Papers by Keyword: SrTiO₃

Abstract: The electronic structure, band structure, density of states (DOS) and electronic density difference of paraelectric SrTiO₃ in the cubic phase were performed by the first-principles calculation based on the density functional theory (DFT). The energy levels of high symmetry points in the Brillouin zone were listed and Mulliken population analysis was performed for valence bond structures. The top valence band of SrTiO₃ is at the R point, and the minimum of the conduction bands is at the Γ point. The calculated value for indirect band gap is 1.84eV in the Brillouin zone. As in other perovskite ABO₃ ferroelectrics, the population analysis, DOS and electron density difference show that there is a very strong hybridization between the Ti3d and O2p states in the valence bands, which is responsible for the ferroelectricity tendency.

Abstract: Oxygen diffusivity in strontium titanate films with intentional non-stoichiometry, Sr_1+xTi_1+yO_δ with (1+x)/(1+y) = 0.71.5, was studied to reveal degradation behavior of Sr_1+xTi_1+yO_δ films. In order to utilize isotope tracer diffusion measurement, so-called gas/solid exchange method using ¹⁸O₂ gas, at relatively low temperature, very thin YSZ layer deposited on top of Sr_1+xTi_1+yO_δ film was used as catalyst for enhanced ¹⁸O[gas]/¹⁶O[solid] exchange at lower temperature. As a result, very high oxygen diffusivity at 673 K in Sr_1+xTi_1+yO_δ films with (1+x)/(1+y)>>1 was observed.

Abstract: Cold-pressed pellets were prepared from the mixtures SrTiO₃ - 0.03MF₂ - 0.12LiF, M = Ca, Sr or Ba, then air-fired at 950 °C for 2 hours. The obtained samples were investigated by X-ray diffraction and scanning electron microscopy. Each ceramic is a perovskite single phase and the relative density reaches 95% for all ceramics. The addition of 3 mol. % of the eutectic composition 1MF₂ - 4LiF (T_m ~ 760 °C) to SrTiO₃ lowers the sintering temperature of pure strontium titanate from 1400 °C to 950 °C and induces a strong modification in the SrTiO₃ cubic phase: superlattice reflections are detected and the unit cell symmetry becomes orthorhombic. Dielectric measurements were performed between - 150 °C and 200 °C in the frequency range 20 Hz - 10⁹ Hz. The dielectric permittivity ε_r showed no maximum in the temperature range investigated and is almost frequency independent. At room temperature, the dielectric losses tanδ are less than 0.1 %. Moreover, the frequency dependence of the complex permittivity exhibits a resonance in the microwave region. These fluorinated ceramics could be of interest in piezoelectric devices and are promising dielectrics for class I capacitors manufactured at low temperature.

Abstract: A sol-gel processing technology was employed to synthesize fine SrTiO₃ powder by using strontium nitrate (Sr(NO₃)₂) , butyl titanate (Ti(OC₄H₉)₄) as precursors, citric acid (C₆H₈O₆) as complexing agent and (ethylene) glycol (C₂H₆O₂) as stabilizer. Prepared the precursors in liquid phase and then calcined the precursors to achieve nano-sized SrTiO₃ powders. The microstructure and composition of the as-synthesized samples were characterized by X-ray diffraction (XRD), Scan Electron Microscopy (SEM), and the photocatalytic activity was studied. The results showed that all the SrTiO₃ particles were identified as perovskite phase. The pH values, the content of the acetic acid, the heat treat temperature play important roles on the synthesis of the SrTiO₃ photocatalyst. When the pH=1, n(Sr(NO₃)₂ ) : n(Ti(OC₄H₉)₄) = 1 : 1, n(Sr(NO₃)₂ ) : n(C₆H₈O₆)=1:1.7, the product has better rate of photodegradation for the Methylene Blue under the ultraviolet condition.

Abstract: In this article, Co-doped SrTiO₃ nanofibres have been prepared by electrospinning from a sol-gel precursor and the following calcination at 923K. XRD results confirmed that no second phase was formed, and Co ions successfully occupied the Ti sites. By annealing in hydrogen, oxygen vacancies and (Co-H-Co) were formed, which both contributed to the magnetic ordering in SrTi_1-xCo_xO₃ nanofibres are at 300K.

Abstract: Integrating germanium on Si is one of the major challenges of epitaxial growth and presents important applicative interest. Recently, SrTiO₃ was adopted as a buffer layer to accommodate the mismatch between Ge and Si. Germanium can take its bulk lattice parameter as soon as the growth begins without threading defects on SrTiO₃ surface. However, the details of Ge adsorption on SrTiO3 surface are not clear. In present work, the electronic structures of Ge deposited on the SrTiO₃ (001) 2×1 Double Layer (DL) TiO₂ surfaces were investigated by means of density functional theory calculations. Several stable adsorption sites are identified. It is found that the germanium adsorption shows site selectivity and causes noticeable surface distortion. The charge transfer from germanium atom to surface contributes to the formation of strong Ge-O bondings and surface metallization.

Abstract: A simple polyacrylamide gel method was used to prepare strontium titanate (SrTiO₃) nanoparticles. X-ray diffraction (XRD) analysis reveals that single-phase SrTiO₃ can be synthesized at a calcination temperature of 550 °C. Scanning electron microscope (SEM) observation shows that the as-prepared SrTiO₃ particles are regularly shaped like spheres and highly uniform in size with an average diameter of ~55 nm. Ultraviolet (UV)-visible diffuse reflectance spectrum analysis indicates that the bandgap of the sample is 3.23 eV. The Brunauer-Emmett-Teller (BET) specific surface area of the sample is measured, by the N2 adsorption-desorption technique, to be 26.4 m²•g⁻¹. The photocatalytic experiments reveal that the SrTiO₃ nanoparticles can effectively degrade various dyes including congo red (CR), rhodamine B (RhB), methyl orange (MO), and methylene blue (MB) under UV irradiation.

Abstract: A new technology, gel solid-state reaction, for synthesis of SrTiO₃ ceramic composite powders is described in the present paper. This process, valuable for a great number of composite powders, is of great simplicity and has a relative low synthetic temperature of powders. The calcined lumped powder is porous and easy to grind. XRD and d-spacing-component figures of the solid solution powder demonstrated that the compounds were mutually miscible in the solid solutions, and SEM showed that they were uniform and substantially spherical with an average size of 1.0 μm in diameter. The remarkable advantages of the method are high yields, solvent-free, and environmental benign.

Abstract: Bismuth potassium titanate – strontium titanate (1-x)Bi_0.5K_0.5TiO₃-(x)SrTiO₃ (BKT-ST) lead-free ceramics when x = 0-0.20 were synthesized by the solid state reaction method with normal sintering. The ferroelectric phase transition was studied by X-ray diffraction (XRD). All compositions showed a single phase perovskite structure with tetragonal symmetry at room temperature and the phase structure transformed from ferroelectric tetragonal – paraelectric cubic in the range of x ³ 0.10 . Dielectric study revealed that the dielectric relaxor behavior was induced with increasing ST and transition temperature (T_m@ε_{r max}) of ST-doped BKT had a tendency to decrease with increasing ST. The Bi_0.5K_0.5TiO₃ – SrTiO₃ system was expected to be a new and promising candidate for lead-free capacitors.

Abstract: Superlattice structure of SrTiO₃ and Nb-doped SrTiO₃ have been epitaxially grown on atomically flat surface of LaAlO₃ substrates by ion beam deposition method. Epitaxial superlattices were grown at 800 °C in the presence of partial oxygen pressure under optimizing growth conditions. The Nb-doped SrTiO₃ layers were varied from 2 to 15 unit cell thickness approximately, while SrTiO₃ layers are maintained at 15 unit cell thickness with 10 periods. The superlattices with various Nb-doped SrTiO₃ layer thicknesses were investigated using X-ray diffractometer (XRD) and atomic force microscope (AFM), in order to clearly understand structural properties and surface structure, which are significant for fabrication of the high quality superlattice structure.