Erchao Meng

Mg Alloys Development and Surface Modification for Biomedical Application

The development of biodegradable implants has grown into one of the important areas in medical science (Mani et al., 2007), since they can be gradually dissolved, absorbed, consumed or excreted in human body environment, and then disappear spontaneously after the bone tissues heal. The biodegradable materials available in the current market are mainly made of polymeric or ceramic materials, while these implants have an unsatisfactory mechanical strength when used for load-bearing parts (Staiger et al., 2006). Compared with the currently approved biomaterials, Mg alloys have a lot of advantages (Song et al., 2007; Witte et al., 2007; Witte et al., 2005; Song et al., 2007). First, with high strength/weight ratio, Mg alloy exhibits an appropriate mechanical integrity and is more suitable for load-bearing implantation. Its fracture toughness is higher than ceramic biomaterials (e.g. HA), and the elastic modulus and compressive yield strength of magnesium are closer to those of natural bone than other metallic implants (Table 1.1). Thus it will help to reduce or avoid “stress shielding effects” that can lead to reducing stimulation of new bone growth and remodeling. Moreover, magnesium has little toxicity to human body. Magnesium is essential to human metabolism and is naturally found in bone tissue. It is the fourth most abundant cation in the human body, with an estimated 1 mol of magnesium stored in the body of a normal 70 kg adult. Approximately, half of the total physiological Mg is stored in bone tissue. Thirdly,

Shape modification of Si nanowires by using faceted silicide catalysts nucleated in Au-Si catalyst solution during the growth

The shape modification of Si nanowires is demonstrated using faceted solid silicide catalysts. The Si nanowires were grown on Si(111) substrates covered with Au as a catalyst using MnCl2 and Si powders as source materials. The solid silicide catalysts were nucleated and formed in the Au-Si catalyst solution at the top of the nanowires during the growth. The faceted solid silicides grew larger with increased growth time and played a role as a solid catalyst. The faceted shape of the catalyst defines the shape of the faceted Si nanowire. The squared Si nanowires were grown with the growth direction of Si[111] and the sidewalls of {110} and {211} planes. The growth evolution of the faceted Si nanowires occurs by a vapor-liquid-solid mechanism followed by the silicide vapor-solid-solid mechanism.

Synthesis and structural control of silicon and silicide nanowires/microrods using metal chloride sources

Si and silicide nanowires/microrods were synthesized using metal-chloride-based sources by the CVD technique. In the case of Si nanowire synthesis, the morphological structures of the products depended on the distance from the source material. The distance dependence of the morphological and structural properties of the nanostructures was investigated to systemically clarify the shape modification phenomena in Si nanowire synthesis. The modification of the cross-sectional shape of the Si nanowires/microrods was successfully demonstrated. The triangular nanowire has a sawtooth faceting structure on its sidewall. In addition, the synthesis of Mn-silicide phases over the entire range of the Mn–Si system was examined. The effect of adding an impurity to the source materials on the structural modifications of the resulting nanowires/microrods was also investigated. It is expected that the morphological and structural control of the nanowires/microrods will be improved by a simple thermal treatment using a metal chloride as the source material.