Materials Science Forum Vol. 750

Abstract: The photo-electrochemical properties of photocatalytic titanium dioxide thin films obtained by reactive RF magnetron sputtering were investigated. The pressure during the sputtering deposition affected the photocatarytic activity and photo-induced hydrophilicity, and at 2 Pa the highest activities were observed. The photocurrent under ultraviolet light illumination corresponded to the photo-induced activities of the films. It is suggested that the band bending near the surface is one of the most crucial factors determines the activities of photo-induced behavior of the titanium dioxide thin films.

Abstract: We investigated the formation of nanopores on top Si layers of silicon on insulator substrates by CH₃SiH₃ pulse jet chemical vapor deposition. Nanopores were obtained by chemical etching of the buried oxide layer below the pits which were introduced during the SiC growth. The high nanopore density was obtained when the SiC growth temperature was set at 925 °C. The nanopore density gradually decreased with increasing the temperature at higher SiC growth temperature. The pore size increased with increasing the SiC growth temperature. These results suggest that pore density and size strongly depend on the SiC growth temperature.

Abstract: In this paper, McEvily’s fatigue crack growth equation is modified and newly combined with the modified Goodman equation to estimate the fatigue strength with arbitrary mean stress, σ_m. Firstly, based on McEvily’s equation, the threshold stresses for fatigue crack initiation and propagation with stress ratio R = –1 or σ_m = 0 are predicted with reasonable accuracy. Then, a simple calculation is presented to predict the fatigue strength with arbitrary mean stress. The adequacy of present method is examined based on the comparison with the available experimental data in the literature.

Abstract: Effect of hydrogen-charging was investigated with respect to the tensile properties of three types of cast irons: JIS FCD400, FCD450 and FCD700. In this study, hydrogen charging led to a marked ductility loss in all the cast irons. The thermal desorption spectroscopy and the hydrogen microprint technique revealed that, in the hydrogen-charged specimens, most of solute hydrogen was diffusive and mainly segregated at graphite, graphite/matrix interface zone and pearlite. In the fracture process of non-charged specimen, neighboring graphites were interconnected with each other mainly by ductile dimple fracture. On the other hand, in the fracture process of hydrogen-charged specimen, the graphites were interconnected by cracks. The difference in the fracture morphology between the non-charged and the hydrogen-charged specimens is attributed to the presence of diffusive hydrogen in graphite and graphite/matrix interface. During early stage of fracture process in hydrogen-charged specimen, the interspace between graphite and matrix is filled with hydrogen gas, which leads to the ductility loss of matrix in the vicinity of graphite. Even after the initiation of crack from graphite, hydrogen is continuously outgassed from graphite and supplied to the crack tip. Therefore, concerning the hydrogen effect on the strength of cast irons, a role of subsurface graphite as a “local hydrogen supplier” should be taken into consideration.

Abstract: Effect of occasional mode II loading on subsequent mode I fatigue crack growth behavior was investigated by using a thin-walled tube made of 7075-T6511 aluminum alloy. Careful observation of crack growth behavior revealed that the occasional mode II loading has two contradictory effects for crack growth behavior. The first is a retardation effect that is associated with the plastic deformation near crack tip. However, this effect is negligibly small for the crack growth life as a whole. The second is an acceleration effect caused by mode II fatigue crack growth itself. It was found that under relatively high ΔK level, the mode II crack growth was about an order magnitude faster than mode I crack growth. Therefore, to properly evaluate the effect of occasional shear loading in the 7075 alloy, the mode II crack growth should be taken into account.

Abstract: Depending on the oxidation extent and the surface roughness after coating manufacturing, CoNiCrAlY coating displays different performance in thermal barrier coating systems. In this study, plasma-sprayed CoNiCrAlY coatings were treated by dry-ice blasting. Various treatment approaches of dry-ice blasting were assessed by means of comparing the coating quality in terms of cross-sectional structure, oxide concentration, surface morphology and roughness, and microhardness. The results showed that CoNiCrAlY coating including decreased oxides and smoother internal texture can be obtained in comparison with that deposited by conventional APS, no matter what type of treatment. The oxidation content differ after using different jet angle and installation location of dry-ice blasting. To some extent, dry-ice blasting has some cleaning effect on those splashed particles. However, no much change can be recognized in the top-surface morpholgy and surface roughness, because of the relatively small hardness of dry-ice pellets. There is a slight decrease in hardness probably resulting from the decrease in the oxide.

Abstract: Ultrathin Al films have been prepared in modulated Al/AlN multilayer structures by DC magnetron sputtering. The surface morphology was observed by scanning probe microscopy (SPM). SPM images show that ultrathin Al films consist of equiaxial in-plane grains. The surface roughness for 20 nm-thick Al film which is more than 1 nm is quite large. Full (2θ, ψ)-scan by 2 dimensional XRD (2D - XRD) confirms that ultrathin Al film has highly (111) texture. Residual stress of ultrathin Al films was observed by applying sin²Ψ method. It is found that 5 nm-thick and 10 nm-thick Al films are subjected to compressive stress due to the influence of Volmer-Weber growth mechanism during sputtering. All samples are subjected to tensile stress after annealing. The unstrained interplanar spacing of Al films expands when the thickness is reduced below 10 nm and contracts with increasing annealing temperature.

Abstract: The aim of our research is to realize a high-sensitivity three-dimensional (3-D) shape measurement of target with an optical microscope. On the conventional method – shape from focus, it’s to regulate focal length of microscope to obtain 3-D shape information of the specimen. However, the method is used the vertical stratification method to obtain 3-D shape information of total surface of target, and the variable focal length of zoom lens is limited therefore the measurement accurate is confined on the vertical direction. In order to solve these problems, we propose a high-sensitivity 3-D shape measurement method based on microscope and laser projection. The method is based on the slit pattern projection technique and 3-D image processing technique. The proposal method can be used for medicine, pharmacy, life science, and material science.

Abstract: In order to verify an alternative metallurgical process of phosphorus removal for solar grade silicon (SOG-Si), slag treatment of metallurgical grade silicon (MG-Si) was conducted followed by acid leaching in the present study. MG-Si containing certain amount of phosphorus and calcium was equilibrated at 1723 and 1773 K with several compositions of the CaO-CaF2 slags and phosphorus in molten silicon was confirmed to be removed into slag phase also by reducing reaction as a form of phosphide ion, P3-, in addition to the phosphate ion, PO43-. These contents were separately determined by a wet chemical analysis method developed by ourselves. Although the distribution ratio of phosphorus could not exceed the highest reported values of 3, subsequent leaching brought about considerably high fraction of P removal. The removal fraction of 95.6% was attained when 5 g of silicon was treated with 10 g of the slag at 1773 K followed by the acid leaching, which would be much higher than that expected by the ordinary oxidizing slag treatment. Although the possibility of reducing dephosphorization by slag treatment was clarified, more effective condition should be pursued by changing slag composition, calcium content of silicon, temperature, etc.

Abstract: The superconducting Ba_1-xK_xFe₂As₂ (x = 0.4) single crystals were prepared by the so-called FeAs self-flux method. The critical temperature by ac susceptibility measurement was estimated to be about 36 K. The magnetic field and temperature dependences of critical current densities were investigated by an ac inductive measurement (Campbell’s method). Unlike the phenomenon of co-existence of the global and local critical current densities observed in many polycrystalline Fe-based superconducting pnictides, it was found that only a uniform critical current density (J_c) flows through the whole sample. The value of J_c at 20 K and 1 T was about 5×10⁸ A/m², which is much smaller than the local critical current density observed in polycrystalline samples. This result implies that a dissimilarity of flux pinning mechanism exists between these two kinds of materials. The force-displacement characteristic of fluxoids in sample was investigated. The Labusch parameter was found to increase monotonously with increasing magnetic field, while the interaction distance was proportional to the fluxoid spacing. These results are consistent with the prediction based on a simple flux pinning mechanism.