The grain-size-dependent behaviors of nano-grained ferroelectric polycrystals: a phase-field study

In this work, the electromechanical behaviors of nano-grained ferroelectric ceramics are numerically studied with different grain sizes. The material under investigation is nano-grained barium titanate ceramic, which shows noticeable grain size dependence of the electromechanical characteristics. A 2D polycrystal phase-field model is developed to investigate the hysteresis loops and the process of the microstructural evolution upon applied bipolar electric field. A core–shell model has been selected to treat the grain interior and the grain boundary with separate ferroelectric properties. Such treatment is analogous to the consideration of a grain-boundary-affected zone as in the study of nano-crystalline plasticity. The grain size dependence of the ferroelectric properties, e.g., coercive electric field and remnant polarization, is examined based on this model, and the mechanism of the grain size dependence is analyzed.     

溶胶-凝胶法制备PbTiO3铁电玻璃陶瓷的高频晶粒尺寸效应

首次报道了用溶胶－凝胶法制备的钛酸铅铁电玻璃陶瓷的晶粒尺寸对其高频介电性能的影响。研究表明,铁电玻璃陶瓷的高频介电弛豫主要是由材料内部的畴壁共振引起的。晶粒尺寸增加,畴壁宽度增大,介电弛豫向低频移动;反之,晶粒尺寸减小,畴壁宽度变小,介电弛豫向高频移动,弛豫频率提高,介电损耗变小。     

Grain size effects on the dielectric properties of ferroelectric Bi2VO5.5 ceramics

Dielectric properties and microstructural characteristics of ferroelectric bismuth vanadate (Bi₂VO_5.5) ceramics exhibiting grain sizes of 7, 10, 20 and 25 m have been studied. Microstructural studies indicate the presence of ferroelectric 90° domain patterns on the surface as well as in the bulk of the coarse-grained ceramics. The dielectric constant and the loss tangent both at room temperature and in the vicinity of the Curie temperature have been found to increase with increasing grain size. The Curie temperature (725 K) is found to shift slightly (by about 7 K) towards higher temperatures as the grain size increases (7–25 m). The magnitude of the dielectric anomaly around 725 K is found to be higher for coarse-grained ceramics. The dielectric constant and the loss have been found to decrease with increase in frequency (1–100 kHz) for all the ceramics studied. The increase in dielectric constant with increasing grain size is attributed to a decrease in thickness of the relatively more insulating grain boundary layer.     

铁电体中相变临界尺寸的研究现状

从铁电薄膜和铁电颗粒两大方面回顾了处理尺寸效应的理论研究方法(包括宏观的热力学唯像理论、微观的横场Ising模型和第一性原理计算)和铁电相变临界尺寸的理论预测. 总结了从50年代以来实验中观测到的各种铁电材料的临界尺寸, 简单介绍了铁电纳米陶瓷的一些尺寸效应, 并叙述了近几年才开始报道的铁电纳米线中的铁电相变. 最后指出了目前研究现状中存在的一些问题.     

(Bi0.5Na0.5)0.935Ba0.065TiO3-xBiScO3陶瓷储能及应变性能研究

本研究采用BiScO₃组分对固相烧结工艺制备的(1-x)(Bi_0.5Na_0.5)_0.935Ba_0.065TiO₃-xBiScO₃(BNBT-xBS)无铅陶瓷进行改性, 考察了BiScO₃掺杂含量对陶瓷的微观结构、储能、场致应变和介电等性能的影响。结果表明: 随着BiScO₃掺杂含量的增加, BNBT-xBS陶瓷的相结构由三方相与四方相共存演变为伪立方相, 无杂相形成, 且平均晶粒尺寸略有增大; BiScO₃组分的引入破坏了BNBT陶瓷铁电畴的长程有序, 表现出弱极化, 且伴随有铁电相到弛豫铁电相的相变过程。BiScO₃组分提高了储能和应变性能, 在70 kV/cm电场下其最大储能密度为0.46 J/cm³, 电致应变达到0.25%。介电常数随着掺杂含量的增加逐渐降低, 其介电行为也表明陶瓷具有弛豫铁电体特征; BNBT-xBS陶瓷表现出负温度系数效应, 且在450℃以下具有较好的绝缘性。     

Domain switching in polycrystalline ferroelectric ceramics

Ferroelectric ceramics are widely used as sensors and actuators for their electro-mechanical properties, and in electronic applications for their dielectric properties. Domain switching--the phenomenon wherein the ferroelectric material changes from one spontaneously polarized state to another under electrical or mechanical loads--is an important attribute of these materials. However, this is a complex collective process in commercially used polycrystalline ceramics that are agglomerations of a very large number of variously oriented grains. As the domains in one grain attempt to switch, they are constrained by the differently oriented neighbouring grains. Here we use a combined theoretical and experimental approach to establish a relation between crystallographic symmetry and the ability of a ferroelectric polycrystalline ceramic to switch. In particular, we show that equiaxed polycrystals of materials that are either tetragonal or rhombohedral cannot switch; yet polycrystals of materials where these two symmetries co-exist can in fact switch.     

纳米铌酸钾钠陶瓷的制备及性能

以新型溶胶-凝胶法制备的平均晶粒尺寸为30 nm的铌酸钾钠粉体为原料, 采用放电等离子体烧结工艺, 在烧结温度为900℃, 压力30 MPa, 烧结时间1 min的条件下, 制备得到纯正交相, 相对密度高达99%以上, 平均晶粒尺寸为40 nm的纳米铌酸钾钠陶瓷, 并对该陶瓷的相结构、微观形貌、介电性能和铁电性能进行了研究。结果表明, 与普通微米晶陶瓷不同, 纳米铌酸钾钠陶瓷的室温介电常数仅为341, 并且随温度变化不明显, 表现出明显的介电弛豫现象, 弥散因子γ为1.60, 并具有明显的电滞回线, 矫顽场强度为13.5 kV/cm, 剩余极化为1.5 μC/cm²。尺寸降低所引起的纳米铌酸钾钠陶瓷中晶界相所占的比例增大是其性能变化的主要原因, 并且可以推断, 如果铌酸钾钠陶瓷具有“临界尺寸”, 那么其值应该在40 nm以下。     

Multiaxial behavior of ferroelectric single and polycrystals with pressure dependent boundary effects

The external electric and mechanical fields applied at angles to the initial poled direction of the ferroelectric ceramics produce a significantly different nonlinear behavior to that of external fields applied parallel to the poling direction. This angle dependent response of ferroelectric single and polycrystals are predicted by the model proposed based on irreversible thermodynamics and physics of domain switching. The dissipation associated with boundary constraints in thin ferroelectric single crystals are incorporated in the model. As well, the pressure dependent constraints imposed by the surrounding grains on the grain of interest at its boundary during domain switching is correlated with the resistance experienced by a ferroelectric single crystal on its boundary during domain switching. Taking all the domain switching possibilities, the volume fractions of each of the variants in a grain are tracked and homogenized for macroscopic behavior. Numerical simulations were carried out for the multiaxial behavior of ferroelectric single and polycrystals under electrical, mechanical and electromechanical loading conditions.     

Synthesis and Sintering of Bi2Ti4O11

The conditions of Bi₂Ti₄O₁₁ formation are studied. It is shown that the process is diffusion-controlled and depends significantly on the heating rate. The optimal conditions for producing Bi₂Ti₄O₁₁ ceramics are determined. Cold pressing of Bi₂Ti₄O₁₁ ceramic powder at high pressures is shown to influence the unit-cell parameters and volume, positional parameters of Bi and Ti, and bond distances in the ferroelectric ceramics produced by subsequent sintering. The ferroelectric transition temperature decreases steadily with increasing densification pressure.     

退火处理对铅基弛豫型铁电体介电、压电性能的影响

铅基弛豫型铁电体的峰值介电常数和压电性能在退火后有较大提高．在PMN－PT陶瓷中,k_p由58％提高到66％,峰值介电常数由30900提高到37200,d_(33)达到530pC／N.在PZN－PT－BT陶瓷中,k_p由35％提高到51％,峰值介电常数km由11400提高到29000,d_(33)由347PC／N提高到624pC／N．这种改善可能与晶界玻璃相的消除以及畴壁运动等因素有关     

Quantitative dependence of the properties of Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 ferroelectric ceramics on porosity

Porous Pb_0.99(Zr_0.95Ti_0.05)_0.98Nb_0.02O₃ ferroelectric ceramics with a pore size of the order of the crystalline grain size were prepared and the microstructure and the properties were investigated. Based on this microstructure, the net porosity of the ceramics can be attributed to the intentionally introduced extrinsic porosity and thus the quantitative dependence of ferroelectric and dielectric properties of the ceramics on the porosity can be established respectively. A good agreement with experimental measurements was obtained. Our work represents the first attempt to tailor the properties of ferroelectric ceramics via varying the porosity from the viewpoint of application.     

EFFECTS OF CRACK LENGTH, NOTCH ROOT RADIUS AND GRAIN SIZE ON FRACTURE TOUGHNESS OF FINE CERAMICS

Generally, fracture toughness and fracture stress of ceramics depend on crack length, notch root radius and grain size. These three parameters are most important when assessing the integrity of structural ceramic members and developing high-performance ceramics. A new failure criterion called the process zone size failure criterion, has been proposed based on the existence of a crack-tip process zone. Using this criterion, it is shown that theoretical values are in good agreement with many test results quoted from many papers. It is concluded that this failure criterion is useful when evaluating crack length and notch root radius problems. The effect of grain size on both the fracture toughness and on the toughening mechanism is also considered.     

The grain sizes of mixed oxide and chemically prepared PLZT-powders

PLZT-powders were prepared applying usual chemical and oxide processes. The grain size of both chemically prepared and milled calcined mixed oxide powders as the pre-state materials for the fabrication of ferroelectric transparent ceramics were investigated by transmission and scanning electron microscopy (TEM,SEM). The grain diameters evaluated by these quasi-absolute techniques are compared to the results obtained by other methods supplying dispersity quantities. Their relations to the absolute grain sizes, given by form factors, are discussed.     

Nb掺杂Bi_4Ti_3O_(12)陶瓷的微结构与介电性能研究

采用固相烧结工艺制备了Nb掺杂的Bi4Ti3O12(BIT)铁电陶瓷.用XRD和AFM对其微观结构进行了分析,研究了Nb掺杂对材料微观结构和介电性能的影响.结果发现,微量Nb的掺入并未改变BIT的晶体结构,但可减小陶瓷的晶粒尺寸并降低材料的居里温度.同时,Nb的掺入大大降低了BIT陶瓷的介电常数并削平了BIT陶瓷的介电损耗峰,适当掺杂Nb可明显降低BIT陶瓷的介电损耗.     

Effects of sintering temperature on microstructure and electrical properties of 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 modified with CuO

The structural, dielectric and ferroelectric properties of Pb(Mg_1/3Nb_2/3)_0.9Ti_0.1O₃ (PMNT) ceramics prepared with CuO as a sintering aid at various sintering temperatures between 950 °C and 1150 °C are investigated. The lattice parameters slightly increase with the sintering temperature >1050 °C. A significant increase in the grain size is observed when the sintering temperature is increased from 1000 °C to 1050 °C. The maximum dielectric constant reaches the highest value of ∼22,000 for the ceramic sintered at 1050 °C. For the ceramics sintered at >1050 °C, the temperature of maximum dielectric constant and the diffuseness parameters tend to increase with the increasing sintering temperature. The optimal sintering temperature for this ceramic is 1050 °C, which displays significant improvements in ferroelectric properties at room temperature, i.e. the increase in the remanent polarization and the ferroelectric loop squareness.     

Effect of sintering temperature on the grain growth and electrical properties of barium zirconate titanate ferroelectric ceramics

In the present work, structural, dielectric and ferroelectric properties of barium zirconate titanate ferroelectric ceramics have been investigated. The specimens were synthesized using a solid state reaction technique. The XRD analysis reveals that the synthesized compound was formed with no secondary phases. As the sintering temperature increases from 1,200 to 1,300 °C, the average grain size is observed to increase from ~0.39 to ~6.15 μm. The dielectric measurements as a function of temperature show a decrease in Curie temperature (T_C) on increasing the sintering temperature. The decrease in Curie temperature is attributed to the substitution of Zr⁺⁴ whose ionic radius is larger than Ti⁺⁴. A large increase in the dielectric constant with the increase in grain size is observed. The remanent polarization is also observed to increase with the increase in grain size.     

Effects of Nb5+ doping on sintering and electrical properties of lead-free (Bi0.5Na0.5)TiO3 ceramics

The Nb⁵⁺ doped (Bi_0.5Na_0.5)TiO₃ (BNT) ceramics were manufactured by a conventional solid state reaction method. The influence of Nb⁵⁺ doping on the sintering, microstructure and various electrical properties of BNT ceramics was investigated. The results of X-ray diffraction show that the solubility limit of Nb⁵⁺ in the BNT lattice is less than 3%. Additionally, Nb⁵⁺ doping produces significant effects on the densification and grain growth of BNT ceramics. Various electrical properties of BNT ceramics are obviously changed with doping a small amount of Nb⁵⁺. The ferroelectric and piezoelectric properties display enhanced values at a low doping level. The formation of A-site vacancies is considered as the reason for the changed ferroelectric and electromechanical behavior.     

Dielectric properties of Y2O3 donor-doped Ba0.8Sr0.2TiO3 ceramics

Ba_0.8Sr_0.2TiO₃ ceramics doped with Y₂O₃ from 0 to 0.10 mol% exhibit normal ferroelectric phase transition, while the ceramics doped with Y₂O₃ from 0.20 to 0.30 mol% show a giant dielectric constant behavior with loss less than 0.15 at 1 kHz from −40 °C to 140 °C, which is suggested due to semiconductive grain and the Maxwell–Wagner effect by structure disordering in grain boundary. The analyses of unipolar charge for the semiconductive grain indicate three kinds of dielectric processes: thermally stimulated process of unipolar hopping, dispersion process of dielectric constant with frequency, and phase transition process accompanied with disappearance of giant dielectric constant in cubic phase. The XPS results confirm that some of the barium ions are in low energy state to form e-Ba²⁺ and to provide hopping sites for electrons. The ceramics doped with Y₂O₃ from 0.50 to 0.75 mol% recover the normal ferroelectricity. The possible mechanics are relevant to binding effect of cation vacancies on electrons.     

Effects of MnO2 and sintering temperature on microstructure, ferroelectric, and piezoelectric properties of Ba0.85Ca0.15Ti0.90Zr0.10O3 lead-free ceramics

Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ + xmol% MnO₂ lead-free ceramics have been prepared by a conventional sintering method and the effects of MnO₂ and sintering temperature on microstructure, ferroelectric, and piezoelectric properties of Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ lead-free ceramics have been studied. The addition of 0.25 mol% MnO₂ promotes grain growth, improves the ferroelectricity of the ceramics and strengthens ferroelectric tetragonal–ferroelectric orthorhombic phase transition near 40 °C. Because of the coexistence of tetragonal and orthorhombic phases and the combinatory effects of soft and hard doping of Mn ions, the ceramic with x = 0.25 exhibits the optimum piezoelectric properties (d ₃₃ = 306 pC/N and k _p = 42.2 %, respectively). Excess MnO₂ inhibits the grain growth and degrades the ferroelectric and piezoelectric properties of the ceramics. Sintering temperature has an important influence on the microstructure, tetragonal–orthorhombic phase transition near 40 °C, ferroelectric and piezoelectric properties of the ceramics. The increase in sintering temperature leads to large grains and more noticeable tetragonal–orthorhombic phase transition near 40 °C, enhances ferroelectricity and thus improves effectively the piezoelectricity of the ceramics. The Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ ceramic sintered at 1350 °C possesses the optimum piezoelectric constant d ₃₃ value of 373 pC/N.     

On the feasibility of synthesizing complex perovskite ferroelectric ceramics via a B-site oxide mixing route

Several relaxor-based complex perovskite ferroelectric ceramics were synthesized by conventional solid-state reaction method via different routes, i.e., the B-site oxide mixing route, the straight calcination method and the columbite/wolframite precursor method. Structure analysis and physical properties measurements prove that the B-site oxide mixing route is efficient and feasible in the synthesizing of complex perovskite ferroelectrics since such a technique is superior in suppressing pyrochlore phases, stabilizing the perovskite structure, and improving the amount of perovskite phase as compared to the straight calcination method and the columbite/wolframite precursor method. The dielectric properties are improved correspondingly, namely exhibiting the largest value of dielectric maximum, the sharpest dielectric response peaks and the least frequency dispersion of dielectric properties in the ferroelectric ceramics prepared via the B-site oxide mixing route, which is considered as correlating with the amount of pyrochlore phases, ceramic density, grain size, and microstructure morphology of the ceramics synthesized.