Nonstoichiometry in Acceptor-Doped BaTiO3

The effect of added Al, as an acceptor impurity, on the equilibrium electrical conductivity of large-grained, polycrystalline BaTiO₃, is consistent with a previously proposed defect model which involves only doubly ionized oxygen vacancies, electrons, holes, and acceptor impurities. The behavior is an extension of that of undoped BaTiO₃, in which an accidental net acceptor excess already plays an important role. Comparison of the derived active acceptor content with the amount of added Al indicates that Al is <50% effective in creating acceptor levels. The magnitude of a small Po₂-independent conductivity component, necessary to fit the observed conductivity minima, increases with added Al content. This is consistent with a contribution from extrinsic oxygen vacancy conduction.     

Impedance Spectroscopy of Undoped BaTiO3 Ceramics

The electrical properties of ceramic BaTiO₃ were investigated by ac impedance spectroscopy over the ranges 25°-330°C and 0.03 Hz-1 MHz. Results are compared with those obtained from fixed-frequency measurements, at 1 kHz and 100 kHz. Fixed-frequency Curie-Weiss plots show deviations from linearity at temperatures well above t _c. The ac measurements show that grain boundary impedances influence Curie-Weiss plots in two ways: at high temperatures, they increasingly dominate the fixed-frequency permittivities; at lower temperatures, closer to T ^c, the high-frequency permittivity contains a contribution from grain boundary effects. Methods for extraction of bulk and grain boundary capacitances from permittivity and electric modulus complex plane plots are discussed. The importance of selecting the appropriate equivalent circuit to model the impedance response is stressed. A constriction impedance model for the grain boundary in BaTiO₃ ceramics is proposed: the grain boundary capacitance is neither temperature-independent, nor shows Curie-Weiss behavior. The grain boundary is ferroelectric, similar to the grains, but its impedance is modified by either air gaps or high-impedance electrical inhomogeneity in the region of the necks between grains; the activation energy of the constriction grain boundary impedance differs from that of the bulk, suggesting differences in defect states or impurity levels.     

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.     

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.     

PTCR Effect in BaTiO3

Calculations of bulk and surface defect energies were used to develop a model of the grain structure of doped BaTiO₃, Segregation processes are expected as a consequence of thermo-dynamic and kinetic factors and result in the development of n-i-n junctions at intergranular contacts, thus leading to the observed PTCR behavior of polycrystalline materials.     

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%.     

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.     

Crystal Chemistry and Dielectric Properties in the Systems BaTiO3-UO2 and BaTiO3-BaUO3

Polycrystalline barium titanate fired in nitrogen at 1300° to 1400°C accommodates up to 3 mole % UO₂ in solid solution; its structure is then cubic at room temperature. With BaUO₃ additions the structure becomes disordered and quasi-cubic. In air, about 1 mole % UO₂ goes into solid solution in BaTiO₃ but the structure remains tetragonal. Diffraction peaks of a new phase, possibly a ternary oxide of barium, uranium, and titanium, appear in patterns of specimens containing more than 2 mole % UO₂. The dielectric constant of BaTiO₃ ceramics fired in air, steam, or oxygen increases with up to about 0.5 mole % UO₂ but declines rapidly above this level. The dielectric constant of BaUO₃ is about two orders of magnitude lower than that of BaTiO₃, and additions of BaUO₃ invariably lower the dielectric constant of BaTiO₃.     

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.     

Field-Induced Strain in Single-Crystal BaTiO3

The electric-field-induced strain in single-crystal BaTiO₃ was investigated. For crystals relatively free of twinning, a longitudinal strain of 0.35% can be induced just above the ferroelectric-paraelectric phase transition temperature (T_c1) primarily by field-forced paraelectric-ferroelectric phase transition. For heavily twinned crystals, 90° domain reorientation under the applied electric field plays an important role in the induced strain below T_c1, and an induced strain of 0.6% is observed a few degrees below T_c1. Above T_c1, the electrostrictive property measured by a weak excitation field is purely intrinsic. When the excitation field is large, so that a field-forced paraelectric-ferroelectric phase transition is involved, the x₃₃/P²₃ value (where x₃₃ and P₃ are the induced strain and polarization along the z axis, respectively) is intrinsic at higher temperature, but may be modified at temperatures just at and slightly above T_c1 by residual 90° twin structure.     

High Tc in Electrospun BaTiO3 Nanofibers

BaTiO₃ nanofibers were prepared by electrospinning. The morphology of synthesized BaTiO₃ nanofibers was investigated under different heat treatment conditions. The phase transformations in BaTiO₃ nanofibers were monitored using Raman spectroscopy. It has been found that the Curie temperature of BaTiO₃ nanofibers increased to 220°C, which is notably higher than the bulk BaTiO₃ ceramics.     

Solubility of BaO in BaTiO3

The solubility and mode of incorporation for BaO in BaTiO₃ were studied by X-ray powder diffraction, scanning and transmission electron microscopy, electron probe microanalysis, and equilibrium electrical conductivity measurements. The presence of barium orthotitanate, Ba₂TiO₄, as a second phase for samples containing >0.1 mol% excess BaO was confirmed by direct microscopic examination. There was no evidence to support the incorporation of excess BaO into BaTiO₃ by a Ruddlesden-Popper type of superlattice ordering mechanism. Measurement of the equilibrium electrical conductivity showed no detectable shift in the conductivity profile resulting from excess BaO, thus setting an upper limit of 100 ppm for the solubility of BaO in BaTiO₃.     

Defect Chemistry of BaTiO3 with Additions of CaTiO3

The substitution of up to 5% Ca²⁺ for Ba²⁺ in BaTiO₃ results in a shift in the oxygen pressure dependence of the equilibrium electrical conductivity that is in the same direction as that caused by addition of acceptor impurities such as Al³⁺ or Ca²⁺ substituted for Ti⁴⁺. In contrast to the latter effect, however, the shape of the conductivity plot is not changed, the conductivity value at the conductivity minimum is not affected, and the amount of the shift increases with decreasing temperature of measurement. It is shown that the shift is primarily due to an increase in the enthalpy of reduction and a decrease in the enthalpy of oxidation as increasing amounts of Ba²⁺ are replaced by Ca²⁺.     

Electrical Properties of Cerium-Doped BaTiO3

The high-temperature equilibrium electrical conductivity of Ce-doped BaTiO₃ was studied in terms of oxygen partial pressure, P (O₂), and composition. In (Ba_1−xCe _x )TiO₃, the conductivity follows the −1/4 power dependence of P (O₂) at high oxygen activities, which supports the view that metal vacancies are created for the compensation of Ce donors, and is nearly independent of P (O₂) where electron compensation prevails at low P (O₂). When Ce is substituted for the normal Ti sites, there is no significant change in conductivity behavior compared with undoped BaTiO₃, indicating the substitution of Ce as Ce⁴⁺ for Ti⁴⁺ in Ba(Ti_1−yCe _y )O₃. The Curie temperature ( T _c) was systematically lowered when Ce³⁺ was incorporated into Ba²⁺ sites, whereas the substitution of Ce⁴⁺ for Ti⁴⁺ sites resulted in no change in this parameter.     

Defect Chemistry of Donor-Doped BaTiO3

Donor additions to BaTiO₃ up to a few tenths atom percent are compensated by electrons and the resulting electrical conductivity is independent of temperature and P o₂ at 700° to 1000°C. The conductivities are impurity-insensitive at very low Poz and high temperature where reduction is the major source of defects. Variation in the site occupation ratio (A/B in ABO₃ has a small effect on the conductivities for donor additions in the 100 ppm range. Nb is more effective as a donor than is Al as an acceptor, and Nb can compensate approximately 2½ times as much Al on an atomic basis.     

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.     

Initial Sintering of BaTiO3 Compacts

Experimental measurements were made of the rate of initial shrinkage of high-purity BaTiO₃ compacts in air. The time dependence of the shrinkage rate was consistent with a model based on grain boundary vacancy diffusion. The apparent activation energy for the shrinkage rate in the range 700° to 1000°C is 112 ± 9 kcal per mole. Comparison with other data indicates that oxygen ion vacancy diffusion controls the initial sintering rate.     

Etch Pits in BaTiO3 Single Crystals

Several new etch-pit features in BaTiO₃ single crystals are shown and discussed. Etch-pit resolution was facilitated by using a 4% HNO₃, 2% HF etchant and by prepolishing the crystals. Etch-pit patterns in plastically deformed regions suggested that the preferred mode of slip in cubic BaTiOs is the 110, (110) direction. An etch-pit structure possibly associated with twins was also observed.     

Conversion of SiO2 Diatom Frustules to BaTiO3 and SrTiO3

Diatom frustules were used as bio-templates to synthesize functional ceramics via solid–gas displacement reactions. Silica-based frustules were exposed to TiF₄ at 330°C to form TiOF₂, which was later converted to TiO₂ (anatase) by heat treatment in air at 600°C. The TiO₂ frustules were then exposed to molten Ba(OH)₂ or Sr(OH)₂ to form BaTiO₃ or SrTiO₃, respectively. In both cases, near-complete conversion was achieved while retaining the morphology of the original silica frustules. BaTiO₃ and SrTiO₃ frustules exhibit nearly phase pure, nanocrystalline perovskite structure.