Shun-ichiro Tanaka

Nanostructure of wetting triple line in a Ag–Cu–Ti/Si3N4 reactive system

Abstract Nanometer scale structures around wetting triple lines were studied in a Ag–Cu–Ti/Si 3 N 4 reactive system. Changes in the contact angle and radius of the molten metal on the substrate as a function of time were also measured in the system as macroscopic wetting behaviors. The macroscopic wetting behaviors showed two wetting stages and double layered reaction products consisting of upper Ti 5 Si 3 and lower TiN layers were observed in both first and second stages. The reaction product always lay in front of the triple line defined as a triple junction of Ag–Cu–Ti alloy/Ti 5 Si 3 /atmosphere. At the front of the reaction product, a dominant phase changed from TiN in the first stage to Ti 5 Si 3 in the second stage. It is considered that the structural change is one of the reasons why the macroscopic wetting behavior changed, and that the structural change was caused by a decrease of Ti activity as the reactive wetting progressed.

Atomic morphology and chemical reactions of the reactive wetting front

Abstract Morphology at the reactive wetting front during spreading was first observed in situ at an atomic scale using a high resolution transmission electron microscope. Two typical nanomorphologies of an Ag–Cu–Ti molten alloy were observed on a SiC substrate depending on the nucleation position of the reaction product, TiC. In the case of the precursor atomic thin layer spreading ahead of the droplet of the molten alloy, the TiC nucleated in the thick region of the precursor, which was a few nm behind the tip of the precursor layer. In contrast, molten alloy spreading with TiC growing simultaneously at the tip was also observed. In this case, the precursor layer on SiC did not exist. Nucleation and growth rate of the reaction product were related to the tip morphology.

Three-dimensional islands of Si and Ge formed on SiO2 through crystallization and agglomeration from amorphous thin films

The crystallization process was examined for amorphous thin films of silicon (a-Si) and germanium (a-Ge) on quartz glass (SiO2) substrate. Three-dimensional crystalline islands were formed through crystallization and agglomeration. These islands indicated a bimodal size distribution. The mechanism of crystalline island (c-Si, c-Ge) formation was discussed on the basis of thermodynamics. In studying the crystallization of the thin films, the influence of the film-substrate interfacial energy should be taken into consideration. It was found that the thickness of the as-deposited amorphous films is an essential factor in determining the crystallization behavior and in controlling island size. Above all, a high size uniformity of crystalline islands could be obtained under moderate thermal annealing conditions. q 1999 Elsevier Science S.A. All rights reserved.

Structure of wetting front in the Ag-Cu-Ti/SiC reactive system

The nanostructure around the wetting triple line in the Ag-Cu-Ti/SiC reactive wetting system was studied. The reaction product consisted of an upper separated Ti5Si3 layer more than 20 nm thick, and a lower TiC layer less than 10 nm thick was observed exceeding the front line of Ag-Cu-Ti brazing metal. At the top of the reaction product, the lower TiC layer was observed to project beyond the front line of the upper Ti5Si3 layer. In Ag-Cu-Ti/Si3N4 system, structural change around the wetting triple line as reactive wetting progressed has been reported, however, in the Ag-Cu-Ti/SiC system, the structure around the triple line did not change. The difference between the two systems was explained from the viewpoint of a stable phase change with the decrease of Ti activity as reactive wetting progressed.

In-situ high temperature X-ray diffraction study of Ni/SiC interface reactions

In-situ experiments on the Ni/SiC interface reaction were carried out with a high temperature X-ray diffractometer capable of measuring the X-ray diffraction pattern in 1–2 s using an imaging plate. The kinetic formation processes of the interface reaction layer were measured in short-period exposure experiments with the apparatus. The time-temperature phase diagram of Ni/SiC in N2 was determined. δ-Ni2Si and Θ-Ni2Si (high temperature phase of δ-Ni2Si) were formed at the Ni/SiC interface between 1072 K and 1418 K in N2. The formation of δ-Ni2Si obeyed the parabolic rate law. The value of the activation energy suggests that the diffusion of Ni through δ-Ni2Si controls the rate of formation. The results of thermal expansion coefficient measurements suggest that when a sample is cooled to room temperature, compression caused by δ-Ni2Si occurs on SiC.

In-situ high temperature X-ray diffraction study of Fe/Al2O3 interface reactions

In-situ experiments on the Fe/Al2O3 interface reaction were carried out with a high temperature X-ray diffractometer capable of measuring the X-ray diffraction pattern in 1–4s, using an imaging plate. The kinetic formation processes of the interface reaction layer were measured in short-period exposure experiments using the apparatus. The time-temperature phase diagram of Fe/Al2O3 in air was determined. Fe/Al2O4 was formed at the FeAl2O3 interface between 1595 K and 1675 K in air. The formation of FeAl2O4 obeyed the parabolic rate law. The value of the activation energy suggests that the diffusion of Al into FeAl2O4 controls the rate of formation. The results of thermal expansion coefficient measurements suggest that when a sample is cooled to room temperature, compressive strain caused by FeAl2O4 occurs on Al2O3.

Assignment of the Raman active vibration modes of β-Si3N4 using micro-Raman scattering

The vibrational modes of the β-Si3N4 Raman active bands appearing in the wave number region between 110 and 1100 cm−1 were determined by the polarization and the crystallographic orientation dependence of the Raman intensities of the bands using micro-Raman spectroscopy. The laser beam was focused onto a single grain with the shape of a regular hexagon or elongated hexagon on the sintered ceramic plate. The three intense bands appearing at about 185, 208, and 230 cm−1 were attributed to the vibrational modes of E2g, Ag, and E1g, respectively. The remaining peaks were also assigned to the irreducible representations.

Phase transformation and bonding of ceramic nanoparticles in a TEM

Abstract Remarkable nanoscopic changes were observed in-situ in metastable oxide particles on the room temperature stage of a TEM under as electron beam intensity of 1019 e/cm2sec. Particles of 10–15 nm diameter α-Al2O3 and 5nm diameter Al metal were obtained from 100nm metastable θ-Al2O3 particles by a process dependent on the electron beam intensity. Al nanoparticles were bonded by continuous irradiation via surface diffusion of Al atom and the formation of 2 nm sized Al clusters The effects of the electron beam are the irradiation induced increase of the oxygen mobility and localized heating.

FORMATION OF SI ISLANDS FROM AMORPHOUS THIN FILMS UPON THERMAL ANNEALING

The mechanism of crystalline Si island formation from an amorphous film was discussed in relation to the free energy of the Si/SiO2 system. Agglomeration of the Si crystallite occurred forming islands, thus preventing a further increase in free energy. Crystal growth proceeded in two steps. At first, the size distribution of Si islands was unimodal but finally became bimodal during the crystallization process.The mechanism of crystalline Si island formation from an amorphous film was discussed in relation to the free energy of the Si/SiO2 system. Agglomeration of the Si crystallite occurred forming islands, thus preventing a further increase in free energy. Crystal growth proceeded in two steps. At first, the size distribution of Si islands was unimodal but finally became bimodal during the crystallization process.

Formation and coalescence of tungsten nanoparticles under electron beam irradiation

Abstract Powder particles of tungsten oxide (WO 3 ) were irradiated by an electron beam with an energy of 200 keV in a transmission electron microscope (TEM) atroom temperature. A number of nanocrystals were induced inside the single crystal particle as a result of irradiation damage. Electron energy loss spectroscopy (EELS) analysis revealed a decrease in the oxygen content in the particle which suggested decomposition of the tungsten oxide. In the vicinity of the irradiated area, a number of spherical ultrafine particles of 2–6 nm in diameter were observed. It is revealed that these ultrafine particles are oxide free tungsten. These nanoparticles migrated under the subsequent electron beam irradiation. A few nanoparticles which came into contact were observed to coalesce each other to form a single crystal. The mobility of the atoms was enhanced by the electron beam irradiation under ultrahigh vacuum in a TEM.