Preparation, Characterization, and Properties of Dy-Doped Small-Grained BaTiO3 Ceramics

The effects of Dy doping and sintering parameters on the properties of BaTiO₃ ceramics were studied. The average grain size decreases with increasing Dy content and is controlled at ∼1.5 μ m by 0.8 at.% Dy. The Curie temperature change, associated with ≤1.2 at.% Dy, is <3°C. The dielectric constant is ∼2600 for specimens doped with 0.8 at.% Dy, calcined at 1200°C, and sintered at 1450°C. The dielectric constant variation with ambient temperature is much less than that of conventional BaTiO₃ ceramics. Lattice constant c decreases with increasing Dy concentration whereas a increases slightly. The effects of grain size on dielectric constant, lattice parameters, and linear thermal expansion coefficient are more pronounced than the chemical effects of Dy doping in the ferroelectric state, whereas in the paraelectric state, these characteristics are almost independent of grain size as well as Dy concentration. The decrease in the frequency of occurrence of 90° twins with decreasing grain size implies that internal stress, which develops when BaTiO₃ ceramics are cooled below T_c , strongly influences the effects of grain size.     

Influence of Rare-Earth Dopants on Barium Titanate Ceramics Microstructure and Corresponding Electrical Properties

In this article, ytterbium and erbium oxides are used as doping materials for barium titanate (BaTiO₃) materials. The amphoteric behavior of these rare-earth ions leads to the increase of dielectric permittivity and decrease of dielectric losses. BaTiO₃ ceramics doped with 0.01–0.5 wt% of Yb₂O₃ and Er₂O₃ were prepared by conventional solid-state procedure and sintered at 1320°C for 4 h. In BaTiO₃ doped with a low content of rare-earth ions (0.01 wt%) the grain size ranged between 10 and 25 μm. With the higher dopant concentration of 0.5 wt%, the abnormal grain growth is inhibited and the grain size ranged between 2 and 10 μm. The measurements of capacitance and dielectric losses as a function of frequency and temperature have been carried out in order to correlate the microstructure and dielectric properties of doped BaTiO₃ ceramics. The temperature dependence of the dielectric constant as a function of dopant amount has been investigated.     

Effect of the ac Field Level on the Aging of the Dielectric Response in Polycrystalline BaTiO3

BaTiO₃ ceramics with grain sizes from 0.6 to 60 μm and relative densities of 89% to 92% were prepared by hot forging and conventional sintering from very pure-oxalate-derived powder. The aging of both the dielectric constant and the dielectric loss was examined at weak and strong fields with respect to grain size and frequency. It was concluded that the main aging mechanism is the aging of hysteretic domain wall motion for coarse-grained ceramics. At grain size of less than 1 μm, the lack of frequency and E _AC dependence, along with a lower aging rate, suggests that domain motions or hysteretic domain wall motions are restricted in finegrained ceramic BaTiO₃ and contribute little to the aging.     

Growth of BaTiO3 Seed Grains by the Twin-Plane Reentrant Edge Mechanism

Two different types of BaTiO₃ seed particles, normal and twinned seeds of ∼30 μm on the average, were prepared from crushed sintered specimens. Normal seeds were obtained from the usual BaTiO₃ sintered compacts, while twinned seeds containing a double twin were obtained from BaTiO₃ compacts sintered with 2 mol% of SiO₂. The BaTiO₃ powder compacts were again prepared with 5 wt% of seed grains and sintered under various conditions. The microstructural evolution was quite different in the two cases: the growth of normal seed grains was ultimately limited but that of the twinned seeds continued extensively. The observed difference is discussed in terms of the growth mechanism and the atomic structure of interfaces.     

Microcrystalline BaTiO3 by Crystallization from Glass

The properties and composition of glasses suitable for crystallization of BaTiO₃ are described. The crystallization of certain glasses results in a nearly complete recovery of BaTiO₃, besides the feldspar BaAl₂SiO₃ as a minor phase. The mechanism of crystallization was investigated by thermal analysis, viscosity, and grainsize measurements as a function of the temperature whereas density data were used for evaluation of the BaTiO₃ content. Within the range 30 to 60% by volume of BaTiO₃ at about 1μ grain size, the measured dielectric constant increased from 100 to 1200. The calculated partial dielectric constant of the Titanate phase at this grain size was about 3500. As the grain size approached 0.1μ, the dielectric constant decreased and became nearly independent of the temperature because of the predominance of surface states. Other effects were attributed to special structural characteristics, such as absence of porosity and clamping of the titanate particles within the microcrystalline matrix. Data are also presented on dielectric constant and loss tangent at different frequencies, dc breakdown strength, dc resistivity, and ferroelectric properties as a function of the grain size of the crystallized material.     

Molten-Salt Synthesis of Ba1–xPbxTiO3 in the System BaTiO3–PbCl2

Ba_{1– x} Pb _x TiO₃ powder with a fixed composition was prepared by the reaction of BaTiO₃ powders with molten PbCl₂at various PbCl₂/BaTiO₃ molar ratios at 600° and 800°C in a nitrogen atmosphere. When 0.1 μm powder was used, the reaction was finished when x = 0.9. Two phases of BaTiO₃and a solid solution of Ba_{1– x} Pb _x TiO₃ coexisted, but the final phase gave a solid solution of Ba_{1– x} Pb _x TiO₃ at 800°C. When 0.5 μm powder was used, the two phases coexisted in the products at 600°C at PbCl₂/BaTiO₃= 1.0. A sintered compact of Ba_{1– x} Pb _x TiO₃ powders solid solution was prepared by hot isostatic pressing, and its dielectric constant was measured in the temperature range 20°–550°C.     

Grain-Boundary Migration and Dielectric Properties of Semiconducting SrTiO3 in the SrTiO3-BaTiO3-CaTiO3 System

Chemically induced grain-boundary migration and its effects on the interface and dielectric properties of semiconducting SrTiO₃ have been investigated. Strontium titanate specimens that had been doped with 0.2 mol% of Nb₂O₅ were sintered in 5H₂/95N₂. The sintered specimens were diffusion annealed at 1400°C in 5H₂/95N₂ with BaTiO₃ or 0.5BaTiO₃-0.5CaTiO₃ (mole fraction) packing powder. The grain boundaries of the annealed specimens were oxidized in air. In the case of BaTiO₃ packing, grain-boundary migration occurred with the diffusion of BaTiO₃ along the grain boundary. The effective dielectric constant of the specimen decreased gradually as the temperature increased but showed two peaks, possibly because of barium enrichment at the grain boundary and an oxidized Sr(Ba)TiO₃ layer. In the case of 0.5BaTiO₃-0.5CaTiO₃ packing, although barium and calcium were present at the grain boundary of the specimen, no boundary migration occurred, as in a previous investigation. With the diffusion of barium and calcium, the resistivity of the specimen increased and the variation of the effective dielectric constant with temperature was much reduced, in comparison to those without solute diffusion. These enhanced properties were attributed to the solute enrichment and the formation of a thin diffusional Sr(Ba,Ca)TiO₃ layer at the grain boundary.     

Electrophoretic Deposition and Characterization of Micrometer-Scale BaTiO3 Based X7R Dielectric Thick Films

Uniform and relatively dense BaTiO₃ thick films of 1–5 μm were prepared by an electrophoretic deposition process using submicrometer BaTiO₃ powders (mean particle size: ∼0.2 μm). Two different BaTiO₃ powders and solvent media were used to investigate the film quality and thickness control. The surface charge mechanism of BaTiO₃ particles was explained according to the observed results. The microstructures were examined by means of SEM. The experimental results show that the thickness could be controlled independently of suspension concentration by keeping a constant applied voltage and a constant current drop in a given suspension. BaTiO₃ thick films have good insulation resistance and dielectric properties such as a dielectric constant and a dissipation factor that are compatible with the data from conventional tape-cast BaTiO₃ thin layers.     

Effect of Rare-Earth Ions on Electrical Conductivity of BaTiO3 Ceramics

Lattice constants, grain size, electrical conductivity, and luminescence were measured for sintered BaTiO₃ ceramics doped with 0 to 1.2 at.% rare-earth ions. BaTiO₃ doped with low levels of rare-earth ions contains grains 10 μm in size and has lattice constants nearly equal to those of undoped ceramics. In this case, rare-earth ions occupy Ba²⁺ sites and yield donors. When grain growth is inhibited by high doping levels or by insufficient sintering, the lattice constants change, the rare-earth ions occupy both Ba²⁺ and Ti⁴⁺ sites, and, consequently, BaTiO₃ becomes insulative because of charge compensation.     

Preparation of 0.95Bi1/2Na1/2TiO3·0.05BaTiO3 Ceramics by an Aqueous Citrate-Gel Route

This report details development of a route to solution-derived (1− x )Bi_1/2Na_1/2TiO₃· x BaTiO₃ powders. The method developed was the citrate-gel method—an evaporative, aqueous technique. When applied to 0.95Bi_1/2Na_1/2TiO₃·0.05BaTiO₃ (BNBT-5), the method produced perovskite phase powders that readily densified in the temperature range of 1000°C. The grain size of the sintered materials was on the order of 1 μm, and the weak-field dielectric properties at 1 MHz were similar to those reported for conventionally prepared materials sintered at higher temperatures (e.g., 1200°C).     

Preparation of Dense BaTiO3 Ceramics with Submicrometer Grains by Spark Plasma Sintering

Dense BaTiO₃ ceramics consisting of submicrometer grains were prepared using the spark plasma sintering (SPS) method. Hydrothermally prepared BaTiO₃ (0.1 and 0.5 µm) was used as starting powders. The powders were densified to more than similar/congruent95% of the theoretical X-ray density by the SPS process. The average grain size of the SPS pellets was less than similar/congruent1 µm, even by sintering at 1000-1200°C, because of the short sintering period (5 min). Cubic-phase BaTiO₃ coexisted with tetragonal BaTiO₃ at room temperature in the SPS pellets, even when well-defined tetragonal-phase BaTiO₃ powder was sintered at 1100° and 1200°C and annealed at 1000°C, signifying that the SPS process is effective for stabilizing metastable cubic phase. The measured permittivity was similar/congruent7000 at 1 kHz at room temperature for samples sintered at 1100°C and showed almost no dependence on frequency within similar/congruent10⁰-10⁶ Hz; the permittivity at 1 MHz was 95% of that at 1 kHz.     

Improvement in the Temperature Stability of a BaTiO3-Based Multilayer Ceramic Capacitor by Constrained Sintering

In this study, a self-constrained BaTiO₃ material system, composed of a low-fire BaTiO₃-based X7R (Δ C / C ±15% within −55° to 125°C) MLCC dielectric and a high-fire, BaTiO₃-based X7R MLCC dielectrics eliminated sintering aid that are laminated on both sides of the BaTiO₃-based X7R MLCC, has been developed. The temperature dependence of capacitance of the BaTiO₃-based X7R MLCC is significantly improved to satisfy X8R requirements over the entire temperature range studied (Δ C / C ±15% within –55° to 150°C) using the self-constrained sintering. The curie temperature of BaTiO₃-based X7R MLCC increases on increasing the thickness of the constraining layers. Compared with the specimen fired by free sintering, a substantial reduction in grain size, leading to a decrease in dielectric constant, was observed in the specimen fired by constrained sintering with the thickness of the constraining layer being 170 μm. The increase in Curie temperature and decrease in grain size of the specimen fired by constrained sintering can be explained in terms of the presence of in-plane tensile stress. The in-plane tensile stress that introduces a friction force between the multilayer matrix and the nonshrinkage constraining layers to suppress the in-plane shrinkage and to inhibit the rate of grain growth was formed during constrained sintering.     

Effects of Grain Growth on the Distribution of Nb in BaTiO3 Ceramics

Measurements of Nb diffusion into large- and small-grained BaTiO₃ disks show a high ratio of grain boundary to bulk diffusivity. Well defined X-ray diffraction lines are found in Nb-doped BaTiO₃ only when significant grain growth occurs during sintering. When grain growth of a 0.65 μm grain size powder is limited at ∼1 μm, excess line broadening results. This is attributed to the simultaneous presence of Nb-free and Nb-rich regions. Because of its low bulk diffusivity little Nb penetrates into the original BaTiO₃ grain cores, and a solid solution forms only in the regions of recrystallization. When grain growth is limited, the "sintering reaction" results in a non-homogeneous system; when appreciable grain growth occurs, most of the original grain cores are eliminated and an essentially uniform system is obtained.     

Effect of Internal Stress on Physical Temperature Characteristics of Cerium-Doped and Gadolinium-Doped Barium Titanate Ceramics

The effect of internal stress on the physical properties (the temperature dependence of the dielectric constant and the lattice constant, and the diffuseness (gamma)) of solid solutions of 0.97BaTiO₃-0.03Gd₂O₃:1.5TiO₂ and 0.97BaTiO₃-0.03CeO₂:1.5TiO₂ was investigated using various particle sizes of the starting BaTiO₃, pressure-dielectric spectroscopy, and high-pressure X-ray analysis. The grain size was almost proportional to the particle size of the starting BaTiO₃ and was inversely proportional to the internal stress. A sudden change in internal stress occurred at grain sizes of ∼2 µm for cerium-doped BaTiO₃ and ∼3 µm for gadolinium-doped BaTiO₃. With an increase in external pressure and a decrease in grain size, the lattice anomalies of the cubic-to-tetragonal transformation and two other factors (the tetragonal-to-orthorhombic and orthorhombic-to-rhombohedral transformations) shifted to lower and higher temperatures, respectively. The temperature dependencies of the dielectric constant and the lattice constant were less sensitive to external pressure in cerium-doped core-shell-grained BaTiO₃ than in gadolinium-doped homogeneous-grained BaTiO₃; this difference was attributable to internal stress. In cerium-doped BaTiO₃, gamma was related to internal stress.     

Preparation of Dense BaTiO3 Ceramics from Sol-Gel-Derived Monolithic Gels

Dense, small-grained BaTiO₃ ceramics, with a grain size around 1 μm and a relative sintered density >98%, were obtained at 1100°C from sol-gel-derived gel monoliths without using any sintering additives. The monolithic gels asprepared had a relative density of about 50% and consisted of ultrafine pseudo-cubic BaTiO₃ particles (<50 nm). These gels, with a significantly high density compared with that of previous ones (∼30%), have been synthesized at room temperature from a sol solution with a concentration of equimolar mixture of titanium isopropoxide and barium ethoxide (0.8 mol/L), using the methanol/2-methoxyethanol mixed-solvent system. Microstructural development of the gel monoliths with increasing sintering temperature and the dielectric properties of the obtained dense BaTiO₃ ceramic have been investigated.     

Control of Liquid-Phase-Enhanced Discontinuous Grain Growth in Barium Titanate

Barium titanate shows discontinuous grain growth in the presence of small amounts of the liquid eutectic Ba₆Ti₁₇O₄₀-BaTiO₃ at T > 1312°C. The exaggerated grain growth in the presence of the liquid phase can be explained in terms of a solution-segregation process. The grain size distribution of sintered BaTiO₃ ceramics and the average grain size are strongly modified by addition of "seed grains" to BaTiO₃ powder. The addition of seed grains seems to be a general method of controlling the discontinuous grain growth of ceramics.     

Microstructure and Electrical Properties of Sc2O3-Doped, Rare-Earth-Oxide-Doped, and Undoped BaTiO3

Polycrystalline BaTiO₃ prepared from alkoxy-derived high-purity submicron powders was studied. Highly dense bodies with uniform grain size were obtained typically by uniaxial cold-pressing at 3000 psi and isostatic pressing at 30,000 psi followed by sintering at 1300° to 1350°C in air for 0.5 to 1 h. Using the same consolidation parameters and intimate mixing of residual concentrations of highly active fine-particulate rare-earth oxides to act as grain-growth inhibitors, nearly theoretically dense bodies with a uniform microstructure and 1 to 1.5 μm grain size were obtained. Typical microstructures with well-defined 90° and 180° domain patterns characteristic of BaTiO₃: were observed. Also, an example of a checkerboard pattern resulting from a 〈111〉 ingrown twin plane in the structure which is independent of the Curie temperature was found. Electrical measurements on the undoped material indicated room-temperature dielectric constant and tan δ values of 5000±500 and 4×10⁻³, respectively. Very high k values and dissipation factors were observed with the La₂O₃- and Nd₂O₃-doped samples.     

Synthesis of Micron-Scale Platelet BaTiO3

Micron-scale platelet barium titanate was synthesized using a twostep molten salt and topochemical technique. Plate-like BaBi₄Ti₄O₁₅ was first synthesized as a precursor by molten salt synthesis. Then, Bi³⁺ in the precursor was replaced by Ba²⁺ and formed perovskite-structure BaTiO₃ through a topochemical reaction. The BaTiO₃ single crystals have an average size of 5–10 μm and a thickness of 0.5 μm. The purpose of this article is to control the particle shape with desired structure. High aspect ratio BaTiO₃ platelets are suitable templates to obtain textured ceramics (especially Pb(Mg_1/3Nb_2/3)O₃–32.5PbTiO₃) by the templated grain growth process.     

Effect of Internal Stress on the Strength of BaTiO3

The strength of BaTiO₃ which had been hot-pressed to ∼99% of theoretical density and tested above the Curie temperature was—9000 psi greater than at room temperature. This measured difference in strength is attributed to internal stress from the cubic→tetragonal phase transformation. Correction of measured strength to zero porosity predicts internal stresses of ∼11,000 psi, in good agreement with the dielectric theory for fine-grain BaTiO₃. The internal stress is nearly independent of grain size in the range ∼1.5 to 150 μm, showing that 90° domains do not reduce internal stresses causing failure. Lower internal stress in commercial BaTiO₃ is attributed to greater porosity, large flaw sizes, and possibly the effects of additives.     

Preparation of Nano BaTiO3-Based Ceramics for Multilayer Ceramic Capacitor Application by Chemical Coating Method

The base-metal-electrode multilayer ceramic capacitors (BME MLCCs) for future application require much thinner dielectric layers (<1 μm). Therefore, the grain size and uniformity of BME MLCC powders should be effectively controlled. In this paper, the nanosized BME MLCC powders were prepared by chemical coating method. The well-coated BaTiO₃ particles were obtained by adjusting an appropriate pH value. Transmission electron microscopy, energy-dispersive spectroscopy, X-ray photoelectron spectrum were utilized for the study of microstructures and element analysis. The high-performance X7R dielectric ceramics were produced in reducing atmosphere by "two-step" sintering method at a low temperature of 950°C. The dielectric constant at room temperature could reach ∼2400, with low dielectric loss below 1.0% and high insulation resistivity ∼10¹²Ω·cm. The ceramic grains were very homogenous with the average size below 150 nm.