Papers by Author: Tatsuki Ohji

Abstract: Porous nano-scale ZnO particle film was rapidly synthesized from a mixing solution of zinc acetate dihydrate – acetone at about 90 °C. The crystal structure and morphology　were clarified by the X-ray diffraction and a field emission scanning electron microscope. The evaporation of higher vapor pressure acetone resulted in the hydrolysis of zinc salt in itself crystalline water. Three morphologies of ZnO (dispersed nanoparticles, dispersed bread chip-shaped particles and flower-like bread chip-shaped assemblies) were observed.

Abstract: Anhydrous zinc acetate pre-coated on the FTO substrate was served as a template layer, and ZnO nanowhisker arrays were simply and successfully fabricated by its forced-hydrolysis-initiated-nucleation of the layer in an aqueous solution at low temperature below 100 °C. The technique doesn’t need any expensive metal catalyst and high-temperature treatment. The density, diameter and length of whiskers were controllable by changing the deposition time. As-grown ZnO nanoarrays showed photo-induced electrical property. Enhanced photo-induced current (2.0~3.0  10-5 A) was detected under laser irradiation after DNA molecules labeled with dye molecules were loaded on the ZnO nanowhisker arrays.

Abstract: Length-tailored, monodisperse, highly orientated, single-crystalline hexagonal and aligned ZnO nanowhiskers were grown onto F-doped SnO2 conductive glass (FTO) substrate at 88 °C using an aqueous solution. The aqueous solution for growth of ZnO nanowhiskers included zinc nitrate hexahydrate, hexamethylenetetramine and polyethylenimine. The addition of branched polyethylenimine, which may be adsorbed on the nonpolar surface of ZnO crystals, improved the growth of ZnO nanowhiskers along the c-axis.

Abstract: Exergy is a measure which can commonly deal with the quantification of the variety of resources, products and energy coming in and going out the manufacturing systems. In this study, exergy analysis was conducted on ceramics and steel heater protection tube used in aluminum casting, and the amount of exergies consumed through their life cycle were calculated. In the production stage, ceramic heater tube consumes much higher exergy than the steel one does, however, analysis throughout the life cycle including production, operation and waste in seven years shows that exergy consumption for the ceramic tube is less than that of the steel tube.

Abstract: Six kinds of silicon nitrides with different microstructures were fabricated by changing the amount of sintering additives, Al2O3 and Y2O3. The hardness decreased with an increase in the amount of additives, whereas the indentation fracture resistance increased in the samples with large amount of additives due to the formation of coarse and elongated grains. The wear property of various Si3N4 ceramics was investigated in the sliding contact test without lubricant and was compared with the mechanical properties. The specific wear rate varied notably from 4x10-4 to 6x10-6 mm-3N-1m-1 depending on the compositions, which was difficult to explain directly from the hardness and fracture resistance. An indentation fracture model for material removal could correlate the wear properties with a function of hardness and fracture resistance of the materials, suggesting that the indentation model was likely to be valid for analyzing the wear behavior in this study.

Abstract: Semi-crystalline mesoporous titania films were prepared by the reactions of titanium isopropoxide (TTIP) in acidic aqueous solutions containing EO106PO70EO106 triblock copolymer (F127) and 1,3,5-triisopropylbenzene (TIPBz). Adsorption experiments of dye molecules over the mesoporous films calcined at different temperatures and prepared with different TTIP/F127 molar ratios showed that adsorption property of the dye molecules would be related to the crystallinity of the frameworks and the macroporosity at the surfaces.

Abstract: Highly conductive and well-aligned ZnO nanowhisker films which can be used to optoelectronic devices have been fabricated on conductive F-doped SnO2 coated glass substrates using aqueous solutions with the addition of polyethylenimine at a lower temperature. ZnO nanowhiskers with well crystalline and hexagonal morphology were grown with high orientation along the c-axis. The electrical properties were investigated by a low intensity red laser irradiation (6 mW), after DNA molecules labeled with dye molecules were absorbed on the nanowhiskers. Enhanced photocurrent was detected.

Abstract: The influence of two measuring conditions, the elapsed time after indentation and the condition of edge of an indenter, on the indentation fracture toughness of silicon nitrides was assessed. No slow crack-growth after unloading was confirmed by optical microscopic observation of a crack tip induced by the indentation, which led to the negligible difference in fracture toughness measured at 1 and 30 min after the indentation. Measurements with relatively new and used indenters gave almost the same fracture toughness data, indicating that the crack lengths were hardly affected by the slight damage of the corner of the indenter. It was suggested that the large scattering of the indentation fracture toughness reported by the round-robin tests such as VAMAS was not originated from these factors.

Abstract: Effect of microstructure of silicon nitride on the fracture toughness, KIc evaluated by the IF method was studied with various indentation loads ranging from 49 N to 490 N, since practical assessment of fracture toughness of small Si3N4 parts is needed in the ceramic ball bearing market. The plot of KIc against the as-indented crack length revealed the rising R-curve behavior for the coarse Si3N4 and slight R-curve for the fine Si3N4. By comparing KIc estimated from the SEPB and IF methods using 4 different equations, it was revealed that the IF equation which gave the nearest value to KIc from SEPB was different depending on the microstructures. These results were discussed in conjunction with their R-curve behavior and the effective crack length in the SEPB specimens.

Abstract: In this study, porous Si3N4 ceramics were fabricated by carbothermal reduction reaction between silicon dioxide and carbon. The influences of different starting powders and sintering additives on microstructure and mechanical properties were investigated. XRD analysis demonstrated the formation of single-phase β-Si3N4 except for glass phase and minor of α-Si3N4 phase. SEM analysis showed that the resultant porous Si3N4 ceramics occupied fine microstructure and uniform pore structure. The samples with fine starting powder showed fine, high aspect ratio of β-Si3N4 grains and good mechanical properties. The addition of Al2O3 accelerated the densification of porous Si3N4 ceramics. With an increasing in the sintering additive content, the porosity decreased, the flexural strength increased.