Growth mechanism of carbon nanotubes from Co-W-C alloy catalyst revealed by atmospheric environmental transmission electron microscopy.

High-melting point alloy catalysts have been reported to be effective for the structure-controlled growth of single-wall carbon nanotubes (SWCNTs). However, some fundamental issues remain unclear because of the complex catalytic growth environment. Here, we directly investigated the active catalytic phase of Co-W-C alloy catalyst, the growth kinetics of CNTs, and their interfacial dynamics using closed-cell environmental transmission electron microscopy at atmospheric pressure. The alloy catalyst was precisely identified as a cubic η-carbide phase that remained unchanged during the whole CNT growth process. Rotations of the catalyst nanoparticles during CNT growth were observed, implying a weak interfacial interaction and undefined orientation dependence for the solid catalyst. Theoretical calculations suggested that the growth kinetics are determined by the diffusion of carbon atoms on the surface of the η-carbide catalyst and through the interface of the catalyst-CNT wall.

First-principle atomistic thermodynamic study on the early-stage corrosion of NiCr alloy under fluoride salt environment.

The atomic morphology change in the NiCr alloy surface induced by fluorine-chemisorption was investigated by the ab initio atomistic thermodynamic method to elucidate early-stage corrosion processes of nickel-based alloys in strong oxidizing environment. The surface phase diagrams of Cr-doped Ni(111) surface as a function of fluorine chemical potential were obtained to track the surface structures that are most likely to be fostered in various temperature and pressure conditions. The adsorption of fluorine on the top site of Cr in the alloy surface was the most energetically favorable one. With increasing fluorine chemical potential, more fluorine atoms started to agglomerate in the trapping sink of Cr. Fluorine-fluorine repulsion interaction coupled with strong F-Cr bonding could facilitate a decided morphology modification of the metal substrate. Moreover, an insight into the desorption pathways for potential species revealed that in the presence of fluorine, the dissociation of Cr predominantly stems from the relatively easy desorption in the form of CrF2/CrF3 molecules from the non-passivated Ni-based alloy surface.

First-Principles Study of Vacancies in Ti3SiC2 and Ti3AlC2.

MAX phase materials have attracted increased attention due to their unique combination of ceramic and metallic properties. In this study, the properties of vacancies in Ti3AlC2 and Ti3SiC2, which are two of the most widely studied MAX phases, were investigated using first-principles calculations. Our calculations indicate that the stabilities of vacancies in Ti3SiC2 and Ti3AlC2 differ greatly from those previously reported for Cr2AlC. The order of the formation energies of vacancies is VTi(a) > VTi(b) > VC > VA for both Ti3SiC2 and Ti3AlC2. Although the diffusion barriers for Ti3SiC2 and Ti3AlC2 are similar (~0.95 eV), the properties of their vacancies are significantly different. Our results show that the vacancy-vacancy interaction is attractive in Ti3AlC2 but repulsive in Ti3SiC2. The introduction of VTi and VC vacancies results in the lattice constant c along the [0001] direction increasing for both Ti3SiC2 and Ti3AlC2. In contrast, the lattice constant c decreases significantly when VA are introduced. The different effect of VA on the lattice constants is explained by enhanced interactions of nearby Ti layers.

Amorphization and Directional Crystallization of Metals Confined in Carbon Nanotubes Investigated by in Situ Transmission Electron Microscopy.

The hollow core of a carbon nanotube (CNT) provides a unique opportunity to explore the physics, chemistry, biology, and metallurgy of different materials confined in such nanospace. Here, we investigate the nonequilibrium metallurgical processes taking place inside CNTs by in situ transmission electron microscopy using CNTs as nanoscale resistively heated crucibles having encapsulated metal nanowires/crystals in their channels. Because of nanometer size of the system and intimate contact between the CNTs and confined metals, an efficient heat transfer and high cooling rates (∼10(13) K/s) were achieved as a result of a flash bias pulse followed by system natural quenching, leading to the formation of disordered amorphous-like structures in iron, cobalt, and gold. An intermediate state between crystalline and amorphous phases was discovered, revealing a memory effect of local short-to-medium range order during these phase transitions. Furthermore, subsequent directional crystallization of an amorphous iron nanowire formed by this method was realized under controlled Joule heating. High-density crystalline defects were generated during crystallization due to a confinement effect from the CNT and severe plastic deformation involved.

Mechanical properties of Si nanowires as revealed by in situ transmission electron microscopy and molecular dynamics simulations.

Deformation and fracture mechanisms of ultrathin Si nanowires (NWs), with diameters of down to ~9 nm, under uniaxial tension and bending were investigated by using in situ transmission electron microscopy and molecular dynamics simulations. It was revealed that the mechanical behavior of Si NWs had been closely related to the wire diameter, loading conditions, and stress states. Under tension, Si NWs deformed elastically until abrupt brittle fracture. The tensile strength showed a clear size dependence, and the greatest strength was up to 11.3 GPa. In contrast, under bending, the Si NWs demonstrated considerable plasticity. Under a bending strain of <14%, they could repeatedly be bent without cracking along with a crystalline-to-amorphous phase transition. Under a larger strain of >20%, the cracks nucleated on the tensed side and propagated from the wire surface, whereas on the compressed side a plastic deformation took place because of dislocation activities and an amorphous transition.

Mechanical properties of bamboo-like boron nitride nanotubes by in situ TEM and MD simulations: strengthening effect of interlocked joint interfaces.

Understanding the influence of interfacial structures on the nanoarchitecture mechanical properties is of particular importance for its mechanical applications. Due to a small size of constituting nanostructural units and a consequently high volume ratio of such interfacial regions, this question becomes crucial for the overall mechanical performance. Boron nitride bamboo-like nanotubes, called hereafter boron nitride nanobamboos (BNNBs), are composed of short BN nanotubular segments with specific interfaces at the bamboo-shaped joints. In this work, the mechanical properties of such structures are investigated by using direct in situ transmission electron microscopy tensile tests and molecular dynamics simulations. The mechanical properties and deformation behaviors are correlated with the interfacial structure under atomic resolution, and a geometry strengthening effect is clearly demonstrated. Due to the interlocked joint interfacial structures and compressive interfacial stresses, the deformation mechanism is switched from an interplanar sliding mode to an in-plane tensile elongation mode. As a result of such a specific geometry strengthening effect, the BNNBs show high tensile fracture strength and Young's modulus up to 8.0 and 225 GPa, respectively.