Seong-Ihl Woo

Phase transition in ABi4Ti4O15 (A=Ca,Sr,Ba) Aurivillius oxides prepared through a soft chemical route

A series of compounds, CaBi4Ti4O15 (CBT), SrBi4Ti4O15 (SBT), and BaBi4Ti4O15 (BBT), belonging to the Aurivillus-type structure (four layers) has been prepared by a modified chemical route. Different oxalates were precipitated from their respective nitrate solution onto the surface of TiO2 powders. The room temperature x-ray diffraction study reveled that the compounds were having orthorhombic symmetry. Considering the tolerance factor, a significant deformation of the perovskite block is observed and that decreases with increase in ionic radius of A site atom. Temperature dependent dielectric study showed normal ferroelectric to paraelectric transition well above the room temperature except for BBT. The BBT ceramic showed a relaxorlike behavior near phase transition. The quantitative characterization and comparison of relaxor behavior were based on empirical parameters γ and ΔTres. The dielectric relaxation rate follows the Vogel–Fulcher relation with activation energy=0.02 eV and freezing temperature=362...

Characterization of zeolite-supported Pt-Cu bimetallic catalyst by xenon-129 NMR and EXAFS

Samples of platinum supported on NaY zeolite (Pt/NaY), Cu/NaY, and bimetallic PtCu/NaY have been prepared by using a scheme that consists of ion exchange of the metalammine cations, activation in flowing O2, and subsequent reduction in H2. These samples have been characterized by 129Xe NMR and extended X-ray absorption fine structure (EXAFS). The chemical shift in 129Xe NMR of the xenon adsorbed on PtCu/NaY has been found to be significantly lower than that on Pt/NaY with the same Pt content under the same experimental conditions, and therefore bimetallic PtCu clusters are formed in supercages of NaY zeolite in both the cases of a sequential clustering method and a simultaneous ion exchange preparation method. The peak intensities at Pt-Pt coordination distance in the Fourier transforms of EXAFS spectra for Pt/NaY and PtCu/NaY are very similar, which indicates that the reduction of Cu precursor has resulted in no appreciable changes in the PtPt coordination structure of the Pt cluster. It thus appears, from these results, that during the reduction of Cu precursor the bimetallic PtCu clusters are formed with a model of a Cu covering on Pt clusters in the supercage of NaY zeolite.

129Xe NMR proof for the distribution of platinum species during Pt/NaY preparation by H2PtCl6 impregnation and Pt(NH3)42+ cation exchange methods

Abstract 129 Xe NMR spectroscopy was used to measure the distribution of platinum species during the preparation of Pt/NaY catalysts by the impregnation of H 2 PtC l6 or the cation exchange of Pt(NH 3 ) 4 2+ into a NaY zeolite. The location of platinum species during the impregnation was found to be affected by the impregnation time and the drying mode. More uniform distributions of platinum species in the impregnation were achieved by longer thermal treatments in a 100% relative humidity chamber before calcination. However, the cation exchange method gave uniform distribution of Pt species in the zeolite channel throughout the ion exchange, calcination, and reduction. This result was consistent with the rate of H 2 PtCl 6 adsorption on the NaY zeolite in aqueous solution and the platinum metal dispersion of the Pt/NaY samples.

Selective Alkylation of Aniline With Methanol Over Boronsilicate Molecular Sieve

The selective alkylation of aniline with methanol catalyzed by boronsilicates was investigated. Correlation of acidity of ZSM-5, (B)ZSM-5 and ion-exchanged (B)ZSM-5s with the selectivity towards methylated anilines suggested that strong acid sites T m 600K, medium acid sites 450T m 600K, and weak acid sites (T m 450) are active sites to produce C-alkylate and coke, N, N-dimethyl aniline (NNDMA) and N-methyltoluidine (NMT), and N-methylaniline (NMA), respectively. However, Cu exchanged (B)ZSM-5 having only weak acid sites is selective to NMT and does not deactivate during the reaction.

Electrical properties of Bi2Mg2∕3Nb4∕3O7 (BMN) pyrochlore thin films deposited on Pt and Cu metal at low temperatures for embedded capacitor applications

200-nm-thick BMN films were deposited on Pt∕TiO2∕SiO2∕Si and Cu∕Ti∕SiO2∕Si substrates at various temperatures by pulsed laser deposition. The dielectric constant and capacitance density of the films deposited on Pt and Cu electrodes show similar tendency with increasing deposition temperature. On the other hand, dielectric loss of the films deposited on Cu electrode varies from 0.7% to 1.3%, while dielectric loss of films on Pt constantly shows 0.2% even though the deposition temperature increases. The low value of breakdown strength in BMN films on Pt compared to films deposited on Cu electrode was attributed to the increase of surface roughness by the formation of secondary phases at interface between BMN films and Pt electrodes.

Effect of thickness on electrical properties of bismuth-magnesium niobate pyrochlore thin films deposited at low temperature

Bi2Mg2∕3Nb4∕3O7 (BMN) pyrochlore thin films were deposited at 25 and 100°C on Cu∕Ti∕Si substrates by pulsed laser deposition. Dielectric and leakage current properties of BMN films are investigated as a function of film thickness. The critical thicknesses showing the thickness dependence of dielectric constant are approximately 50 and 70nm in BMN films deposited at 25 and 100°C, respectively. The capacitances of interfacial layers in the films deposited at 25 and 100°C are approximately 5.5 and 3.9pF, respectively. The thickness dependence of leakage current characteristics was attributed to the copper diffusion into the BMN films. An intrinsic conduction of BMN films was controlled by Schottky emission and the barrier height was estimated as 0.9–1.2eV in the temperature range from 25to100°C. Film thickness in terms of leakage current characteristics is limited above 100nm for embedded capacitor applications.

Improvement of photocatalytic oxidation activity on a WO3/TiO2 heterojunction composite photocatalyst with broad spectral response

WO3/TiO2 that can efficiently use ultraviolet and visible light simultaneously was designed and investigated for photocatalytic O2 evolution. It was found that a remarkable synergistic effect on the photocatalytic activity is obtained between WO3 and TiO2 under UV-visible light illumination. The optimum photocatalyst WO3/TiO2 shows an evolution rate of 281 μmol h−1 g−1 of oxygen, which is even higher than the summation of the photocatalytic activities of WO3 and TiO2. The UV-visible spectra indicate that WO3/TiO2 have a broad spectral response. XRD and Raman spectra demonstrated the formation of the bulk and surface phases of WO3. The HRTEM image shows a close contact at the interface of WO3 and TiO2, indicating the formation of heterojunctions. Thus, it is proposed that the broad light absorption and efficient electron transfer via heterojunctions of WO3/TiO2 account for the higher photocatalytic activity of WO3/TiO2.

Polyolefin/Layered Silicate Nanocomposites Prepared by In Situ Polymerization

This chapter briefely overviews the polymer-layered silicate nanocomposites, including the structure and properties of the layered silicates and their modifications. It also addresses the developments in synthesizing polyolefin/clay nanocomposites by olefin polymerization under catalysis of clay intercalated precatalysts. It mainly focuses on the synthetic routes, structural characterization, and properties of polyolefin/clay nanocomposites.

Improvement of Leakage Current Characteristics by Plasma Treatment in Bi2Mg2 ∕ 3Nb4 ∕ 3O12 Dielectric Thin Films

Bismuth magnesium niobate (BMN) thin films deposited on Cu/Ti/Si substrates at 100°C by pulsed laser deposition were investigated for the effect of plasma treatment on dielectric and leakage current characteristics. The dielectric constant and dissipation factor of 60 nm thick films slightly decrease with increasing oxygen content in atmosphere of plasma treatment. The leakage current densities were improved with increasing oxygen content. The capacitance density and breakdown voltage of the films treated with N 2 :O 2 = 100:100 seem (standard cm 3 /min) were approximately 520 nF/cm 2 and 3 V, respectively. On the other hand, 100 nm thick BMN films treated by plasma at N 2 :O 2 = 100:100 sccm exhibit a leakage current density of approximately 6 X 10 -7 A/cm 2 at 10 V and a breakdown voltage above 10 V which is possible for embedded capacitor applications.

Characterization and Catalytic Properties of Pt-Ir Small Bimetallic Cluster in NaY

Abstract The formation of small Pt-Ir bimetallic clusters (about 1 nm) by the reducton of Pt and Ir co-ion-exchanged NaY with H 2 at 300°C after calcined with oxygen at 300°C was evidenced by temperature programmed oxidation/reduction (TPO/R), 129 Xe NMR, hydrogen chemisorption and catalytic probe reaction. These small Pt-Ir bimetallic clusters inside the supercage were more stable than Ir monometallic cluster and remained inside the supercage after re-oxidation treatment below 400°C. Temperature programmed oxidation/reduction (TPO/R) results suggested that the interaction between Pt and Ir takes place in the calcination step prior to hydrogen reduction. The Pt-Ir/NaY bimetallic catalyst showed a higher activity and better activity maintenance than those of Pt/NaY catalyst in n-heptane reforming reaction.