Stacking faults are an important class of crystal defects commonly observed in nanostructures of close packed crystal structures. They can bridge the transition between hexagonal wurtzite (WZ) and cubic zinc blende (ZB) phases, with the most known example represented by the "nanowire (NW) twinning superlattice". Understanding the formation mechanisms of stacking faults is crucial to better control them and thus enhance the capability of tailoring physical properties of nanomaterials through defect engineering. Here we provide a different perspective to the formation of stacking faults associated with the screw dislocation-driven growth mechanism of nanomaterials. With the use of NWs of WZ aluminum nitride (AlN) grown by a high-temperature nitridation method as the model system, dislocation-driven growth was first confirmed by transmission electron microscopy (TEM). Meanwhile numerous stacking faults and associated partial dislocations were also observed and identified to be the Type I stacking faults and the Frank partial dislocations, respectively, using high-resolution TEM. In contrast, AlN NWs obtained by rapid quenching after growth displayed no stacking faults or partial dislocations; instead many of them had voids that were associated with the dislocation-driven growth. On the basis of these observations, we suggest a formation mechanism of stacking faults that originate from dislocation voids during the cooling process in the syntheses. Similar stacking fault features were also observed in other NWs with WZ structure, such as cadmium sulfide (CdS) and zinc oxide (ZnO).
Metal-matrix nanocomposites (MMNCs) have great potential for a wide range of applications. To provide high performance, effective nanoparticle (NP) dispersion in the liquid and NP capture within the metal grains during solidification is essential. In this work, we present the novel synthesis and structural characterization of surface-clean titanium diboride (TiB2) NPs with an average particle size of 20 nm, by ultrasonic-assisted reduction of fluorotitanate and fluoroboride salts in molten aluminum. The high-intensity ultrasonic field restricts NP growth. Using a master nanocomposite approach, the as-prepared TiB2 NPs are effectively incorporated into A206 alloys during solidification processing because of their clean surface, showing partial capture and significant grain refinement.
We report a novel synthesis of Ti5Si3 nanoparticles (NPs) via the magnesio-reduction of TiO2 NPs and SiO2 in eutectic LiCl-KCl molten salts at 700 °C. The Ti5Si3 particle size (∼25 nm) is confined to the nanoscale due to the partial dissolution of Mg and silica in the molten salts and a subsequent heterogeneous reduction on the surface of the TiO2 NPs.
ZnO nanostructured powders have been prepared at low temperature and atmospheric pressure by thermal decomposition of solid precursors. Two alternative methods have been explored by using: (i) commercial zinc acetate dihydrate and (ii) a solid mixture obtained after the evaporation of a solution of zinc acetate in aqueous ammonia at 90°C. Powder X-ray diffraction characterization shows that in both cases crystalline ZnO was obtained after precursor treatment at 90°C over 150 h. According to attenuated total reflection Fourier transform infrared analysis, powders obtained by both routes retain a small percentage of adsorbed acetate. Elemental analysis proves that almost 98 wt.-% of acetate
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