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.     

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.     

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.     

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%.     

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 Silver and Palladium in BaTiO3

Silver, palladium, and their alloys are frequently used as electrode materials for BaTiO₃ (BT) based dielectrics. However, the electrodes and dielectrics usually are cofired at high temperatures, and silver and palladium can dissolve into the BT during cofiring. In the present study, the solubility of silver and palladium into BT after cofiring was determined. Three measurement techniques were used to determine solubility: chemical analysis, structural analysis, and dielectric analysis. The solubility of the silver in the BT was low, 450 ppm, after cofiring at 1290°C for 2 h in air. The diffusion distance of the silver ions into the BT was >5 μm. The solubility of the palladium in the BT was even lower, 50 ppm at 1290°C, and the diffusion distance was ∼1 μm. The solubility of both the silver and the palladium in the BT decreased as the oxygen partial pressure of the sintering atmosphere decreased. These results demonstrated that both silver and palladium solutes act as acceptors for BT.     

Compensating Defects in Highly Donor-Doped BaTiO3

Nb-doped BaTiO₃ has been prepared with various Ba/(Ti + Nb) ratios such that single-phase products will be obtained if the charged donor center is assumed to be compensated in turn by Ba vacancies, Ti vacancies, equal concentrations of the two cation vacancies, oxygen interstitials, or electrons. For air-Fired samples, examination by transmission electron microscopy showed that only the composition adjusted for compensation by titanium vacancies was single phase. The other compositions contained a Ti-rich second phase in order to achieve a matrix with the appropriate concentration of titanium vacancies. When sintered in a reducing atmosphere, compensation was by electrons, and a Ba-rich second phase was present hi the composition adjusted to give compensation by titanium vacancies. The results indicate that for donor concentrations greater than ∼0.5 mol% in BaTiO₃, charge compensation is achieved by Ti vacancies under oxidizing conditions, and by electrons (as is well-known) under reducing conditions.' The effect of compensating defects on grain growth is also discussed.     

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.     

The Solid Solubility of Holmium in BaTiO3 Under Reducing Conditions

The solid solubility of Ho³⁺ in BaTiO₃ fired under the reducing conditions of a nitrogen atmosphere containing 10% of hydrogen was studied by quantitative electron-probe microanalysis using wavelength-dispersive spectroscopy. The solubility was found to depend mainly on the starting composition. In the TiO₂-rich samples, the solubility of the Ho³⁺ donors at the Ba lattice sites was measured to be ∼4 mol% Ho_Ba (Ba_0.96Ho_0.04TiO_3−δ at 1400°C), which is much higher than after sintering in air. In the BaO-rich compositions, the solubility of the Ho³⁺ acceptors at the Ti sites (solubility limit     at 1500°C) decreased compared with that in air. Like with sintering in air, under reducing conditions, the highest solubility of holmium in BaTiO₃ was determined when it was incorporated at both lattice sites (solubility limit     ).     

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.     

Solid Solubility of Holmium, Yttrium, and Dysprosium in BaTiO3

The solid solubility of R ions (R = Ho³⁺, Dy³⁺, and Y³⁺) in the BaTiO₃ perovskite structure was studied by quantitative electron-probe microanalysis (EPMA) using wavelength-dispersive spectroscopy (WDS), scanning electron microscopy (SEM), and X-ray diffractometry (XRD). Highly doped BaTiO₃ samples were prepared using mixed-oxide technology including equilibration at 1400° and 1500°C in ambient air. The solubility was found to depend mainly on the starting composition. In the TiO₂-rich samples a relatively low concentration of R incorporated preferentially at the Ba²⁺ lattice sites (solubility limit ∼Ba_0.986R_0.014Ti_0.9965(V^"_Ti^")_0.0035O₃at 1400°C). In BaO-rich samples a high concentration of R entered the BaTiO₃ structure at the Ti⁴⁺ lattice sites (solubility limit ∼BaTi_0.85R_0.15O_2.925(V_O^••)_0.075at 1500°C). Ho³⁺, Dy³⁺, and Y³⁺incorporated preferentially at the Ti⁴⁺ lattice sites stabilize the hexagonal polymorph of BaTiO₃. The phase equilibria of the Ho³⁺–BaTiO₃ solid solutions were presented in a BaO–Ho₂O₃–TiO₂phase diagram.     

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.     

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.     

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.     

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.     

Solubility of KUO3 in BaUO3

The formation of a solid solution between cubic perovskne-type KUO₃ and pseudocubic BaUO₃ was investigated. The reaction begins at 550°C, and the solubility of KUO₃ reaches more than 30 mol% KUO₃ in BaUO₃ at 750°C. The region in which a single-phase solid solution exists was determined. The variation of the lattice parameter of the reacted samples was caused by solid solution formation and by oxygen absorption. The electrical conductivities of the samples varied with composition and showed a distinct maximum. The activation energy for electric conduction was very low compared to that for UO_z+x, or U₃O₈.     

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.