Takayoshi Yagasaki

Thermoelectric Generating Properties of Aurivillius Compounds

Thermoelectric generator is expected as an independent source, an energy converter for co-generation with Refuse Derived Fuel (RDF) and so on. Thermoelectric materials were required high Seebeck coefficient, low electrical resistivity and low thermal conductivity. Thermoelectric oxides are suitable at the high temperature range because of chemical stability. Authors focus attention on Aurivillius compounds. The Aurivillius compounds consist of Perovskite layers and Bi-O layers. It is expected that nano-layered structure shows high Seebeck coefficient due to the quantum confinement of electron in Perovskite layers. It was reported that the Seebeck coefficient of Aurivillius phase Bi2VO5.5 was high value of -28.3 mVK-1 at 1010 K, and the electrical resistivity of the one was also high value of 0.033 Ωm at 1010 K. We investigated about element substitution effects at V site on thermoelectric properties. Bi2V1-xMxO5.5 (M=Cu, Cr, x=0, 0.05, 0.1, 0.2) were prepared by solid-state reaction and hot pressing. From the results of the electrical resistivities and the Seebeck coefficients, Cu and Cr behaved as acceptor to Bi2VO5.5. Cr was effective for reducing the thermal conductivity of Bi2VO5.5. The maximum value of dimensionless figure of merit for Bi2VO5.5 was 0.06 at 910 K.

Thermoelectric Properties and Thermal Stability of BiCuSeO

Although BiCuSeO has become of interest as a high performance thermoelectric material, it has been reported to be unstable in air above 548 K. However, the details of the thermal degradation of BiCuSeO have not yet been determined. In this study, the time dependence of the thermoelectric properties of BiCuSeO at 550 K was assessed along with the accompanying thermal degradation behavior. Variations in ρ and α over time at 550 K were determined using the direct current four-terminal method and small temperature difference method, respectively. In addition, the crystalline phases of the specimen after thermoelectric measurements were identified by x-ray diffraction (XRD) analysis, and the surface chemical states were analyzed by x-ray photoelectron spectroscopy (XPS). The results of thermoelectric property measurements over time showed that ρ abruptly increased between 135 min and 182 min then decreased from 182 min to 210 min, while the value of α exhibited the opposite trend. XPS and XRD data suggested that BiCuSeO was pyrolyzed upon heating at 550 K for 6 h in air. In contrast, thermogravimetric analysis demonstrated that BiCuSeO was stable below 800 K under inert gas flow, thus the pyrolysis observed at 550 K was the result of reaction with atmospheric oxygen.

The Synthesis of In-Se by Vapor Transport Method

The α and β phases of In2Se3 are attractive thermophotovolβtaic (TPV) materials. For example, they have a direct transition and an optimal bandgap for TPV systems. We investigated about the influence of the growth conditions on the crystal forms of In-Se prepared by vapor transport. Firstly, only bulk In2Se3 was encapsulated in a quartz ampoule and crystals were grown by vapor transport. Hollow hexagonal cylinders that were approximately 10 μm in diagonal dimension were obtained. They were identified as a mixed phase of In2Se3 by XRD. Secondly, the deposition surface configuration was varied by using an ampoule with a smaller internal diameter and a silicon oxide substrate. Hollow hexagonal cylinders that were approximately 20 μm in diagonal dimension were obtained on the inner wall of the ampoule with a smaller internal diameter. Solid hexagonal crystals of approximately 10 μm in diagonal dimension grew perpendicular to the silicon oxide substrate, whereas hollow hexagonal cylinders grew on the inner wall of the ampoule. Finally, the surface energy of the deposition surface was varied for silicon substrate. Nanoparticles with diameters of approximately 50 nm were sparsely deposited on the textured silicon surface.

Evaluation of Corrosion Characteristics of Plasma-Spray Ceramic Coated Steel

Evaluation of corrosion characteristics of plasma-sprayed ceramic coating was conducted by employing various electrochemical methods. Then, the corroded surface of ceramic coating and interfaces between ceramic top coat and undercoated NiCrAlY layers were examined by SEM. As a result, for the degradation process of plasma-sprayed ceramic coating in aqueous solution, the anodic dissolution in the boundary regions between ZrO2 top coat and NiCrAlY under coat layers is indicated to be important. Open pores and micro-cracks in the top coat layer play the important role as the path through which corrossive media comes into the boundary layers.

AN EVALUATION OF STATIC FATIGUE CHARACTERISTICS OF GLASS CERAMICS IN WATER

An evaluation of static fatigue crack growth characteristics of glass ceramics in water was conducted through employing a double cantilever beam specimen with side grooves. It was recognized that glass ceramics have superior resistance to static fatigue crack propagation than do soda-lime glass. Then, the reasons why glass ceramics are superior to soda-lime glass in static fatigue characteristics were investigated from the viewpoints of the reaction rate of glass structure with water molecules, the difference in process zone sizes and the morphologies of propagating cracks. Also, the effectiveness of the procedures of delayed fracture test using Vikers indented double cantilever beam specimens with side grooves was indicated.

Dependency of the initial stage of stress corrosion cracking behavior of austenitic stainless steel on the stress level

ABSTRACT In this paper, the initial stage of stress corrosion cracking behavior of austenitic stainless steel SUS304 was investigated in boiling 42% magnesium chloride aqueous solution. Nucleation and initial propagation of stress corrosion micro cracks were analyzed both in surface and in fracture surface from microstructural aspect in a wide range of stress from around threshold stress where no plastic deformation occurred macroscopically to 0.2% proof stress. The main results are summarized as follows; (1) Stress corrosion cracking behavior at the initial stage is dependent upon the stress level. Namely, at higher stress levels, intercrystalline cracking originating from triple grain boundaries occurred, but as the stress level was lowered, it was replaced with transgranular one accompanied with corrosion grooves. (2) Micro crack growth from the small corrosion grooves is an important factor determining the value of the threshold stress of material used. (3) At a stress level near 0.2% proof stress, the stress corrosion crack nucleats from broken point of the passive film on triple grain boundaries where the material is concentrically strained and it propagates to interconnect triple grain boundaries.