Enhancement of ferromagnetic and ferroelectric properties in calcium doped BiFeO3 by chemical synthesis

Calcium (Ca)-doped bismuth ferrite (BiFeO₃) thin films prepared by using the polymeric precursor method (PPM) were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), polarization and magnetic measurements. Structural studies by XRD and Rietveld refinement reveal the co-existence of distorted rhombohedral and tetragonal phases in the highest doped BiFeO₃ (BFO) where enhanced ferroelectric and magnetic properties are produced by internal strain. A high coercive field in the hysteresis loop is observed for the BiFeO₃ film. Fatigue and retention free characteristics are improved in the highest Ca-doped sample due to changes in the crystal structure of BFO for a primitive cubic perovskite lattice with four-fold symmetry and a large tetragonal distortion within the crystal domain.     

Study of Mn doped multiferroic DyFeO3 ceramics

Structural, Raman, room temperature and temperature dependent leakage current density, dielectric, magnetization and room temperature Mossbauer studies of Mn doped DyFeO₃ (i.e., DyFe_1−xMn_xO₃; x=0 to 0.5) polycrystalline materials prepared through sol-gel route are reported in this paper. From Rietveld refinement of x-ray diffraction (XRD) patterns it is found that all the samples are formed in single phase without any detectable impurity. The Raman modes with doping are consistent with literature of such doped orthoferrites. From the room temperature (RT) leakage current density (J-E) measurements, it is observed that leakage current density increases with Mn doping concentration, which is explained in terms of microstructure. The leakage current density is found to decrease with the decrease of temperature in each sample as observed from low temperature leakage current density (J-E) measurements. Further, activation energy is calculated from the temperature dependent J-E data. The dielectric loss data is observed to exhibit frequency dependence and the activation energy obtained indicate the contribution from space charges. From temperature dependent magnetization data, it is found that with the increase of Mn content, the spin reorientation (SR) transition temperature (T_SR) moves towards higher temperature. From M-H curves at 10 K and 300 K with different Mn doping concentrations, it is found that saturation Magnetization (M_S) decreases with increase of Mn doping. Room temperature Mossbauer data shows the presence of Fe³⁺ state and the gradual decrease of internal hyperfine filed with increase of Mn content.     

Effects of Different Sintering Methods on the Structural and Electrical Properties of Ca0.9Sr0.1MnO3

Nano-crystalline Ca_0.9Sr_0.1MnO₃ powders were synthesized through citrate sol-gel method. Two different sintering techniques, i.e., conventional sintering and microwave sintering were employed to sinter the prepared samples at 1123 K. The high value of relative density was achieved for both the samples. Phase confirmation, surface morphology, and elemental composition were carried out by powder x-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy, respectively. DC four-probe measurement results revealed that the microwave sintered sample has higher electrical conductivity values than the conventional sintered sample. The conventional sintered sample exhibited semiconductor nature, whereas the microwave sintered sample indicated semi-metallic properties from room temperature to 673 K.     

Enhancement of extrinsic magnetoresistance behavior in BiFeO3 doped La0.8Ag0.2MnO3 ceramic

BiFeO₃ (BFO) doped La_0.8Ag_0.2MnO₃ ceramics were prepared by the conventional solid-state synthesis method to investigate the effect of BFO on transport and magnetic properties as well as its influence on magnetoresistance (MR). X-ray diffraction analysis showed a decrease in unit cell volume indicating partial substitution of some BFO into the La_0.8Ag_0.2MnO₃ (LaAgMO) lattice while results of scanning electron microscope (SEM) showed improvement in grains connectivity and formation of more regular shaped grains with BFO doping. Energy Dispersive X-ray emission (EDX) analysis showed BFO mainly segregates at the grains boundaries of LaAgMO. Resistivity and susceptibility measurements showed both metal–insulator transition temperatures, T_MI and paramagnetic-to-ferromagnetic transition temperature, T_c decreased with increased BFO content indicating weakening of the double exchange, DE mechanism. The temperature dependence of MR showed a small peak around T_MI for all samples which is ascribed to the intrinsic MR effect. Below the MR peak, the MR increased almost linearly with decreasing temperature for all samples and this is ascribed to the phenomena of extrinsic MR. The highest MR% (at 40 K) was observed for the x=1.5% sample which showed a MR of more than twice that of the undoped (x=0%) sample. This extrinsic effect is suggested to be related to improved spin polarized tunneling of conduction electrons between grains under external field as a result of improved spin alignment. It is suggested that BFO induced some kind of magnetoelectric coupling between BFO and LaAgMO leading to the enhancement process.     

Effect of Zn and Ti Co-doping on structure and electrical properties of BiFeO3 ceramics

In this paper, the effect of Zn and Ti co-doping on the structure, dielectric, ferroelectric and magnetic properties of BiFeO₃ (BFO) ceramics have been studied. Co-doped BFO ceramics with different doping concentrations are prepared by using the sol-gel method. The structure, morphology and electrical properties of co-doped BFO ceramics are studied by using X-ray diffraction (XRD), scanning electron microscopy, impedance analyzer and ferroelectric test analysis system. Furthermore, the vibrating sample magnetometer (VSM) is used to investigate the magnetic properties. The structural transition from rhombohedral to tetragonal-like phase is observed with a high level of co-doping (x = 2.5%). In addition, higher content of Zn and Ti results in enhanced dielectric constant (ε) and reduced dielectric loss (tanδ). The highest remnant polarization of 0.4 μC/cm² and coercive electric field of 15.5?kV/cm is demonstrated at x = 2.5%. The enhanced dielectric and ferroelectric properties can be attributed to the formation of defect complexes due to the Zn and Ti substitution.     

Effect of thermal treatment on the electrical properties of the sol–gel-derived (Zr,Sn)TiO4 thin films

The effect of heating temperatures on the electrical properties of sol–gel-derived (Zr,Sn)TiO₄ thin films deposited on a p-type (1 0 0) Si substrate was studied. The leakage currents of films with two different heating temperatures chosen to burn-out the solvent as a function of applied voltage were measured at different temperatures. The activation energies obtained from the Arrhenius plot of the leakage current density versus measured temperature for (Zr,Sn)TiO₄ films were then extracted. Additionally, microstructures of films with two different heating temperatures chosen to burn-out the solvent were analyzed by a conductive atomic force microscope (AFM) and an X-ray diffraction (XRD). Finally, the conductive mechanisms of leakage current and leakage current correlated to microstructures were also discussed.     

Tunable multiferroic properties of Mn substituted BiFeO3 thin films

The current study explored the influence of Mn substitution on the electrical and magnetic properties of BiFeO₃ (BFO) thin films synthesized using low cost chemical solution deposition technique. X-ray diffraction analysis revealed that pure rhombohedral phase of BiFeO₃ was transformed to the tetragonal structure with P4mm symmetry on Mn substitution. A leakage current density of 5.7×10⁻⁴ A/cm² which is about two orders of magnitude lower than pure BFO was observed in 3% Mn doped BFO thin film at an external electric field >400 kV/cm. A well saturated (p-E) loops with saturation polarization (P_sat) and remanent polarization (2P_r) as high as 60.34 µC/cm² and 25.06 µC/cm² were observed in 10% Mn substituted BFO thin films. An escalation in dielectric tunability (n_r), figure of merit (K) and quality factor (Q) were observed in suitable Mn doped BFO thin films. The magnetic measurement revealed that Mn substituted BFO thin films showed a large saturation magnetization compared to pure BFO thin film. The highest saturation ~31 emu/cc was observed for 3% Mn substituted BFO thin films.     

Microstructures,magnetic, and dielectric properties of Ba-doped BiFeO3 nanoparticles synthesized via molten salt route

As the paradigm of magnetoelectric multiferroic materials, BiFeO₃ (BFO) has potential applications in spintronics, memory devices, sensors, and actuators. However, its large leakage current and small magnetism at room temperature restrict its practical applications. It is demonstrated that the substitutions of Bi by alkali earth elements at A-site of BFO can significantly reduce the leakage current and enhance the remanent magnetization of BFO. In this work, Ba-doped BFO nanoparticles Bi_1-xBa_xFeO₃ (x = 0, 0.05, 0.10, 0.15 and 0.20) were synthesized via molten salt route. X-ray diffraction patterns revealed that with increasing the Ba-doped content the formation of the impurity phase was depressed and the rhombohedral distortions of these nanoparticles were suppressed, as confirmed by Raman spectra. X-ray photoelectron spectroscopy measurements reveal that the Fe element in the nanoparticles exists in the dual valence states of Fe³⁺ and Fe²⁺, and two kinds of oxygen atoms (lattice oxygen atoms and the adsorbed oxygen atoms) exist in the nanoparticles. With increasing the Ba-doped content, the content ratios of Fe³⁺ to Fe²⁺ ions were generally increased, whereas the oxygen vacancy concentrations were decreased. The average particle sizes of the Ba-doped BFO nanoparticles were decreased as compared with that of nondoped BFO nanoparticles. In contrast, the room temperature magnetization of the Ba-doped BFO nanoparticles was greatly enhanced by Ba-substitution, as confirmed by the M-H loops. At room temperature, the remanent magnetization and coercive field of the Bi_0.8Ba_0.2FeO₃ nanoparticles were 0.51 emu/g and 1130 Oe, respectively. Furthermore, the leakage current density was reduced by one order of magnitude at x = 0.2 and the dielectric properties are also improved by Ba-substitution. The improvements on the remanent magnetization, leakage current density as well as dielectric properties of the Ba-doped BFO nanoparticles make them promising candidates for spintronics and dielectric energy storages.     

Structural,dielectric and piezoelectric properties of xBiFeO3–(1−x)BaTi0.9Zr0.1O3 ceramics

Effect of BiFeO₃ (BFO) content on the microstructure and electrical properties of BaTi_0.9Zr_0.1O₃ (BTZ) ceramics prepared by the solid-state reaction technique was investigated. X-ray diffraction analyses show that BFO diffused into the lattice of BTZ to form a solid solution with perovskite structure. The relative density of the BTZ ceramics is increased by the introduction of BFO. The dielectric study reveals that the dielectric constant and the average dielectric loss of the solid solution decreased simultaneously with an increase in BFO content. The materials undergo a diffuse type ferroelectric phase transition. The diffusivity increases with increase in BFO contents in the studied composition range. On the other hand, the piezoelectric coefficient and electromechanical coupling coefficient decrease simultaneously with increasing the BFO content, whereas the mechanical quality factor increases gradually. The structure–property relationship and the mechanism associated with the change of the electrical properties are discussed intensively.     

Effect of non-magnetic Al3+ doping on structural,optical, electrical,dielectric and magnetic properties of BiFeO3 ceramics

Pure BiFeO₃ (BFO) and Al doped BFO samples were synthesized via citrate precursor method and sintered at 500 °C for two hours. Effect of Al doping on the structural, optical, electrical, dielectric and magnetic properties were investigated. X-ray diffraction (XRD) confirmed the distorted rhombohedral structure without any merging of peaks which indicates no structural transformation. Average crystallite size was found to be in the range 28–39 nm. Field emission scanning electron microscopy (FESEM) images illustrated the dense, agglomerated, spherically shaped with reduced grain size nanoparticles. Increased value of dielectric constant with low dielectric tangent loss was observed for the Al doped BFO samples. The value of dielectric constant was found to be 51 at 100 kHz for x = 0.1 sample. Temperature dependent dielectric constant showed a dielectric anomaly, indicating the antiferromagnetic transition. The remanent polarization and the corresponding coercive field for x = 0.1 was found to be 0.0625 µC/cm² and 56.154 kV/cm at an operating voltage of 1000 V. The improved electrical properties with low leakage current density were ascribed to the stabilization of the pervoskite structure and reduced oxygen vacancies.     

Preparation and characterization of nano-sized LiFePO4 by low heating solid-state coordination method and microwave heating

The precursors of LiFePO₄ were prepared by low heating solid-state coordination method using lithium acetate, ammonium dihydric phosphate, ferrous oxalate and citric acid as raw materials. Olivine phase LiFePO₄ as a cathode material for lithium-ion batteries was successfully synthesized by microwave heating in a few minutes. X-ray diffraction (XRD) and transmission electron microscope (TEM) were used to characterize its structure and morphology. Cyclic voltammetry (CV) and charge-discharge cycling performance were used to characterize its electrochemical properties. The results showed that the grain size of the optimal sample was about 40-50 nm, and the as-prepared particles were homogeneous. The nano-sized LiFePO₄ obtained has a high electrochemical capacity (125 mAh g⁻¹) and stable cycle ability.     

Synthesis of TS-1 by microwave heating of template-impregnated SiO2–TiO2 xerogels

TS-1 was prepared by microwave heating of a SiO₂–TiO₂ xerogel dry-impregnated with the template, TPAOH. A highly crystalline product was obtained within 30 min after microwave irradiation with yields over 90%. These are significant advantages over the TS-1 obtained by conventional oven heating using alkoxide precursors in liquid phase, which requires 1–2 day crystallization time with low product yields. Characterization of the TS-1 obtained was carried out using XRD, SEM, FT-IR, and UV-vis spectroscopy, and catalytic activity was examined for 1-hexene epoxidation using H₂O₂ as oxidant. These studies revealed that the material obtained by microwave heating of the mixed oxide gel shows essentially identical physicochemical properties to those prepared by conventional means.     

Rapid synthesis of ZnO nanostructures through microwave heating process

In this paper, polycrystalline zinc oxide (ZnO) nanostructures have been prepared by a hydrothermal synthesis through rapid microwave heating (180 s). The structure, composition and optical properties of the products were examined by scanning electron microscopy (SEM), energy dispersive x-ray spectrum (EDS), ultraviolet–visible spectroscopy (UV–vis), x-ray diffraction (XRD), photoluminescence spectroscopy (PL) and Raman spectroscopy. Typically, the synthesized nanostructures were zinc-rich with diameter ranging from 20 nm to 200 nm in length. From the Raman spectroscopy and PL measurements, it was found that the as-deposited films contain vacancy defects that originated from the rapid synthesis process.     

Stability properties and microstructure properties of microwave-sintered CeO2 doped zirconia ceramics

Nanosized CeO₂–ZrO₂ powders prepared by atmospheric pressure pyrolysis were used as raw materials to prepare CeO₂–ZrO₂ ceramics using microwave sintering. The samples were characterised using bulk density measurements, X-ray diffraction (XRD), Fourier Transform Infrared Spectrometer (FT-IR), Raman, and scanning electron microscopy (SEM). The purpose was to determine the optimised microwave sintering process for CeO₂–ZrO₂ ceramics and reveal the corresponding mechanism. The results show that with a CeO₂ addition content above 5 mol%, the tetragonal phase peak appeared obviously in the sample. The results show that the tetragonal phase peak appears when the CeO₂ content is more than 5 mol%. The dopants, namely CeO₂, have reduced the solid solution's phase transformation temperature with the assistance of microwave heating. Additionally, the grain size of the CeO₂–ZrO₂ ceramics has shown a negative relationship with Ce content at a temperature of 900 °C. The reason is that the rapid sintering due to microwave sintering and the oxygen vacancies generated by CeO₂ can effectively inhibit grain growth. The regulation mechanism on microwave sintering of CeO₂–ZrO₂ ceramic was clarified, and the technical prototype of controlled prepared CeO₂–ZrO₂ ceramics by microwave sintering was constructed.     

Rapid synthesis of nanocrystalline Co3O4 by a microwave-assisted combustion method

A facile and rapid microwave-assisted combustion method was developed to synthesize the nanocrystalline Co₃O₄. The study suggested that application of microwave heating to produce the homogeneous porous Co₃O₄ was achieved in a few minutes. The structure and morphology of the as-prepared nanocrystalline Co₃O₄ were investigated by means of X-ray diffraction (XRD), Fourier transform infrared spectra (FTIR), Raman spectra, UV-vis absorbance spectra and scanning electron microscopy (SEM). The XRD, FTIR and Raman spectra confirmed the formation of spinel structural Co₃O₄, and the SEM results indicated the porous surface characteristic of the products. Magnetic measurement was carried out using a vibrating sample magnetometer (VSM). The field dependence of the magnetization at room temperature showed a tiny hysteresis loop with a coercivity of 56.7 Oe.     

Synthesis,electrical and magnetic characteristics of Nd-modified BiFeO3

The polycrystalline Nd-modified bismuth ferrite BiFeO₃ (Bi_1−xNd_xFeO₃ (BNFO) (x=0, 0.05, 0.15, and 0.25)) were prepared in a single-phase using a standard and cost effective solid-state reaction method. In order to check the quality and formation of the compounds x-rays diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDAX) techniques were used. Preliminary structural analysis indicates that the crystal structure of BNFO is rhombohedra for its low content of Nd (x=0, 0.05, 0.15) whereas for higher content (x=0.25) it is tetragonal. The dielectric and ferroelectric properties of BiFeO₃ (BFO) were dramatically enhanced on the above Nd-substitutions. Study of the frequency dependence of ac conductivity suggests that the materials obey Jonscher׳s universal power law. An increase in Nd-content in BNFO results in the enhancement of spontaneous magnetization of BFO because of the collapse of spin cycloid structure.     

LiFePO4/C cathode materials synthesized by co-precipitation and microwave heating

LiFePO₄/C cathode materials were synthesized by a combination of co-precipitation and microwave heating using polyethylene glycol (PEG) as a carbon resource and the influence of microwave heating time on the structure and electrochemical performance of the materials was also discussed. The samples were characterized by X-ray diffraction (XRD), TEM, particle-size analysis and constant current charge-discharge experiment. The results show that the LiFePO₄/C heated for 9 min has a pure olive-type phase and excellent electrochemical performance. The initial discharge capacities of this sample are 154.3, 139.7, 123.9 mAh/g at the rates 0.1C, 0.2C, 1C at room temperature, respectively, and after 20 cycles remain 152.3, 134.3, 118.5 mAh/g, respectively. __________ Translated from New Chemical Materials, 2008, 36(2): 21–24 [译自:化工新型材料]     

Preparation and dielectric properties of dense and amorphous alumina film by sol–gel technology

Dense and crack-free aluminum oxide films were fabricated by sol–gel spin-coating technology. Aluminum nitrate (Al(NO₃)₃.9H₂O) was used as the precursor material. X-ray diffraction shows that the fabricated films are amorphous. X-ray photoelectron spectroscopy confirms that the thin films are alumina (Al₂O₃). Field-emission scanning electron microscopy images of the films reveal that the films are compact with a dense cross section. Dielectric measurements were carried out on samples with a metal–insulator–metal structure. The electrical characteristics of the films were affected by the thermal sintering temperature of the films. The leakage current density of the films decreases with the increase in the sintering temperature and increases with the increase in the measuring temperature. The leakage current shows a linear dependence on the voltage in the low-electric field-regime. The current density ascends to higher values due to the effect of space charges in the high-electric-field regime. The ionization energy of the top-electrode metals (Au, Pt or Ti–Au) has a strong effect on the leakage current.     

Structural,dielectric, magnetic,and ferroelectric properties of bismuth ferrite (BiFeO3) synthesized by a solvothermal process using hexamethylenetetramine (HMTA) as precipitating agent

Multiferroic BiFeO₃ (BFO) has been synthesized by the solvothermal technique for 4 h at 180 °C using Hexamethylenetetramine (HMTA) as a precipitating agent. Optimizations on HMTA concentration were performed to attain BFO in a narrow possible time using the solvothermal method. The effect of HMTA concentration on structural properties was investigated by XRD, FE-SEM, FT-IR, and Raman techniques. Moreover, the magnetic, ferroelectric, dielectric, and optical properties were studied by VSM, Ferroelectric analyser, Dielectric spectrometer, and UV–Vis–NIR spectrometer, respectively. The XRD data has unveiled the significant role of HMTA, as a precipitating agent, in obtaining the pure phase BFO at a particular concentration. A high concentration of HMTA (6 M) is found to be more favourable for producing pure phase BFO within the short reaction time of 4 h without any impurity phase formations. The FE-SEM images have shown the formation of granular structures that are inhomogeneously distributed throughout the powdered BFO with porosities. In addition, the magnetic measurements confirmed that the BFO synthesized with varying HMTA concentration exhibits a reasonable week ferromagnetic behaviour. However, the pure BFO powder synthesized at 6 M HMTA shows high magnetization value (0.72 emu/g) compared to the other samples synthesized at low concentrations of HMTA. Further, the dielectric measurements of all the synthesized BFO samples have shown a decrease in dielectric loss with an increase in frequency. Whereas the dielectric behaviour exhibited by the pure BFO synthesized at 6 M HMTA has shown a high dielectric constant value with moderate dielectric loss. A ferroelectric analyser studies the ferroelectric behaviour of BFO powder. The study revealed that pure phase BFO synthesized with 6 M HMTA exhibits a high value of remanent polarization with unsaturated P-E loops due to the high leakage current in the sample. The UV–Vis data shows that the BFO samples exhibited an excellent optical absorption in the visible range with narrow optical band gaps. Therefore, all the characterizations related to magnetic, structural, dielectric, ferroelectric, and optical properties of synthesized pure BFO have proven the significance of precipitating agent HMTA in the solvothermal synthesis method. Based on our findings, the synthesized pure phase BFO can be an excellent semiconducting photocatalyst for multiferroic based photocatalysis applications.     

Microwave assisted low temperature synthesis of MnZn ferrite nanoparticles

MnZnFe₂O₄ ferrite nanoparticles were prepared by co-precipitation method using a microwave heating system at temperature of 100 °C. X-ray diffraction reveals the samples as prepared are pure ferrite nanocrystalline phase, transmission electron microscopy image analysis shows particles are in agglomeration state with an average size of about 10 nm, furthermore, crystal size of samples are increased with longer microwave heating.