The phase transitions and ferroelectric behavior of dense nanocrystalline BaTiO3 ceramics fabricated by pressure assisted sintering

Dense nanocrystalline BaTiO₃ ceramics with uniform grain sizes of 30 nm was obtained by pressure assisted sintering. The phase transitions were investigated by Raman scattering at temperatures ranging from −190 to 200 °C. With increasing temperature, similar to 3 μm BaTiO₃ normal ceramics, the successive phase transitions from rhombohedral to orthorhombic, orthorhombic to tetragonal, tetragonal to cubic were also observed in 30 nm BaTiO₃ ceramics. Especially, the coexistence of ferroelectric tetragonal and orthorhombic phases was found at room temperature. The ferroelectric behavior was further characterized by P-E hysteresis loop. The experimental results indicate that the critical grain size of the disappearance of ferroelectricity in nanocrystalline BaTiO₃ ceramics fabricated by pressure assisted sintering is below 30 nm.     

纳米BaTiO3陶瓷的超高压烧结

将10 nm钛酸钡粉在6 GPa超高压条件下进行烧结,得到了晶粒大小约为30 nm的钛酸钡陶瓷.用扫描电子显微镜和原子力显微镜观测了样品的微观结构.研究表明,由于超高压能够压碎纳米粉体中的团聚体,而且能增加烧结的驱动力,降低成核的势垒,从而使成核速率增加;同时由于扩散能力的降低而使生长速率减小,所以超高压烧结能在较低的...     

Effect of Annealing on Ferroelectric Properties of Nanometre BaTiO3 Ceramics Prepared by High Pressure Sintering Method

Dense nanocrystalline BaTiO3 ceramics with a grain size of 5Onto are prepared under 6 GPa at 1273K using a high pressure sintering method. The sintered bulk is uniform and the relative density is above 97%. We anneal the ceramic samples in oxygen with various temperatures and for the annealing, several broadened peaks can be observed at different times without apparent grain growth. After about 378K（ by dielectric measurements. However, these peaks are very different from those of coarser-grained ceramics. It is indicated that both the elimination of oxygen vacancies and the release of residual stresses caused by high pressure greatly improve the overall ferroelectric properties of BaTiO3 ceramics. The observation of nearly linear polarization hysteresis loop after anneal provides the solid evidence of ferroelectricity in these nano-sized BaTiO3 ceramics. It is believed that the absence of 90° domains and the existence of poor-permittivity nonferroelectric grain boundaries contribute to the slim loop.     

高压烧结法合成致密纳米BaTiO3陶瓷结构和铁电性能研究

 在6 GPa压力、1 000 ℃温度条件下制备了致密的纳米BaTiO₃陶瓷，合成样品的平均晶粒尺寸为50 nm，理论密度在97%以上。通过介电测量，观察到了样品宽化的相变峰，它与粗晶陶瓷的相变峰大不相同。由于90°电畴的减少和退极化场的存在，观察到了细长的电滞回线，它是样品铁电性存在的有力证据，表明钛酸钡陶瓷的临界尺寸在50 nm以下。     

Electrospinning of complex oxide nanofibers

Electrospinning is a versatile process for drawing fibers of diverse materials including polymers, ceramics, and composites. We demonstrate here its application in the synthesis of complex ceramic oxide materials. The phase formation and morphology of BaTiO₃ nanofibers synthesized via electrospinning is investigated as a function of heat treatment conditions. Fully crystallized BaTiO₃ nanofibers with the perovskite structure are obtained after annealing at 750 °C and show an average grain size of about 30 nm. Tetragonal crystal structure of the fibers is indicated by XRD peak splitting (calculated c/a ratio=1.007), and confirmed by Raman spectroscopy. Furthermore, the advancement in heat treatment of the electrospun fibers yields single crystalline BaTiO₃ nanofibers with 50 nm in diameter and lengths up to 1 μm.     

表面修饰的钛酸钡的拉曼光谱

采用水热法制得表面包裹有十二烷基苯磺酸(DBS)和硬脂酸(St)的BaTiO₃纳米粒子，平均粒径均为60nm左右.拉曼光谱检测结果表明，样品的确为表面包裹有表面活性剂(DBS或St)的BaTiO₃纳米粒子集合体.与未包裹的相比，各光学声子模对应的拉曼振动模式峰均发生了蓝移.还发现不同的包裹体，所导致的蓝移值也不同.从纳米粒子的表面结构及包裹层的压力出发对此现象进行了定性解释和讨论. 关键词： 拉曼光谱 3纳米粒子')" href="#">DBS/ BaTiO₃纳米粒子 蓝移     

Effect of sintering temperature on phase formation,microstructure and dielectric properties of nanogold modified barium titanate ceramics

In this work, BaTiO₃ ceramics modified with 0.5 mol% Au nanoparticles were fabricated by using a combination of the solid-state reaction and pressureless-sintering techniques. By employing a combination of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), Archimedes principle and dielectric measurement techniques, it was found that no phases other than tetragonal BaTiO₃ were observed in all ceramics. In contrast to the tetragonality, the relative density, grain size and maximum dielectric constant at Curie temperature of the ceramics were found to increase with sintering temperature. In addition, it has been found that, under suitable sintering temperature, dense perovskite nanogold modified BaTiO₃ ceramics with fine-grained microstructure (∼1 μm) and better dielectric properties than those of gold-free ceramics can be produced.     

Synthesis and ion conductivity of (Bi2O3)0.75(Dy2O3)0.25 ceramics with grain sizes from the nano to the micro scale

Using (Bi₂O₃)_0.75(Dy₂O₃)_0.25 nano-powder synthesized by reverse titration co-precipitation method as raw material, dense ceramics were sintered by both Spark Plasma Sintering (SPS) and pressureless sintering. According to the predominance area diagram of Bi-O binary system, the sintering conditions under SPS were optimized. (Bi₂O₃)_0.75(Dy₂O₃)_0.25 ceramics with relative density higher than 95% and an average grain size of 20 nm were sintered in only 10 min up to 500 °C. During the pressureless sintering process, the grain growth behavior of (Bi₂O₃)_0.75(Dy₂O₃)_0.25 followed a parabolic trend, expressed as D² − D₀² = Kt, and the apparent activation energy of grain growth was found to be 284 kJ mol^− 1. Dense (Bi₂O₃)_0.75(Dy₂O₃)_0.25 ceramics with different grain sizes were obtained, and the effect of grain size on ion conductivity was investigated by impedance spectroscopy. It was shown that the total ion conductivity was not affected by the grain size down to 100 nm, however lower conductivity was measured for the sample with the smallest grain size (20 nm). But, although only the δ phase was evidenced by X-ray diffraction for this sample, a closer inspection by Raman spectroscopy revealed traces of α-Bi₂O₃.     

Heat capacity of nanostructured BaTiO3 ceramics

The temperature dependence of the heat capacity of nanostructured BaTiO₃ ceramics produced by the solid-phase method has been studied; before sintering, the synthesized charge is subjected to a severe action in Bridgman anvils in combination with shear deformation. It has been found that the ferroelectric transition is substantially smeared and the phase transition temperature decreases nonlinearly with increasing applied pressure. It has been shown that the defect structure plays a dominant role in the formation of physical properties of the ceramics.     

Dielectric and magnetic properties of low-temperature fired NiCuZn-BaTiO3 composites

The composition effects on the dielectric and magnetic properties of NiCuZn-BaTiO₃ composites fired at low temperature were investigated. The coexistence of perovskite BaTiO₃ and spinel ferrite phases in the composites were observed; no significant chemical reactions occurred between BaTiO₃ and NiCuZn ceramics during sintering. The nanosized BaTiO₃ powders favored a decrease in grain size. The saturation magnetization, remanent magnetization and real permeability continuously decreased with increasing BaTiO₃ content. And the real permittivity continuously increased with the BaTiO₃ content. The Q-factor (quality factor) exhibited relatively high values with 20-30 wt% BaTiO₃. All composite materials exhibited a low dielectric loss below 100 MHz. Synthetically considerations, the composites with 20-30 wt% BaTiO₃ could obtain relatively high real permeability and real permittivity values, and the magnetic and dielectric losses were relatively low, so they were the best candidates to produce LC-integrated chip elements.     

Effect of low-temperature high-pressure sintering on BiFeO3 density,electrical magnetic and structural properties

Single-phase multiferroic BiFeO₃ (BFO) powders were prepared by hydrothermal microwave synthesis and dense BiFeO₃ ceramics were fabricated for the first time by the low-temperature high-pressure (LTHP) sintering technique. Effect of sintering temperature ranging from 400 to 800 °C (3 min and 10 min) and pressure of 3–8 GPa on structural, microstructural, electric and magnetic properties were investigated through X-ray diffraction, scanning electron microscope (SEM), electrochemical impedance spectroscopy (EIS), density and magnetic measurements. The results highlighted that LTHP sintering method, thanks to the high pressure involved, requires lower temperature and shorter time than other techniques, avoiding BiFeO₃ phase degradation. SEM images show that for short experimental time (t = 3 min) the average grain size of the sintered samples was approximately the same size of raw powder. Extending the sintering time up to 10 min the grain growth phenomena occurred. Moreover the results indicate that the best obtained density value was around 98% of theoretical density. The dielectric behavior of BiFeO₃ ceramics was not significantly influenced by the LTHP sintering conditions. Magnetic measurements showed that ceramic BiFeO₃ is weakly ferromagnetic at room temperature.     

Magnetic properties of xNi0.53Cu0.12Zn0.35Fe1.88O4+(1−x)BaTiO3 nanocomposites

The nanocrystalline Ni_0.53Cu_0.12Zn_0.35Fe_1.88O₄ and BaTiO₃ powders were prepared using Microwave-Hydrothermal (M-H) method at 160 °C/45 min. The as synthesized powders were characterized using the X-ray diffraction (XRD) and Transmission Electron Microscope (TEM). The size of the powders that were synthesized using M-H system was found to be ∼30 and ∼50 nm for ferrite phase and ferroelectric phases, respectively. The powders were densified using microwave sintering method at 900 °C/30 min. The ferrite and ferroelectric phases were observed from XRD and morphology of the composites was observed with the Scanning Electron Microscope (SEM).The magnetic hysteresis loops were recorded using the Vibrating Sample Magnetometer (VSM).The frequency dependence of real (μ′) and imaginary (μ″) parts of permeability was measured in the range of 1 MHz-1.8 GHz. The permeability decreases with an increase of BaTiO₃ content at 1 MHz. The transition temperature (T_C) of ferrite was found to be 245 °C. The T_C of composite materials decreases with an increase in BaTiO₃ content.     

Enhanced magnetization and suppressed current leakage in BiFeO3 ceramics prepared by spark plasma sintering of sol-gel derived nanoparticles

BiFeO₃ (BFO) ceramics of different grain size have been synthesized by spark plasma sintering of sol-gel derived nanoparticles. It was found that with decreasing grain size there occurs an enhancement in magnetization and a simultaneous suppression in current leakage. According to systematic materials characterization, the enhanced magnetization is attributed to the enriched grain boundaries where the missing structural order perturbs the spin helix structure of BFO and thus generates uncompensated spins, while the reduced current leakage is ascribed to fewer conduction paths provided by the compacted grain structure.     

Ferroelectric aging behaviors of BaTi0.995Mn0.005O3 ceramics: grain size effects

We report the ferroelectric aging effect of dense BaTi_0.995Mn_0.005O₃ ceramics with grain size varying from 2000 nm to 150 nm. Given the identical aging process, it is revealed that the significant aging effect with clear double-hysteresis loop, observed in coarse-grain sample, is substantially suppressed with decreasing grain size. This suppression can be attributed to the reduction of tetragonal distortion and the grain boundary barrier effect in fine-grain sample. Consequently, the weak thermodynamic driving force and the limited kinetic migration are unfavorable to a reversible domain switching, resulting in a normal hysteresis loop in small grained samples.     

Core-shell structure and size effect in barium titanate nanoparticle

A quantitative core-shell structure model on the grain size effect of nano-BaTiO₃ ceramics has been proposed on the basis of the Ginsburg–Landau–Devonshire thermodynamic theory. In this study, we have considered the existence of surface effects, which leads to the core-shell structure in BaTiO₃ nanoparticles consisting of inner tetragonal core, gradient lattice strain layer, and surface cubic layer. The size effects on the spontaneous polarization, Curie temperature, c/a ratio, and electrocaloric and dielectric properties of BaTiO₃ ferroelectric nanoparticle are successfully explained. Theoretical and experimental results for BaTiO₃ nanoparticle are compared and discussed.     

Co:BaTiO3/Si(100)纳米复合薄膜制备、微结构及其紫外光电子能谱研究

采用脉冲激光气相沉积(PLD)方法,在Si(100)晶面上制备了Co:BaTiO₃纳米复合薄膜.采用X射线衍射(XRD)结合透射电镜(TEM)方法研究了两种厚度Co:BaTiO₃纳米复合薄膜的晶体结构,当薄膜厚度约为30 nm时,薄膜为单一择优取向;当薄膜厚度约为100nm时,薄膜呈多晶结构.原子力显微镜(AFM)分析表明,当膜厚为30nm时,薄膜呈现明显的方形晶粒.采用紫外光电子能谱(UPS)研究了Co的价态和Co:BaTiO₃纳米复合薄 关键词： 3')" href="#">BaTiO₃ 纳米复合薄膜 紫外光电子能谱     

Co:BaTiO3/Si(100)纳米复合薄膜制备、微结构及其紫外光电子能谱研究

采用脉冲激光气相沉积(PLD)方法，在Si(100)晶面上制备了Co:BaTiO₃纳米复合薄膜.采用X射线衍射(XRD)结合透射电镜(TEM)方法研究了两种厚度Co:BaTiO₃纳米复合薄膜的晶体结构，当薄膜厚度约为30 nm时，薄膜为单一择优取向；当薄膜厚度约为100nm时，薄膜呈多晶结构.原子力显微镜(AFM)分析表明，当膜厚为30nm时，薄膜呈现明显的方形晶粒.采用紫外光电子能谱(UPS)研究了Co的价态和Co:BaTiO₃纳米复合薄     

Structural and optical characterization of sol–gel derived Tm-doped BaTiO3 nanopowders and ceramics

Nanocrystalline Tm³⁺(5%)-doped BaTiO₃ (BT-Tm) has been synthesized by the sol–gel method. The morphology, structure, and optical properties of powders and ceramics were characterized. The average grain size of the gel precursor annealed at 700 and 900 °C was 20 nm and 30 nm, respectively. These powders were single phase and crystallized with a cubic structure while the BT-Tm sintered ceramics were crystallized with the tetragonal BaTiO₃ structure. The photoluminescence spectra showed typical transitions of Tm³⁺ ions and a structure consistent with the Tm³⁺ ions incorporation in the BaTiO₃ crystalline lattice. Thermoluminescence peaks recorded at 300 °C (for annealed samples) or at 230 °C for the ceramic sample were assigned to the recombination of the Tm²⁺-electron traps located mainly at the surface of the nano-crystals or inside the microcrystals, respectively.     

Dielectric and ferroelectric properties of NBT-BT systems

ABSTRACT

The (1-x)Na_0.5Bi_0.5TiO₃- xBaTiO₃ ceramics (x = 0.1, 0.135 and 0.17) were fabricated by a conventional solid phase sintering process. The bulk density of the obtained samples exceeded 95% of the theoretical relative density as determined by Archimedes method. Dielectric and ferroelectric measurements of these ceramics were performed. Measurements of the ferroelectric properties show that, above the depolarization temperature T_d, the shape of the hysteresis loops approaches that of linear dielectrics. The dielectric study results correlate with the hysteresis loops measurements. The relaxor-like behavior of the investigated materials was revealed.     

Nanocrystalline BaTiO3, doped with Eu and Cr: Fluorescence spectroscopy and spontaneous ordering effects

The results of optical studies of Cr³⁺ and Eu³⁺ ions doped nanocrystalline ferroelectric BaTiO₃ produced by the sol-gel process (particle size 20-40 nm) are reported. The Cr³⁺ impurity ions ²E-⁴A₂ (R-lines) fluorescence spectra in BaTiO₃ revealed significant differences from that reported in literature for the bulk material. At least three types of Cr³⁺ centers were found in the spectra. The temperature dependence of optical second harmonic generation in nanocrystalline BaTiO₃: Eu³⁺ shows a strong hysteresis in C_4v-O_h ferroelectric phase transition region, which was explained by the ordering effects in the system of electric dipole moments of dipole nanocrystals. The temperature dependencies of radiative lifetime of Eu³⁺⁵D₀ excited level reveal some hysteresis too. The possibility of influence of the ordering in the system of BaTiO₃:Eu³⁺ nanocrystals on the effective index of refraction of the medium and thus on the Eu³⁺⁵D₀ radiative lifetime, due to the local field effects is discussed.