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.     

Effects of Grain Growth on the Distribution of Nb in BaTiO3 Ceramics

Measurements of Nb diffusion into large- and small-grained BaTiO₃ disks show a high ratio of grain boundary to bulk diffusivity. Well defined X-ray diffraction lines are found in Nb-doped BaTiO₃ only when significant grain growth occurs during sintering. When grain growth of a 0.65 μm grain size powder is limited at ∼1 μm, excess line broadening results. This is attributed to the simultaneous presence of Nb-free and Nb-rich regions. Because of its low bulk diffusivity little Nb penetrates into the original BaTiO₃ grain cores, and a solid solution forms only in the regions of recrystallization. When grain growth is limited, the "sintering reaction" results in a non-homogeneous system; when appreciable grain growth occurs, most of the original grain cores are eliminated and an essentially uniform system is obtained.     

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.     

Initial Specific Surface Area and Graln Growth in Donor-Doped Barium Titanate

The grain growth of donor-doped BaTiO₃ prepared from BaTiO₃ powders with different initial specific surface areas was studied. Results show that a higher initial surface area and, consequently, a smaller critical grain size at the phase boundary drastically increase the critical amount of donor dopant, causing the grain size anomaly during sintering.     

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.     

Two-Step Sintering of Ceramics with Constant Grain-Size, II: BaTiO3 and Ni–Cu–Zn Ferrite

We investigated the preparation of bulk dense nanocrystalline BaTiO₃ and Ni–Cu–Zn ferrite ceramics using an unconventional two-step sintering strategy, which offers the advantage of not having grain growth while increasing density from about 75% to above 96%. Using nanosized powders, dense ferrite ceramics with a grain size of 200 nm and BaTiO₃ with a grain size of 35 nm were obtained by two-step sintering. Like the previous studies on Y₂O₃, the different kinetics between densification diffusion and grain boundary network mobility leaves a kinetic window that can be utilized in the second-step sintering. Evidence indicates that low symmetry, ferroelectric structures still exist in nanograin BaTiO₃ ceramics, and that saturation magnetization is the same in nanograin and coarse grain ferrite ceramics.     

Effects of La2O3/ZnO Additives on Microstructure And Microwave Dielectric Properties of Zr0.8Sn0.2TiO4 Ceramics

Dielectric ceramics of Zr_0.8Sn_0.2TiO₄ containing La₂O₃ and ZnO as sintering aids were prepared and investigated for microstructure and microwave dielectric properties. Low-level doping with La₂O₃ and ZnO (up to 0.30 wt%) is good for densification and dielectric properties. These additives do not affect the dielectric constant and the temperature coefficient. Dielectric losses increase significantly at additive levels higher than 0.15 wt%. The combined additives La₂O₃ and ZnO act as grain growth enhancers. With 0.15 wt% additives, a ceramic having a dielectric constant, a quality factor, and a temperature coefficient of frequency at 4.2 GHz of 37.6, 12 800, and –2.9 ppm/°C, respectively, was obtained. The quality factor was considerably improved by prolonged sintering.     

Effect of Rare-Earth Ions on Electrical Conductivity of BaTiO3 Ceramics

Lattice constants, grain size, electrical conductivity, and luminescence were measured for sintered BaTiO₃ ceramics doped with 0 to 1.2 at.% rare-earth ions. BaTiO₃ doped with low levels of rare-earth ions contains grains 10 μm in size and has lattice constants nearly equal to those of undoped ceramics. In this case, rare-earth ions occupy Ba²⁺ sites and yield donors. When grain growth is inhibited by high doping levels or by insufficient sintering, the lattice constants change, the rare-earth ions occupy both Ba²⁺ and Ti⁴⁺ sites, and, consequently, BaTiO₃ becomes insulative because of charge compensation.     

Grain Growth of ZnO during Bi2O3 Liquid-Phase Sintering

Grain growth of ZnO during the liquid-phase sintering of binary ZnO–Bi₂O₃ ceramics has been studied for Bi₂O₃ contents from 3 to 12 wt% and sintering from 900° to 1400°C. The results are considered in combination with previously published studies of ZnO grain growth in the ZnO–Bi₂O₃ system. For the Bi₂O₃ contents of the present study, the rate of ZnO grain growth is found to decrease with increasing Bi₂O₃. Activation analysis, when combined with the results of similar analyses of the previous studies, reveals a change in the rate-controlling mechanism for ZnO grain growth. Following a low-Bi₂O₃-content region of nearly constant activation energy values of about 150 kJ/mol, further Bi₂O₃ additions cause an increase of the activation energy to about 270 kJ/mol. consistent with accepted models of liquid-phase sintering, it is concluded that the rate-controlling mechanism of ZnO grain growth during liquid-phase sintering in the presence of Bi₂O₃ changes from one of a phase-boundary reaction at low Bi₂O₃ levels to one of diffusion through the liquid phase at about the 5 to 6 wt% Bi₂O₃ level and above.     

Core-Shell Structure of Acceptor-Rich, Coarse Barium Titanate Grains

Heavily doped barium titanate (BaTiO₃) powders, one with a donor-rich and the other with an acceptor-rich composition, were prepared. After sintering, the donor-rich specimen exhibited a fine-grained microstructure but significant grain growth occurred in the acceptor-rich specimen. A stable dielectric behavior was observed only in the donor-rich fine-grained specimen over the temperature range studied. However, in both specimens, an undoped core region several hundred nanometers in size was detected. The core-shell structure appeared to be maintained in BaTiO₃ under the conventional sintering conditions_.     

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.     

Influence of the Addition of Bi2O3 on the Grain Growth and Magnetic Permeability of MnZn Ferrites

Additions of Bi₂O₃ were used to promote grain growth and to increase magnetic permeability during sintering of MnZn ferrites. The results showed that small additions of Bi₂O₃ of <0.05 wt% remarkably increase the permeability of MnZn ferrites. On the other hand, addition of 0.05 wt% Bi₂O₃ induced the formation of a microstructure composed of giant grains with trapped pores embedded in a normal microstructure. The permeability of these samples showed a pronounced secondary maximum in permeability. At still higher Bi₂O₃ concentrations, above 0.2 wt%, the grain growth was retarded and a normal microstructure appeared; however, the magnetic permeability was strongly reduced.     

Grain-Boundary Migration and Dielectric Properties of Semiconducting SrTiO3 in the SrTiO3-BaTiO3-CaTiO3 System

Chemically induced grain-boundary migration and its effects on the interface and dielectric properties of semiconducting SrTiO₃ have been investigated. Strontium titanate specimens that had been doped with 0.2 mol% of Nb₂O₅ were sintered in 5H₂/95N₂. The sintered specimens were diffusion annealed at 1400°C in 5H₂/95N₂ with BaTiO₃ or 0.5BaTiO₃-0.5CaTiO₃ (mole fraction) packing powder. The grain boundaries of the annealed specimens were oxidized in air. In the case of BaTiO₃ packing, grain-boundary migration occurred with the diffusion of BaTiO₃ along the grain boundary. The effective dielectric constant of the specimen decreased gradually as the temperature increased but showed two peaks, possibly because of barium enrichment at the grain boundary and an oxidized Sr(Ba)TiO₃ layer. In the case of 0.5BaTiO₃-0.5CaTiO₃ packing, although barium and calcium were present at the grain boundary of the specimen, no boundary migration occurred, as in a previous investigation. With the diffusion of barium and calcium, the resistivity of the specimen increased and the variation of the effective dielectric constant with temperature was much reduced, in comparison to those without solute diffusion. These enhanced properties were attributed to the solute enrichment and the formation of a thin diffusional Sr(Ba,Ca)TiO₃ layer at the grain boundary.     

Effect of Grain Boundary Structure on Diffusion-Induced Grain Boundary Migration in BaTiO3

The effect of grain boundary structure, either rough or faceted, on diffusion-induced grain boundary migration (DIGM) has been investigated in BaTiO₃. SrTiO₃ particles were scattered on the polished surfaces of two kinds of BaTiO₃ samples with faceted and rough boundaries and annealed in air for the samples with faceted boundaries and in H₂ for those with rough boundaries. In the BaTiO₃ samples with rough boundaries, an appreciable grain boundary migration occurred. In contrast, grain-boundary migration hardly occurred in the BaTiO₃ samples with faceted boundaries. The migration suppression observed in the sample with faceted boundaries was attributed to a low boundary mobility. The present experimental results show that DIGM is strongly affected by the boundary structure and can be suppressed by structural transition of boundaries from rough to faceted.     

Grain Growth of ZnO in ZnO-Bl2O3 Ceramics with Ai2O3 Additions

Grain growth of ZnO during liquid-phase sintering of a ZnO-6 wt% Bi₂O₃ ceramic was investigated for A1₂O₃ additions from 0.10 to 0.80 wt%. Sintering in air for 0.5 to 4 h at 900° to 1400°C was studied. The AI₂O₃ reacted with the ZnO to form ZnAl₂O₄ spinel, which reduced the rate of ZnO grain growth. The ZnO grain-growth exponent was determined to be 4 and the activation energy for ZnO grain growth was estimated to be 400 kJ/mol. These values were compared with the activation parameters for ZnO grain growth in other ceramic systems. It was confirmed that the reduced ZnO grain growth was a result of ZnAl₂O₄ spinel particles pinning the ZnO grain boundaries and reducing their mobility, which explained the grain-growth exponent of 4. It was concluded that the 400 kJ/mol activation energy was related to the transport of the ZnAl₂O₄ spinel particles, most probably controlled by the diffusion of O^2- in the ZnAl₂O₄ spinel structure.     

Effect of Silver on the Sintering and Grain-Growth Behavior of Barium Titanate

Silver and its alloys frequently are used as electrode material for BaTiO₃-based dielectrics. In the present study, a small amount of fine silver particles have been intimately mixed with BaTiO₃ powder. The sintering and grain-growth behavior of the silver-doped BaTiO₃ in air are investigated. The solubility of silver in BaTiO₃, as revealed by lattice-parameter measurement, electrical measurement, and electron probe microanalysis, is <300 ppm. The densification of BaTiO₃ is slowed slightly by the addition of silver inclusions. However, the presence of a small amount (<0.3 wt%) of silver increases the amount and size of abnormal grains. When the silver content is >0.3 wt%, the grain growth of BaTiO₃ then is prohibited by the silver inclusions.     

Anomalous Grain Growth in Donor-Doped Barium Titanate with Excess Barium Oxide

Grain growth and semiconductivity of donor-doped BaTiO₃ceramics with an excess of BaO and additions of SiO₂or B₂O₃were studied. The microstructures and electrical measurements on sintered samples revealed that their electrical properties are related to the microstructure development of the sintered samples. Samples heated with an excess of BaO developed a normal microstructure during sintering, as a consequence of normal grain growth (NGG), and were yellow and insulating. In contrast, samples with an excess of BaO and an addition of SiO₂or B₂O₃exhibited anomalous grain growth (AGG) and were dark blue and semiconducting after sintering. When some BaTiO₃seed grains were embedded in a sample of donor-doped BaTiO₃with an excess of BaO (without SiO₂or B₂O₃), AGG was observed, i.e., some seed grains grew into large grains and were blue and semiconducting. An explanation is given for why AGG is responsible for the oxygen release and the formation of semiconducting grains in donor-doped BaTiO₃and not NGG.     

Interfacial Segregation in Perovskites: III, Microstructure and Electrical Properties

A model is proposed to relate segregation of dopants with the development of fine ceramic microstructures and electrical properties in polycrystalline donor-doped BaTiO₃. As the average dopant concentration is increased, the dopant concentration at the grain boundary increases compared with the bulk. This has two important effects: (1) dopant incorporation at the grain boundary shifts from electronic to vacancy compensation and the formation of high-resistive layers, and (2) grain-boundary mobility is impeded and grain growth retarded with increased dopant additions. Thus, for high dopant concentrations, donor-doped BaTiO₃ becomes insulating. The model discusses in detail the nonequilibrium heterogeneous defect chemistry as a function of thermal-processing and accounts for barrier layer phenomena.     

Effects of Additives on Microstructure and Properties of Alumina-Silicon Carbide Nanocomposites

Al₂O₃/5-vol%-SiC nanocomposites have been fabricated by using pressureless sintering with MgO and/or Y₂O₃ sintering aids and post-hot isostatic pressing (HIPing), which circumvents the limitations of hot pressing. Al₂O₃/SiC nanocomposites that have been doped with 0.1 wt% MgO and 0.1 wt% MgO + 0.1 wt% Y₂O₃ show an increased sintering density and a homogeneous microstructure, as well as a high fracture strength (1 GPa) after HIPing. In contrast, using Y₂O₃ as a dopant has a negative impact on the microstructure and the fracture strength. The results suggest that MgO, as a sintering additive, has a key role in improving the densification and controlling the microstructure of Al₂O₃/SiC nanocomposites.     

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.