Grain Growth Control and Dopant Distribution in ZnO-Doped BaTiO3

ZnO additions to BaTiO₃ have been studied in order to determine the role of this dopant on sintering and microstructure development. As a consequence of a better initial dopant distribution, samples doped with 0.1 wt% zinc stearate show homogeneous fine-grained microstructure, while a doping level of 0.5 wt% solid ZnO is necessary to reach the same effect. When solid ZnO is used as the dopant precursor, ZnO is redistributed among the BaTiO₃ particles during heating. Since no liquid formation has been detected for temperatures below 1400°C in the system BaTiO₃-ZnO, it is proposed that dopant redistribution takes place by vapor-phase transport and grain boundary diffusion. Shrinkage and porosimetry measurements have shown that grain growth is inhibited during the first step of sintering for the doped samples. STEM-EDX analysis revealed that solid solubility of ZnO into the BaTiO₃ lattice is very low, being strongly segregated at the grain boundaries. Grain growth control is attributed to a decrease in grain boundary mobility due to solute drag. Because of its effectiveness in controlling grain growth, ZnO appears to be an attractive additive for BaTiO₃ dielectrics.     

Grain Growth During Sintering of Donor-Doped BaTiO3

Grain growth of donor-doped BaTiO₃ in the presence of KF b'quid phase was studied. The results showed that the composition of the liquid phase present during sintering had a pronounced influence on grain growth and on the formation of semiconducting donor-doped BaTiO₃.     

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.     

Growth of Single Crystals of BaTiO3 by Exaggerated Grain Growth

Grain-growth experiments with steep temperature gradients and with polycrystalline aggregates seeded with single crystals were carried out to explore the feasibility of growing large crystals of BaTiO₃ in the solid state. It is shown that both methods resulted in a large increase in size of crystals normally obtained by sintering but that the rapid grain-boundary motion resulted in high porosity. Definite orientation dependence of grain-boundary velocity is demonstrated, and growth in a 〈110〉 direction was found to be at least as fast as 2 mm per hour.     

Transmission Electron Microscopy at Grain Boundaries of PTC-Type BaTiO3 Ceramics

Transmission electron microscopy of the grain boundaries of Ti-rich semiconducting PTC-type BaTiO₃ ceramics is described. At a width of 2 to 10 nm, there were no indications of intergranular second-phase layers covering the grains. Second phase was segregated only at the contacts of three or more grains; electron diffraction confirmed the amorphous structure of this phase. Observed ferroelectric domains at the grain boundaries do not indicate a PTC-specific orientation of ferroelectric domains due to the negative grain surface charge.     

Influence of Grain Growth on Dielectric Properties of Nb-Doped BaTiO3

Effects of grain size and grain growth in Nb-doped BaTiO₃ on temperature and frequency dependencies of the dielectric constant were investigated. When 0.65 μm powder is sintered to an average grain size of 1 μm, two dielectric constant peaks indicate the presence of Nb-free BaTiO₃ and of Nb-containing material. Single peaks are observed above room temperature after additional grain growth or when 0.07 μm powder is sintered to an average grain size of 1 μm. The Curie point of pure BaTiO₃ with 1 μm grains is 4 to 6°C lower than that of material with grains >10 μm. Thermodynamically, this behavior is accounted for by a phase inversion stress ∼ the room-temperature stress.     

Oxygen Partial Pressure and Grain Growth in Donor-Doped BaTiO3

The grain growth of donor-doped BaTiO₃ at different oxygen partial pressures was studied. Results showed that the oxygen pressure had a pronounced influence on the grain growth and related effects. A model for the grain size anomaly during sintering of donor-doped BaTiO₃ in the presence of a TiO₂-rich liquid phase is proposed.     

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.     

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.     

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.     

Abnormal Grain Growth and Intergranular Amorphous Film Formation in BaTiO3

The effect of abnormal grain growth on the formation of amorphous films at grains boundaries was studied in a model system BaTiO₃. 0.4 mol% TiO₂-excess BaTiO₃ powder compacts were sintered at 1380°C for various times up to 16 h. During the sintering, abnormal grains formed. With the growth of the abnormal grains, amorphous films formed and eventually thickened up to 19.2 nm at grain boundaries. The film formation is attributed to the accumulation of Ti solutes at grain boundaries with the grain growth, while the film thickening is mostly caused by the redistribution of liquid at triple junctions. Extended annealing of the 16-h-sintered sample at 1350°C for 15 days resulted in a thinning of the film to nearly 1.7 nm without a change in the grain size, showing an equilibrium thickness. This result demonstrates that the film thickness observed during the growth of the grain may not be the equilibrium thickness. The result further suggests that the shape of the abnormal grains, even when equiaxed, can differ from the equilibrium shape.     

Effect of Ba6Ti17O40/BaTiO3 Interface Structure on {111} Twin Formation and Abnormal Grain Growth in BaTiO3

A structural transition of Ba₆Ti₁₇O₄₀/BaTiO₃ interfaces from faceted to rough was induced by reducing oxygen partial pressure in the atmosphere. As the oxygen partial pressure decreased, the number densities of {111} twins and abnormal grain decreased. TEM observation showed that the twin formation was governed only by the faceting of the interface. Experimental evidence of {111} twin-assisted abnormal growth of faceted BaTiO₃ grains was also obtained.     

Direct Examinations of PTC Action of Single Grain Boundaries in Semiconducting BaTiO3 Ceramics

R-T and I-V characteristics of single grains and grain boundaries in large-grained BaTiO₃ PTC ceramics were studied with a two-probe technique using a micromanipulator and fine Al wire. The PTC originates in the grain boundary only and behaves differently in each boundary. Even below T_c , the ceramic resistance depends almost entirely on the boundary. I-V characteristics in the boundary follow Ohm's law and conduction by a space-charge-limited current with a trap, using different applied voltages. The PTC anomaly relates to activation of the trap in the boundary, not to barrier height. A band model in the intergranular layer, with dielectric BaTiO₃ and the trap, is proposed.     

Faceting of High-Angle Grain Boundaries in Titanium-Excess BaTiO3

The grain boundaries in BaTiO₃ with excess Ti of 0.5, 0.3, and 0.1 at.% sintered at 1300° or 1250°C have been examined by scanning electron microscopy (SEM), electron backscattered diffraction pattern (EBSP), and transmission electron microscopy (TEM). In the 0.1% Ti-excess specimen, large grains growing abnormally form high-angle grain boundaries when they impinge on each other as verified by EBSP. A large fraction of these grain boundaries are faceted with hill-and-valley shapes. In the 0.5% Ti-excess specimen, large grains growing abnormally are elongated in the directions of their {111} double twins. These grains often form flat grain boundaries parallel to their {111} planes with the fine matrix grains, and the grain-boundary segments between the large impinging grains with high misorientation angles are often also parallel to the {111} planes of one of the grains. These grain boundaries are expected to be singular. Most of the grain boundaries between the randomly oriented fine-matrix grains in the 0.3 at.% Ti-excess specimen are also faceted with hill-and-valley shapes at finer scales when observed under TEM. The facet planes are parallel to {111}, {011}, and {012} planes of one of the grain pairs and are also expected to be singular. These high-angle grain boundaries lying on low index planes of one of the grain pairs are similar to those observed in other oxides and metals.     

Abnormal Grain Growth Occurring at the Surface of a Sintered BaTiO3 Specimen

When a BaTiO₃ powder compact was sintered in air, the abnormal grain growth (AGG) was observed to occur at the surface of the specimen. The BaO evaporation from the surface and consequent formation of eutectic liquid is suggested to be the cause of AGG. When the evaporation of BaO during sintering was prevented by embedding the specimen in the same powder, no AGG was observed to occur.     

Grain Growth in Sintered ZnO and ZnO-Bi2O3 Ceramics

Grain growth in a high-purity ZnO and for the same ZnO with Bi₂O₃ additions from 0.5 to 4 wt% was studied for sintering from 900° to 1400°C in air. The results are discussed and compared with previous studies in terms of the phenomenological kinetic grain growth expression: G ⁿ— G ⁿ₀= K ₀ t exp(— Q/RT ). For the pure ZnO, the grain growth exponent or n value was observed to be 3 while the apparent activation energy was 224 ± 16 kJ/mol. These parameters substantiate the Gupta and Coble conclusion of a Zn²⁺ lattice diffusion mechanism. Additions of Bi₂O₃ to promote liquidphase sintering increased the ZnO grain size and the grain growth exponent to about 5, but reduced the apparent activation energy to about 150 kJ/mol, independent of Bi₂O₃ content. The preexponential term K ₀ was also independent of Bi₂O₃ content. It is concluded that the grain growth of ZnO in liquid-phase-sintered ZnO-Bi₂O₃ ceramics is controlled by the phase boundary reaction of the solid ZnO grains and the Bi₂O₃-rich liquid phase.     

Effects of Ba6Ti17O40 on the Dielectric Properties of Nb-Doped BaTiO3 Ceramics

The dielectric properties, including the DC breakdown strength, of 1 mol% Nb⁵⁺-doped BaTiO₃ ceramics with different quantities of excess TiO₂ have been investigated. The breakdown strength was found to decrease with increasing TiO₂ content, but could not be readily explained by relative density and grain size effects. The decrease in the breakdown strength from a stoichiometric BaTiO₃ composition to samples with excess TiO₂ is believed to be due to the field enhancement effect (up to a factor of 1.40) at the BaTiO₃ matrix because of the presence of a Ba₆Ti₁₇O₄₀ second phase. The thermal expansion coefficient mismatch between the BaTiO₃ matrix phase and the Ba₆Ti₁₇O₄₀ phase may also result in a low breakdown strength. The dielectric properties of the pure Ba₆Ti₁₇O₄₀ phase were also investigated and are reported herein.