Synthesis and Characterization of Nanocrystalline SrTiO3

The synthesis and densification of fairly dense nanocrystalline SrTiO₃ and its characterization are described in this paper in detail. Significant grain growth was avoided by the application of a two-stage sintering process using hot pressing. High-resolution transmission electron microscopy and electron energy loss spectroscopy characterizations indicate pure material with no detectable chemical inhomogeneities. The electrical measurements indicate the disappearance of bulk contribution to the electrical conduction due to the overlap of depleted space charge layers if the grain size is below 100 nm. Owing to the overlap, the capacitance appears as bulk-like rather than due to space charge polarization. Non-contact sub-millimeter optical spectroscopy measurements reveal strong suppression of the dielectric constant values at low temperature.     

Electrochemical Synthesis of Small SrTiO3 Particles

Small strontium titanate (SrTiO₃) particles with an average particle size of about 100 nm have been successfully synthesized by a simple and fast electrochemical route, in which Ti metal plates are used as electrodes and an aqueous solution of KOH and Sr(OH)₂ as an electrolyte. Anodic sparks played a key role, and KOH concentration was one of the most significant factors that affected the appearance of anodic sparks in this method. XRD patterns indicated that the powders obtained in our study were a pure perovskite phase SrTiO₃ with a cubic structure, whose size and morphology were subsequently studied by transmission electron microscopy. UV-VIS result showed that the products had better UV absorption and visible light response than TiO₂-P25.     

Variability of the Sr/Ti Ratio in SrTiO3

TiO₂ is observed as a second phase in SrTiO₃ having Sr/Ti ≤ 0.995 (≥0.5 mol% excess TiO₂). The effect of excess TiO₂ on the equilibrium electrical conductivity at 1000°C is consistent with a solubility of <0.1 mol% TiO₂ with the formation of unassociated oxygen vacancies. More Ti0₂ is retained in solid solution when samples are quenched from the sintering temperature rather than furnace-cooled. The effect of excess SrO on the equilibrium electrical conductivity also indicates some solid solubility.     

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.     

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.     

Oxidation Kinetics of Single-Crystal SrTiO3

The oxidation kinetics were determined for single-crystal SrTiO₃ by measuring the time and temperature dependence of the weight gain of reduced crystals. The oxidation can be described as a diffusion-controlled process. The calculated diffusion coefficients between 850° and 1460°C are represented by D = 0.33 exp (-22.5 ± 5.0 kcal/ RT ) cm²/sec. Directly measured oxygen ion diffusion coefficients in the same temperature interval reported earlier are interpreted as being extrinsic and can be represented by D = 5.2 × 10⁻⁶ exp (-26.1 ± 5.0 kcal/ RT ) cm²/sec, where the activation energy is for mobility only. Assuming that the calculated diffusion coefficients are for vacancy diffusion and the two activation energies are equivalent within experimental error, a vacancy concentration (fraction of vacant lattice sites), [O□], fixed by impurities in the fully oxidized crystal is calculated to be 1.6 × 10⁻⁵ by virtue of the relation between the oxygen self-diffusion coefficient, D_02-, and the oxygen vacancy diffusion coefficient, D_o□ ; D _o2-= [O□] D _o□ where the oxygen ion concentration [O^2-] is taken as unity.     

Preparation and Characterization of Alkoxy-Derived SrZrO3 and SrTiO3

High-purity strontium, zirconium, and titanium alkoxides were synthesized and characterized as precursors for complex oxides. Simultaneous hydrolytic decomposition either of strontium and zirconium alkoxides or of strontium and titanium alkoxides was used to obtain nearly stoichiometric, ideally mixed SrZrO₃ or SrTiO₃ powders of high surface activity. As-prepared helium-dried SrTiO₃ is crystalline before calcination. An ultraviolet radiation technique demonstrates the nucleation and growth of SrZrO₃ crystallites in the calcination temperature range to 350°C. The experimental results are supported by ir, TGA, and X-ray diffraction data. The high degree of control over purity, mixing uniformity, and crystallite size demonstrates the value of the alkoxide precursor approach for the solution of reproducibility problems encountered in the synthesis of electrical-quality ceramics.     

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.     

Conversion of SiO2 Diatom Frustules to BaTiO3 and SrTiO3

Diatom frustules were used as bio-templates to synthesize functional ceramics via solid–gas displacement reactions. Silica-based frustules were exposed to TiF₄ at 330°C to form TiOF₂, which was later converted to TiO₂ (anatase) by heat treatment in air at 600°C. The TiO₂ frustules were then exposed to molten Ba(OH)₂ or Sr(OH)₂ to form BaTiO₃ or SrTiO₃, respectively. In both cases, near-complete conversion was achieved while retaining the morphology of the original silica frustules. BaTiO₃ and SrTiO₃ frustules exhibit nearly phase pure, nanocrystalline perovskite structure.     

Microstructure of SrTiO3 Boundary-Layer Capacitor Material

SiTiO₃ capacitor material with indiffused Bi₂O₃ was studied using SEM, ESCA combined with Ar⁺ ion-etching, and TEM equipped with EDX. The apparent thickness of a second-phase layer observed with SEM was found to be influenced by in-depth effects. ESCA and TEM results show that only a 10- to 100-nm thick layer of second phase is present between the SrTiO₃ grains. In addition, it was found with TEM that the outer part of each grain contained Bi (at most 2 at.%), representing a diffusion layer. These results have implications for the boundary-layer model proposed to explain the dielectric properties .     

Chemical Control of the Grain-Boundary Migration of SrTiO3 in the SrTiO3─BaTiO3─CaTiO3 System

The grain-boundary migration induced by chemical instability has been studied in SrTiO₃. Sintered SrTiO₃ has been packed with various BaTiO₃/CaTiO₃ powder mixtures and annealed at 1400°C for various times. At the surface region of most SrTiO₃ specimens, the grain boundary has migrated with the diffusion of Ba and Ca ions. The ratio of Ba and Ca ions present in the migrated region varies with the depth from the surface and the packing-powder composition, and is not equal to the cation ratio of packing powder. When the Ba/Ca ratio of packing powder is 1, cessation of migration has been observed. The composition of the area without migration is estimated to be approximately equal to the composition for the matching of crystal lattices between the original grain (SrTiO₃) and the layer formed by diffusion of Ba and Ca ions. The coherency strain energy induced by diffusion of solute atoms is believed to be the major driving force for the grain-boundary migration of SrTO₃ under the chemical instability. The grain-boundary migration can be accordingly controlled by lattice parameter change and matching.     

Dislocation Dependence of Cationic Diffusion in SrTiO3

Cation self-diffusion was measured in single-crystal strontium titanate. Nonhomogeneous diffusion kinetics were observed and were interpreted by the Fisher model. The composite diffusion coefficient, P , of the Fisher model is shown to have a dependence on dislocation density, ρ, in single crystals at 1875°C given by:
for strontium, and
for titanium.
It has previously been shown that oxygen ions have enhanced diffusivities in dislocation pipes; therefore, in SrTiO₃ all ions exhibit enhanced dislocation diffusion. Based on an assumed maximum value for lattice diffusion, the temperature dependence of strontium dislocation pipe diffusion is given by:
where h ² is the dislocation core area.     

Phase Equilibria in the System BaTiO3–SrTiO3

Phase equilibria in the binary system BaTiO₃–SrTiO₃ were studied above 1200°C. This system is characterized by a complete series of solid solutions with a minimum at 2.5 mole % SrTiO₃ and 1585°C. The hexagonal BaTiO₃ phase is suppressed to a region extending no farther than 0.5% SrTiO₃ at 1600°C. No immiscibility gap was found.     

Hole Mobility in Acceptor-Doped, Monocrystalline SrTiO3

The oxidation reaction equilibrium constant and the holes mobility in p -type-semiconducting SrTiO₃ single crystals were determined directly by simultaneous Hall and conductivity measurements between 500° and 1000°C. This information and the SrTiO₃ defect model were used to interpret the electrical measurements taken on this substance which represents a model substance for semiconducting perovskites. Measurements with varying oxygen partial pressures showed that shifts of the crystal defect equilibrium give rise to changes in the carrier concentration with unchanged carrier mobility.     

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.     

Control of Microstructure and Orientation in Solution-Deposited BaTiO3 and SrTiO3 Thin Films

Columnar and highly oriented (100) BaTiO₃ and SrTiO₃ thin films were prepared by a chelate-type chemical solution deposition (CSD) process by manipulation of film deposition conditions and seeded growth techniques. Randomly oriented columnar films were prepared on platinum-coated Si substrates by a multilayering process in which nucleation of the perovskite phase was restricted to the substrate or underlying layers by control of layer thickness. The columnar films displayed improvements in dielectric constant and dielectric loss compared to the fine-grain equiaxed films that typically result from CSD methods. Highly oriented BaTiO₃ and SrTiO₃ thin films were fabricated on LaAlO₃ by a seeded growth process that appeared to follow a standard "two-step" growth mechanism that has been previously reported. The film transformation process involved the bulk nucleation of BaTiO₃ throughout the film, followed by the consumption of this matrix by an epitaxial overgrowth process originating at the seed layer. Both BaTiO₃ and PbTiO₃ seed layers were effective in promoting the growth of highly oriented (100) BaTiO₃ films. Based on the various processing factors that can influence thin film microstructure, the decomposition pathway involving the formation of BaCO₃ and TiO₂ appeared to dictate thin film microstructural evolution.     

Densification and Microstructural Development of SrTiO3 Sintered with V2O5

Precipitation of TiO₂ occurs during the sintering of SrTiO₃ with V₂O₅ added as a liquid-phase sintering agent. Satisfactory densification can be obtained at 1250°C when using a high content of V₂O₅ during sintering. However, a microstructure of fine grains and large pores results along with the precipitation of TiO₂. The precipitation of TiO₂ can be repressed by the addition of excess SrO. A well-sintered microstructure with superior densification can thus be obtained at 125O°C from specimens sintered with a low content of V₂O₅ and an appropriate amount of excess SrO.     

Topochemical Synthesis of a High-Aspect-Ratio Platelet NaNbO3 Template

A high-aspect-ratio platelet sodium niobate (NaNbO₃) was synthesized by a topochemical reaction. Plate-like layered-perovskite Bi_2.5Na_3.5Nb₅O₁₈ (BNN5) as a precursor was first prepared, and then Bi³⁺ in the precursor phase was replaced by Na⁺ through topotactic conversion and perovskite NaNbO₃ formed at 950°C in NaCl molten salt. NaNbO₃ crystals had an average length of 15 μm and a thickness of 0.5 μm. Scanning electron microscope and high-resolution transmission electron microscope analyses indicated that the retro-synthesis process of the desired perovskite NaNbO₃ retained the morphological and structural features of the BNN5 precursor. It may be possible to synthesize other perovskite-structured templates using the same method.