Electrical Conduction of Rare-Earth-Doped BaTiO3 Single Crystals

The effect of 0.2 to 0.8 at.% rare-earth ions on the conductivity of melt-grown BaTiO₃ crystals was studied by optical absorption, luminescence, Hall coefficient, and electrical conductivity measurements. The conductivity is proportional to the dopant concentration in these crystals; such is not the case in BaTiO₃ ceramics. A rare-earth ion substitutes on the Ba²⁺ site and yields a donor.     

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.     

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.     

Structure Evolution in Hydrothermally Processed (<100°C) BaTiO3 Films

Thin films of cubic BaTiO₃ were processed hydrothermally at 40°–80°C by reacting thin layers of titanium organo metallic liquid precursors in aqueous solutions of either Ba(OH)₂ or a mixture of NaOH and BaCl₂. All films (thickness ∼1 μm) were polycrystalline with grain sizes ranging from nano- to micrometer dimensions. BaTiO₃ formation was facilitated by increasing [OH^-], [Ba²⁺], and the temperature. The film structure was related to the nucleation and growth behavior of the BaTiO₃ particles. Films processed at relatively low [OH^-], [Ba²⁺], and temperatures were coarse grain and opaque, but increasing [OH^-], [Ba²⁺], and temperature caused the grain size to decrease, resulting in transparent films.     

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.     

Interaction between Dissolved Ba2+ and PAA-NH4 Dispersant in Aqueous Barium Titanate Suspensions

The interaction between dissolved Ba²⁺ and dissociated ammonium salt of poly(acrylic acid) (PAA-NH₄) in aqueous suspension has been studied. The dissolved Ba²⁺ causes flocculation of dissociated PAA-NH₄, thus degrading its dispersing effectiveness in aqueous BaTiO₃ suspensions. The concentration of PAA-NH₄ required to stabilize aqueous BaTiO₃ suspension increases with increasing Ba²⁺ concentration at a given pH. A stability map, which is determined by a rheological study, is constructed to describe the amount of PAA-NH₄ required to have well-dispersed aqueous BaTiO₃ suspensions as a function of Ba²⁺ concentrations at different pH values.     

Novel Doping Mechanism for Very-High-Permittivity Barium Titanate Ceramics

Barium titanate (BaTiO₃) can be doped with La³⁺ ions via partial substitution for Ba²⁺ ions; charge balance is maintained by the creation of Ti⁴⁺ vacancies. Samples processed in an atmosphere of 1 bar O₂ and a temperature of 1350°C are insulating and free from electronic defects associated with either O₂ loss or reduction of Ti⁴⁺ to Ti³⁺. The Curie temperature ( T _c) decreases approximately linearly as the lanthanum content increases and, at the same time, an increase in the permittivity (ɛ') maximum at T _c occurs. For the composition Ba_{1- x} La _x Ti_{1- x /4}O₃, where x = 0.05, ɛ' has a maximum value of 19000 at 18°C, compared with a typical value of 10000 at 130°C in undoped BaTiO₃ ceramics. This value is the highest value reported for A-site-doped BaTiO₃ and is linked to the mechanism of combined A-site doping and Ti-vacancy creation.     

Influence of Interdiffusion on Solid Solution Formation and Sintering in the System BaTiO3-SrTiO3

Formation of BaTiO₃-SrTiO₃ solid solution during sintering of powder mixtures is characterized by preferential diffusion of Ba²⁺ ions. As a consequence, several nonequilibrium phases are temporarily formed; they were identified by X-ray and microprobe analysis. Eutectic liquid appears below 1300°C, which may explain exaggerated grain growth during sintering of BaTiO₃-SrTiO₃ mixtures. Disturbance in neck growth and Kirkendall-type porosity hamper densification in the heterogeneous system as compared with the pure titanates.     

Interfacial Segregation in Perovskites: II, Experimental Evidence

Experimental evidence determined by scanning transmission electron microscopy, scanning Auger electron spectroscopy, and lattice parameter measurements is reported for interfacial segregation in pure and doped perovskite materials, e.g., BaTiO₃ and SrTiO₃. The segregation behavior for isovalent dopants (e.g., Sr²⁺, Ca²⁺, or Pb²⁺ at a Ba²⁺ site) is mainly associated with a strain energy contribution from ionic misfit. However, both strain energy and space charge contributions are important in determining the segregation behavior of aliovalent ions (e.g., La³⁺ or Nd³⁺ at a Ba²⁺ site and Fe³⁺ or Ni³⁺ at a Ti⁴⁺ site). Segregation depths for aliovlent dopants are typically 15 to 20 nm thick. Besides dopants, constituent alkaline-earth ions also segregate to interfaces. Their segregation behavior is promoted by high temperatures, low partial pressures of oxygen, and the presence of acceptors. The results are explained in terms of a proposed theory of segregation presented in part I of the present series.     

Defect Structure and Phase Relations of Highly Lanthanum-Doped Barium Titanate

Incorporation of La³⁺ into the BaTiO₃ lattice was studied on samples of various composition, using quantitative WDS microanalysis (EPMA) in combination with scanning electron microscopy (SEM) and X-ray powder diffractometry (XRPD). Direct determination of solid-solution formulas by microanalysis supports the structure model of a solid solution with La³⁺ on Ba²⁺ sites and a deficient Ti⁴⁺ sublattice, independent of the starting composition. Solid solution extends on the tie line, which points from BaTiO₃ to La₄Ti₃O₁₂, to the composition of approximately Ba_0.70- La_0.38Ti_0.925( V "")_0.075O₃. On the basis of these results, the BaTiO₃-rich part of the BaO-La₂O₃-TiO₂ phase diagram was constructed.     

Effect of Ba2+ Substitution on Dielectric and Electric-Field-Induced Strain Properties of PMN–PZ–PT Ceramics

Effect of Ba²⁺ substitution for Pb²⁺ on the dielectric and electric-field-induced strain characteristics of the PMN–PZ–PT ceramics has been investigated in the compositions of the tetragonal-rich 0.2PMN–0.36PZ–0.44PT and rhombohedral-rich 0.2PMN–0.4PZ–0.4PT ceramics. The phase approached cubic structure from the tetragonal and rhombohedral, and grain size was reduced when the Ba²⁺ cation was substituted. As Ba²⁺ content increased, frequency-dependent relaxor-like behavior of the dielectric constant was observed at temperatures below the dielectric maximum ( T _max) for compositions with 20 and 25 mol% Ba²⁺. Electric-field-induced strain was maximized in the 12 mol% Ba²⁺-substituted 0.2PMN–0.4PZ–0.4PT specimen ( S _max= 0.15%), and maximum piezoelectric, d ₃₁, was 300 in the 14 mol% Ba²⁺-substituted 0.2PMN–0.4PZ–0.4PT specimen.     

Compensation Effect in Semiconducting Barium Titanate

Donor-doped, stoichiometric BaTiO₃ sintered at 1350°C for 1 h exhibits a maximum room-temperature conductivity at [La³⁺]∼0.15 mol%. Elements of lower valence than Ba²⁺ or Ti⁴⁺, when incorporated into semiconducting BaTiO₃, are regarded as poisoning impurities, i.e., acceptors. They tend to increase the room-temperature resistivity of the semiconducting BaTiO₃. For insulating BaTiO₃ resulting from high Mg²⁺ acceptor doping levels, the semiconductivity can be restored by introducing higher La³⁺ donor-dopant concentrations. This behavior is interpreted as a compensation effect based on the defect chemistry of the acceptor- and donor-doped BaTiO₃.     

Phase Transition and Physical Characteristics of Nanograined BaTiO3 Ceramics Synthesized from Surface-Coated Nanopowders

Nanograined BaTiO₃ ceramics prepared from 40-nm-size BaTiO₃ nanopowders exhibited the cubic as well as the tetragonal phase, while nanograined BaTiO₃ ceramics prepared from BaTiO₃ nanopowders coated with Mn had only the tetragonal phase. The dielectric constant of the latter was 10 times larger than that of the former; the latter exhibited PTCR behavior with a resistivity jump ratio of about 5.0 × 10⁴. These physical properties of the BaTiO₃ ceramics appeared to be significantly affected by the strain near grain boundaries; such strain resulted in a phase transition from the cubic to the tetragonal phase in the nanograined BaTiO₃ ceramics, even though the grain size was about 40 nm.     

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.     

Relaxor Behavior of the 0.9BaTiO3–0.1La(Mg1/2Ti1/2)O3 Solid Solution

Low-frequency dielectric response of air- and oxygen-sintered ceramics with the composition 0.9BaTiO₃–0.1La(Mg_1/2Ti_1/2) O₃ (0.9BT–0.1LMT) has been studied in the temperature range of 12–550 K. In comparison with pure BT, in 0.9BT–0.1LMT the dielectric permittivity maximum is shifted by almost 300 K toward lower temperatures. Both real and imaginary parts of dielectric permittivity of the solid solution, in the range 12–150 K, show a strong frequency-dependent behavior, which is typical of relaxors. On the basis of the model of exponential cluster size distribution and the Cole–Cole equation, the degree of interaction between the polar clusters was estimated. It was shown that the oxygen vacancies arising during sintering at high temperatures did not affect noticeably the relaxor properties of the material. The role of heterovalent La³⁺/Ba²⁺ and Mg²⁺/Ti⁴⁺ substitutions in the relaxor behavior formation is discussed.     

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.     

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⁺.     

Effect of Reoxidation on the Grain-Boundary Acceptor-State Density of Reduced BaTiO3 Ceramics

To investigate the effect of reoxidation on the grain-boundary acceptor-state density of reduced barium titanate, n -doped BaTiO₃ ceramics are sintered in a reducing atmosphere (2% H₂+ 98% N₂) and then annealed in oxygen. After annealing at 1150°C for different times, the experimental results show a relationship between temperature-averaged acceptor-state density and annealing time as N _s= N _so Bt ^1/n with n between 2 and 3. An inherent acceptorstate density N _so= 4.25 × 10¹² cm⁻² is obtained with an increase rate B = 4.8 × 10¹² cm⁻². min^−1/3, when n reaches 3. The inherent grain-boundary acceptor states in the reduced n -doped BaTiO₃ ceramics are believed not due to adsorbed oxygen ions.     

Dispersion Properties of BaTiO3 Colloids with Amphoteric Polyelectrolytes

An amphoteric water-soluble copolymer, i.e., poly(acrylamide/(α- N,N -dimethyl- N -acryloyloxyethyl) ammonium ethanate) (PAAM/DAAE), was evaluated as a novel dispersant for aqueous BaTiO₃ (BT) slurries. The dispersing property of this copolymer was examined by means of rheology, sedimentation, particle size, green density, zeta potential, and leached Ba²⁺ concentration measurements. The results indicate that PAAM/DAAE could reduce the viscosity of slurries greatly, cause BT particle sizes to shift to smaller values, and make green compacts more consolidated. Compared with a commercial dispersant, ammonium salt of poly(methacrylic acid) (PMAA-NH₄), it is as effective or even better in preparing stabilized suspensions. More importantly, PAAM/DAAE could lessen the leached Ba²⁺ concentration. This is related to the adsorption behavior of this copolymer onto BT particles, and the interaction between the adsorbed dispersant and dissolved barium ions.     

Lanthanum-Magnesium and Lanthanum-Manganese Donor-Acceptor-Codoped Semiconducting Barium Titanate

BaTiO₃ powder doped with La donor and codoped with Mn or Mg acceptor was sintered at 1350°C/1 h in air. For Ladoped BaTiO₃, the room-temperature resistivity decreased to a minimum at [La³⁺] ∼ 0.15 mol%. For La-Mn-codoped BaTiO₃, the minimum resistivity occurred at [La³⁺] - 2[Mn²⁺] ∼ 0.15 mol%. When the ceramic was changed to a fine-grained insulator by high donor doping ([La³⁺] >0.15 mol%), its semiconductivity was restored, and the relatively homogeneous, coarse-grained microstructure recurred by codoping with either Mg or Mn acceptor, with the transition at [La³⁺] - 2[Mg²⁺] = 0.15 mol% or [La³⁺] - 2[Mn²⁺] = 0.15 mol%. The analogy of a compensation effect between La-Mn- and La-Mg-codoped BaTiO₃ suggested that Mn acceptor added to BaTiO₃ exists as Mn²⁺ ion in the bulk grain region; its influence on the positive temperature coefficient of resistivity behavior is then discussed.