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[en] In order to investigate the electrochemical properties of Si electrodes deposited on a roughened current collector (substrate), the electrochemical etching method was applied to a Cu current collector. The surface roughness of a Cu current collector could be controlled with an electrochemical etching time and the roughest surface was obtained at 20 min etched Cu foil. After the etching, a thin Cu2O film was formed on the surface due to high reactivity with air. Amorphous Si films were deposited on flat and rough substrates and their morphologies were considerably affected by those of substrates. A Si electrode with a rough current collector exhibited remarkably improved cycle performance and maintained a discharge capacity over 1200 mA h g-1 even after the 50th cycle. The formation of large cracked Si tiles was caused by the improved adhesion between Si film and Cu substrate.
[en] Electrochemical properties of Si/Ni/Cu film electrodes were investigated with different Ni film thicknesses (50-600 nm) and annealing temperatures (200-500 0C). The morphology of amorphous Si film was considerably affected by surface features of the Ni under-layer. The best cycle performance of 90.2% retention after 70 cycles was obtained from Si/Ni/Cu film with 300 nm thick Ni film and after 400 0C annealing. For the annealed Si/Ni/Cu electrode, the surface morphology (protrusions) and the formation of compounds among Si, Ni and Cu prohibited the volume expansion of Si during the charge-discharge process and enhanced adhesion between films and substrate, respectively.
[en] We report the phenomenon of size segregation and the experimental evidence for the presence of correlated areas mediated by dipolar interactions in three-dimensional Fe nanoparticle assemblies. Iron nanoparticles dispersed in ethanol assemble into tabular whiskers (8 μm × 40 μm in cross section with lengths up to 10 cm) due to dipolar interactions. Magnetic force microscopy observations on iron nanoparticle compact assemblies prove the local magnetic correlation of the Fe nanoparticles due to dipolar coupling and the formation of domain-like structures in expanded dimensions. Magnetic measurements show that the coercivity and the low field magnetic susceptibility of the Fe nanoparticle assemblies increase while the saturation magnetization decreases with the increasing inter-particle distance. (condensed matter: electronic structure, electrical, magnetic, and optical properties)
[en] Highly luminescent boron nitride (BN) was synthesized by pyrolysis of molecular adduct prepared by chemical mixing of boric acid and melamine with a small amount of cerium nitrate. After pyrolysis, Ce ions were substitutionally incorporated to B3N3 hexagonal layer of turbostratic BN at a very low doping level which was estimated to be less than 7 x 1018 atoms/cm3. Ordered stacking of hexagonal layers along c-axis of BN or transformation of turbostratic BN to hexagonal BN was induced by addition of Ce ions even though the amount of Ce ions surpassed substitutional doping limits. The intense photoluminescence (PL) spectrum of Ce-doped BN was detected at 390 nm by excitation of UV light (λ = 314 nm) and PL intensity was improved with increasing level of Ce addition following the enhanced crystallinity of turbostratic BN. The origin of UV-emission was ascribed to the transition from the crystal field components of the 5d level to the 4f ground states of the Ce3+ ion. (orig.)
[en] A Pd-Si amorphous phase was formed at a palladium/silicon oxide (Pd/SiO_x) interface at room temperature by electron irradiation at acceleration voltages ranging between 25 kV and 200 kV. Solid-state amorphization was stimulated without the electron knock-on effects. The total dose required for the solid-state amorphization decreases with decreasing acceleration voltage. This is the first report on electron irradiation induced metallic amorphous formation caused by the electronic excitation at metal/silicon oxide interface
[en] Irradiation-induced crystallization of an amorphous phase was stimulated at a Pd-Si amorphous/silicon oxide (a(Pd-Si)/SiO_x) interface at 298 K by electron irradiation at acceleration voltages ranging between 25 kV and 200 kV. Under irradiation, a Pd-Si amorphous phase was initially formed at the crystalline face-centered cubic palladium/silicon oxide (Pd/SiO_x) interface, followed by the formation of a Pd_2Si intermetallic compound through irradiation-induced crystallization. The irradiation-induced crystallization can be considered to be stimulated not by defect introduction through the electron knock-on effects and electron-beam heating, but by the electronic excitation mechanism. The observed irradiation-induced structural change at the a(Pd-Si)/SiO_x and Pd/SiO_x interfaces indicates multiple structural modifications at the metal/silicon oxide interfaces through electronic excitation induced by the electron-beam processes.
[en] Memory of texture in hydrogenation–disproportionation–desorption–recombination (HDDR) processed NdFeB magnets was directly observed using electron backscatter diffraction (EBSD). By precise control of dynamic HDDR processing conditions, shape changes of samples were minimized by proper modifications of dimensions of samples subjected to HDDR treatment. EBSD analysis shows the preservation of texture before and after the HDDR process and biaxial texture in initial Nd_2Fe_1_4B grains after the dynamic HDDR treatment at a hydrogen pressure of 30 kPa. It is clearly demonstrated that the c-axis of initial Nd_2Fe_1_4B matches the c-axis of recombined Nd_2Fe_1_4B and it recovers after an anisotropic HDDR process.
[en] The interfacial microstructure of Sn-37Pb solder with immersion Au/electroless Ni-P under bump metallization (UBM) was studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A Ni3Sn4 intermetallic compound (IMC) layer was formed at the interface between the solder and Ni-P UBM upon reflow. However, after isothermal aging, AuSn4 containing a certain amount of Ni dissolved in it, i.e. (Au,Ni)Sn4, appeared above the Ni3Sn4 layer. Two distinctive layers, P-rich and Ni-Sn-P, were additionally found from the TEM observation. Analytical studies using energy dispersive spectrometer (EDS) equipped in TEM revealed that the averaged composition of the P-rich layer is more close to that of a mixture of Ni3P and Ni, while that of the Ni-Sn-P layer is analogous to the P-rich layer but containing a small amount of Sn in it. The growth of the Ni3Sn4 IMC layer thickness obeyed a parabolic growth law which implies that the interfacial IMC growth is mainly controlled by volume diffusion of the elements to the reaction layer. The shear force of the joints decreased during isothermal aging at 373 K. The decrease of the shear force could be mainly due to the coarsening effect of the microstructure within the solder. The failure behavior could be well explained with the results of finite element modeling (FEM) analyses
[en] Ferromagnetic Mn-Al alloy powders were fabricated by mechanical milling and heat treatment with gas-atomized powders. Different processes, i.e., heat treatment before ball milling and ball milling before heat treatment, result in different microstructures and magnetic properties of the powders. It was found that Hc increased and Mr decreased with the size reduction regardless of the sequence of heat treatment and ball milling. However, tendency of the change in Hc and Mr depended on the sequence. Further annealing of the powders ball-milled after heat treatment resulted in slight decrease of Hc and large increase of Mr. The magnetic properties, Mr = 41.2 emu/g, Hc = 3.1 kOe, were obtained from the powders ball-milled for 5 h after heat treatment at 650 deg. C for 20 min, and subsequent annealing at 280 deg. C for 20 min.
[en] The microstructural evolution of Nd–Fe–B magnets during a hydrogenation–disproportionation–desorption–recombination (HDDR) process was investigated, with particular focus on the effects of the desorption–recombination (DR) stage of the process. Samples that went through the DR process under different conditions were compared to examine the texture development during the reaction. Even though the same hydrogenation–disproportionation (HD) treatment was carried out on all samples before the DR reaction, variations in conditions of the latter significantly affected the development of texture in the samples. In consideration of the microstructural evolution, magnetic properties, and thermodynamics, nucleation of recombined Nd_2Fe_1_4B grains was found to occur not only at the NdH_2/Fe_2B interfaces but also at the NdH_2/α-Fe interfaces, and it was affected by the desorption of hydrogen. Preferential growth of nuclei at the NdH_2/Fe_2B interfaces, which led to a highly textured Nd_2Fe_1_4B phase, could be induced by slow desorption and recombination with a low driving force. Hydrogen desorption at a slower rate was important for achieving high magnetic anisotropy in the HDDR-processed Nd–Fe–B powders. - Highlights: • The DR condition significantly affected magnetic anisotropy of HDDR powder. • Mechanism of texture development during the DR stage was suggested. • Recombined Nd_2Fe_1_4B could be nucleated at both NdH_2/Fe_2B and NdH_2/α-Fe interfaces. • Hydrogen pressure during DR reaction affects the preference of nucleation site. • Slow DR process is important for high magnetic anisotropy.