Ferroelectric and magnetic properties in (1−x)BiFeO3–x(0.5CaTiO3–0.5SmFeO3) ceramics

Multiferroic ceramics were prepared and characterized in (1?x)BiFeO₃–x(0.5CaTiO₃–0.5SmFeO₃) system by a standard solid‐state reaction process. The structure evolution was investigated by X‐ray diffraction and Raman spectrum analyses. The refinement results confirmed the different phase assemblages with varying amounts of polar rhombohedral R3c and nonpolar orthorhombic Pbnm as a function of the substitution content. In the compositions range of 0.2≤x≤0.5, polar R3c and nonpolar Pbnm coexisted, which was referred to polar‐to‐nonpolar morphotropic phase boundary (MPB). According to the dielectric and DSC analysis results, the ceramics with x≤0.2 changed to diffused ferroelectric, and the ferroelectric properties were enhanced significantly. Two dielectric relaxations were detected in the temperature range of 200‐300 K and 500‐700 K, respectively. The high‐temperature dielectric relaxation was attributed to the grain‐boundary effects. While the low temperature dielectric relaxation obtained in the samples with x=0.3‐0.5 was related to the charge transfer between Fe²⁺ and Fe³⁺. The magnetic hysteresis loops measured at different temperature indicated the enhanced magnetic properties in the present ceramics, which could be attributed to the suppressed cycloidal spin magnetic structure by Ti ions. In addition, the rare‐earth Sm spin moments might also affect the magnetic properties at relatively lower temperature.     

Magnetocrystalline anisotropy in the Co/Fe codoped Aurivillius oxide with different perovskite layer number

Perpendicular magnetic anisotropic materials are of great interest for their huge potential to realize high‐density nonvolatile memory and logic chips. Designing such materials with lower cost and better magnetoelectric coupling still remains major challenges. In this work, aurivillius oxide ceramics with highly [00l]‐oriented grains were prepared by a facile pressureless sintering method, and the perovskite layer number was modified by varying the cobalt content. Afterwards, the unique perpendicular magnetic anisotropy and the intriguing anisotropic ferroelectric properties have been observed. The magnetic properties of the Aurivillius oxides with different layer numbers have been carefully investigated by field cooling, zero field cooling, and the magnetization with a varying field. The magnetic anisotropy in the oriented ceramics is demonstrated to be caused by the magnetocrystalline anisotropy with the easy magnetization direction along the c‐axis, which possibly arises from the unquenched 3d orbitals combined with the special layered crystal structures. The magnetocrystalline anisotropy becomes weaker in the ceramics with lower numbered perovskite layers, whereas the orientation degree of the ceramics and the ferroelectric anisotropy show quite opposite trends. Furthermore, the weak magnetoelectric coupling is also observed in the ceramics. This special anisotropic multiferroic properties may open up a new window for the Aurivillius materials.     

软磁铁氧体纳米粉的制备

介绍了一种制备纳米级软磁铁氧体的新方法——催化相转化水热法。     

Strontium Zirconate and Strontium Titanate Ceramics for High-Voltage Applications: Synthesis, Processing, and Dielectric Properties

The synthesis, processing, and electrical properties of SrZrO₃ and SrTiO₃ materials have been examined. Phase-pure powders of SrZrO₃ and SrTiO₃ materials were synthesized using the Pechini method. Powder processing routes that used water and 2-propanol as carrier fluids were developed to achieve high green densities, which resulted in sintered densities of >99% of the theoretical density. The relative permittivity and average breakdown strength for carefully processed SrZrO₃ were 60 and 40 V/μm; the corresponding values for SrTiO₃ were 400 and 35 V/μm. The higher breakdown strengths suggest that these materials can be used in high-voltage capacitor applications.     

Ferroic ordering and charge‐spin‐lattice order coupling in Gd‐doped Fe3O4 nanoparticles relaxor multiferroic system

We investigated the effect of gadolinium doping (1‐5 at.%) on the magnetic and dielectric properties of Fe₃O₄ nanoparticles, synthesized by the chemical co‐precipitation technique, primarily to understand the onset of multifunctional properties such as ferroelectricity and magnetodielectric coupling. The substitution of larger Gd³⁺ ions at smaller Fe³⁺ octahedral sites in inverse spinel Fe₃O₄ has significantly influenced the morphology, average crystallite size, and more importantly, the magneto‐crystalline anisotropy and saturation magnetization. The magneto‐crystalline anisotropy and the saturation magnetization decreases substantially, however, significant increase in the average crystallite size is observed upon Gd doping. Furthermore, temperature‐dependent dielectric studies suggest that these nanoparticle systems exhibit relaxor ferroelectric behavior, with much pronounced ferroelectric polarization moment recorded for 5 at.% Gd doped Fe₃O₄ as compared to its undoped counterpart.     

水热法制备BiFeO3粉体

以FeC l3.6H2O和B i(NO3)3.5H2O为原料,氨水为沉淀剂,KOH为矿化剂,采用共沉淀法制备前驱物,水热法合成了纯相的B iFeO3粉体。X射线衍射结果表明,在160℃,碱浓度仅为0.15 mol/L的水热条件下,即可合成纯相的B iFeO3粉体。该工艺大大降低了水热温度,减小了碱浓度,从而节约了能源,降低了成本,减轻了碱对水热设备的腐蚀。扫描电镜显示,前驱沉淀物陈化时间为1 d时,水热制备的B iFeO3粉体中有发育良好的六方短柱状晶体形成;陈化时间增加到3 d时,所得B iFeO3粉体呈双层板状。差热-失重分析表明,所得B iFeO3粉体的尼尔温度(TN)为301℃,居里温度(TC)为828℃,分解温度为964℃。     

Thermally activated giant piezoelectricity and enhanced interface elastic strain-mediated magnetoelectric coupling

Perovskite materials with compositions in the vicinity of the steep morphotropic phase boundary (MPB) exhibit various intriguing properties including giant piezoelectricity and large dielectric constant. Aside from composition, the phase configuration of the perovskites is also strongly related to the ambient temperature. Here, we report a giant piezoelectricity of 10 980 pm/V at 93°C in the 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (PMN-PT) single crystals which is more than five times larger than that at room temperature. The enhanced piezoelectricity can be attributed to the instability of the thermally induced tetragonal phase which can be converted to the orthorhombic phase by the external electric field in the <011> oriented single crystal. The transverse piezoelectricity has been investigated by measuring the electric-field-dependent ferromagnetic resonance (FMR) field in the CoFeB/PMN-PT magnetoelectric (ME) heterostructures. The ME coupling coefficient has been increased from 49.3 to 476 Oe cm/kV as temperature increased from 25 to 90°C. The findings reveal that both longitudinal and transverse piezoelectricity in the PMN-PT single crystals can be greatly enhanced by proper setting of ambient temperature, indicating an effective route for the design of strain-mediated tunable devices with ultralow driving voltage.     

超细钛酸锶陶瓷粉体的湿化学制备技术

随着钛酸锶电子陶瓷应用日益广泛,对其性能要求不断提高,所以制备超细钛酸锶粉体的湿化学制备技术越来越受到重视,制备超细钛酸锶粉体的湿化学方法主要包括:沉淀法,金属醇盐水解法,溶胶-凝胶法,微乳液法,水热法,燃烧合成法.本文简要阐述这6种主要湿化学合成工艺的研究进展,介绍了它们的优缺点,对各种方法进行了比较.     

Processing and Properties of Strontium Bismuth Vanadate Niobate Ferroelectric Ceramics

The processing conditions, microstructure, and dielectric properties of strontium bismuth niobate vanadate ceramics, SrBi₂(V _x Nb_{1− x} )₂O₉ (SBVN, with 0 ≤ x ≤ 0.3), were systematically studied. A relative density of >90% was obtained for all the samples using a two-step sintering process. XRD showed that a single phase with the layered perovskite structure of SrBi₂Nb₂O₉ (SBN) was formed with a vanadium content of up to 30 at.%. SEM revealed that the average grain size decreased gradually with an increase in vanadium content. The Curie point was found to gradually increase from ∼418°C for SBN to ∼459°C for SBVN with 30 at.% vanadium. Dielectric constants at room temperature and their respective Curie points were found to peak at a composition with 10–15 at.%; vanadium. Moreover, a high concentration of vanadium (>5 at.%) resulted in a significant increase in tangent loss at low frequencies (<1000 Hz). The relationships between chemical composition, processing condition, microstructure, and dielectric properties of SBVN ferroelectric ceramics are discussed.     

Room-temperature multiferroicity in NiFe2O4 and its magnetoelectric coupling intensified through defect engineering

The well-known ferromagnetic oxide, NiFe₂O₄, was studied as a potential candidate for room-temperature Type II magnetoelectrics. A spin canting as one of the essential requirements for Type II multiferroics was induced by breaking the stoichiometry, that is, intentionally subtracting Fe ions. We observed that Fe ions were first subtracted exclusively from the tetrahedral sites, leading to an increase in the magnetoelectric coupling owing to an increasing degree of spin canting. The enhancement in the magnetoelectric coupling culminated beyond the subtraction level of ~30 at.%, at which Fe ions started to be removed from the octahedral sites. Alongside, we observed that the subtraction of Fe ions gives rise to a ferroelectricity due to the formation of defect complexes that establish an internal bias field.     

Improved Performance of YIG (Y3Fe5O12) Films Grown on Pt‐Buffered Si Substrates by Chemical Solution Deposition Technique

High‐quality Yttrium iron garnet (YIG) films with crack‐free surface and improved magnetic performance were grown on platinum (Pt)‐buffered Si substrates by chemical solution deposition technique. The saturation magnetization of obtained YIG films can reach 124 emu/cm³, which was the 88% theoretical value of YIG single crystal. The effects of annealing condition were also discussed. When annealed at 750°C for 1 h, YIG films showed a very small coercive field of 12 Oe and the peak‐to‐peak ferromagnetic resonance linewidth can be as low as 95 Oe at 9.10 GHz. The results demonstrated that YIG films prepared on Pt‐buffered Si substrates can be beneficial to the application of YIG films to integrated devices.     

钡钛醋酸盐凝胶制备BaTiO3粉体

本文报道了用ｓｏｌ－ｇｅｌ法制备钡醋酸盐凝胶，并分别作高灼烧和水热处理制备ＢａＴｉＯ３粉体。通过对产物的表征，比较了这两种处理过程，进而对水热条件下陶瓷粉体的形成机理作了探讨。由于水热条件，凝胶在反应介质中溶解，进而结晶形成晶粒，因此所形成的ＢａＴｉＯ３晶粒线度小；线度分布范围窄。晶粒形貌完整，明显优于高温灼烧处理后得到的粉体。     

Production of Fine Ceramic Powders from Chloromethylsilanes Using Pulsed Excimer Radiation

Ceramic powders have been produced by the photodecomposition of chloromethylsilanes using a pulsed excimer laser operating at 193 nm. These feedstocks are cheaper and easier to handle than SiH₄, the main feedstock used in earlier work using continuous–wave CO₂ lasers as the irradiation source. Powders were produced from photolysis of silanes containing methyl groups without the addition of any coreagents, while the two silanes tested containing only chlorine and hydrogen ligands, SiCI₃H and SiCI₄, required the addition of H₂ as a coreagent for significant yields of powder to be produced. The powders produced were found to be low in chlorine, with amorphous Si powders or β-SiC being produced depending on feedstock. Particle sizes measured from electron micrographs varied from less than 10 nm to 1.5 μm.     

用特殊液相沉淀法制备纳米碳酸锶

根据反应胶粒析出机理及实验原理,采用快速高强度机械混合沉淀法,并控制溶液最终pH≥10,让沉淀爆发性快速生成,然后用NH4OH、NH4HCO3清洗液过滤,洗涤,制备了碳酸锶纳米粉体,经TEM和XRD表征,获得的碳酸锶纳米粉体平均粒径为29．3nm,分散性良好,晶型为斜方晶系。     

Direct visualization of magnetic‐field‐induced magnetoelectric switching in multiferroic aurivillius phase thin films

Multiferroic materials displaying coupled ferroelectric and ferromagnetic order parameters could provide a means for data storage whereby bits could be written electrically and read magnetically, or vice versa. Thin films of Aurivillius phase Bi₆Ti_2.8Fe_1.52Mn_0.68O₁₈, previously prepared by a chemical solution deposition (CSD) technique, are multiferroics demonstrating magnetoelectric coupling at room temperature. Here, we demonstrate the growth of a similar composition, Bi₆Ti_2.99Fe_1.46Mn_0.55O₁₈, via the liquid injection chemical vapor deposition technique. High‐resolution magnetic measurements reveal a considerably higher in‐plane ferromagnetic signature than CSD grown films (M_S=24.25 emu/g (215 emu/cm³), M_R=9.916 emu/g (81.5 emu/cm³), H_C=170 Oe). A statistical analysis of the results from a thorough microstructural examination of the samples, allows us to conclude that the ferromagnetic signature can be attributed to the Aurivillius phase, with a confidence level of 99.95%. In addition, we report the direct piezoresponse force microscopy visualization of ferroelectric switching while going through a full in‐plane magnetic field cycle, where increased volumes (8.6% to 14% compared with 4% to 7% for the CSD‐grown films) of the film engage in magnetoelectric coupling and demonstrate both irreversible and reversible magnetoelectric domain switching.     

Low-Temperature Preparation and Size Effect of Strontium Barium Niobate Ultrafine Powder

Ultrafine strontium barium niobate ((Sr _x Ba_{1− x} )Nb₂O₆, SBN) powders were prepared using sol–gel and hydrolysis aging processes. Their structural properties were examined via X-ray diffractometry and Raman spectroscopy. Our results indicated that the SBN powders crystallize to a pure tetragonal phase at annealing temperatures as low as 700°C. The crystallite size of SBN powders subjected to different thermal treatments was in the range of several tens of nanometers. Differential scanning calorimetry revealed that the heat flow had a broadened peak. The transition temperature from the ferroelectric phase to the paraelectric phase and the relative dielectric constant of the SBN50 powder were less than the corresponding values of the bulk ceramic and the single crystal.     

纳米铁酸锌粉体的制备研究进展

铁酸锌粉体是一种重要的磁性材料,不仅具有良好的气敏和光催化特性,而且具有良好的光电转换性能。由于具有这些特殊的性能,人们越来越重视铁酸锌粉体的制备以及应用。本文对纳米铁酸锌粉体的主要制备技术进行了简单介绍,并分析了各种制备方法的发展现状、优势及不足。     

Suppression of Liquid Film Migration and Improvement of Dielectric Properties in Niobium-Doped Strontium Titanate

In an attempt to improve the dielectric properties of SrTiO₃-based boundary-layer capacitors (BLCs), the effects of infiltrant composition on the liquid film migration and dielectric properties in 0.2-mol%-Nb₂O₅-doped SrTiO₃ were investigated. Powder compacts were sintered at 1480°C for 5 h in 5H₂·95N₂ and infiltrated with 80Bi₂O₃·20( x CaO–(1 – x )BaO) at 1300°C for various times in air. When the value of x was 0, 0.2, 0.7, and 1.0, liquid film migration occurred, which formed a new solid-solution layer containing solute species. On the other hand, when x = 0.5, no liquid film migration was observed. The effective dielectric constant was the highest in the sample with x = 0.5 (no liquid film migration), and it decreased as the migration distance increased. In addition, the loss tangent was the lowest, <1%, in the sample with x = 0.5. Agreement between the estimated effective dielectric constants and the measured values showed that the suppression of liquid film migration improved the dielectric constant. The agreement further indicated that the prediction of the dielectric constant in SrTiO₃-based BLCs was possible using a two-layer model with a liquid-infiltrated layer and a SrTiO₃-based oxidized layer.     

Charge-Compensating Defect in Ceria-Doped Strontium Barium Niobate by Electron Energy-Loss Spectroscopy and X-ray Photoelectron Spectroscopy

Undoped and CeO₂-doped Sr_0.5Ba_0.5Nb₂O₆ (SBN) powders were synthesized using the solid-state reaction method. The lattice parameters of undoped and CeO₂-doped SBN were evaluated using X-ray diffractometry. The valence state of the cerium ion in Ce-doped SBN was identified using X-ray photoelectron spectroscopy (XPS), and the valence of 3+ was confirmed. The charge-compensated defect of the cerium doping in SBN ceramics, i.e., excessive oxygen ions occupying the vacant O(4) or O(5) site, was further evidenced using electron energy-loss spectroscopy (EELS) and XPS. The relationships of charge-compensated defect, structure, binding energy, and temperature of the maximum of the relative permittivity ( T _m) were discussed.     

热水溶液中钛酸锶晶粒的结晶过程

研究了以TiCl4水解得到的H4TiO4胶体作为钛源,在热水溶液中制备SrTiO3晶粒的结晶过程,TEM和XRD结果表明,其结晶过程主要包括3个阶段,第1阶段主要是SrTiO3晶核的形成过程;第2阶段主要是SrTiO3晶粒的配向聚集生长过程;第3阶段主要是SrTiO3晶粒的溶解结晶过程,其中第1阶段的控制步骤为H4TiO4胶体的溶解,第2阶段中晶粒的配向聚集生长在短时间内即可完成,很快进入第3阶段小晶粒的溶解和大晶粒的重结晶过程,该阶段反应速度慢,需要较长时间才能得到结晶完好的晶粒,在热水溶液中以H4TiO4胶体作为钛源制备SrTiO3晶粒关键在于控制结晶过程的第1阶段和第2阶段。