Hirotaro Mori

Hollow oxide formation by oxidation of Al and Cu nanoparticles

The formation of hollow metal oxide nanoparticles through the oxidation process at low temperatures from 295 to 423 K has been studied by transmission electron microscopy for Cu, Al, and Pb. For Cu and Al, hollow oxide nanoparticles are obtained as a result of vacancy aggregation in the oxidation processes, resulting from the rapid outward diffusion of metal ions through the oxide layer during the oxidation process. On the other hand, Pb nanoparticles turn to solid PbO because the diffusivity difference DPb<DO in PbO does not lend itself to the formation of vacancy clusters. The oxide growth behavior of Cu and Al nanoparticles of a larger size at 423 K are summarized as follows: (i) for Al, the rapidly forming oxide layer on its surface stops growing once it reaches a critical thickness of about 1.5 nm, (ii) the growth of Cu2O continues until hollow Cu2O of a certain thickness is formed. This suggests the occurrence of two different diffusion processes in the formation of hollow oxides: the rapid outward ...

Microwave-assisted size control of CdS nanocrystallites

The size and size-distribution of CdS nanocrystallites is controlled by the microwave-assisted reaction of cadmium acetate with thiourea in N,N-dimethylformamide (DMF). The absorption onset of the CdS nanocrystallites shifts to longer wavelength with increasing irradiation time, indicating particle size growth under prolonged irradiation. However, when the microwave irradiation of the solution is periodically interrupted and then repeated, keeping the solution at ambient temperature before each irradiation, the absorption onset remains at the same wavelength and only the optical density of the absorption band is increased, suggesting that the particle growth occurs only during the continuous irradiation and stops once the system is cooled down. Photoluminescence observed for the CdS nanocrystallites changes with the irradiation time and the repetition of the irradiation, suggesting a structural change of the nanocrystallite surface due to the consumption of excess Cd2+.

Microstructural characterization of TiB in in situ synthesized titanium matrix composites prepared by common casting technique

Abstract TiC reinforced titanium matrix composites were produced by non-consumable arc-melting technology utilizing the self-propagation high-temperature synthesis (SHS) reaction between titanium and graphite. X-Ray diffraction (XRD) was used to identify the phases in the composites. Microstructures of the composites were observed by optical microscope (OM) and transmission electron microscope (TEM). The results show that there are two phases in the composites: TiC and titanium matrix alloy. TiC has two different shapes: dendritic shape, equiaxed or near-equiaxed shape. The in situ synthesized TiC grows by dissolution–precipitation. Analysis of the binary phase diagram determined that the solidification path undertook the following three stages: primary TiC, binary eutectic β-Ti+TiC and solid transformation. Primary TiC grows in dendritic shape due to the formation of composition undercooling. Binary eutectic TiC grows in equiaxed or near-equiaxed shape. A small quantity of TiC may form twin structure during nucleation and growth. The twin plane is the (111) plane.

Temperature Dependence of Electron-Irradiation Induced Amorphization of NiTi Alloys

Electron-irradiation induced amorphization of NiTi alloys has been investigated in a temperature range from 50 to 393 K by high voltage electron microscopy with the following results: A crystalline-amorphous transition occurs when the total dose of electrons reaches a certain value. The critical value decreases with decreasing irradiation temperature and becomes almost constant with about 2×1025 e/m2 below 100 K. Process of the transition is also revealed.

Amorphous transition in intermetallic compounds induced by electron irradiation

It was recently found that a crystalline to amorphous (C-A) transition could be induced by in-situ electron irradiation in a high voltage electron microscope in such intermetallic compounds as Fe3B and NiTi. These findings are of considerable importance in that production of cascades is proved not to be a necessary condition for the C-A transition in crystals induced by particle irradiation. A question arises as to whether or not a similar C-A transition can be produced in other intermetallic compounds, because studies of electron irradiation damage in intermetallic compounds have so far failed to reveal such a transition. In this paper, a series of intermetallic compounds with different physical properties were prepared and irradiated with 2 MeV electrons and the relative ease of amorphous formation has been compared. Analysis of the results shows the tendency toward amorphization has a rough correlation with the extent of solubility of the particular compounds.

Microstructure and tensile properties of in situ (TiB+TiC)/Ti6242 (TiB:TiC=1:1) composites prepared by common casting technique

In the present work, (TiBw+TiCp)/Ti6242 composites with TiB:TiC=1:1 were produced by common casting and hot-forging technology utilizing the SHS reactions between titanium and B4C, C powder. The microstructures of composites were examined using optical microscopy (OM) and transmission electron microscopy (TEM). The X-ray diffraction (XRD) was used to identify the phases that were present in the composites. There are three phases — TiB, TiC and titanium matrix alloy. TiB grows in short-fiber shape, whereas TiC grows in dendritic, equiaxed or near-equiaxed shape. TiB whiskers were made to align the longitudinal direction and TiC dendritic was broken up after hot-forging. The reinforcements are distributed uniformly in matrix alloy. The interfaces between reinforcements and titanium matrix alloy are very clean. The tensile strength (yield strength and ultimate tensile strength) and the Youngs modulus improve with the addition of TiB whiskers and TiC particles although some reduction in ductility is observed. (TiBw+TiCp)/Ti6242 composites with TiB:TiC=4:1 will fracture on a lower level of applied stain due to deformation restraint of TiB whiskers on titanium matrix alloy. The (TiBw+TiCp)/Ti6242 composites with TiB:TiC=1:1 show higher tensile strength and ductility. The addition of graphite not only improves the tensile strength and the Youngs modulus but also increases the ductility. The improved Youngs moduli and increased tensile strengths of the composites are explained using shear lag and rule-of mixtures theories. The Youngs moduli of the composites were found in good agreement with that calculated from Tsai–Halpin equation applied for discontinuous-reinforced composites.

Zinc oxide/n‐Si junction solar cells produced by spray‐pyrolysis method

Zinc oxide (ZnO)/n‐Si junction solar cells were fabricated by a spray‐pyrolysis method and high short‐circuit photocurrent densities and relatively high photovoltages were obtained by adjusting the conditions of the deposition and the post‐deposition heat treatment. Consequently, relatively high conversion efficiencies ranging between 6.9% and 8.5% were obtained. The efficiency of the solar cells with ZnO/n‐Si structure decreases slowly with time when they are kept in air in the dark because of the increase in the thickness of the silicon oxide layer between Si and the ZnO film. This degradation can be avoided by forming an indium‐tin‐oxide (ITO) overlayer on the ZnO film, indicating that the silicon oxide layer grows through the reaction of Si with oxygen diffusing from the atmosphere, not with ZnO. The efficiency of the ZnO/n‐Si junction solar cells decreases rapidly with the illumination time. Capacitance‐voltage measurements show that this degradation is caused by a decrease in the work function of th...

Spontaneous alloying of copper into gold atom clusters

Abstract A double-source evaporator which can be set in the specimen chamber of a high voltage electron microscope has been constructed, With the use of the evaporator, isolated gold clusters of 5 ~ 10 nm in size were first prepared on a supporting film of amorphous carbon, and then copper atoms were deposited onto the film (and of course onto the gold clusters as well), which was kept at room temperature. Upon the deposition of copper, the gold clusters changed into highly concentrated, homogeneously mixed Au—Cu alloy clusters. The copper atom diffusivity in gold clusters deduced from the present observations is at least 9 orders of magnitude faster than the copper solute diffusion in bulk gold.

The relationship between the chemical and topological disorder in the intermetallic compound Cu4Ti3

Abstract Using in situ measurements of the Bragg-Williams long-range order parameter, S, in a HVEM, the relationship between electron irradiation induced chemical disordering and the crystalline to amorphous (C-A) transition is studied in the line compound Cu4Ti3. The existence of critical temperatures above which the attainable chemical disordering is critically reduced and above which the C-A transition does not occur is shown, and it is found that the two critical temperatures are coincident. Below the critical temperature, significant chemical disordering is produced and the C-A transition always occurs; above the critical temperature, the attainable degree of chemical disordering is severely limited, the C-A transition does not occur, and secondary defects are formed. Thermodynamical consideration is developed in which chemical disordering raises the energy of the crystal lattice until it becomes unstable with respect to the amorphous state. In Cu4Ti3, the necessary degree of chemical disordering corresponds to an S value of approx. 0.3. The thermodynamical consideration is supported by a HREM study of the crystalline-amorphous interface, the observation of a preferred C-A transition at antiphase domain boundaries and energy comparisons. Finally, the role of point defects in the C-A transition is discussed in terms of the temperature dependence of the chemical disordering rate and the existence of the critical temperature.

Oxidation behaviour of Ni nanoparticles and formation process of hollow NiO

The oxidation behaviour of Ni nanoparticles at temperatures from 573 to 673 K and the formation process of hollow oxide particles were studied by transmission electron microscopy. In the course of oxidation, a single large void was observed at one site of the interface between inner Ni and outer NiO layer due to vacancy clustering, which occurs during the oxidation process resulting from the rapid outward diffusion of Ni ions through the NiO layer. This suggests that supersaturated vacancies generated at the interface migrate to the site over a long-range distance and aggregate at the site. Ni nanoparticles were fully oxidized to become hollow NiO, in which nano-holes in the form of vacancy clusters were located at the off-centred positions. The de-centring of the voids in hollow NiO is probably due to the large mobility of vacancies inside Ni during oxidation. 1l. (a) 44 567 (1977)