Bai An

Surface oxidation of a Nb(100) single crystal by scanning tunneling microscopy

Abstract The surface structures of a Nb(100) single crystal in the initial stages of oxidation have been investigated by ultrahigh vacuum (UHV) scanning tunneling microscopy (STM). A ladder-like (3×10) structure was observed on the surface after numerous cycles of Ar-ion sputtering and flash annealing at 1973 K. The ladder-like structure could be identified as a NbO 2 overlayer on a Nb(100) surface due to the segregation of oxygen on the surface. The ladder-like structure gradually changed to a disordered structure upon exposure to oxygen at room temperature and was restored by annealing at above 853 K in UHV. The oxidation process on the Nb(100) surface is discussed.

Pore structure analysis of activated carbon fiber by microdomain-based model

The pore structures of commercial pitch and PAN-based activated carbon fibers (ACFs) were investigated. The pore size and pore size distribution of pitch-based ACFs were measured by nitrogen adsorption isotherms and 129Xe NMR spectroscopy and compared with each other. Scanning tunneling microscopy showed that the ACFs were composed of spherical microdomain units the size of a few nanometers. The activation mechanism of ACFs was considered and explained by novel hypothesis; the concept of microdomain structure of ACFs was considered and explained to overcome limitation of the conventional fractal hypothesis. Whereas micropores were generated on each microdomain, the origin of mesopores was interdomain pores, resulting from the microdomain hypothesis.

Surface Superstructure of Carbon Nanotubes on Highly Oriented Pyrolytic Graphite Annealed at Elevated Temperatures

Carbon nanotubes deposited on highly oriented pyrolytic graphite (HOPG) are annealed in ultra high vacuum. The effect of annealing temperature on the surface morphology of the carbon nanotubes on HOPG is examined by scanning tunneling microscopy. The ring-like surface superstructure of (√ 3×√ 3)R30° of graphite is found on the carbon nanotubes annealed above 1593 K. The tips of the carbon nanotubes are destroyed and the stacking misarrangement between the upper and the lower walls of the tube join with HOPG resulting in the superstructure.

Single pentagon in a hexagonal carbon lattice revealed by scanning tunneling microscopy

The electronic structure of a single pentagon in a hexagonal carbon lattice has been revealed on an atomic scale by scanning tunneling microscopy. The pentagon is located at the apex of the conical protuberance of the graphitic particle. The enhanced charge density localized at each carbon atom in the pentagon is identified, and the ringlike pattern of the (∛×∛)R30° superstructure of graphite is clearly observed around the pentagon.

Hydrogen Effects on Localized Plasticity in SUS310S Stainless Steel Investigated by Nanoindentation and Atomic Force Microscopy

The effect of hydrogen on the localized plasticity of an industrial material, SUS310S stainless steel, has been investigated using a combination of nanoindentation and atomic force microscopy (AFM). The load versus displacement curves show that 73.4 wt.ppm hydrogen in the SUS310S sample causes a 41% decrease in the first excursion load but has little effect on the first excursion depth, which indicates that hydrogen enhances dislocation nucleation and promotes dislocation emission and multiplication. The AFM images of the pile-ups around the indentation indicate that hydrogen causes two types of slip step in the {111} slip planes, indicating that hydrogen promotes slip planarity and localized plasticity.

Surface Structure on Ar+-Ion Irradiated Graphite by Scanning Probe Microscopy

The surface structure of a highly oriented pyrolytic graphite (HOPG), irradiated by Ar+ ions with an ion energy of 0.5–1.0 keV at doses below 5×1011 ions/cm2 during annealing, was characterized by scanning probe microscopy. The ion-induced hillocks were observed by both scanning tunneling microscopy (STM) and atomic force microscopy (AFM) after the ion irradiation, the heights of which, measured by STM, were larger than that measured by AFM in the tapping mode. The hillocks were recovered distinguishably by annealing above 470 K. Almost 85% of the hillocks disappeared after annealing at 1270 K and they disappeared completely after annealing above 1770 K. The behavior of defects produced by ion-irradiation in HOPG during annealing is discussed.

Surface superstructure of Ar+-bombarded highly oriented pyrolytic graphite during recrystallization

The recrystallization process of highly oriented pyrolytic graphite (HOPG) surface bombarded by argon ions with an ion energy of 2.0–2.5 keV at doses of 2–5×1016 ions/cm2 was examined by scanning tunneling microscopy. The hillocks formed by the ion bombardment of HOPG coalesced to form larger grains with increasing annealing temperature. Three types of ringlike superstructures of (3×3)R30° of graphite were found on the curved surface of the grains after annealing in the temperature range of 1823–2673 K. The specimen surface became flat above 2773 K, where the ringlike superstructure disappeared and the triangular structure of graphite appeared in its place. The effect of the curved surface of the grains on the formation of the ringlike superstructure during graphitization is discussed.

Surface structure of niobium-dioxide overlayer on niobium(100) identified by scanning tunneling microscopy

The surface structures of an oxide overlayer on Nb(100) single crystal formed by numerous cycles of Ar-ion sputtering and flash annealing at 1973 K in an ultra-high-vacuum chamber have been investigated by scanning tunneling microscopy. Flat terraces with a monatomic step height of Nb(100) are observed; where two orthogonal domains are alternately evident. A ladder-like (3×10) structure of Nb(100) with no long-range order is identified at atomic resolution in the domain on the terrace. The ladder-like (3×10) structure is interpreted to be a modulated NbO2(010) overlayer on the Nb(100) surface and an atomic structural model of the ladder-like (3×10) structure is proposed.

Structure and electrochemical applications of boron-doped graphitized carbon nanofibers

Boron-doped graphitized carbon nanofibers (CNFs) were prepared by optimizing CNFs preparation, surface treatment, graphitization and boron-added graphitization. The interlayer spacing (d₀₀₂) of the boron-doped graphitized CNFs reached 3.356 Å, similar to that of single-crystal graphite. Special platelet CNFs (PCNFs), for which d₀₀₂ is less than 3.400 Å, were selected for further heat treatment. The first heat treatment of PCNFs at 2800 °C yielded a d₀₀₂ between 3.357 and 3.365 Å. Successive nitric acid treatment and a second heat treatment with boric acid reduced d₀₀₂ to 3.356 Å. The resulting boron-doped PCNFs exhibited a high discharge capacity of 338 mAh g⁻¹ between 0 and 0.5 V versus Li/Li⁺ and 368 mAh g⁻¹ between 0 and 1.5 V versus Li/Li⁺. The first-cycle Coulombic efficiency was also enhanced to 71-80%. Such capacity is comparable to that of natural graphite under the same charge/discharge conditions. The boron-doped PCNFs also exhibited improved rate performance with twice the capacity of boron-doped natural graphite at a discharge rate of 5 C.

Micromechanisms of Hydrogen-Assisted Cracking in Super Duplex Stainless Steel Investigated by Scanning Probe Microscopy

Understanding the micromechanisms of hydrogen-assisted fracture in multiphase metals is of great scientific and engineering importance. By using a combination of scanning electron microscopy (SEM), scanning tunneling microscopy (STM), atomic force microscopy (AFM) and magnetic force microscopy (MFM), the micromorphology of fracture surface and microcrack formation in hydrogen-precharged super duplex stainless steel 2507 are characterized from microscale to nanoscale. The results reveal that the fracture surfaces consist of quasi-brittle facets with riverlike patterns at the microscale, which exhibit rough irregular patterns or remarkable quasi-periodic corrugation patterns at the nanoscale that can be correlated with highly localized plastic deformation. The microcracks preferentially initiate and propagate in ferrite phase and are stopped or deflected by the boundaries of the austenite phase. The hydrogen-assisted cracking mechanisms in super duplex stainless steel are discussed according to the experimental results and hydrogen-enhanced localized plasticity theory.Copyright