Papers by Keyword: Perovskite Oxide

Abstract: Radiation effects of ion beams in perovskite oxide memristors are analyzedand linked to absorbed dose values, calculated from simulations of ion transport. Several ion species were used in simulations, chosen to represent certain commonly encountered radiation environments. Results indicate that considerable formation of oxygen ion - oxygen vacancy pairs, as well as advent of displaced rare earth and alkaline atoms, is to be expected. Oxygen vacancies can lead to a decrease or increase of active layer resistance, depending on applied voltage polarity. The loss of vacancies from the device is bound to impair the performance of the memristor. Calculated absorbed dose values in the memristor for various incident ion beams are typically on the order of several kGy.

Abstract: Perovskites with formula A_1-xSr_xBO_3-δ (A = La, Nd, Sm; B = Fe or Co) have been synthesized by solid state reaction method and have been studied as materials for oxygen sensor application. The obtained powders were used for making thick film pastes that were later screen printed as sensing electrodes on yttria stabilized zirconia (YSZ) substrates. Sensing electrodes were exposed to low concentration oxygen gas at elevated temperatures. Measurements of the electromotive force of electrochemical cells with the fabricated electrodes were carried out as a function of temperature. The investigation indicated that all of the synthesized materials show Nernstian behavior in a relatively wide temperature range, although the most promising for oxygen electrochemical sensor application is strontium doped samarium cobaltite.

Abstract: Electric-current induced electroresistance effect has been investigated in epitaxial Pr_0.7Sr_0.3MnO₃ thin film grown on the (100) SrTiO₃ substrate. A significant change ~38% in the ratio of the peak resistance at different currents with a current density up to ~3.3×10³ A/cm² was achieved. Such an ER effect is more remarkable in comparing with that reported in other manganite oxides with similar current densities. Compared with that of the as-grown films, the electroresistance of the post-annealed films is smaller, while the ER effect (~14%) is obtained. Although the nature behind such an electroresistance effect has not been well understood, the field tunability of the metal/insulator transition and the electroresistance effect induced by currents might be of potential for various applications such as filed effect devices.

Abstract: Barium zirconate (BaZrO₃) was prepared by the composite-hydroxide-mediated (CHM) approach at low temperature. The CHM method is based on chemical reactions of materials in the eutectic hydroxide melt. In this method, the eutectic point at an sodium hydroxide (NaOH) : potassium hydroxide (KOH) molar ratio of 51.5 : 48.5 is about 165°C. In the present work, the low-temperature preparation and morphology of BaZrO₃ nanocrystals were investigated. Zirconium tetra-n-butoxide ((CH₃CH₂CH₂CH₂O)₄Zr) and/or zirconium dioxide (ZrO₂) were used as a zirconium source. X-ray diffraction measurements confirmed that the BaZrO₃ had a perovskite structure. Barium hydroxide (Ba (OH)₂) was used as a barium source. BaZrO₃ nanoparticles were obtained when (CH₃CH₂CH₂CH₂O)₄Zr was used as a raw material. On the other hand, BaZrO₃ nanocubes were formed when ZrO₂ was used as a raw material. Scanning electron microscopy and transmission electron microscopy observations indicated that BaZrO₃ nanocrystals were formed.

Abstract: Barium titanate (BaTiO₃) / Strontium titanate (SrTiO₃) accumulation ceramics with necking structure of SrTiO₃ nanocubes were prepared using a solvothermal method. XRD measurements confirmed SrTiO₃ and BaTiO₃ with a perovskite structure. The XRD peak intensities of the BaTiO₃ increased significantly as the solvothermal reaction was repeated. After heat treatment of the SrTiO₃ at 900°C, the particles grew larger and formed necked contacts with each other. As the solvothermal reaction was repeated in order to coat with BaTiO₃, the density increased, the porosity decreased, and the thickness of the BaTiO₃ layer increased.

Abstract: The catalysts La_0.8K_0.2FeO₃ (LKFO), La_0.8K_0.2Fe_0.7Mn_0.3O₃ (LKFMO) and La_0.8K_0.2Fe_0.67Mn_0.3Pt_0.03O₃ (LKFMPO) were prepared by the citrate-gel process and the catalyst-coated honeycomb ceramic devices were prepared by the citrate-gel assisted dip-coating method. All the catalysts have a high performance on the simultaneous removal of NO_x and soot at a temperature range of 200 to 400°C under the practical diesel exhaust emission. The obvious catalytic improvement is largely due to the effects of ions substitution, pore structure and microstructural characteristics of the catalysts. The catalytic performance order is LKFMPO > LKFMO > LKFO. Among them the LKFMPO catalyst shows the best catalytic properties, especially in the removal of NO_x, with a maximum conversion rate of NO_x (21.2%).

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.

Abstract: In complex perovskite-type oxides which have been studied as cathode materials, the thermal expansion coefficient increases with the increase in the oxygen ionic conductivity. In the present study, with the aim to explain such a behavior, a research has been carried out from a chemical bond point of view. For oxides A1-xA′xB1-yB′yO with perovskite structure, the ionicity of the individual bond, A-O and B-O, and the thermal expansion coefficient of mixed compounds were estimated by using semiempirical methods. It has been shown that the thermal expansion coefficient and the oxygen ionic conductivity decrease with the increase in the difference of the ionicity between A-O and B-O bonds. It is also found that the tolerance factor and the specific free volume are linearly correlated with the difference of ionicity.

Abstract: In this study, we successfully fabricated the Schottky junctions consisting of Pt electrode and high concentration Nb-doped (0.5 wt%) SrTiO3 (001) single crystal by sputtering process. The carrier concentrations of Nb-0.5wt%-doped SrTiO3 were determined as 1020 /cm3 order by Hall effect measurement. The electrical properties of junctions were investigated by measuring their current-voltage (I-V), capacitance-voltage (C-V) characteristics at temperature range from 80K to 400K. The hysteresis feature was observed that indicating the alteration of barrier height in junctions especially at lower temperature. The donor concentration and built-in potentials calculated from C-2-V data showed large discrepancy from Hall effect measurement indicating that the junctions deviate from the ideal Schottky diode model.

Abstract: Single crystals of PbTiO3 (PT) were grown by a self flux method, and effects of lattice defects on the leakage current properties were investigated. While PT crystals annealed in air at 700 oC showed a leakage current density of the order of 10-5 A/cm2, annealing under a high oxygen partial pressure of 35 MPa increased leakage current density to 10-4 A/cm2. The increase in leakage current by the oxidation treatment provides direct evidence that electron hole is a detrimental carrier for the leakage current property of PT at room temperature. The vacancies of Pb are suggested to act as an electron acceptor for generating electron holes.