Necessary Conditions for the Formation of {111} Twins in Barium Titanate

The experimental conditions for {111} twin formation in BaTiO₃ were investigated. When BaTiO₃ compacts without excess TiO₂ were sintered either in an oxidizing atmosphere (air) or in a reducing atmosphere (95N₂–5H₂), no {111} twins formed within the BaTiO₃ grains and no abnormal grain growth occurred. In contrast, many {111} twins were present within the abnormally grown grains in the excess-TiO₂-containing BaTiO₃ samples sintered in air, while no twins were observed in the excess-TiO₂-containing samples sintered in 95N₂–5H₂. X-ray diffraction analysis showed that excess TiO₂ forms a Ba₆Ti₁₇O₄₀ phase during sintering with the space group A 2/ a in air and a Ba₆Ti₁₇O_{40− x} phase with the space group C in 95N₂–5H₂. It appears therefore that excess TiO₂ and an oxidizing atmosphere are necessary for {111} twin formation in BaTiO₃. These results may also indicate that the interface structure between BaTiO₃ and Ba₆Ti₁₇O₄₀ influences the twin formation.     

Anomalous Grain Growth in Donor-Doped Barium Titanate with Excess Barium Oxide

Grain growth and semiconductivity of donor-doped BaTiO₃ceramics with an excess of BaO and additions of SiO₂or B₂O₃were studied. The microstructures and electrical measurements on sintered samples revealed that their electrical properties are related to the microstructure development of the sintered samples. Samples heated with an excess of BaO developed a normal microstructure during sintering, as a consequence of normal grain growth (NGG), and were yellow and insulating. In contrast, samples with an excess of BaO and an addition of SiO₂or B₂O₃exhibited anomalous grain growth (AGG) and were dark blue and semiconducting after sintering. When some BaTiO₃seed grains were embedded in a sample of donor-doped BaTiO₃with an excess of BaO (without SiO₂or B₂O₃), AGG was observed, i.e., some seed grains grew into large grains and were blue and semiconducting. An explanation is given for why AGG is responsible for the oxygen release and the formation of semiconducting grains in donor-doped BaTiO₃and not NGG.     

Effect of Dislocations on Grain Growth in Strontium Titanate

Two series of experiments were done to investigate the effect of dislocations on grain growth in SrTiO₃. In the first series, we observed the growth behavior of a single-crystal plate toward a TiO₂-excess SrTiO₃ powder compact, containing different dislocation densities on two equivalent {100} surfaces. The surface with a higher dislocation density exhibited faster growth, showing interface mobility enhancement by dislocation. In the second series, a polycrystalline SrTiO₃ sample which had been plastically deformed by hot pressing was embedded in a TiO₂-excess SrTiO₃ powder compact, and its growth behavior toward the powder compact was compared with that of a sample without hot pressing. As with a single crystal, the grains with the higher dislocation density in the plastically deformed sample grew faster. In addition, some grains in the plastically deformed sample showed the characteristic of abnormal grain growth. The transmission electron microscopy (TEM) observations on a sintered TiO₂-excess SrTiO₃ showed that the abnormally large grains contained many dislocations while the fine matrix grains contained practically no dislocations. This result suggests that the uneven distribution of dislocations between grains is possibly one of the causes of the abnormal grain growth in polycrystalline SrTiO₃.     

Genesis of the (111) Twin in Barium Titanate

On the basis of the topotaxy between BaTiO₃ and Ba₆Ti₁₇O₄₀ found recently, a model of a nonconservative (111) twin in TiO₂-rich BaTiO₃ was constructed. The model consists of several (001) layers of Ba₆Ti₁₇O₄₀ intergrown between (111) layers of BaTiO₃, the core of the twin being a slightly modified double layer of Ba₆Ti₁₇O₄₀ containing face-sharing octahedra. Using this model, anomalous grain growth below the eutectic temperature and preferential growth of (111) twins in a reducing atmosphere were explained, as well a nucleation of butterfly twins.     

Grain-Boundary Chemistry of Barium Titanate and Strontium Titanate: I, High-Temperature Equilibrium Space Charge

Direct observations using scanning transmission electron microscopy (STEM) of the grain-boundary chemistry of selectively doped SrTiO₃ and BaTiO₃ show the predominant solute segregation in both systems to be that of acceptors (negative effective charge). Appreciable donor segregation is not observed even at lattice concentrations as high as 10 mol%. Donor and acceptor codoped materials show segregation of the acceptor only. The results are consistent with a grain-boundary space-charge distribution consisting of a positive boundary and negative space charge. All grain boundaries examined also show an excess of Ti relative to the A-site cations, suggesting that the positive boundary charge is at least partially accommodated by an excess of Ti ions. The sign and magnitude of the electrostatic potential appear to be remarkably insensitive to changes in lattice defect structure with solute doping. Grain-boundary chemistry appears dominated by space-charge segregation, in contrast with the predictions of recent atomistic simulations which neglect the space-charge potential.     

Templated Grain Growth of Barium Titanate Single Crystals

BaTiO₃ single crystals were grown via templated grain growth (TGG), which is a process in which a single-crystal "template" is placed in contact with a sintered polycrystalline matrix and then heated to migrate the single-crystal boundary into the matrix. Millimeter-sized, stoichiometric single crystals of BaTiO₃ were produced by heating polycrystalline matrix with a relative density of 97% and a Ba/Ti ratio of <1.00, which was bonded to a BaTiO₃ single crystal, at temperatures above the eutectic temperature. Growth rates of 590–790, 180–350, and 42–59 μm/h were observed for {111}-, {100}-, and {110}-oriented single-crystal templates, respectively. Lower-surface-energy facets were formed for {111}- and {100}-oriented templates, whereas {110} crystals maintained a {110} growth front, which indicated that this plane orientation was the lowest-energy surface in this system. SrTiO₃ also was shown to be a suitable substrate for TGG of BaTiO₃.     

Effect of Ba6Ti17O40/BaTiO3 Interface Structure on {111} Twin Formation and Abnormal Grain Growth in BaTiO3

A structural transition of Ba₆Ti₁₇O₄₀/BaTiO₃ interfaces from faceted to rough was induced by reducing oxygen partial pressure in the atmosphere. As the oxygen partial pressure decreased, the number densities of {111} twins and abnormal grain decreased. TEM observation showed that the twin formation was governed only by the faceting of the interface. Experimental evidence of {111} twin-assisted abnormal growth of faceted BaTiO₃ grains was also obtained.     

钛酸锶钡结构与制备工艺研究进展

钛酸锶钡[(Ba,Sr)TiO3,SBT]是1种重要的电子陶瓷材料,具有高介电常数、低漏电流、高热释电系数,高的介电可调等特性,广泛用于动态随机存储器、热释电探测器、介质移相器等电子元件的制备。综述了钛酸锶钡[(Ba,Sr)TiO3,SBT]的结构和制备工艺研究进展,评述了制备工艺和掺杂等对钛酸锶钡的结构和性能的影响,并指出了钛酸锶钡尚待解决的问题。     

Grain-Boundary Chemistry of Barium Titanate and Strontium Titanate: II, Origin of Electrical Barriers in Positive-Temperature-Coefficient Thermistors

Scanning transmission electron microscopy (STEM) of positive-temperature-coefficient (PTC) BaTiO₃ thermistors shows that the grain-boundary oxygen content in as-received (oxidatively cooled) materials is slightly enriched compared to quenched samples, and the acceptor-rich space-charge present at high temperatures is retained upon cooling. The defect density of the space charge is approximately equal to the acceptor state density at PTC boundaries determined by electrical measurements. Accordingly, it is proposed that the electrical barrier forms when acceptor defects already segregated in the ionic space charge at high temperature become active interface states when compensating donor defects in the grain-boundary core are oxidized. These acceptor defects appear to be primarily barium vacancies, but need not form upon cooling in the manner proposed by Daniels and Wernicke. Acceptor solutes when present can also contribute to barrier formation through space-charge segregation; the increase in interface state density upon addition of Mn is consistent with the magnitude of the expected segregation.     

Phase Equilibria in the System Barium Titanate—Strontium Titanate

Phase equilibria in the system BaTiO₃–SrTiO₃ (with 0 to 7 mol% SrTiO₃) were studied at temperatures above 1600°C in air. Quenching experiments were performed using high-purity starting materials, and run products were examined by X-ray diffraction and differential scanning calorimetry to determine phase composition and Sr concentration. Melting involves a binary loop intersected by the invariant reaction hexagonal (Ba, Sr)TiO₃( ss ) ⇌ cubic (Ba, Sr)TiO₃( ss ) + liquid. In contrast with earlier work, these results indicate that there is no depression of the melting point with Sr addition and no congruent melting point in this compositional range.     

Flextensional Barium Strontium Titanate Actuators

This paper describes the performance of the cymbal flextensional transducer using Dy-doped barium strontium titanate (BST) as the driver material. BST was first characterized for its dielectric and loss behavior as a function of temperature and electrical bias field. With no electrical bias, the transition temperature was measured to be near 20°C and have a dielectric constant >20 000. The strain of a BST disk was then measured and compared with other ceramics. At room temperature the strain and average effective piezoelectric d ₃₃ of this non-lead composition was slightly larger than Navy type I lead zirconate titanate (PZT-4) ceramic. The strain/field behavior was also measured as a function of temperature. Cymbal capped BST ceramic was found to have an amplified displacement of 28×, also very similar to type I ceramic. The stiffness of BST was found to be tunable by dc voltage and 2 to 3 times larger than that of PZT. This material has promise for applications in actuators and transducers with large generative force.     

钛酸锶钡微晶玻璃的铈掺杂效应研究

通过X射线光电子能谱、紫外-可见漫反射光谱、傅里叶变换红外光谱和拉曼光谱研究了(Ba,Sr)TiO3微晶玻璃的铈掺杂效应.玻璃体系为BaO-SrO-TiO2-Al2O3-SiO2(Ce摩尔分数为0％、1％、2％、3％).随着Ce摩尔含量增加,Ce在(Ba,Sr)TiO3晶格中的占位从Ce4+取代Ti4+位的形式逐渐变成...     

Initial Specific Surface Area and Graln Growth in Donor-Doped Barium Titanate

The grain growth of donor-doped BaTiO₃ prepared from BaTiO₃ powders with different initial specific surface areas was studied. Results show that a higher initial surface area and, consequently, a smaller critical grain size at the phase boundary drastically increase the critical amount of donor dopant, causing the grain size anomaly during sintering.     

钛酸锶钡陶瓷材料的掺杂改性

钛酸锶钡(BST)是一种重要的具有钙钛矿结构的铁电材料。顺电态下,其介电损耗较小,结构稳定。因此对顺电态下的BST进行掺杂改性是近年来铁电材料的研究热点之一。本文简要介绍了目前国内外科研工作者利用稀土、碱土氧化物进行掺杂时对BST的微观结构、介电损耗、介电常数以及可调率方面的影响。     

Electrical and Structural Characterization of a Low-Angle Tilt Grain Boundary in Iron-Doped Strontium Titanate

The atomistic structure and electrical properties of a symmetrical 5.4° [001] tilt grain boundary in Fe-doped SrTiO₃ have been investigated, respectively, by means of various transmission electron microscopy (TEM) techniques and impedance spectroscopy. In weak-beam dark-field images, the grain boundary is revealed to consist of a periodic array of dislocations; high-resolution TEM images show that the dislocation cores are separated by regions of strained lattice. The impedance response of the bicrystal has been measured in the frequency range 20 Hz ≤ f ≤ 10⁶ Hz as a function of temperature and oxygen partial pressure. The transport of charge across the array of dislocations that form the grain boundary is strongly hindered. Analysis of the impedance data in terms of a double-Schottky-barrier model yields a space-charge potential that exhibits a weak dependence on temperature and oxygen partial pressure and is ∼0.55 V in the investigated regime.     

Effect of Green States on Sintering Behavior and Microstructural Evolution of High-Purity Barium Titanate

Compacts prepared from three differently agglomerated powders were studied. Hg-penetration results and SEM observations were employed to compare the uniformity of powder compacts and to investigate the pore-size evolution and the microstructural development during sintering. It was found that the more nonuniform the powder compact, the higher the degree of pore growth in the initial and at the beginning of intermediate stages of sintering. Moreover, a higher sintering temperature and a nonuniform microstructure with larger grains could not be avoided. Microstresses might develop because of the differential shrinkage, but they would be released thereafter via the change of grain morphology. It was observed that the aggregate and pore-boundary separation might not be the primary reason for the initiation of discontinuous grain growth.     

Temperature Dependence of the Coarsening Behavior of Barium Titanate Grains

The coarsening behavior of large seed particles during the sintering of BaTiO₃ ceramics has been investigated. At 1350°C, the grains are faceted, and the seed particles grow extensively. At 1380°C, however, the grains are spherical, and coarsening of the seed particles is limited. The observed difference is discussed in terms of the growth mechanism and the atomic structure of the interfaces.     

Effect of Silver on the Sintering and Grain-Growth Behavior of Barium Titanate

Silver and its alloys frequently are used as electrode material for BaTiO₃-based dielectrics. In the present study, a small amount of fine silver particles have been intimately mixed with BaTiO₃ powder. The sintering and grain-growth behavior of the silver-doped BaTiO₃ in air are investigated. The solubility of silver in BaTiO₃, as revealed by lattice-parameter measurement, electrical measurement, and electron probe microanalysis, is <300 ppm. The densification of BaTiO₃ is slowed slightly by the addition of silver inclusions. However, the presence of a small amount (<0.3 wt%) of silver increases the amount and size of abnormal grains. When the silver content is >0.3 wt%, the grain growth of BaTiO₃ then is prohibited by the silver inclusions.