Microstructural evolution and hardening phenomenon caused by aging of AlSi10Mg alloy by laser powder bed fusion.

Microstructures and age-hardening phenomena of directly aged (artificial aged) AlSi10Mg alloys fabricated by laser powder bed fusion (LPBF) were characterized using scanning transmission electron microscopy, atom probe tomography, and Vickers hardness testing. The microstructure derived from overlapping melt pools has a full cellular structure consisting of eutectic Si walls surrounding α-Al cells. In the initial stage of aging, solute clusters with density on the order of 1024/m3 were formed in α-Al cells. By prolonging the aging time further, fine Si particles of about 50 nm in diameter precipitated. Before Si precipitation, the hardness of the aged sample was clearly greater than that of the as-built state. With further aging time, the hardness increased further because of the Si precipitation. Cluster analysis revealed that the number density and the size of clusters increased from as-built state by aging, whereas the types of the solute clusters remained almost unchanged by aging. The results indicate that the nanoscale clusters within the α-Al cells, which increase via aging, produce age-hardening effect.

Synthesis of a Mesoporous SnO2 Catalyst Support and the Effect of Its Pore Size on the Performance of Polymer Electrolyte Fuel Cells.

The aim of this study was to clarify the effectiveness and challenges of applying mesoporous tin oxide (SnO2)-based supports for Pt catalysts in the cathodes of polymer electrolyte fuel cells (PEFCs) to simultaneously achieve high performance and high durability. Recently, the focus of PEFC application in automobiles has shifted to heavy-duty vehicles (HDVs), which require high durability, high energy-conversion efficiency, and high power density. It has been reported that employing mesoporous carbon supports improves the initial performance by mitigating catalyst poisoning caused by sulfonic acid groups of the ionomer as well as by reducing the oxygen transport resistance through the Pt/ionomer interface. However, carbon materials in the cathode can degrade oxidatively during long-term operation, and more stable materials are desired. In this study, we synthesized connected mesoporous Sb-doped tin oxides (CMSbTOs) with controlled mesopore sizes in the range of 4-11 nm and tested their performance and durability as cathode catalyst supports. The CMSbTO supports exhibited higher fuel cell performance at a pore size of 7.3 nm than the solid-core SnO2-based, solid-core carbon, and mesoporous carbon supports under dry conditions, which can be attributed to the mitigation of the formation of the Pt/ionomer interface and the better proton conductivity within the mesopores even at the low-humidity conditions. In addition, the CMSbTO supports exhibited high durability under oxidative conditions. These results demonstrate the promising applicability of mesoporous tin oxide supports in PEFCs for HDVs. The remaining challenges, including the requirements for improving performance under wet conditions and stability under reductive conditions, are also discussed.

Model-based deconvolution for particle analysis applied to a through-focus series of HAADF-STEM images.

This paper presents an approach for determining the sizes and three-dimensional (3D) positions of nanoparticles from a through-focus series of high-angle annular dark-field scanning transmission electron microscopy images. By assuming spherical particles with uniform density, the sizes and 3D positions can be derived via Wiener deconvolution using a series of kernels prepared by the convolution of the 3D point spread function of the electron beam and the 3D density distribution of spheres with different radii. This process is referred to as a model-based deconvolution. Four 3D datasets with a volume size of 148 × 148 × 560 nm3 were obtained from the four sets of 256 high-angle annular dark-field scanning transmission electron microscopy images of 256 × 256 pixels taken from the same field of view under the through-focus condition. The 3D positions and radii of 14 particles in each 3D dataset were derived using the model-based deconvolution for ∼8 min. The observation errors of the 3D position were estimated as σx ≅ σy ≅ 0.3 nm and σz < 1.6 nm.

A practical method for determining film thickness using X-ray absorption spectroscopy in total electron yield mode.

Thin films formed on surfaces have a large impact on the properties of materials and devices. In this study, a method is proposed using X-ray absorption spectroscopy to derive the film thickness of a thin film formed on a substrate using the spectral separation and logarithmic equation, which is a modified version of the formula used in electron spectroscopy. In the equation, the decay length in X-ray absorption spectroscopy is longer than in electron spectroscopy due to a cascade of inelastic scattering of electrons generated in a solid. The modification factor, representing a multiple of the decay length, was experimentally determined using oxidized Si and Cu with films of thickness 19 nm and 39 nm, respectively. The validity of the proposed method was verified, and the results indicated that the method can be used in the analysis of various materials with thin films.

Electrochemical CO2 reduction over nanoparticles derived from an oxidized Cu-Ni intermetallic alloy.

Oxide-derived Cu-Ni (3-32 at%-Ni) alloy nanoparticles with a size of 10 nm enhance selectivity for ethylene and ethanol formation over oxide-derived Cu nanoparticles by electrochemical CO2 reduction. X-ray absorption spectroscopy measurements suggest that Ni (generally recognized as an element to avoid) is in a mixed phase of oxidized and metallic states.

Inelastic mean free path measurement by STEM-EELS technique using needle-shaped specimen.

Inelastic mean free path (IMFP) was determined by electron energy loss spectroscopy (EELS) to estimate the accurate thickness of TEM thin foil using needle-shaped specimen. From EELS measurements performed for 99.99% Al, Si wafer and 99.99% Fe, linear relationships were confirmed between the thickness of the TEM thin foil and the ratio of the total intensity of EELS spectrum to the total intensity of zero-loss spectrum for all samples. By weighted least-square fitting, the IMFP was estimated to be 143-150 nm for Al, 159-165 nm for Si with amorphous layer, and 92-94 nm for Fe with the collection semi-angle ß = 11.9 - 35.7 mrad, and accelerated voltage of 200 kV. Thus, the dependence of IMFP on ß is not dominant. The accuracy depends on the roundness of the cross-section of the needle-shaped specimen, and is observed to be low in terms of percentage in this work.

Photoelectrochemical water-splitting over a surface modified p-type Cr2O3 photocathode.

Cr2O3 is a p-type semiconductor with a negative conduction band minimum position suitable for photocathodic H2 generation. Therefore, Cr2O3 is a candidate photocathode material for photoelectrochemical (PEC) water-splitting. However, Cr2O3 has not yet been applied for the purpose of H2 generation because the efficiency and stability of the photocurrent generated by a Cr2O3 electrode are poor, due to high defect and vacancy concentrations. In the present work, the Cr2O3 surface was modified with n-type TiO2 after which Pt particles were deposited to catalyse H2 production. The TiO2 overlayer passivated the Cr2O3 surface states that otherwise cause deleterious interactions with the Pt particles. This layer also improved charge separation from the conduction band of Cr2O3 to the Pt co-catalyst, by forming a p-n junction. As a result of the TiO2 insertion, the cathodic photocurrent resulting from light absorption by Cr2O3 was enhanced and stabilized. This represents the first-ever use of Cr2O3 as a light-absorbing material in a multi-layered electrode to accomplish PEC water-splitting for H2 generation.

Solute clustering and supersaturated solid solution of AlSi10Mg alloy fabricated by selective laser melting.

α-Al grains surrounded by Al-Si eutectic as the substructures in AlSi10Mg alloy fabricated by selective laser melting (SLM) were investigated in detail using three-dimensional atom-probe and transmission electron microscopy. Quantitative analysis of α-Al revealed (1) the formation of Mg clusters and Mg-Si co-clusters with a density in the order of 1023 /m3 in α-Al of the as-built state caused by the complicated thermal history, and (2) a supersaturated Si content that significantly exceeded the maximum solubility due to rapid solidification, during the SLM process.