No abstract

We report on the existence of MoS2 hollowed microtubes, several mm in lengths and less than 0.1 μm wall thickness, grown from the vapor phase. Scanning electron microscopy studies reveal that instability of thin weakly bonded sheets against folding causes the tube growth directly or indirectly beyond the formation of turbulent gas flow. Electron diffraction of a single tube wall proves a parallel growth mode of successive layers while the crystal lattices of both tube walls are mutually rotated. A spiral growth mode is proposed.

A systematic study of texturing mechanisms in pure Ag is presented, the goal being to obtain a cube textured {100}001 tape that can be used as a substrate for superconducting coated conductors. It is shown that a sharp cube texture can be produced after pre-heated deformation and recrystallization. Optimal parameters are detailed. 99.99% pure Ag powder is used as starting material. Fine grained (15 µm) Ag ingots are pre-heated at 100 °C before standard cold rolling. The effect of pre-heating on deformation texture is to change the main texture from the brass orientation {110}112 to the Goss {110}001 one. The presence of a copper-type texture with relatively high values of the ODF (oriented distribution function) after the 100 °C pre-heated deformation can be understood as a slight increase of the stacking fault energy of Ag during the pre-heated deformation. The optimal annealing is achieved at 700 °C for 30 min in a primary vacuum. This leads to a sharp cube texture. The FWHM values of the three x-ray pole figures are, at no more than 10°, the smallest ever reported for cube textured Ag tapes. The formation of cube texture is interpreted in terms of orientation nucleation and orientation growth theories that play a common role during the texturing process. The stability of cube texture at high temperature up to 900 °C indicates that the tapes are suitable for epitaxial growth of superconducting oxides.

Recently it was found that instability of thin weakly bonded MoS 2 sheets against folding can lead to a growth of hollowed microtubes with several mm lengths, 1–20 μm diameter and less than 0.1 μm wall thickness.1 Bent sheets can directly roll up into the tube or they can produce in their microfolds a turbulent gas flow of transported molecules which promotes a tubelike growth mode. These uncommon stable features of inorganic crystals reveal a great similarity to much smaller carbon fullerences and especially to inorganic ( MoS 2, WS 2) nested fullereness-nanotubes.2 Scanning electron microscopic studies of cylindrical microtubes and twisted microribbons are combined with high resolution electron microscopic observations of stacking faults.

Texture development in pure Ag was investigated in order to obtain {110}⟨uvw⟩ textured Ag ribbons that can be used as a substrate for YBa2Cu3O7 superconducting coated tapes without any buffer layer. The starting material was 99.95% pure commercial Ag foils. A 20% deformation reduction was used at each step of the cold rolling process after which an optimal annealing was achieved at 800 °C for 4 h in a primary vacuum. This process leads to large-grain ribbons with the {110}⟨112⟩ orientation. A sharp and pure texture is obtained with a FWHM smaller than 6° in the three x-ray pole figures that were used to calculate the orientation distribution function. Crystallographic orientation maps achieved by electron back-scattering diffraction show a misorientation from the normal direction of less than 4°. A twinning mechanism is used to explain the formation of the {110}⟨112⟩ texture. The stability of the {110}⟨112⟩ texture is confirmed by further annealings up to 900 °C, a practical temperature for coated tapes preparation. Lengths up to 25 cm with a uniform {110}⟨112⟩ texture have been obtained.