Keh-Chang Chen

D.c. diode ion nitriding behavior of titanium and Ti-6Al-4V

What determines the propagation of the surface layer if nitriding parameters or alloy composition were changed? By adjusting nitriding temperature and nitrogen partial pressure, the surface layers grown on titanium and Ti-6Al-4V were characterized in this study to reveal the propagation of each layer. Ion nitriding was carried out in a d.c. glow discharge. The nitrided specimens were characterized by a number of techniques including optical microscopy, scanning electron microscopy, wavelength dispersive spectroscopy and X-ray diffraction. Surface microhardness measurements were conducted in order to evaluate the effect of the processing parameters on the surface layers. The results show that the aluminum segregated layer forming between the compound layer and diffusion layer hinders the diffusion of nitrogen toward the core of Ti-6Al-4V and reduces the growth rate of the compound layer without affecting diffusion layer thickness. The change in plasma conditions such as H2 gas concentration can only have the effect on a nitrided compound layer. The most efficient ionization occurs at the H2 gas concentration of 17.5% in our case. The activation energy measured is lower than those obtained at higher nitriding temperature. This is believed to be the result of the changes in diffusion path that are affected by the compound layer texture and internal stress.

Using Micro-Arc Oxidation and Alkali Etching to Produce a Nanoporous TiO2 Layer on Titanium Foil for Flexible Dye-Sensitized Solar Cell Application

To increase the specific surface area of a TiO2 layer synthesized by micro-arc oxidation (MAO), an alkali etching process was developed to form a nanoflaky structure in place of the existing microporous morphology of the MAO-TiO2 layer for dye-sensitized solar cell (DSSC) electrode application. An annealing treatment was also carried out to enhance the crystallinity of the nanofeatured TiO2 layer for a higher photovoltaic efficiency. Experimental results show that a 6-µm-thick crystalline porous TiO2 layer was fabricated on a Ti foil by MAO treatment, which consists of major amorphous and anatase phases with a minor rutile phase. As expected, the pores in the MAO-TiO2 layer exhibited micrometer-scale dimensions. The maximum photovoltaic efficiency realized in a device assembled with the MAO-TiO2 layer was only 0.061%. After alkali etching, a nanofeatured layer was developed over the MAO-TiO2 layer surface with numerous pores and nanoflakes of 50 nm size. These nanoflakes were uniformly distributed over the entire surface of the treated layer. The device assembled with the alkali-etched TiO2 layer exhibited an improved photovoltaic efficiency of 0.329%. This fivefold increase of the photovoltaic efficiency for the MAO-TiO2 layer indicates the effectiveness of enlarging the specific surface area by alkali etching. Furthermore, after postannealing, the crystallinity and fraction of the anatase phase in the overall TiO2 layer were enhanced. As a result, the photovoltaic efficiency ultimately reached 2.194%.

Elastic interaction between screw dislocations and a circular surface crack

Abstract The behaviour of a screw dislocation around a circular surface crack was analyzed using the conformal mapping method. The effects of the crack length l and its radius of curvature R on the shielding effect and the strain energy were discussed in detail. It was found that for a dislocation being fixed at a constant distance from the crack tip and located on a plane tangent to crack surface at the tip, there exists a critical crack length l * corresponding to a maximum shielding effect induced by the dislocation on the circular surface crack. The l * increases with R and approaches infinity as the crack becomes planar (i.e. R → ∞). The shielding effect vanishes as the crack disappears. It increases rapidly with l if 0 l ≪ l *, regardless of R . After reaching the maximum, it decreases slightly for larger R but significantly for smaller R as the crack becomes longer. The shielding is more pronounced for larger R than that for smaller R , with the difference increasing with crack length. As a result, a surface microcrack propagates initially in a rather brittle manner then becomes more ductile. When the crack gets longer, it still keeps ductile for a less curved or a planar crack but becomes relatively brittle for a severely curved crack. In addition, the strain energy is also significantly influenced by crack length and increases rapidly with l when l is rather small, regardless of R . With increasing l , it become less affected by l , and the variation with R is still not obvious, with larger R corresponding to a slightly higher strain energy.