Materials Science Forum Vols. 654-656

Abstract: The electrical properties of single ZnO nanowire were researched in the chamber of a scanning electron microscope under high-vacuum conditions using nanomanipulator and measurement system. The result shows that ZnO nanowire resistivity was about 1.4 Ω•cm with Ohmic contact. The local change of electron density induced by Shottky contacts or Ohmic contact with tip and semiconductor/metal materials significantly affects the current transport through the nanowire. Single ZnO nanowire was configured as field effect transistors (FET) and based on metal tantalum (Ta) as electrodes show a pronounced n-type gate modulation with an electron concentration of ~1.0×1019 cm−3 and an electron mobility of ~52 cm2 /V s at a bias voltage of 1 V.

Abstract: Single crystal silicon (sc-Si) wafers are widely used as the precursors to prepare silicon nanowires (SiNWs) by employing a silver-assisted chemical etching process. In this work, we obtained SiNWs arrays using multicrystalline silicon (mc-Si) wafers. Firstly, silver nanoparticles were deposited on the textured solar-grade mc-Si wafer by a galvanic displacement process; secondly, the SiNWs arrays were formed by a silver-assisted chemical etching process conducted in a HF-H2O2 aqueous solution. The etching process indicated that the growth of SiNWs is independent on the orientation of the Si wafer. TEM images showed that the SiNWs have rough and nanoporous structures on the top side along axial directions. The photoluminescence (PL) spectrum of SiNWs showed a broad visible emission centred around 700 nm, which is attributed to the emission properties of silicon nanocrystallites in SiNWs. This work may contribute to the development of SiNWs in application including optoelectronic devices, solar energy conversion devices, chemical sensors, and lithium secondary batteries, etc.

Abstract: Three different kinds of morphologies including spherical, chainlike and wirelike cobalt nanopowders, have been synthesized by chemical reduction of coblat chloride solution with hydrazine hydrate in basic solution. The products were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). It was revealed that the morphologies of the nanopowders depend on the concentration of Co2+ and the way of adding reducing agent (hydrazine hydrate). These two features affected the nucleation sites and the number of nucleuses. When the concentration of Co2+ was low, nucleuses formed in the soft template, while the reducing of the reducing agent added drop wise, a little number of nucleuses was formed. Based on that, a mechanism of formation, as a basis of gram-scale syntheses, was proposed.

Abstract: Nanowires and nanofilms are fundamental building blocks of micro and nano-electronics for both of bottom-up and top-down technologies. Monitoring and recording the mechanical property dynamics at atomic scale are important to understand the atomic mechanism of new and surprising nano-phenomena and design new applications. Through years’ endeavors, we developed tensile and/or bending in-situ atomic-lattice resolution electron microscopy methods and equipments for nanowires and successfully conducted atomic-lattice resolution mechanical tests on individual nano-objects. With this, we observed the brittle materials SiC and Si nanowires (NWs) become highly ductile at room temperature. The crystalline structural evolution processes corresponding to the occurrence of unusual large strain plasticity includes the dislocation initiation, dislocation accumulation and amorphorization as well as the necking of the one dimensional nanowires were fully recorded at atomic scale and in real time. We also expand the experimental methods and equipments to two-dimensional nanofilms. An example of tensile experiment on nano-crystalline Au films is presented. The deformation mechanisms of nano-crystalline gold films were observed at the atomic scale and real-time. At the mean time, an atomic scale the crack blunting behavior was captured and the plastic deformation mechanism of the single nano-crystalline was revealed.

Abstract: In this study, a well-textured AZ31 Mg alloy sheet was friction stir (FS) processed, and the microstructure and texture evolution in various regions of the processed alloy were examined by optical microscopy (OM) and electron back scatter diffraction (EBSD). The results showed that the grain size in the FS zone was significantly refined compared to that in the base material (BM). The average grain size in the thermomechanically affected zone (TMAZ) and heat-affected zone (HAZ) was comparable with that in the BM. There is a gradual change of local texture from BM to FS zone due to plastic flow together with heating input during the FS processing. The <0002> direction was roughly parallel to the cylindrical pin surface normal of the FS zone. The <0002> direction in the HAZ is similar to that in the BM, but more diffuse. The <0002> direction in the TMAZ was tilted 25~30o away from the ND and there is a distinct boundary between the FS zone and TMAZ at the advancing side which introduced an incompatibility between the FS zone and TMAZ. This might explain the fact that the transverse FS processed Mg alloys commonly fracture at the advancing side during tensile tests.

Abstract: It was confirmed in the present study that deep drawing process in multi passes to produce fine wires of copper alloys can act as a kind of severe plastic deformation process. High purity copper was deeply wire-drawn up to an equivalent strain of 6.9, and microstructure evolution and change in mechanical properties were investigated. It was confirmed that deep wire-drawing process produced nanostructures composed of fiber-shaped ultrafine grains with a diameter of about 300 nm. The copper wires having nanostructures showed tensile strength of 480MPa, which was two times higher than that of the starting material. The microstructures and mechanical properties of the deep drawn copper wire were almost equivalent with those of the copper sheets severely deformed by ARB process, though there were some minor differences between two processes.

Abstract: Among the various severe plastic deformation (SPD) processes, high pressure torsion (HPT) has several unique characteristics. These are applicability of very large strain and deformation under high pressure. Due to these abilities of HPT, several unique phenomena have been observed. In the present paper, three topics were reviewed; 1) work-softening in pure Cu, 2) high pressure phase formation in pure Ti and 3) synthesis of Cu-NbC composite. Work softening in pure Cu was observed when low strain rate and high pressure were applied. In Ti high pressure ω phase is obtained after unloading only when the deformation at high pressure was applied. The volume fraction of ω phase increased with the increase in the amount of strain. In pure Fe, high pressure ε phase was not retained at ambient pressure. The bulk Cu-NbC composite was synthesized starting from elemental powders. This demonstrates that HPT is an efficient tool for mechanical alloying and cold consolidation.

Abstract: Commercially pure cast aluminium was subjected to equal channel angular pressing (ECAP) at room temperature using routes A, Bc and C. Microhardness distribution maps were produced on sections of extruded billets after one, two, three and four passes for each of the processing routes. It was found that the mean hardness increased significantly already after the first pass. With subsequent passes, the hardness increase was smaller but the hardness distribution became narrower, indicating increasing homogeneity. For route Bc, a slight decrease in average hardness was observed after the fourth pass. The mean hardness after four passes was highest for the route C sample, followed by the route A and route Bc samples.

Abstract: Equal channel angular extrusion (ECAE) experiments were conducted on as-cast pure Cu, Cu–10%Zn, and Cu–30%Zn, using a 90 die for up to 4 passes. The stacking fault energies (SFEs) of the three materials decrease with increasing Zn content. The microstructures and textures of the processed billets were analyzed using optical microscope and X-ray diffraction, respectively. The results show that the structures of the three materials were all significantly refined after ECAE and a lower SFE led to a finer microstructure with a higher density of shear bands. The textures were considerably weakened after 4 passes, more apparent in the material with a lower SFE. It is concluded that the SFE has significant influences on both the grain refinement and texture evolution during ECAE deformation.

Abstract: Mechanical properties such as hardness, mechanical strength or fatigue resistance are by far the most successful material behaviour improved by the ECAP processed. However, the lack of ductility is the most critical negative effect. Combination of multimodal grain size is one of the most promising solutions to improve the strength/ductility balance. Different characterisation techniques are used here to analyse the properties of ECAP Cu thermal treatments and their mechanical properties influence were also investigated.