Tokimasa Kawabata

Room temperature gas sensing of p-type TeO2 nanowires

Tellurium dioxide (TeO2) nanowires with a tetragonal structure have been grown by thermally evaporating tellurium metal at 400°C in air. The nanowires produced have diameters ranging from 30to200nm and have lengths of several tens of micrometers. Gas sensors were fabricated using the obtained TeO2 nanowires. The sensing behavior to NO2, NH3, and H2S gases at room temperature showed typical characteristics of a p-type semiconductor. The results demonstrate the potential to develop TeO2 nanowire based gas sensors with low power consumption.

HAADF-STEM study ofβ′-type precipitates in an over-aged Al–Mg–Si–Ag alloy

Coarse, rod-shaped precipitates growing along ⟨100⟩Al directions in an Al–1.0 wt% Mg2Si alloy with 0.5 wt% Ag additions were investigated by high-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). All investigated precipitates had complex structures, being composed of domains separated by anti-phase resembling boundaries. The domains consist of a modified hexagonal β′-type structure that contains a considerable amount of Ag. Based on HAADF-STEM images, an average atomic model with space group P-62 m (189) and composition Al3Mg3Si2Ag is proposed, having Al incorporation and Ag replacing certain Si atomic columns. Co-existence with the Ag-free β′-Mg9Si5 phase has been observed for some precipitates. The boundaries may be described as full or half units of the orthorhombic U2-AlMgSi precipitate phase. The HAADF-STEM images indicate partial replacements of Al atoms by Ag, in both the β′-type domains and the U2-type boundaries. Ag enrichment of the Al matrix near the precipitate/Al interface was observed for all the investigated precipitates

High-resolution elemental maps for three directions of Mg2Si phase in Al-Mg-Si alloy

Elemental maps of the Mg and Si sub-lattices of the Mg2Si phase in an Al-1.0mass% Mg2Si alloy were produced using an energy-filtering transmission electron microscope (EFTEM). Low magnification elemental maps were obtained using both low and high energy loss edges, and the intensities of the high energy loss edges were sufficiently high to allow the Mg2Si phase to be observed at high magnification. High-resolution core-loss images of Mg and Si-K edges were taken parallel to [001], [111] and [110] of the Mg2Si phase. In the [110] direction, Mg and Si atoms were successfully identified as sub-lattices. The Mg atoms formed a 0.39 nm diamond network, whereas the Si atoms formed a 0.32 nm by 0.22 nm rectangular network. This result is in good agreement with the projected potential of the Mg2Si phase in the [110] direction. This is the first report of magnesium and silicon atoms in the Mg2Si phase being successfully identified at the atomic level by EFTEM.

Hexagonal tabular β-phase in Al–Mg–Si–Cu alloy

Abstract A hexagonal plate-shaped β-phase was discovered in Al–0.70at.%Mg–0.36at.%Si–0.21at.%Cu alloy. This phase has the same crystal lattice as β-Mg 2 Si, commonly formed in Cu-free Al–Mg–Si alloys, but a different habit plane, and contains less than 2.0at.%Cu.

HRTEM Observation of the Precipitates in Heat-Resistant Mg-Gd-Y-Zr Alloy

The high resolution transmission electron microscope (HRTEM) observation was carried out to investigate the microstructure of precipitates in Mg-Gd-Y-Zr alloy. In the early stage of aging at 423K, monoatomic layers were observed along the (1 1 00)Mg plane. After 64h aging at 423K, the contrast of β” phase based on ordered D019 hexagonal super lattice structure was appeared. The monoatomic layers, two layers spaced by (1 1 00) Mg planes and β’ phase based on bco-structure were observed in the specimen aged for 128h. In the peak- aged specimen, the β’ phase and monoatomic layers were observed.

HRTEM Observation of Precipitates at Early Stage of Aging in Mg-Gd-Zr Alloy

High resolution transmission electron microscopy（HRTEM）observation was performed to clarify morphology of precipitates at early stage of aging in Mg-Gd-Zr alloy. In the specimen aged at 423K, monolayer was observed on {1100}Mg planes. With increasing aging time, the monolayer and β” phase grew along the <1120>Mg directions. Arrangement of bright dots corresponding to {020}β’ except for the monolayer and β” phase was observed before peak aging stage. β’ phases formed at the same aging stage. Thus, these precipitates co-existed before peak aging stage. The monolayer, β” phase and β’ phase still co-existed in the peak aged specimen. The β’ phase was dominated in the peak hardness. It is revealed that the monolayer lying on {1100}Mg act as precursor of β” phase and β’ phase.

Localization of laser-induced breakdown in aggregates of silver nanoshells

Abstract Nanoshells are useful in near-infrared (NIR) applications because of the red shift of the Mie resonance into NIR. Silica/gold/silver nanoshells were prepared via gold anchoring and characterized by Mie-theoretical calculations and transmission electron microscopy, which indicated a broad and enhanced NIR absorption and a partially coated, mottled Ag/Au structure on the silica core. The mottled nanoshells were found to spontaneously aggregate in solution to form irregular islands. Their surface roughness was examined by the fractal dimension analysis. When the aggregates were irradiated with 1064-nm laser pulses, highly localized emission due to laser-induced breakdown was observed, which is attributed to the non-uniform metal distribution over the aggregate surface and the localization of the optical field induced by the disordered arrangement of the nanoshells in the aggregate.

Crystallographic Orientation Relationship between Discontinuous Precipitates and the Matrix in AM-Series Alloys

Magnesium alloys have received considerable attention because of their lightweight and recyclability. AM-series and AZ-series Mg-Al alloys have been used for industrial products widely, particularly for AM-series alloys because of better toughness and impact absorption properties than AZ-series alloys. The Mg17Al12 intermetallic compound is the only precipitate formed during ageing after the solution heart treatment. Discontinuous precipitates exist in grain boundary randomly, and continuous precipitates exist in the matrix. However, there is few report about the orientation relationship between the discontinuous precipitates and the matrix. The purpose of this study is to investigate the orientation relationship between the discontinuous precipitates and the matrix of AM-series magnesium alloys. Hardness measurement, SEM observation, the electron backscattered diffraction (EBSD) techniques were preformed in order to understand the relationship between the discontinuous precipitate and crystallographic orientation of grains in AM-series magnesium alloy. TEM samples with discontinuous precipitates were prepared using the focused ion beam (FIB). And TEM observation was performed to investigate the discontinuous precipitates and crystallographic orientation in the matrix.

Effect of Al and Mn Contents on Microstructure in AM-Series Magnesium Alloys

Magnesium alloys containing aluminum have been used for industrial materials due to their lightweight and recyclability. And the Mg-Al based alloys are usually used for the industrial production. The Mg17Al12 phase is reported as the precipitate formed in the Mg-Al alloys during aging after the solution heat treatment, which is the discontinuous precipitate in the grain boundary and continuous precipitate in the matrix. The Mg-Al alloys are divided into AZ-series alloys with the addition of Zn, and AM-series alloys with the addition of Mn, respectively. AM-series and AZ-series Mg-Al alloys have been used for industrial products widely, particularly for AM-series alloys because of better toughness and impact absorption properties than AZ-series alloys. However, there is few report about the effect of Al and Mn contents on age-hardening behavior and microstructure of AM-series alloys. The propose of this study is to investigate the difference of the age-hardening behavior and microstructures of AM-series alloys using hardness test and scanning electron microscopy (SEM) observation.

Effect of Zn Contents on Microstructure in AZ-Series Magnesium Alloys

Magnesium alloys containing Al have been used for industrial materials due to their lightweight and recyclability. The Mg-Al alloys are usually used for the industrial production. The Mg-Al alloys are divided into AZ-series alloys with the addition of Zn, and AM-series alloys with the addition of Mn, respectively. The addition of Zn to the Mg-Al alloy system reduces the solid solubility of Al in Mg, increases the amount of precipitate phases after ageing and thus causes a moderate increase in strength. The Mg17Al12 phase (γ) is reported as the precipitate formed in the Mg-Al alloys during aging after the solution heat treatment, which is the discontinuous precipitate in the grain boundary and continuous precipitate in the matrix. However, there is few report about the effect of Zn contents on age-hardening behavior and microstructure of AZ-series alloys. The propose of this study is to investigate the difference of the age-hardening behavior and microstructures of AZ-series alloys using hardness test and scanning electron microscopy (SEM) observation.