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.     

Effect of External Electric Field on the Grain-Growth Behavior of Barium Titanate

The effect of an external electric field on the grain-growth behavior of acceptor Mg-doped, undoped, and donor Nb-doped BaTiO₃ ceramics was investigated. The acceptor-doped and undoped specimens showed enhanced grain growth at the positive-biased region. On the other hand, for the highly donor-doped specimens, grain growth was enhanced in the negative-biased region. The results have been explained in terms of defect polarization and the consequent change in the boundary potential. It has been suggested that liquid penetration into grain boundaries is critically dependent on the boundary potential.     

Evaporation of Silver during Cofiring with Barium Titanate

Silver and Ag-Pd alloy are cofired with BaTiO₃-based dielectrics during the manufacturing of capacitors. The diffusion of silver into BaTiO₃ during sintering is very slow. However, the vapor pressure of silver is high at temperatures greater than the melting point of silver (960°C). The effect of the evaporation of silver during the sintering with BaTiO₃ at 1200°, 1250°, and 1290°C is investigated in the present study. The silver vapor can transport through the pore channel of the dielectrics to a distance of a few hundred micrometers. The melting point of Ag-Pd alloy is higher than that of silver; therefore, the transportation of silver vapor from Ag-Pd alloy is hardly observed at temperatures >1200°C.     

Water-Induced Degradation of Barium Titanate Ceramics Studied by Electrochemical Hydrogen Charging

Water-induced degradation of barium titanate (BTO) ceramics has been investigated using electrochemical hydrogen charging, in which the silver electrodes of BTO ceramic specimens are made cathodes in a 0.01 M NaOH solution to evolve hydrogen by electrolysis of water. After 80 h of treatment, the resistance of BTO decreases by ∼3 orders of magnitude, and the dielectric loss obviously increases. The degradation can be explained by the reduction reaction of atomic hydrogen with BTO. Hydrogen acts as a donor in BTO, and electrons are formed by the reduction. It is proposed that the reduction reaction of atomic hydrogen at ambient temperature by electrolysis of water is an important origin for degradation of BTO-based ceramic devices.     

Tetragonal Nanocrystalline Barium Titanate Powder: Preparation, Characterization, and Dielectric Properties

Nanocrystalline tetragonal-BaTiO₃ powder was prepared using a hydrothermal method, under moderate conditions, and with a high precursor concentration. Characterization via X-ray diffractometry and differential scanning calorimetry confirmed that the average particle size and tetragonal content of the prepared powder were 70 nm and 80%, respectively. The sintered sample made from the prepared powder had a room-temperature dielectric constant of 6900, which was high for BaTiO₃.     

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.     

Electrical Properties of PTCR Barium Titanate

When properly doped, barium titanate ceramics display positive temperature coefficient resistance (PTCR) behavior. This has been proved to be a Schottky barrier type of grain-boundary effect. However there has not yet been a complete point-to-point comparison between the experimental data and theory for the entire set of the material nonlinear dielectric properties. In this study, a methodology has been developed which allows the study of the depletion layer dielectric properties while the PTCR effect is being investigated. An equivalent dielectric constant, the value of which is to be determined from this experiment, is treated as an average of the dielectric properties of the depletion layer and is used to analyze the grain-boundary resistance and capacitance data based on a simple double-depletion-layer model. The theoretical relationship between this equivalent dielectric constant and the material dielectric properties is also explored in this study.     

Microwave Dielectric Properties of Gallium-Doped Hexagonal Barium Titanate Ceramics

High-permittivity and low-loss ceramics with composition BaTi_0.92Ga_0.08O_2.96 have been prepared in the BaO–Ga₂O₃–TiO₂ system using the mixed-oxide route. This compound forms as the hexagonal polymorph (6 H ) of BaTiO₃ with the space group P 6₃/ mmc . The dielectric properties of dense ceramics have been studied, at microwave frequencies, with the ceramics fired at 1450°C under flowing oxygen gas; the results are a relative permittivity, ɛ_r, of ∼74 and a quality factor, Q · f _r, of ∼7815 at 5.5 GHz. The quality factor is strongly influenced by the sintering conditions (temperature and atmosphere), whereas the relative permittivity is not influenced significantly by ceramic processing for pellets ≥93% of the theoretical X-ray density. To our knowledge, this is the first report of microwave dielectric resonance in a perovskite-type BaTiO₃-based ceramic.     

Dopant Effect on Local Dielectric Properties in Barium Titanate Based Electroceramics Determined by Transmission EELS

Local electronic excitations of Nb-doped BaTiO₃ electroceramic were investigated using low-loss electron energy loss (EEL) spectroscopy with a transmission electron microscope. The variations in electronic structure of the BaTiO₃ were monitored as a function of Nb content by using Kramers-Krönig analysis of the low-loss EEL spectra. We found that the valence state of Nb (+5) as compared with that of the Ti (+4) introduces changes in the valence and conduction band states of BaTi_{1− x} Nb _x O₃ samples. The real part of the dielectric function, Re(1/ɛ), displays an increasingly less negative character with higher amounts of dopant and the valence plasmon exhibits "quasi-plasmon" characteristics with the addition of Nb (at 0.0–1.8 at.%). Further, the plasmon energy shifts (by about 0.5 eV) to higher values with Nb additions. Significant changes take place in oscillator strengths of excitations in local (nanometer-scale) regions of the perovskite samples. This investigation demonstrates a method to quantitatively assess electronic properties, at the submicrometer scale, of doped ceramics used in electronic and electrooptical applications.     

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.     

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.     

Core-Shell Structure of Acceptor-Rich, Coarse Barium Titanate Grains

Heavily doped barium titanate (BaTiO₃) powders, one with a donor-rich and the other with an acceptor-rich composition, were prepared. After sintering, the donor-rich specimen exhibited a fine-grained microstructure but significant grain growth occurred in the acceptor-rich specimen. A stable dielectric behavior was observed only in the donor-rich fine-grained specimen over the temperature range studied. However, in both specimens, an undoped core region several hundred nanometers in size was detected. The core-shell structure appeared to be maintained in BaTiO₃ under the conventional sintering conditions_.     

Effects of Zirconia on Microstructure and Dielectric Properties of Barium Titanate Ceramics

Dielectric properties and structural characteristics of BaTiO₃ ceramics are significantly influenced by small addition (2 wt%) of ZrO₂. SEM and TEM observations revealed enhanced microstructural uniformity and retarded grain growth depending on sintering temperature. Above 1320°C, Zr diffusion into the BaTiO₃ lattice resulted in a chemical modification of the tetragonal structure and the development of core–shell grains. Below 1320°C, TEM analysis showed ZrO₂ at the grain boundaries as discrete particles (∼0.03μm). X-ray diffraction analysis revealed a decrease in the axial (c/a) ratio with decreasing grain size. A corresponding decrease in the spontaneous polarization, and twinned domain structures, were also observed in the fine-grained ceramics. These samples also showed a flattened permittivity response with temperature and significantly lower losses.     

Effect of the Liquid-Phase Characteristic on the Microstructures and Dielectric Properties of Donor- (Niobium) and Acceptor- (Magnesium) Doped Barium Titanate

Changes in the microstructure and dielectric properties with the variation of the donor/acceptor ratio in BaTiO₃ ceramics were investigated. In donor-rich specimens, a liquid that appeared during sintering did not penetrate into grain boundaries. However, in the acceptor-rich specimens, the grains were separated by a liquid film during sintering. The much higher mobility of the liquid film than that of the grain boundaries was suggested to cause extensive grain growth in acceptor-rich BaTiO₃. The macroscopic homogenization of dopants because of grain growth in acceptor-rich specimens resulted in changes in the dielectric properties.     

钽掺杂对钛酸钡基陶瓷介电性能影响的研究

研究了微量钽掺杂对钛酸钡基陶瓷介电常数、介电损耗以及温度稳定性等介电性能的影响.利用XRD、SEM等现代分析手段分析了材料的显微结构,分析了显微结构与材料介电性能的关系,得到了性能较为优良的电容器基料配方.     

Effect of Aliovalent Substituents on the Ferroelectric Properties of Modified Barium Titanate Ceramics - Relaxor Ferroelectric Behavior

Structure–property relationships were investigated to reveal the generic influence of various substituents on ferroelectric BaTiO₃. Compositional modifications of ferroelectric BaTiO₃ were systematically investigated for La³⁺ and Nb⁵⁺ substituents. A generic influence of charged point defects in breaking down the ferroelectric domains into polar clusters was demonstrated. In both types of modified BaTiO₃, relaxor ferroelectric behavior was induced for higher concentrations of substituents.     

Tunable Microwave Properties of Barium Titanate-Based Ferroelectric Glass-Ceramics

BaTiO₃-based ferroelectric glass-ceramics with the composition 0.65(Ba_{1− X} Sr _X )TiO₃·0.27SiO₂·0.08Al₂O₃ ( X = 0.2–0.6) were fabricated, and their tunable dielectric properties were measured at microwave frequency. Major crystalline phases that precipitated during thermal treatment up to 1000°C were (Ba,Sr)TiO₃, Ba₂TiSi₂O₈, and BaAl₂Si₂O₈. The Curie temperatures of the heated bulk samples were successfully tuned near room temperature at the composition between X = 0.2 and 0.3. A thick-film sample with X = 0.3 showed 27% tunability at 5 GHz under 10 kV/cm bias voltage.     

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.