Abstract

The synergistic action of local elemental distribution, and in particular Re doping, with interfacial dislocations at the γ/γ′ interface is still one of the most considered and unclear issues during creep of nickel-based single-crystal superalloys. In order to investigate this problem, a detailed characterization of interfacial dislocations in a DD6 superalloy after creep loading for 12 h at high temperature and low stress was carried out using transmission electron microscopy and high-angle annular dark field scanning transmission electron microscopy techniques. In addition, the local elemental distribution near dislocation core regions was determined by energy dispersive X-ray spectroscopy (EDS) mapping. It was found for the first time that three types of interfacial protrusions are formed at the γ/γ′ interface after creep loading for 12 h under conditions of high temperature and low stress and demonstrated that the formation of these features originates from dislocation motion. Additionally, EDS mapping provides evidence for co-segregation of Re with Cr and Co at the tip of the protrusions. Based on this, a model concerning dislocation core structure and dislocation climb was proposed to explain the different morphology of the protrusions. The observations highlight the importance of the coupling between Re segregation and γ/γ′ interfacial dislocations for improving creep properties in nickel-based superalloys. The results of the study will be beneficial for the design of new high-temperature materials and for understanding the origin of the effect of Re additions in nickel-based single-crystal superalloys.

Abstract

Bulk tip-shape ZnO nanowires (TSZONWs) have been synthesized via thermal evaporation method. The carbon nanotubes have been found to play an important role for the synthesis of this structure. The sensing capabilities of the structures have been investigated through electrochemical measurements. The photoluminescence spectra exhibit strong visible emission band due to an increase amount of defects, which leads to a significant role in the sensing applications. The electrochemical measurements prove that the TSZONWs are very sensitive to the pH of the PB solution as well as the bio-chemical species. Our investigation demonstrates that the arrays of TSZONWs can be employed for various applications.

A templated catalytic etching process has been developed to fabricate large-area arrays of silicon nanowires with controlled diameter, length, and density. The figure shows an example of an array constructed by this technique. Etched polystyrene spheres are used as templates to define the lateral dimensions of the array, whereas the length of the nanowires is defined by the duration of the etching process.

A one-dimensional heterojunction is fabricated and characterized. This heterojunction comprises a Ni nanowire, a multiwalled carbon nanotube (MWCNT), and an amorphous carbon nanotube (a-CNT). The three components are in an end-to-end configuration, and form two MWCNT contacts, namely a Ni/MWCNT and an MWCNT/a-CNT contact. The interfacial structures of the two contacts show that multiple outer walls in the MWCNT simultaneously contact the Ni nanowire and the a-CNT, and can simultaneously participate in electrical transport. By investigating the electrical-transport properties of the heterojunctions, the two contacts to the MWCNT in every heterojunction are found to behave as two diodes connected in series face-to-face, at least one of which exhibits the characteristics of a nearly ideal Schottky diode and obeys thermionic-emission theory, wherein only the image force lowers the Schottky barrier. The appearance of this type of nearly ideal diode is attributed to the good contacts to the multiple outer walls of the MWCNTs realized by the heterojunctions' structures.

A novel strategy for preparing large-area, oriented silicon nanowire (SiNW) arrays on silicon substrates at near room temperature by localized chemical etching is presented. The strategy is based on metal-induced (either by Ag or Au) excessive local oxidation and dissolution of a silicon substrate in an aqueous fluoride solution. The density and size of the as-prepared SiNWs depend on the distribution of the patterned metal particles on the silicon surface. High-density metal particles facilitate the formation of silicon nanowires. Well-separated, straight nanoholes are dug along the Si block when metal particles are well dispersed with a large space between them. The etching technique is weakly dependent on the orientation and doping type of the silicon wafer. Therefore, SiNWs with desired axial crystallographic orientations and doping characteristics are readily obtained. Detailed scanning electron microscopy observations reveal the formation process of the silicon nanowires, and a reasonable mechanism is proposed on the basis of the electrochemistry of silicon and the experimental results.

A straightforward metal-particle-induced, highly localized site-specific corrosion-like mechanism was proposed for the formation of aligned silicon-nanowire arrays on silicon in aqueous HF/AgNO₃ solution on the basis of convincing experimental results. The etching process features weak dependence on the doping of the silicon wafers and, thus, provides an efficient method to prepare silicon nanowires with desirable doping characteristics. The novel electrochemical properties between silicon and active noble metals should be useful for preparing novel silicon nanostructures and also new optoelectronic devices.

Neatly scratching the surface: A facile etching technique assisted by a silver-nanoparticle network to produce large-area 1D silicon nanostructure arrays with desired orientation and doping characteristics is demonstrated (see picture). A mechanism for the highly selective etching is proposed on the basis of experimental evidence.

Feine Kratzer auf der Oberfläche: Eine einfache Ätztechnik, die von einem Ag-Nanopartikelnetzwerk unterstützt wird und großflächige 1D-Anordnungen von Siliciumnanostrukturen mit der gewünschten Orientierung und Dotierungscharakteristik liefert, wird vorgestellt (siehe Bild). Aus den experimentellen Befunden wird ein Mechanismus für das hoch selektive Ätzen abgeleitet.

Arrays of heterojunctions of Ag nanowires and amorphous carbon nanotubes have been synthesized. These heterojunctions have ohmic current–voltage characteristics based on the general characteristics of electronic structures of amorphous semiconductors and may provide a wide base for the design of various ohmic contacts in nanodevices. The Figure shows atomic force microscopy characterizations of a nanowire–nanotube array.

This article concerns the detailed investigations on the silver dendrite-assisted growth of single-crystalline silicon nanowires, and their possible self-assembling nanoelectrochemistry growth mechanism. The growth of silicon nanowires was carried out through an electroless metal deposition process in a conventional autoclave containing aqueous HF and AgNO₃ solution near room temperature. In order to explore the mechanism and prove the centrality of silver dendrites in the growth of silicon nanowires, other etching solution systems with different metal species were also investigated in this work. The morphology of etched silicon substrates strongly depends upon the composition of the etching solution, especially the metal species. Our experimental results prove that the simultaneous formation of silver dendrites is a guarantee of the preservation of free-standing nanoscale electrolytic cells on the silicon substrate, and also assists in the final formation of silicon nanowire arrays on the substrate surface.