Takefumi Mitsuhashi

Kinetic analysis of solid-state processes

A simple method for kinetic analysis of solid-state processes has been developed. A criteria capable of classifying different processes is explored here with a view toward visualizing the complexity of solid-state kinetics. They provide a useful tool for the determination of the most suitable kinetic model. The method has been applied to the analysis of crystallization processes in amorphous ZrO 2 and RuO 2 . It is found that the crystallization kinetics of as-prepared sample exhibits a complex behavior under nonisothermal conditions. This is probably due to an overlapping of the nucleation- and crystal-growth processes at the beginning of crystallization. As a consequence, the Johnson–Mehl–Avrami nucleation-growth model cannot be applied. A two-parameter autocatalytic model provides a good description of the crystallization process under isothermal and nonisothermal conditions.

New improved syntheses of LaRuO3 perovskites and their applications in environmental catalysis

Abstract Unsupported and supported LaRuO3 type lanthanum ruthenate perovskites have been synthesized using co-precipitation and other improved techniques, which results in synthesis of LaRuO3 with better physical properties. ‘Freeze-drying’, ‘in situ’ synthesis, co-precipitation and ‘deposition precipitation’ techniques have been used mostly for the first time to synthesize LaRuO3 perovskite with ruthenium in 3+ oxidation state. These innovative synthesis methods have resulted in formation of LaRuO3 with improved, physical and catalytic properties. Alumina supported LaRuO3 has also been prepared which shows remarkable improvement in surface area and catalytic activity for certain reactions of environmental importance. These methods offer non-tedious, easy synthesis of Ru(III) perovskite at relatively lower temperature. LaRuO3 shows reasonably high thermal stability and can be a potential candidate for many catalytic reactions even at elevated temperatures.

Alumina supported, perovskite oxide based catalytic materials and their auto-exhaust application

Abstract Substituted lanthanum manganate type perovskites have been synthesized following co-precipitation and a modified in situ method. These perovskites have been supported on cordierite honeycomb with and without alumina washcoat. Alumina washcoated supports with lanthana pre-coat, have been found suitable for the in situ synthesis of perovskites to avoid reactivity of perovskite precursors with alumina (in absence of lanthana pre-coat). This modified synthesis has resulted in remarkable improvement in surface area while, excellent catalyst adhesion has also been observed. The catalyst composition has also been promoted by a small amount of platinum, which has resulted in improved catalytic activity. These catalysts have been first evaluated using a pure gas laboratory evaluation assembly. Catalysts prepared on alumina washcoated honeycomb having lanthana pre-coat, shows better activity than the samples prepared by co-precipitation method. Catalytic converter prototypes have been prepared using the selected catalyst and evaluated on engine and chassis dynamometers. The converter shows good conversion activity for all the three pollutants, viz. CO, HC and NO x . These results substantiate the possibility of using supported perovskites for automobile applications. The detailed characterization of supported perovskites, however, could not be done conclusively and demand for further investigations, to properly understand the interaction of pre-coat, alumina washcoat and catalyst.

Anatase-type TiO2 thin films produced by lattice deformation

TiO2 thin films were grown onto SrTiO3 substrates by a molecular beam epitaxy (MBE) method using an oxygen radical source. The structure of the thin films obtained was evaluated by X-ray reflection diffractometer (XRD) and reflection high energy electron diffraction (RHEED); TiO2 thin films were determined to be of anatase type and were epitaxially grown in the direction of the c-axis, parallel to the [001] of the substrates. Near the interface, the a-value of thin films increased and the c-value shrank in comparison to that of anatase powder. These results indicated that the anatase phase of TiO2 was induced by a lattice matching process at the interface between SrTiO3 and TiO2.

Photocatalytic Property and Deep Levels of Nb-doped Anatase TiO2 Film Grown by Metalorganic Chemical Vapor Depostion

A Nb-doped anatase TiO2 (Nb: 5 at%) film was grown on a SrTiO3(001) substrate by metalorganic chemical vapor deposition. Deep level transient spectroscopy analysis revealed that this film possessed two kinds of deep levels due to the Nb ions, which were found to exist at 0.32 eV and 0.67 eV below the bottom of the conduction band. The photocatalytic activity of this film was low compared with that of the undoped anatase film, suggesting that the deep levels acted as recombination centers of photoexcited electron-hole pairs.

High-temperature metastability of cubic spinel Si3N4

The metastability of cubic spinel Si3N4 has been investigated at high temperature, up to about 1800 K, and in Ar atmosphere by quench experiments and differential thermal analyses coupled with thermogravimetry. The results indicate that the enthalpy change from spinel-type Si3N4 to β-Si3N4 is −29.2±3.5 kJ/mol and that the transformation starts at 1670 K. The high-temperature metastability of spinel Si3N4 may provide various directions for industrial applications.

Pulse mode effects on crystallization temperature of titanium dioxide films in pulse magnetron sputtering

Pulse mode effects on the crystallization temperature and photocatalytic properties of TiO 2 films were investigated. TiO 2 films were deposited on silica glass substrates using a pulse magnetron sputtering apparatus in the unipolar and the bipolar pulse modes. X-Ray diffraction showed that in the bipolar pulse mode the rutile phase TiO 2 film grew at lower substrate temperature compared with the unipolar pulse mode. The photocatalytic activities obtained from the photoreduction of Ag ions at the surface of TiO 2 films indicated that the anatase phase TiO 2 films grown in the bipolar pulse mode had higher photocatalytic activity compared with those grown in the unipolar pulse mode. This suggests that the bipolar pulse mode is an effective technique to achieve higher nhotocatalytic activity of TiO 2 film.

Crystallization kinetics of amorphous RuO2

Abstract The crystallization kinetics of nanocrystalline t-RuO 2 in amorphous ruthenium oxide was studied by differential scanning calorimetry (DSC). It is shown that this process cannot be described by the Johnson-Mehl-Avrami model and that the two-parameter empirical Sestak-Berggren equation gives a more quantitative description. The reliability of kinetic parameters calculated from nonisothermal DSC data is tested by comparing calculated and experimental isothermal data. It is shown that a very good prediction of isothermal behavior can be obtained.

Deep Level Transient Spectroscopy Analysis of an Anatase Epitaxial Film Grown by Metal Organic Chemical Vapor Deposition

The deep level transient spectroscopy (DLTS) study of anatase-type TiO2 material was performed for the first time. The anatase film was epitaxialy grown on a conductive Nb-doped single-crystalline SrTiO3 (100) substrate by metal organic chemical vapor deposition. The photoluminescence characteristics of this anatase film were identical to those in previous reports, where they were attributed to the radiative recombination of self-trapped excitons. According to the DLTS analysis, it was revealed that this anatase film had a characteristic deep level located at 0.96 eV below the bottom of the conduction band with large concentration (6.5×1016/cm3) and capture cross section (8.3×10-13 cm2). Since the capture cross section of this level appeared to be too large to be caused by point defects, the origin of this deep level was attributed to line defects such as dislocations.

Calorimetric and high-resolution transmission electron microscopy study of nanocrystallization in zirconia gel

The formation of metastable tetragonal zirconia nanophase by thermal treatment of a zirconia gel derived from zirconyl chloride has been studied by high-resolution transmission electron microscopy (HRTEM) and differential scanning calorimetry (DSC). HRTEM observations revealed that a fully crystallized sample consists of nanocrystals, around 13 nm in size. This nanocrystalline t-ZrO2 has practically the same crystal structure as that of the high-temperature tetragonal zirconia phase. The nonisothermal crystallization rate is very fast in as-prepared zirconia gel. DSC data at various heating rates can be described by a two-parameter model which predicts the crystallization kinetics in isothermal conditions very well. The Johnson–Mehl–Avrami (JMA) model can be used, however, in partially crystalline samples (crystallinity >30%) where the rate of crystallization process is considerably slower. The kinetic exponent of the JMA model (m = 1.0 ± 0.1) then corresponds to linear dependence of the crystallization rate as a function of the fraction crystallized.