Rapid Liquid Phase Sintering Research Articles

Structure and multiferroic properties of Eu-substituted BiFeO3 ceramics

Polycrystalline Bi1−xEuxFeO3 (x=0.00–0.25) ceramics were synthesized by the solid state reaction method with the rapid liquid phase sintering process. The effects of Eu substitution on the structure, and ferroelectric and magnetic properties of BiFeO3 ceramics were investigated. X-ray diffraction measurements reveal that the structure of BiFeO3 was changed from rhombohedral to orthorhombic and the impurity phases were decreased both due to Eu substitution. Raman spectra results also confirm that a structure transition occurs in the Eu concentration range of 0.15–0.20. The SEM investigation has suggested that the Eu substitution hinders the grain growth. Vibrating sample magnetometer measurements indicate ferromagnetism in Eu-substituted BiFeO3 ceramics. It is found that the room temperature magnetic moment increases with increasing Eu concentration due to the suppressed or broken cycloid spin structure. Ferroelectric measurements show that Eu substitution enhances the polarization due to the significant decrease of the electric leakage of the samples. Therefore, the Eu-substituted BiFeO3, or more complicated substituted BiFeO3 based on Eu substitution, will have great potential for many practical applications.

Effect of Sm, Co codoping on the dielectric and magnetoelectric properties of BiFeO3 polycrystalline ceramics

Multiferroic Bi0.95Sm0.05Fe1−xCoxO3 (x=0−0.1) ceramics were prepared by the rapid liquid phase sintering method. For all the samples studied, the dielectric constant and dielectric loss decrease with increasing frequency in the range from 1kHz to 1MHz. It shows that the dielectric constant of Bi0.95Sm0.05FeO3 at 10kHz is about forty times larger than that of pure BiFeO3. This dramatic change in the dielectric properties of Bi0.95Sm0.05Fe1−xCoxO3 (x=0−0.1) samples can be understood in terms of the space charge limited conduction associated with crystal defects, which was indicated by the increase of magnetoelectric effect with doping Co3+ under applied magnetic field from 1 to 8kOe. It was believed that the ferroelectric polarization enhancement comes from the exchange interaction between the Sm3+ and Fe3+ or Co3+ ions for Bi0.95Sm0.05Fe0.95Co0.05O3 at room temperature.

Effects of Gd and Co doping on the electrical and ferromagnetism properties of BiFeO3 ceramics

Multiferroic Bi0.95Gd0.05Fe1-xCoxO3 (x= 0, 0.05, 0.1, 0.15, 0.2) ceramics were prepared by rapid liquid phase sintering method. We studied effect of Gd and Co doping on the structure, electrical and ferromagnetism properties of BiFeO3 ceramics. The structure and morphology of BiFeO3 ceramics are characterized by X-ray diffraction (XRD). The results show that all the peaks for Bi0.95Gd0.05Fe1-xCoxO3 (x= 0, 0.05, 0.1, 0.15, 0.2) samples can be indexed according to the crystal structure of pure BiFeO3. And X-ray diffraction analysis reveals a phase transition in Gd-Co codoped BiFeO3 ceramics when x is larger than 0.1.The current densities of all samples measured at room temperature are approximately three orders of magnitude lower than that of BFO ceramic, and the leakage current of the ceramics at room temperature exhibits two distinctive conduction behaviors: Ohmic conduction and space charge limited (SCL) conduction mechanism.For all the samples studied here, the dielectric constant and dielectric loss decrease with the increase of frequency in a range from 1 kHz to 1 MHz. The dielectric constants of Bi0.95Gd0.05Fe1-xCoxO3 (x= 0, 0.05, 0.1, 0.15,0.2) samples are nearly 1.9, 2.68, 3.85, 5.3, and 6 times larger than that of pure BiFeO3 (εr= 61.2) ceramic at 1 kHz, respectively. And the dielectric losses of Bi0.95Gd0.05Fe1-xCoxO3 samples become smaller than that of BFO ceramic.The magnetic measurements show that all the samples possess strong ferromagnetism at room temperature expect BiFeO3 and Bi0.95Gd0.05FeO3 which are weakly ferromagnetic. Under an external magnetic field of 30 kOe, the values of Mr of Bi0.95Gd0.05Fe1-xCoxO3 are 34, 60, 105, 103 and 180 times that of BiFeO3, respectively.

Effects of Europium Substitution on the Microstructure and Electric Properties of Bismuth Ferrite Ceramics

Bi1−xEuxFeO3 (x=0.00–0.15) ceramics were synthesized by solid state reaction method with rapid liquid phase sintering process at different europium content and the microstructure and electrical properties of the samples were investigated. X-ray diffraction (XRD) studies show that europium substitution has changed the structure of BiFeO3 from rhombohedral R3c to orthorhombic Pnma and decreased the impurity phase. Raman spectra results also conform that a structure transition occurs at about x=0.20, and indicate that the Eu substitution at Bi site can obviously affect the Bi–O bond. Impedance analyzer measurements show that both dielectric constant and dielectric loss are strongly dependent on the Eu content. The dielectric constant of the Bi1−xEuxFeO3 increases with increasing Eu content from 0.00 to 0.20, then decreases with increasing Eu content from 0.20 to 0.30. The dielectric constant measured at 100 Hz is 542.0 for the x=0.20 sample, which is about 8.5 times as big as that for unsubsitituted BiFeO3. The dielectric loss can be effectively decreased by the substitution of Eu for Bi. In addition, the leakage current measurements show that the substitution of Eu can effectively reduce the leakage current density of BiFeO3.

Ferroelectric and Magnetic Properties of Hot-Pressed BiFeO3-PVDF Composite Films

The ferroelectric and magnetic properties of hot-pressed BiFeO3- (BFO) polyvinylidene fluoride (PVDF) composite films have been studied. The BiFeO3 (BFO) ceramics have been synthesized by a rapid liquid phase sintering technique. The X-ray diffraction (XRD) studies revealed that the impure phase observed in pure BFO ceramics was significantly reduced in the composite films. The presence of both ferroelectric and magnetic hysteresis loops confirms the multiferroic nature of the composite films at room temperature. A well-saturated ferroelectric hysteresis loop with a remanent polarization (Pr)∼4.8 μC-cm-2 and coercive field (Ec)∼1.55 kV/cm has been observed in composite thin films at room temperature. The magnetic hysteresis loops were traced at room temperature with SQUID. The remanent magnetization (Mr)∼3.0×10−3 emu/gm and coercive field (Hc)∼0.99 kOe was observed in the composite film. The magnetic polarization of the composite films has found to be enhanced as compared to pure BFO and correlated to reduction in BFO impure peak intensity.

Ferroelectric Properties and Raman Scattering Spectra of Bi<sub>1-x</sub>Gd<sub>x</sub>FeO<sub>3</sub> Ceramics

Multiferroic ceramic samples of Bi1-xGdxFeO3 (x = 0, 0.05, 0.1 and 0.15) have been prepared by the rapid liquid phase sintering technique. The effect of Gd substitution on ferroelectric properties of Bi1-xGdxFeO3 ceramics has been investigated. The results of x-ray diffraction (XRD) patterns show that the single-phase BiFeO3 sample has a rhombohedral structure and Gd3+ substitution for Bi3+ has not affected the perovskite structure. Experimental results suggest that Gd3+ substitution for Bi3+ has markedly affected the Raman scattering spectra, and the ferroelectric properties of BiFeO3 are improved by Gd3+ doping. When x = 0.15, saturated ferroelectric hysteresis loop is observed at room temperature with the maximal 2Pr = 1.62 μC/cm2.

Effects of Nd and high-valence Mn co-doping on the electrical and magnetic properties of multiferroic [formula omitted] ceramics

Pure BiFeO 3 (BFO), 15% Nd doped BFO (BNF), and 15% Nd and 2% high-valence Mn co-doped BFO (BNFM) multiferroic ceramics were prepared by a rapid liquid phase sintering technique. A structural transition from rhombohedral R 3 c to orthorhombic P 4 m m occurred in the BNF and BNFM ceramics. The co-doped BNFM ceramics exhibit the largest dielectric constant, the smallest dielectric loss and leakage current, and an improved rectangular-shaped polarization hysteresis loop with a remnant polarization as high as 31 μC/cm 2, which could be attributed to the lower concentration of charge defects and the structural transition. Besides, enhanced ferromagnetism is confirmed in BNFM with a remnant magnetization of 0.075 emu/g and a coercive magnetic field of 5.8 kOe, which might be attributed to the effective suppression of the spatially modulated spin structure by the Nd and Mn co-doping.

Investigation on Electrical and Magnetic Properties of Gd-doped BiFeO3

Multiferroic ceramic samples of Bi1−xGdxFeO3 (x=0, 0.05, 0.1 and 0.15) have been prepared by rapid liquid-phase sintering technique. The effect of Gd substitution on ferroelectric and magnetic properties of Bi1−xGdxFeO3 ceramics has been investigated. The results of X-ray diffraction (XRD) patterns show that the single-phase BiFeO3 sample has a rhombohedral structure and Gd3+ substitution for Bi3+ has not affected its structure. Experimental results suggest that for Bi1−xGdxFeO3 system, the ferroelectric and magnetic properties of BiFeO3 are improved by Gd doping and the loop area increases with the Gd content. When x=0.15, saturated ferroelectric hysteresis loop is observed at room temperature with the maximal 2Pr=1.62 μC/cm2, which is about 578.6% higher than that of BiFeO3.

Properties of Co Substituted Bismuth Ferrite System by Rapid Liquid Phase Sintering

The multiferroic ceramic samples of Co substituted Bi0.9Nd0.1Fe1-xCoxO3 ( x = 0.00, 0.03, 0.05, 0.07, 0.09, 0.10, 0.20 and 0.30) were prepared by a rapid liquid phase sintering technique. As Co content increases, the Bi0.9Nd0.1Fe1-xCoxO3 samples have a structural transformation from rhombohedral (space group R3c) to cubic structure (space group I23). A small amount of Co substitution maintains the ferroelectric properties while high doping content deteriorates the ferroelectric behavior due to the attendant high leakage current caused by the valence fluctuation of Co ions. It is found that the remnant magnetization (Mr) is enhanced greatly from a value of 4.87  10-6 emu/g at x = 0.00 to 0.054 emu/g at x = 0.30.

TEM study of a fluorite-type (1 − x)Bi 2O 3· xFe 2O 3 superstructure in BiFeO 3 ceramics synthesized by the rapid liquid-phase sintering method

A commensurate modulated structure of (1 − x)Bi 2O 3· xFe 2O 3 is found in the BiFeO 3 ceramics synthesized by the rapid liquid-phase sintering method. Transmission electron microscopy studies show that the superstructure is based on fluorite-type δ-Bi 2O 3 and modulated probably by oxygen vacancy ordering. The existence of such a superstructure may increase the leakage current of BiFeO 3 ceramics.

Detailed Phase Analysis and Crystal Structure Investigation of a Bi1−xCaxFeO3−x/2 Perovskite-Related Solid Solution Phase and Selected Property Measurements Thereof

A well-ordered, perovskite-related, Bi1−xCaxFeIIIO3−x/2 solid solution phase is synthesized via a rapid liquid phase sintering technique and shown to exist over the composition range ∼0.20 < x < ∼0.49. Two phase regions are shown to separate this phase from compositionally narrow end-member Bi1−xCaxFeIIIO3−x/2 solid solutions based on rhombohedral BiFeO3 and brownmillerite, Ca2Fe2O5, respectively. The wide range, non-stoichiometric phase is characterized by a well ordered, (in general) incommensurately modulated structure that varies systematically with composition. Mössbauer spectroscopy is used to verify the oxidation state of iron as (III), as well as showing the existence of three separate iron environments across the solid solution field. HRTEM imaging is used to develop a structural model for the phase at one particular composition. The magnetic, dielectric, and impedance properties of this solid solution phase are reported. Magnetic measurements show antiferromagnetic behavior. There is no change in either susceptibility or antiferromagnetic behavior with composition. Attempts at obtaining saturated ferroelectric loops failed due to high intrinsic conductivity. The phase is found to exhibit a low frequency impedance relaxation that is likely due to correlated oxygen vacancy hopping.

Rapid liquid phase sintered Mn doped BiFeO3 ceramics with enhanced polarization and weak magnetization

Single-phase BiFe1−xMnxO3 multiferroic ceramics have been synthesized by rapid liquid phase sintering method to study the influence of Mn substitution on their crystal structure, dielectric, magnetic, and ferroelectric behaviors. From XRD analysis it is seen that Mn substitution does not affect the crystal structure of the BiFe1−xMnxO3 system. An enhancement in magnetization was observed for BiFe1−xMnxO3 ceramics. However, the ferooelectric hysteresis loops were not really saturated, we observed a spontaneous polarization of 10.23μC∕cm2 under the applied field of 42kV∕cm and remanent polarization of 3.99μC∕cm2 for x=0.3 ceramic.

Magnetic field induced phase transition in multiferroic BiFe1−xTixO3 ceramics prepared by rapid liquid phase sintering

Single phase BiFe1−xTixO3 (BFTO) ceramics with varying x up to 0.35 were prepared by a rapid liquid phase sintering method to study the influence of Ti doping on their crystal structure, morphology, and magnetic behavior. Rhombohedral to orthorhombic phase was observed above x⩾0.33. The BFTO ceramics were highly resistive and with uniform morphology. Magnetic measurements revealed an increase in magnetization with the increase in Ti content and was maximum for x=0.2 ceramic. Surprisingly, a magnetic field induced phase transition from weak ferromagnetic to diamagnetic above 50kOe was observed for x=0.30 and 0.315 ceramics.

Processing and properties of Yb-doped BiFeO3 ceramics

The authors prepared Yb-doped bismuth iron oxide ceramics (Bi1−xYbxFeO3, with 0⩽x⩽0.20) by rapid liquid phase sintering method and investigated the material’s structures and electrical properties. The x-ray diffraction measurements showed that the doping of Yb has induced noticeable lattice distortion in the ceramics, and a largest distortion was observed when the concentration of Yb was 15%. By doping electrical resistivity, ferroelectric and dielectric properties of the ceramics were improved. Among all samples, BiFeO3 doped with 15% Yb was found to have the smallest leakage current density (&amp;lt;10−7A∕cm2) and the largest remnant polarization (8.5μC∕cm2).

Preparation of La-doped BiFeO3 thin films with Fe2+ ions on Si substrates

La-doped BiFeO3 thin films with Fe2+ ions have been prepared on Pt∕TiO2∕SiO2∕Si substrates by pulsed laser deposition in order to enhance the ferroelectric and magnetic properties. The targets for the film deposition were synthesized using a rapid liquid phase sintering technique to ensure the low leakage. The dielectric properties at room temperature and above were investigated. It was observed that the La doping greatly enhances the ferroelectric polarization at room temperature by modifying the film structure from rhombohedral to monoclinic. The saturation magnetization was enhanced about two times due to the Fe2+ ions in the thin films.

Preparation and multi-properties of insulated single-phase BiFeO 3 ceramics

Six types of BiFeO 3 ceramic samples, with subtle differences in synthesis conditions, were prepared. The comparison of their phases, electrical resistivity, and porosity revealed that the use of Bi 2O 3 and Fe 2O 3 powders of <1 μm size and a rapid liquid-phase sintering process of 855 °C for 5 min at 100 °C/s is beneficial to synthesize poreless single-phase BiFeO 3 samples with high electrical resistivity of ∼5×10 12 Ω cm. Deoxygenated Bi x Fe y O 1.5 x+1.5 y− δ ( x≠ y, δ≥0) impurities were identified and found to be the main cause of low electrical resistivity and high porosity in the multi-phase samples. Large saturation polarization of 16.6 μC/cm 2 and low leakage current density of 30 mA/m 2, both at a high electric field of 145 kV/cm, were measured in the optimized single-phase samples at room temperature besides a large piezoelectric d 33 coefficient of 27 pC/N and an obvious canted antiferromagnetic behavior.

Magnetic and electrical properties of single-phase multiferroic BiFeO3

We have reported the structural, thermal, microscopic, magnetization, polarization, and dielectric properties of BiFeO3 ceramics synthesized by a rapid liquid-phase sintering technique. Optimum conditions for the synthesis of single-phase BiFeO3 ceramics were obtained. Temperature-dependent magnetization and hysteresis loops indicate antiferromagnetic behavior in BiFeO3 at room temperature. Although saturated ferroelectric hysteresis loops were observed in single-phase BiFeO3 ceramic synthesized at 880 °C, the reduced polarization is found to be due to the high loss and low dielectric permittivity of the ceramic, which is caused by higher leakage current.

Room-temperature saturated ferroelectric polarization in BiFeO3 ceramics synthesized by rapid liquid phase sintering

Single-phased ferroelectromagnet BiFeO3 ceramics with high resistivity were synthesized by a rapid liquid phase sintering technique. Saturated ferroelectric hysteresis loops were observed at room temperature in the ceramics sintered at 880 °C for 450 s. The spontaneous polarization, remnant polarization, and the coercive field are 8.9 μC/cm2, 4.0 μC/cm2, and 39 kV/cm, respectively, under an applied field of 100 kV/cm. It is proposed that the formation of Fe2+ and an oxygen deficiency leading to the higher leakage can be greatly suppressed by the very high heating rate, short sintering period, and liquid phase sintering technique. The latter was also found effective in increasing the density of the ceramics. The sintering technique developed in this work is expected to be useful in synthesizing other ceramics from multivalent or volatile starting materials.