Suppression of Acceptor Solubilities in BaTiO3 Densified in Highly Reducing Atmospheres

The solubilities of acceptor impurities are strongly suppressed when BaTiO₃ is densified in highly reducing atmospheres. This is evidenced by a shift in the minimum in the equilibrium electrical conductivity to higher oxygen activities, a decrease in the ionic contribution to the conductivity, and a decrease in the leakage current and the rate of leakage current degradation under temperature–voltage stress. The normal solubility is restored by subsequent anneals in air that result in substantial grain growth, and the properties then revert to those of BaTiO₃ sintered in air. The solubility suppression is attributed to a mass-action interaction between the processes that generate oxygen vacancies, those that result from the compensation of acceptor centers, and those that result from reduction.     

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.     

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.     

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.     

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%.     

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⁺.     

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.     

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.     

Polymorphism of BaTiO3 Acceptor Doped with Mn3+, Fe3+, and Ti3+

The effect of Mn doping on the cubic to hexagonal phase transition temperature in BaTiO₃ has been determined by quenching samples with different Mn contents from a range of temperatures. Under conditions of equilibrating samples in air over the range 1000°–1400°C, cubic solid solutions BaTi_{1− x} Mn _x O_3−δ form over the range 0≤ x ≤0.015(5), whereas hexagonal solid solutions form for x ≥0.02, depending on the temperature. The results are compared with those on doping BaTiO₃ with Fe³⁺ and observations made concerning acceptor doping with Ti³⁺.     

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.     

Incorporation of Er3+ into BaTiO3

The incorporation of Er³⁺ into BaTiO₃ ceramics was investigated on samples containing 0.25, 0.5, 1, 2, 8, and 10 at.% of dopant, after sintering at 1350–1550°C in air. For Er³⁺ concentrations ≤1 at.%, dense and large-grained ceramics with low room-temperature resistivity (10²–10³Ω·cm) were obtained. The observed properties are largely independent of stoichiometry. Simultaneous substitution of Er³⁺ at both cation sites, with higher preference for the Ba site, is proposed. The behavior of heavily doped ceramics depends on stoichiometry. When Ba/Ti < 1, the electrical properties change from slightly semiconducting to insulating as Er concentration increases from 2 to 8 at.%. The ceramics have tetragonal perovskite structure and contain a large amount of Er₂Ti₂O₇ pyrochlore phase. On the other hand, when Ba/Ti > 1, the ceramics are insulating, fine-grained, and single phase. In this case, incorporation of Er³⁺ predominantly occurs at the Ti site, with oxygen vacancy compensation. Incorporation is accompanied by a significant reduction of tetragonality and by expansion of the unit cell. The different results indicate that Er³⁺ solubility at the Ba site does not exceed 1 at.%, whereas solubility at the Ti site is at least 10 at.%. However, the incorporation of Er³⁺ and the resulting properties are also strongly affected by sintering conditions.