Highly ordered self-organized TiO2 nanotube arrays prepared by a multi-step anodic oxidation process

Highly ordered TiO₂ nanotube arrays were prepared using a self-templating multi-step anodic oxidation process in a fluoride-containing electrolyte. The microstructures, chemical compositions, and phases of the self-organized TiO₂ nanotube arrays were analyzed by FESEM, XPS, and XRD, respectively. Hexagonal packing density in TiO₂ nanotube arrays significantly improved after the the multi-step anodic oxidation. The area densities of the hexagonal TiO₂ nanotube arrays increased approximately 3 times from the first to second step in the anodic oxidation steps process (4.9 μm⁻² to 16.4 μm⁻²), but there was no difference between the second and third step (16.4 μm⁻² to 16.0 μm⁻²). The as-anodized TiO₂ nanotube array had an amorphous structure and it transformed to an anatase phase during the annealing process at 450 °C for 1 h. The as-anodized TiO₂ nanotube arrays adsorbed the fluoride, hydrocarbon groups (CH), hydroxyl groups (OH, C-OH), and carboxyl groups (O = C-OH) on their surfaces.     

Comments on "Robust iterative learning control design is straightforward for uncertain LTI systems satisfying the robust performance condition"

For original paper see ibid., vol.48, p.101-6 (2003). The article comments on the result of the above paper. The authors of the original paper showed that Theorem 1 is only the sufficient condition of the convergence in the sense of the L/sub 2/-norm. In this note, Theorem 1 is proved to be the not only sufficient but also necessary condition using some mathematical manipulation.     

Consolidation of binderless nanostructured TiC by pulsed current activated sintering and its mechanical properties

A dense nanostructured TiC with a relative density of up to 98% was produced with simultaneous application of 80 MPa pressure and pulsed current of 2800 A using the nanopowder of TiC. The effect of the ball milling times on the sintering behavior, grain size and mechanical properties of binderless TiC was investigated.     

Mechanical synthesis and rapid consolidation of nanocrystalline 3Fe<Subscript>0.67</Subscript>Cr<Subscript>0.33</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite by high frequency induction heating

Nanopowders of 3Fe_0.67Cr_0.33 and Al₂O₃ were synthesized from CrO₃ and 2FeAl powders by high-energy ball milling. A highly dense nanocrystalline 3Fe_0.67Cr_0.33-Al₂O₃ composite was consolidated by a high frequency induction heated sintering (HFIHS) method within three minutes from mechanically synthesized powders of Al₂O₃ and 3Fe_0.67Cr_0.33. The average grain size and mechanical properties of the composite were investigated.     

Properties and rapid low-temperature consolidation of nanocrystalline Fe-ZrO<Subscript>2</Subscript> composite by pulsed current activated sintering

Nanopowders of Fe and ZrO₂ were synthesized from Fe₂O₃ and Zr by high-energy ball milling. The powder sizes of Fe and ZrO₂ were 70 nm and 12 nm, respectively. Highly dense nanostructured 4/3Fe-ZrO₂ composite was consolidated by a pulsed current activated sintering method within 1 minute from the mechanically synthesized powders (Fe-ZrO₂) and horizontal milled Fe₂O₃+Zr powders under the 1 GPa pressure. The grain sizes of Fe and ZrO₂ in the composite were calculated. The average hardness and fracture toughness values of nanostuctured 4/3Fe-ZrO₂ composite were investigated.     

Properties and rapid consolidation of nanocrystalline 8MoSi<Subscript>2</Subscript>-Si<Subscript>3</Subscript>N<Subscript>4</Subscript> by high frequency induction heated sintering

Nanopowders of MoSi₂ and Si₃N₄ were synthesized from Mo₂N and Si by high ball milling. Dense nanostructured 8MoSi₂-Si₃N₄ composite was consolidated by high frequency induction heated sintering (HFIHS) method within two minutes from mechanically synthesized powders of MoSi₂ and Si₃N₄. Highly dense 8MoSi₂-Si₃N₄ composite with a relative density of up to 97 % was produced under simultaneous application of 80 MPa pressure and the induced current. The average grain size and mechanical properties (hardness and fracture toughness) of the composite were investigated.     

Characteristics of dynamic brittle fracture captured with peridynamics

Using a bond-based peridynamic model, we are able to reproduce various characteristics of dynamic brittle fracture observed in experiments; crack branching, crack-path instability, asymmetries of crack paths, successive branching, secondary cracking at right angles from existing crack surfaces, etc. We analyze the source of asymmetry in the crack path in numerical simulations with an isotropic material and symmetric coordinates about the pre-crack line. Asymmetries in the order of terms in computing the nodal forces lead to different round-off errors for symmetric nodes about the pre-crack line. This induces the observed slight asymmetries in the branched crack paths. A dramatically enhanced crack-path instability and asymmetry of the branching pattern are obtained when we use fracture energy values that change with the local damage. The peridynamic model used here captures well the experimentally observed successive branching events and secondary cracking. Secondary cracks form as a direct consequence of wave propagation and reflection from the boundaries.