycrystalline round-shaped BiFeO₃ particles with size about 20 μm occurs via aggregation of perovskite-type phase crystallites (38–70 nm). Pycnometric density of BiFeO₃ and the amorphous phase has been determined, and Mossbauer spectra reflecting the state of iron in the phases coexisting during the formation of BiFeO₃ have been analyzed.     

Na3SbS4: A Solution Processable Sodium Superionic Conductor for All‐Solid‐State Sodium‐Ion Batteries

All‐solid‐state sodium‐ion batteries that operate at room temperature are attractive candidates for use in large‐scale energy storage systems. However, materials innovation in solid electrolytes is imperative to fulfill multiple requirements, including high conductivity, functional synthesis protocols for achieving intimate ionic contact with active materials, and air stability. A new, highly conductive (1.1 mS cm^?1 at 25 °C, E_a=0.20 eV) and dry air stable sodium superionic conductor, tetragonal Na₃SbS₄, is described. Importantly, Na₃SbS₄ can be prepared by scalable solution processes using methanol or water, and it exhibits high conductivities of 0.1–0.3 mS cm^?1. The solution‐processed, highly conductive solidified Na₃SbS₄ electrolyte coated on an active material (NaCrO₂) demonstrates dramatically improved electrochemical performance in all‐solid‐state batteries.     

Characterization of a Novel Intrinsic Luminescent Room‐Temperature Ionic Liquid Based on [P6,6,6,14][ANS]

Intrinsically luminescent room‐temperature ionic liquids (RTILs) can be prepared by combining a luminescent anion (more common) or cation with appropriate counter ions, rendering new luminescent soft materials. These RTILs are still new, and many of their photochemical properties are not well known. A novel intrinsic luminescent RTIL based on the 8‐anilinonaphthalene‐1‐sulfonate ([ANS]) anion combined with the trihexyltetradecylphosphonium ([P_6,6,6,14]) cation was prepared and characterized by spectroscopic techniques. Detailed photophysical studies highlight the influence of the ionic liquid environment on the ANS fluorescence, which together with rheological and ¹H NMR experiments illustrate the effects of both the viscosity and electrostatic interactions between the ions. This material is liquid at room temperature and possesses a glass transition temperature (T_g) of 230.4 K. The fluorescence is not highly sensitive to factors such as temperature, but owing to its high viscosity, dynamic Stokes shift measurements reveal very slow components for the IL relaxation.