Dislocation Loops in Pressureless-Sintered Undoped BaTiO3 Ceramics

Dislocation loops in pressureless-sintered undoped BaTiO₃ ceramics have been analyzed by transmission electron microscopy. The Burgers vector of the loops and its sense b =+1/2[010] were determined using the g·b =0 invisibility criteria, combined with the inside–outside contrast technique using ( g·b ) s _g >0 or<0, keeping the deviation parameter s _g >0. The edge-vacancy nature was further ascertained by determining the loop habit plane normal n =[0 1 0]. Weak-beam dark-field imaging reveals that loops contained no stacking fault fringes; they are edge-vacancy partial dislocation loops lying in {020} or {010} where parts of the TiO₂ or BaO layer are vacant. It is suggested that the extrinsic defects of both cations and oxygen vacancies generated by non-stoichiometry have condensed during sintering in air and are responsible for the formation of such vacancy loops.     

Nonstoichiometry in Undoped BaTiO3

Equilibrium electrical conductivity data for large-grained, poly crystalline, undoped BaTiO₃, as a function of temperature, 750° to 1000°C, and oxygen partial pressure, 10⁻²⁰< P _O2<10⁻¹ MPa, were quantitatively fit to a defect model involving only doubly ionized oxygen vacancies, electrons, holes, and accidental acceptor impurities. The latter are invariably present in sufficient excess to control the defect concentrations through the compensating oxygen vacancies, except under the most severely reducing conditions. Singly ionized oxygen vacancies play no discernible role in the defect chemistry of BaTiO₃ within this experimental range. The derived accidental acceptor content has a slight temperature dependence which may reflect some small amount of defect association. Deviation of the conductivity minima from the ideal shape yields a small P _O2-independent conductivity contribution, which is tentatively identified as oxygen vacancy conduction.     

Solubility of Hydrogen Defects in Doped and Undoped BaTiO3

Thermal desorption studies and electrical conductivity measurements have been used to investigate the solubility of hydrogen defects in barium titanate ceramics in oxidizing atmospheres. Hydrogen is found to dissolve in the perovskite lattice of undoped and acceptor-doped BaTiO₃ by the formation of hydroxide ions on regular oxygen ion sites. The hydrogen defects act as mobile donors and may substitute the oxygen vacancies in compensating fixed acceptors. From the dependence of the solubility on the water vapor pressure and the dopant concentration, the incorporation/desorption equilibrium reaction is deduced. A defect model is derived which consistently extends previously proposed models. The influence of Ba₂TiO₄ and Ba₆Ti₁₇O₄₀ second phases on the solubility of hydrogen defects is discussed.     

Investigation of Twin Lamellae in BaTiO3 Ceramics

The microstructure of many electroceramic components based on barium titanate is dominated by anomalous grain growth. At firing temperatures below the eutectic temperature nearly all large crystallites were found to contain lamellae. The crys-tallographic orientation of these lamellae was studied by TEM. By combining image and diffraction information it could be directly shown that the lamellae lie on (111) planes and represent twins, which are ∼0.5 μm thick. The relevance of twin formation for the mechanism of anomalous grain growth is briefly discussed.     

Interfacial Reaction of BaTiO3 Ceramics with PbO–B2O3 Glasses

Interfacial reactions of pure, lead-, and zirconium-substituted BaTiO₃ ceramics with PbOB₂O₃ glasses were studied, with an emphasis on the effect of glass composition. Microstructures were analyzed by scanning electron microscopy and electron-probe microanalysis aided with X-ray diffractometry of powder mixtures in the system BaTiO₃PbOB₂O₃ heated at 850°C. The interfacial microstructures were divided into two types, depending on the glass composition. The first type was characterized by precipitates of TiO₂ dispersed in the glass matrix. Extended heating or limited glass volume resulted in the formation of a continuous layer of BaTi(BO₃)₂. The second type of microstructure was characterized by a lead-rich perovskite phase, which developed at the glass/ceramic interfacial region. Growth kinetics for this phase denied the diffusion-controlled mechanism. The substitution of lead in BaTiO₃ enhanced the penetration of glass into the ceramics along the grain boundaries and developed a coreshell structure.     

Weakly Coupled Relaxor Behavior of BaTiO3–BiScO3 Ceramics

The structural and dielectric properties of (1− x )BaTiO₃– x BiScO₃ ( x =0–0.5) ceramics were investigated to acquire a better understanding of the binary system, including determination of the symmetry of the phases, the associated dielectric properties, and the differences in the roles of Bi₂O₃ and BiScO₃ substitutions in a BaTiO₃ solid solution. The solubility limit for BiScO₃ into the BaTiO₃ perovskite structure was determined to be about x =0.4. A systematic structural change from the ferroelectric tetragonal phase to a pseudo-cubic one was observed at about x =0.05–0.075 at room temperature. Dielectric measurements revealed a gradual change from proper ferroelectric behavior in pure BaTiO₃ to highly diffusive and dispersive relaxor-like characteristics from 10 to 40 mol% BiScO₃. Several of the compositions showed high relative permittivities with low-temperature coefficients of capacitance over a wide range of temperature. Quantification of the relaxation behavior was obtained through the Vogel–Fulcher model, which yielded an activation energy of 0.2–0.3 eV. The attempt characteristic frequency was 10¹³ Hz and the freezing temperature, T _f, ranged from −177° to −93°C as a function of composition. The high coercive fields, low remanent polarization, and high activation energies suggest that in the BiScO₃–BaTiO₃ solid solutions, the polarization in nanopolar regions is weakly coupled from region to region, limiting the ability to obtain long-range dipole ordering in these relaxors under field-cooled conditions.     

Calculation of Ultraslow Dielectric Relaxation of Doped BaTiO3 Ceramics

Using a linear voltage ramp method, ultraslow dielectric relaxation processes of La-doped BaTiO₃ boundary layer (BL) capacitors are measured at room temperature. For the electric polarization model of a double-layer dielectric, a procedure to calculate the electrical parameters of grain and grain boundary has been developed. Calculated results suggest that there is a shell on the surface of the grain. TEM images of the BL capacitor also show a lot of microregions with minor structural modifications. Relaxation times are estimated from the equivalent circuit and are in agreement with fitted results.     

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.     

Grain Boundaries of Semiconducting SrTiO3 and BaTiO3 Ceramics Synthesized from Surface-Coated Powders

The chemical and electrical features of the grain boundaries in polycrystalline SrTi_0.99Nb_0.01O₃ (ST) and BaTiO₃ (BT) ceramics, which were synthesized by hot-press sintering Na- and Mn-coated semiconducting ST and BT powders, respectively, were investigated. Because of the excess negative electric charges formed near grain boundaries, electrostatic potential barriers were formed near the grain boundaries. The electrical features of the grain boundaries in ceramics are very sensitive to the amount of the coating material. When the amount of the coating material was increased from 0 to 5 wt%, the threshold voltage of the ST ceramics and the resistivity jump ratio of the BT ceramics increased from 0.7 to 81.0 V/cm and from 1.0 to 2.0 × 10³, respectively. The electrical features of the grain boundaries are related to their chemical characteristics.     

Preparation of Small-Grained and Large-Grained Ceramics from Nb-Doped BaTiO3

Barium titanate powders with average grain sizes of 0.07 and 0.65 μm were coated with an organic Nb ester, using a liquid-solid mixing process. Small-grained (1 to 3 μm) ceramic dielectrics from both powders and large-grained (>50 μm) dielectrics from the 0.65 μm powder were fired using 0 to 2 at.% Nb. Microstructures, densities, and electrical resistivities were investigated.     

Formation of (111) Twins in BaTiO3 Ceramics

(111) twins are found frequently in the microstructure of BaTiO₃ ceramics prepared by the mixed-oxide technique. The processing steps by which the (111) twins are formed are in question. A BaCO₃-TiO₂ powder mixture is milled and then Calcined at 1100°C for 2 h, where BaTiO₃ is formed by the release of CO₂. After the powder is calcined, it is analyzed by TEM. Image and diffraction pattern data are combined to prove the presence of (111) twins in powder particles; the formation of (111) twins occurred frequently during calcination. The formation mechanism of such twins and their role during anomalous grain growth are discussed.     

High-Energy Density Capacitors Utilizing 0.7 BaTiO3–0.3 BiScO3 Ceramics

A high, temperature-stable dielectric constant (∼1000 from 0° to 300°C) coupled with a high electrical resistivity (∼10¹²Ω·cm at 250°C) make 0.7 BaTiO₃–0.3 BiScO₃ ceramics an attractive candidate for high-energy density capacitors operating at elevated temperatures. Single dielectric layer capacitors were prepared to confirm the feasibility of BaTiO₃–BiScO₃ for this application. It was found that an energy density of about 6.1 J/cm³ at a field of 73 kV/mm could be achieved at room temperature, which is superior to typical commercial X7R capacitors. Moreover, the high-energy density values were retained to 300°C. This suggests that BaTiO₃–BiScO₃ ceramics have some advantages compared with conventional capacitor materials for high-temperature energy storage, and with further improvements in microstructure and composition, could provide realistic solutions for power electronic capacitors.     

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.     

Reaction Rates of BaTiO3 and SrTiO3

The temperature dependence of the reaction rates in a 50(BaTiO₃)-50(SrTiO₃) ceramic mixture (wt%) was studied by high-temperature X-ray diffraction. The data were fitted to two theoretical models and times for complete reactions and activation energy were calculated. Use of the results in electronic material applications is discussed.     

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.     

Effect of Crystal-Axis Orientation on Resonance Mode of BaTiO3 Ceramics

BaTiO₃ ceramics were fabricated by sintering a mixture of fibrous and commercial powders, and the effects of crystal-axis orientation on the resonance characteristics of the poled specimens was investigated. Resonance in the radial direction was observed only for the fibrous BaTiO₃ ceramic disk. The degree of c -axis orientation of the sintered compact decreased with an increase in the initial amount of the commercial powder, and the decrease in the degree of orientation caused the appearance of the resonance in the thickness direction.     

Solubility of TiO2 in BaTiO3

Scanning electron microscopy and electron probe micro-analysis were used to investigate the microstructure of both slow-cooled and quenched polycrystalline BaTiO₃ specimens with a small excess of TiO₂ (Ba/Ti=0.995 to 0.999) or of BaO (Ba/Ti=1.002 and 1.005). The electron micrographs of polished and etched TiO₂-excess BaTiOs samples, and of fracture surfaces of quenched samples, showed a second phase in the grain boundaries and triple-point regions, whereas no second phase was observed in samples having Ba/Ti=1.000. Microprobe analysis of the second phase gave compositions near that of the reported adjacent phase of higher TiO₂ content, Ba₆Ti₁₇O₄₀. The results indicate that the solubility of TiO₂ in BaTiO₃ is <0.1 mol%.     

Release of Oxygen During the Sintering of Doped BaTiO3 Ceramics

The release of oxygen during the sintering of Sb₂O₃-doped BaTiO₃ ceramics containing excess TiO₂ was measured using a mass spectrometer. The amount of oxygen released is proportional to the dopant concentration in the product phase. The evolution of oxygen during sintering was attributed to dissolution of the oxidized form of doped BaTiO₃ in the reacting mixture and simultaneous re-crystallization of the- reduced form.