Synthesis and properties of multiferroic BiFeO3 ceramics

BiFeO₃ was synthesized using a modified sol–gel process, i.e., so-called Pechini method, as evidenced by X-ray diffraction pattern. Optimal conditions for the synthesis of single-phase BiFeO3 ceramics were obtained. This Pechini technique developed in this work is expected to be useful in synthesis of doped BiFeO₃ or BiFeO₃-based solid solution. Conventional sintering and spark plasma sintering processes were used to fabricate BiFeO₃ ceramics. Ferroelectric and magnetic loops were measured at room temperature. The ceramic sample shows a stable dielectric constant and low loss tangent between 100 Hz and 10 MHz.     

Direct and converse magnetoelectric effect at resonant frequency in laminar piezoelectric-magnetostrictive composite

Laminar piezoelectric-magnetostrictive composites using piezoelectric lead zirconate titanate ceramics and the giant magnetostrictive rare-earth-iron alloy Terfenol-D were prepared by epoxy bonding. The direct and converse magnetoelectric (ME) effects at and off the mechanical resonant frequency were characterized and compared to the theoretical modelling. The mechanical resonant frequency of the composites depended on the sample orientation and the magnetic DC bias field. In the longitudinal configuration, the resonant frequency shifted down monotonically with the increasing bias field. When the sample was in the transverse configuration, the resonant frequency decreased with the increasing field at first. However, at higher bias, it shifted up with the increasing bias. A phenomenological model based on the ΔE effect of magnetostrictive materials is proposed to explain the observed phenomena.     

Photocatalytic behaviors observed in Ba and Mn doped BiFeO3 nanofibers

BiFeO₃ nanofibers doped with Ba and Mn have been prepared by an electrospinning method. The influence of Ba doping on the phase composition, microstructure, photocatalytic properties was investigated detailedly. Our results indicate that with the increase of Ba doped contents, the formation of impurity phase can be suppressed in these BiFeO₃ nanofibers. A crystal lattice transformation occurred when 15 % Ba was doped. The degradation of Congo Red proved the enhanced photocatalytic behaviors in visible light region can be observed in these BFO nanofibers.     

Improved electric insulation ability and ferromagnetic property in Nb2O5 doped BiFeO3-based multiferroic ceramics

Journal of Electroceramics - 0.675BiFeO3-0.3BaTiO3-0.025LaFeO3-x mol% Nb2O5 (x?=?0–1.25) multiferroic lead- free ceramics, fabricated by conventional solid-state reaction, were...     

Synthesis and characterization of PZT: CF magnetoelectric composites

Here, we report the structural, dielectric, ferroelectric, piezoelectric and magnetic properties of ME composites of Lead Zirconate Titanate (PZT) and Cobalt ferrite (CF) with compositional formula (1?x) PbZr_0.55Ti_0.45O₃ + (x) CoFe₂O₄, (x = 0.00, 0.05, 0.10, 0.15 and 1.00 by weight). The samples were synthesized by conventional solid state reaction. The formation of perovskite tetragonal and spinel structure in the composites are confirmed by X-ray diffraction studies. Microstructural study of all samples was done by SEM analysis and average grain size was also calculated. The dielectric properties of samples were studied as a function of frequency and as a function of temperature (30°C – 500°C) respectively. P-E hysteresis loop at room temperature and piezoelectric coefficient were measured to study the ferroelectric and piezoelectric properties. To confirm the ferromagnetic behavior of the composites M-H hysteresis loop measurement was carried out. The Magnetoelectric voltage coefficient (dE/dH) was measured as a function of applied DC magnetic field. The maximum ME output of 389μV/ (cm.Oe) was observed for the composite with x = 0.15.     

Electrical properties of conventional and microwave sintered lead free magnetoelectric composites

Abstract

Here we report comparison of dielectric properties of composition synthesized by microwave and conventional sintering. Microwave sintering requires less time and temperature to achieve the same quality of materials as sintered by conventional route. The material sample was prepared by conventional solid state method and sintered in conventional & microwave furnace. Sintered samples were then subjected to XRD analysis. X-ray diffraction revealed the formation of single phase material. The dielectric and ferroelectric properties were recorded for both the samples and properties were found to improve in microwave sintered samples. There is also a significant improvement in density by microwave processing.     

Induced magnetoelectric coupling and photoluminescence response in solution-processed CoFe2O4/Pb0.6Sr0.4TiO3 multiferroic composite film

ABSTRACT

The combined effects of interface (represented by the extrapolation length) and thickness on polarization of ferroelectric thin films derived by sol-gel method are investigated using Gingzburg-Landau-Devonshire theory. The polarization profile, the average and the maximal polarization are numerically modeled and analyzed for various film thicknesses and degrees of interface imperfection. Results show that the polarization near the interface is dramatically suppressed due to the interface effects, and the average and maximal polarizations both decrease significantly with the reduced thickness down to 50 nm or below. The simulation results indicate that the interface effects play a major role behind the polarization reduction when the thickness is decreased.     

Simultaneously improved magnetization and polarization of flexible film of BiFeO3-LaY2Fe5O12 composites

ABSTRACT

BiFeO₃-LaY₂Fe₅O₁₂ (BFO-LYIG) composites and the corresponding flexible films were prepared via solid-sate reaction and spin coating respectively. The composites were used for structures and magnetic properties characterization, while the flexible films were used for multiferroic properties measurement. The results show that the BFO and LYIG phases can coexist in the composite with no impurity phase for certain mole ratio, but with obvious impurity phases for other mole ratios. Compared to the weak magnetic and ferroelectric property of BFO, the magnetization and polarization of the composites can be simultaneously improved due to the high magnetization and resistivity of LYIG.     

BiFeO3薄膜在光伏电池中的应用

介绍了BiFeO3薄膜在光伏电池中应用的基础知识,主要包括它的基本结构、工作原理、用作太阳能电池的要求、加工方法、研究现状及前景.对传统的p-n结太阳能电池和BiFeO3光伏电池的工作原理进行了比较并作了解释.在加工方法部分,一些最新研究进展列举其中.对目前所面临的主要问题进行了分析并简单给出了相应的正在研究的解决方法.     

Effect of BiScO 3 doping on the structure and properties of BiFeO 3 -BaTiO 3 piezoelectric ceramics

High temperature lead-free piezoelectric ceramics 0.67BiFeO3–0.33BaTiO3 doped with 0.35 mol% MnO2 and xBiScO3 (x = 0, 0.005, 0.01, 0.015, 0.02) were prepared by conventional...     

Ferroelectric Property and Crystal Structure of KNbO3 Based Ceramics

Rietveld refinement analysis was carried out to obtain the knowledge on the solid solution structure of the La and Fe co-doped KNbO₃ ceramics. The diffraction data was well fitted with tetragonal space group P4mm, and showed that La and Fe located at K and Nb sites in KNbO₃ perovskite, respectively. The sum of both occupancy ratios agreed almost well with the doping content. Furthermore, the effects of MnO₂ addition on the ferroelectric properties and crystal structure of the La and Fe co-doped KNbO₃ ceramics were investigated. MnO₂ addition strongly affected the ferroelectric and piezoelectric properties, because the valence of Mn ions changed from Mn^{4 +} to some extents of Mn^{3 +} and Mn²⁺, which were suggested by XAFS analysis.     

Ferroelectric electrode interactions in BaTiO3 and PZT thin films

Abstract

We have studied ferroelectric thin films deposited by sol-gel processing onto non-noble metal substrates. The results obtained indicate that by careful control of processing conditions, ferroelectric materials can be deposited onto these substrates, although interdiffusion between the metal and the ferroelectric can significantly degrade the ferroelectric properties of the materials. The effects of this interdiffusion are demonstrated and possible implications of ferroelectric-electrode interactions for fatigue in ferroelectric materials are discussed.     

Dielectric properties of BiFeO3 ceramics obtained from mechanochemically synthesized nanopowders

Dielectric behaviour of BiFeO₃ ceramics, obtained by hot-pressing of nanopowders produced by mechanochemical synthesis from Bi₂O₃ and Fe₂O₃ oxides (weight ratio 2:1), was studied in the temperature range 125?C575?K. The ceramics was found to exhibit step-like dielectric response ??*(T) with high permittivity values, similar to the behaviour of materials with giant dielectric permittivity. Three overlapping relaxation processes contribute to the dielectric response: i) relaxation in the low-temperature range (220?C420?K), characterized by activation energy of 0.4?eV, ii) relaxation in the temperature range 320?C520?K with activation energy of 1.0?eV and iii) broad dielectric anomaly in the vicinity of 420?K, which disappears after 1?h annealing at 775?K. The low-temperature relaxation is ascribed to the carrier hopping process between Fe²⁺ and Fe³⁺ ions. The presence of mixed valence of the Fe ions was proved by X-ray photoelectron spectroscopy. Dielectric relaxation in the middle-temperature range is considered as a result of grain boundary effect and internal barrier layers related to Bi₂₅FeO₄₀ phase as verified by X-ray diffraction. The high-temperature dielectric anomaly we relate to short-range hopping of ordered oxygen vacancies.     

Microwave Dielectric Properties of Ferroelectric BaTiO3 Thin Film

The dielectric properties of c-axis epitaxial BaTiO₃ thin film on LaAlO₃ are investigated at frequencies of 0.5–30 GHz. For the measurements, interdigital capacitors with the Au/Ti electrode configurations of five fingers pairs that are 15 m wide and spaced 2 m apart are prepared by photolithography and lift-off patterning. Finger length varies from 20 to 80 m. The capacitance of epitaxial BaTiO₃ films exhibited no frequency dependence up to 10 GHz with the exception of slightly upward tendency of capacitance in BaTiO₃ film with a finger length of 80 m due to the self resonant frequency at 20 GHz. The Q-factors of the capacitors, defined as Q = 1/CR, are decreased up to 10 GHz with increased frequency. At 10 GHz, the BaTiO₃ film has a tunability [defined as k(V) = [C(0)–C(V)]C(0)] of 1.5% at 15 V, a loss tangent of 0.2 at room temperature. The small tunability can be interpreted as a result of in-plane compressive stress of BaTiO₃ film exhibiting large dielectric anisotropy. For the improvement of tunability and dielectric loss in the interdigital BaTiO₃ capacitor, the tetragonality (c/a) of epitaxial BaTiO₃ film and design of interdigital capacitor should be modified.     

Photovoltaic effect in rhombohedral and tetragonal phase BiFeO3 ferroelectric thin films

The in-plane photovoltaic effect of epitaxially grown rhombohedral and tetragonal BiFeO₃ thin film were studied. The absorption spectrum of tetragonal BiFeO₃ is overall blue shifted compared with that of rhombohedral one, with an absorption onset near 2.5?eV and direct band gap of 3.1?eV. The photocurrent is measured with symmetric platinum electrodes in the plane to eliminate the interfacial energy barriers in rhombohedral and the effect of the depolarization field in tetragonal film. Except for the different sign and magnitude, the photo-current of both rhombohedral and tetragonal films depends on the linearly polarized light onto the plane of the sample along X and Y directions, showing evidence of a strong bulk photovoltaic effect (BPVE). These results indicate that the BPVE in both rhombohedral and tetragonal BiFeO₃ thin films could be further explored for the next generation of solar photovoltaic applications.     

LEAKAGE CURRENT MECHANISM OF POLYCRYSTALLINE BiFeO3 FILMS WITH PT ELECTRODE

ABSTRACT

Leakage current mechanism of the polycrystalline BiFeO₃ film annealing at 923 K with Pt electrodes was discussed based on Schottky-emission conduction, Poole-Frenkel trap limited conduction, Fowler-Nordheim tunneling conduction and space charge limited current (SCLC). The leakage current mechanism at room temperature is as follows; Schottky-emission or Poole-Frenkel was the candidate leakage current mechanism at low electric field and then leakage current mechanism was changed to the SCLC at high electric field. When decreasing the measuring temperature, the leakage current mechanism at low electric field was Poole-Frenkel trap limited conduction, though the electric field region of the PF trap limited conduction was much broader than that of the room temperature.     

Structural,ferroelectric and magnetic properties of BiFeO3-ZnFe2O4 nano-composites

ABSTRACT

Multiferroic BiFeO₃-ZnFeO₃ nano-composites in different composition were prepared by sol-gel method. Detailed investigations were made on the structural, magnetic and ferroelectric properties of these nanocomposites. The X-Ray Diffraction (XRD) pattern confirms the formation of distorted perovskite and spinel phases of BiFeO₃ and ZnFe₂O₄ respectively. Transmission Electron microscopy reveals the particle size and the elemental idea of pure ferrites. The particle sizes calculated using TEM of ZnFe₂O₄ is 20–30 nm and match with XRD result. An enhancement of polarization in nano-composites is observed from Polarization Vs Electric Field loops. Magnetic polarization versus Magnetic field curves indicates the improved magnetic properties.     

Influence of preparation method on phase formation,structural and magnetic properties of BiFeO3

Journal of Electroceramics - Nanocrystalline BiFeO3 was synthesized utilizing two distinct techniques: auto-combustion and ceramic. A unique auto-combustion process employing glycine as a fuel has...     

Micro-structure and ferroelectric properties of BiFeO3 thin films formed on Pt-coated r-plane sapphire substrates

Magnetoelectric BiFeO₃ (BFO) materials exhibit ferroelectric and ferromagnetic properties simultaneously, therefore they have a potential to be applied in magnetic as well as ferroelectric devices. BFO thin films were formed by depositing sol-gel solutions on Pt-coated r-plane sapphire dielectric substrates. We did not observe any secondary phase such as Bi₂Fe₄O₉ on the r-plane sapphire substrates, which is generally observed on Si substrates. We observed small ferroelectric grains of about 0.1 μm on Pt/sapphire structures. The leakage current density in BFO films was found to be decreased dramatically after optimizing process conditions of stoichiometric BFO chemical solution. The leakage current densities were in the range of 10^{− 7} A/cm² at room temperature and 10^{− 9} A/cm² at 80 K under 0.4 MV/cm applied electric field. The main reason for low leakage current is considered to be reduction of oxygen vacancies due to the presence of exclusive Fe^{3 +} valance state in the films. An applied electric field higher than 0.5 MV/cm was required to pole the BFO films, which made it difficult to obtain the saturated polarization at room temperature. We could measure the saturated remanent polarization in the BFO films at 80 K and the obtained remanent polarization was 100 μC/cm².