Processing of Novel Strontium Titanate-Based Thin-Film Varistors by Chemical Solution Deposition

Strontium titanate (SrTiO₃)-based multilayered film with varistor characteristics has successfully been fabricated by a chemical solution deposition (CSD) method. Homogeneous precursor solutions of SrTiO₃ and Nb:SrTiO₃ with long-term stability could be prepared by optimizing the reaction conditions among strontium ethoxide, titanium isopropoxide, and niobium ethoxide. Films were prepared using the precursor solutions on fused silica substrates at 700°C. Triple layered films with SrTiO₃/Nb:SrTiO₃/SrTiO₃ structure were also successfully crystallized on Pt/Ti/SiO₂/Si substrates at 700°C. The current–voltage ( I - V ) curve of the multilayered film was characteristic to varistors and the nonlinear coefficient α of the synthesized film was ∼3.0. The varistor voltages ( E _0.01) of 0.6-μm-thick film were 140 kV/cm for the forward direction and −120 kV/cm for the reverse direction, respectively, at room temperature.     

Grain Boundary Segregation and High Nonlinear I–V Characteristics in Fe-Added Strontium Titanate

The nonlinear current–voltage ( I – V ) characteristics of SrTiO₃ have been improved greatly by inducing nonequilibrium segregation of Fe-acceptor dopant at grain boundaries. When Fe-doped SrTiO₃ samples with different Fe additions (0, 0.2, 0.5, 1, 2, and 3 mol%) were sintered at 1350°C in H₂, metallic Fe particles were precipitated at grain boundaries and triple junctions. During subsequent air-annealing at 1100°C, the metallic Fe in the H₂-sintered samples oxidized as Fe₂O₃ and FeO, and the Fe ions diffused along and segregated at the grain boundaries in the surface region of the sample. The annealed samples showed excellent nonlinear I – V characteristics (α≈173), suggesting that an electrical potential barrier was formed at grain boundaries by the air-annealing. As the amount of Fe increased up to 2 mol%, the nonlinear properties considerably increased as long as the Fe ion diffusion into the bulk grains was not considerable. The present investigation shows the processing and applicability of Fe-doped SrTiO₃ as a new nonlinear device material.     

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.     

Thermophysical Properties of Perovskite-Type Strontium Cerate and Zirconate

Polycrystalline samples of strontium series perovskite-type oxides, SrCeO₃ and SrZrO₃, were prepared and the thermophysical properties were measured. The chemical compositions of the samples do not deviate from the stoichiometric composition. The oxygen-to-metal (O/M) ratios of SrCeO₃ and SrZrO₃ are 3.00±0.03 and 2.98±0.01, respectively. The average linear thermal expansion coefficients are 1.11 × 10⁻⁵ K⁻¹ for SrCeO₃ and 9.69 × 10⁻⁶ K⁻¹ for SrZrO₃ in the temperature range between 300 and 1000 K. The melting temperatures T _m of SrCeO₃ and SrZrO₃ are 2266 and 2883 K, respectively. The longitudinal and shear sound velocities were measured by an ultrasonic pulse–echo method at room temperature in air, which enables to evaluate the elastic moduli and Debye temperature. The heat capacity was measured by using a differential scanning calorimeter in high-purity argon atmosphere. The thermal diffusivity was measured by a laser flash method in vacuum. The thermal conductivities of SrCeO₃ and SrZrO₃ at room temperature are 2.95 and 4.06 W·m⁻¹·K⁻¹, respectively.     

Perovskite-Type Strontium Zirconate as a New Material for Thermal Barrier Coatings

Perovskite-type SrZrO₃ was investigated as an alternative to yttria-stabilized zirconia (YSZ) material for thermal barrier coating (TBC) applications. Three phase transformations (orthorhombic↔pseudo-tetragonal↔tetragonal↔cubic) were found only by heat capacity measurement, whereas the phase transformation from orthorhombic to pseudo-tetragonal was found in thermal expansion measurements. The thermal expansion coefficients (TECs) of SrZrO₃ coatings were at least 4.5% larger than YSZ coatings up to 1200°C. Mechanical properties (Young's modulus, hardness, and fracture toughness) of dense SrZrO₃ showed lower Young's modulus, hardness, and comparable fracture toughness with respect to YSZ. The "steady-state" sintering rate of a SrZrO₃ coating at 1200°C was 1.04 × 10⁻⁹ s⁻¹, which was less than half that of YSZ coating at 1200°C. Plasma-sprayed coatings were produced and characterized. Thermal cycling with a gas burner showed that at operating temperatures ∼1250°C the cycling lifetime of SrZrO₃/YSZ double-layer coating (DLC) was more than twice as long as SrZrO₃ coating and comparable to YSZ coating. However, at operating temperatures >1300°C, the cycling lifetime of SrZrO₃/YSZ DLC was comparable to the optimized YSZ coating, indicating SrZrO₃ might be a promising material for TBC applications at higher temperatures compared with YSZ.     

Atmosphere Control of Interface Migration and Its Effect on Dielectric Property of CuO-Infiltrated Strontium Titanate

The control of interface migration and its effect on dielectric properties of SrTiO₃ based materials have been investigated. SrTiO₃ specimens with or without Nb₂O₅ doping were prepared, and the atmospheres of sintering and heat treatment were varied during the processing. It has been demonstrated that the change in defect structure at the interface of Nb-doped SrTiO₃ grains by atmosphere change was the cause of interface migration observed during CuO infiltration or heat treatment. The interface migration caused by sintering in a reducing atmosphere and infiltration in air could be suppressed by preoxidizing grain interfaces before the CuO infiltration in air. The suppression of migration increased the effective dielectric constant of the material.     

Synthesis of High Aspect Ratio Platelet SrTiO3

A method for synthesis of high aspect ratio platelet seeds by growth of SrTiO₃ on Sr₃Ti₂O₇ core particles is reported. The aim of this study was to identify and control the morphology and size of SrTiO₃ particles via molten salt synthesis. Platelet and tabular morphologies with rectangular faces were obtained using rutile and anatase, respectively. Platelet SrTiO₃ particles with an edge length of 10–40 μm and a thickness of 1–4 μm were obtained. High aspect ratios (edge length to thickness) of 7–10 were measured for platelet particles as opposed to lower aspect ratios of 2–4 for tabular particles. Highly anisotropic platelets are suitable template candidates to achieve textured ceramics.     

Mica-Like Tellurites of Calcium, Strontium, and Cadmium

The compounds CaTe₂O₅, SrTe₂O₅, and CdTe₂O₅ were obtained as mica-like plates by slowly cooling their melts. Dielectric properties were similar to those of mica. Optical and infrared spectra showed appreciable transmittance from 0.33 to 7 μm on with a cutoff at 10 μm. The compounds have refractive indices greater than 2. X-ray diffraction studies showed that only the calcium and cadmium compounds are isomorphous; tentative structures, accounting for the marked cleavage, are proposed for these materials on the basis of the geometric distribution of X-ray scattering.     

Effect of Dislocations on Grain Growth in Strontium Titanate

Two series of experiments were done to investigate the effect of dislocations on grain growth in SrTiO₃. In the first series, we observed the growth behavior of a single-crystal plate toward a TiO₂-excess SrTiO₃ powder compact, containing different dislocation densities on two equivalent {100} surfaces. The surface with a higher dislocation density exhibited faster growth, showing interface mobility enhancement by dislocation. In the second series, a polycrystalline SrTiO₃ sample which had been plastically deformed by hot pressing was embedded in a TiO₂-excess SrTiO₃ powder compact, and its growth behavior toward the powder compact was compared with that of a sample without hot pressing. As with a single crystal, the grains with the higher dislocation density in the plastically deformed sample grew faster. In addition, some grains in the plastically deformed sample showed the characteristic of abnormal grain growth. The transmission electron microscopy (TEM) observations on a sintered TiO₂-excess SrTiO₃ showed that the abnormally large grains contained many dislocations while the fine matrix grains contained practically no dislocations. This result suggests that the uneven distribution of dislocations between grains is possibly one of the causes of the abnormal grain growth in polycrystalline SrTiO₃.     

Phase Equilibria in the System Barium Titanate—Strontium Titanate

Phase equilibria in the system BaTiO₃–SrTiO₃ (with 0 to 7 mol% SrTiO₃) were studied at temperatures above 1600°C in air. Quenching experiments were performed using high-purity starting materials, and run products were examined by X-ray diffraction and differential scanning calorimetry to determine phase composition and Sr concentration. Melting involves a binary loop intersected by the invariant reaction hexagonal (Ba, Sr)TiO₃( ss ) ⇌ cubic (Ba, Sr)TiO₃( ss ) + liquid. In contrast with earlier work, these results indicate that there is no depression of the melting point with Sr addition and no congruent melting point in this compositional range.     

Dielectric Properties of Strontium Titanate Solid Solutions Containing Niobia

SrTiO₃ solid solutions containing Sr_0.5NbO₃ were studied by X-ray diffraction and dielectric measurements. Single-phase perovskite solid solutions were formed up to the composition containing 30 mole % Sr_0.5 NbO₃. The addition of niobium resulted in a symmetry change from cubic (SrTiO₃) to tetragonal (solid solution) and the c/a ratio increased with increasing niobium content. The dielectric constant for the solid solutions was lower than that for SrTiO₃. Pronounced dielectric relaxation peaks and dispersion in permittivity were observed for the composition 7SrTiO₃:3Sr_0.5NbO₃. The activation energy for the relaxation process is about 0.42 ev. It is suggested that the existence of the A-site vacancy in the perovskite ABO₃ lattice introduced by the niobium substitution distorts the oxygen octahedra, producing more than one possible "noncentral" site for the Ti⁴⁺ ion. The relaxation arises from the thermal motion over potential barriers separating these alternative sites.     

Synthesis of Strontium Titanate Nanometer Crystallites Using a Peroxide-Based Route

Nanocrystalline strontium titanate (SrTiO₃) particles were prepared using a peroxide-based route. The reaction between hydrogen peroxide and β-titanic acid yielded a peroxytitanic solution. An amorphous precipitate containing strontium and titanium was formed when a solution of strontium nitrate was added at pH 9. The precipitate was filtered, washed, and calcined. The precipitate was characterized by thermogravimetry, infrared spectroscopy, and X-ray diffraction. Powder calcined at temperatures of about 750°C within 1 h showed a well-crystallized cubic SrTiO₃ perovskite phase. Applying this synthesis procedure for SrTiO₃ nanoparticles yields powders with a grain size of nearly 50 nm.     

Fabrication and Characterization of Colossal Electroresistance Chip Devices Composed of Polycrystalline Lanthanum-Doped Strontium Titanate and Palladium Electrodes

A colossal electroresistance (CER) multilayered chip device composed of polycrystalline 0.8-at% La-doped SrTiO₃ and Pd electrodes has been successfully fabricated. Polycrystalline SrTiO₃ devices exhibit large hysteresis in their current–voltage ( I – V ) characteristics after the forming process. Further, their resistance states can be switched by applying voltage pulses above ±50 V, and the resistance changes by approximately two orders of magnitude (from ∼600 Ω to ∼80 kΩ). These resistance-switching behaviors demonstrate that even ceramics can exhibit resistance changes as large as thin-film devices and provide the possibility of new switching devices with the memory effect composed of ceramics.     

Synthesis of Monosized Strontium Titanate Particles with Tailored Morphologies

Monosized strontium titanate (SrTiO₃) particles with tailored morphologies have been successfully synthesized using ultrasound irradiation. Characterization techniques such as X-ray powder diffraction analysis, scanning electron microscopy, energy-dispersive analysis of X-rays, and laser particle size analysis were used to investigate the products. The results showed that all the as-prepared powders have a cubic SrTiO₃ structure. Reaction conditions such as reactant concentrations, sonication time, and sonication power were found to have important effects on the morphology of SrTiO₃ particles.     

Preparation of Strontium Titanate Thin Films by the Hydrothermal-Electrochemical Method in a Solution Flow System

SrTiO₃ thin films have been prepared on titanium substrates using strontium acetate solutions in newly constructed flow-system equipment by the hydrothermal-electrochemical method. The synthesis parameters (temperature of 120°-200°C and flow rate of 1-40 mL/min) allow fabrication of dense, single-phase films with grain sizes in the range of 80-340 nm by controlling nucleation and/or growth rates. The flow can be closed, enabling easy recycling of the solutions used. This processing route may serve as an inexpensive and environmentally friendly way of fabrication of single-phase SrTiO₃ thin films as well as functionally graded ferroelectric materials.     

Growth of Strontium Titanate Thin Films of Controlled Thickness by the Hydrothermal–Electrochemical Method

Strontium titanate thin films were grown on titanium electrodes up to ca. 2 μm in thickness by a hydrothermal–electrochemical method using electrolysis performed potentiostatically by the three-electrode cell technique. The film thickness increased monotonically with an increase in the quantity of electricity passed through the Ti electrode, and could be controlled accurately by this factor. Grown films were composed of a surface layer having an invariant thickness of ca. 0.2 μm and an inner layer grown by electrolysis. The lattice parameter of the cubic SrTiO₃ film was analyzed to be 3.919 A.     

Hydrothermal Strontium Titanate Films on Titanium: An XPS and AES Depth-Profiling Study

SrTiO₃ films in the 100-nm thickness range were grown on Ti foils by a 2–4 h hydrothermal treatment at 225°C in an aqueous solution of Sr(OH)₂. X-ray diffraction showed the principal reflections of cubic SrTiO₃. Photoelectron analysis revealed the presence of carbon contamination and suggested the presence of hydroxyl groups on the surface, which disappeared in the film bulk. The Auger depth profiles showed three distinct zones. The outermost layer, besides the common carbon contamination, had a large number of OH groups. The second zone was a clean, almost carbon-free SrTiO₃ film. The third zone was a diffuse interface region where the Sr and O concentrations slowly decreased toward the substrate.     

Atomic Resolution Imaging of Au Nanocluster Dispersed in TiO2, SrTiO3, and MgO

Gold nanoclusters dispersed in single crystal TiO₂, MgO, and SrTiO₃ have been prepared by ion implantation at 300–975 K and subsequent annealing at 1275 K for 10 h. High-resolution transmission electron microscopy and high-angle annular dark field (HAADF) imaging in aberration corrected scanning transmission electron microscope (STEM) have been used to characterize the microstructure of the gold nanoclusters dispersed materials. STEM-HAADF imaging with atomic resolution has directly revealed for all three materials that Au atoms partially occupy cation lattice positions. Cavities up to several tens of nanometers were observed in MgO and SrTiO₃. The cavities and gold clusters are spatially associated in MgO and SrTiO₃, indicating a strong interaction between the Au cluster and cavities. For MgO and SrTiO₃, the faceting planes appear to be the same for both nanometer-sized cavity and the Au cluster, demonstrating that both the surface energy and the interfacial energy between Au cluster and the matrix are lowest on these planes.     

Surprising Results of a Study on the Plasticity in Strontium Titanate

Oxides such as SrTiO₃ are expected to fail via brittle fracture at low temperatures. Surprisingly, in the present study, SrTiO₃ single crystals could be plastically deformed in compression in two temperature ranges: from 78 K to ∼1050 K and from 1500 K to 1800 K. SrTiO₃ was brittle at temperatures between these two ranges (∼1050–1500 K). This phenomenon of a ductile–brittle–ductile transition, together with microstructural investigations, suggests that the role of dislocations in the plasticity of ceramics must be reconsidered.     

High-Temperature Defect Structure of Acceptor-Doped Strontium Titanate

The defect structure of acceptor-doped (Fe, Al, Cr) polycrystalline strontium titanate was investigated by measuring the equilibrium electrical conductivity as a function of oxygen activity (10⁻²¹≤a_o2≤1) and temperature (85°C≤ T ≤ 1050°C). The electrical conductivity was n type with ∼−1/4 dependence on a ₀₂ for a₀₂<10⁻¹⁰. For a₀₂>10⁻⁸, the observed data for Fe- or Al-doped samples were proportional to ∼1/4.5 power of a₀₂. In this region for Cr-doped samples, the value of m in.σp αa₀₂^+1/m varies from ∼4.80 to ∼9.00 as the concentration of Cr is increased from 460 to 10 000 ppm. The onset of p -type conductivity depends on the amount. of acceptor impurity added to the sample. The absolute values of the conductivity in the acceptor-doped samples were lower in the n-type region than those for undoped SrTiO₃. The conductivity minima shift toward lower oxygen activity with increasing acceptor concentration. For the entire oxygen activity rangae used in this study, the defect structure of SrTiO₃ is dominated by the added acceptor impurities.