Ultrafast synthesis of SiC nanowire webs by floating catalysts rationalised through in situ measurements and thermodynamic calculations

This work presents the synthesis of SiC nanowires floating in a gas stream through the vapour-liquid-solid (VLS) mechanism using an aerosol of catalyst nanoparticles. These conditions lead to ultrafast growth at 8.5 μm s^-1 (maximum of 50 μm s^-1), which is up to 3 orders of magnitude above conventional substrate-based chemical vapour deposition. The high aspect ratio of the nanowires (up to 2200) favours their entanglement and the formation of freestanding network materials consisting entirely of SiCNWs. The floating catalyst chemical vapour deposition growth process is rationalised through in situ sampling of reaction products and catalyst aerosol from the gas phase, and thermodynamic calculations of the bulk ternary Si-C-Fe phase diagram. The phase diagram suggests a description of the mechanistic path for the selective growth of SiCNWs, consistent with the observation that no other types of nanowires (Si or C) are grown by the catalyst. SiCNW growth occurs at 1130 °C, close to the calculated eutectic. According to the calculated phase diagram, upon addition of Si and C, the Fe-rich liquid segregates a carbon shell, and later enrichment of the liquid in Si leads to the formation of SiC. The exceptionally fast growth rate relative to substrate-based processes is attributed to the increased availability of precursors for incorporation into the catalyst due to the high collision rate inherent to this new synthesis mode.

Enhanced mechanical and wear properties of Al6061 alloy nanocomposite reinforced by CNT-template-grown core-shell CNT/SiC nanotubes

Novel one-dimensional template-grown coaxial SiC@carbon nanotubes (SiC@CNTs) were fabricated using a chemical vapor deposition method. To facilitate the formation of SiC on CNT template, a molecular-level mixing process was used to coat the surface of commercial multiwalled carbon nanotubes (MWCNTs) by Fe2O3. These Fe-CNTs were transformed into SiC@CNT nanotubes, which were then mixed with Al6061 alloy and consolidated by spark plasma sintering to obtain Al6061-SiC@CNT nanocomposites. The addition of 5 vol% SiC@CNT resulted in 58% enhancement in Young's modulus and 46% enhancement in yield strength. Furthermore, the friction coefficient was reduced by 31% and the specific wear rate was reduced by 45%. The enhancement effect of Al6061-SiC@CNT on the mechanical and tribological properties was much greater than those of traditional nanoparticles, nanowires, and whiskers of SiCs. The extraordinary strengthening behavior of SiC@CNT, when compared with that of other SiC analogues, is attributed to the coaxial structure consisting of a SiC shell and CNT core. This coaxial structure enhanced the mechanical and tribological properties beyond that attainable with traditional SiC-derived reinforcements.

Selective Growth of Stacking Fault Free ⟨100⟩ Nanowires on a Polycrystalline Substrate for Energy Conversion Application

Cubic semiconductor nanowires grown along ⟨100⟩ directions have been reported to be promising for optoelectronics and energy conversion applications, owing to their pure zinc-blende structure without any stacking fault. But, until date, only limited success has been achieved in growing ⟨100⟩ oriented nanowires. Here we report the selective growth of stacking fault free ⟨100⟩ nanowires on a commercial transparent conductive polycrystalline fluorine-doped SnO₂ (FTO) glass substrate via a simple and cost-effective chemical vapor deposition (CVD) method. By means of crystallographic analysis and density functional theory calculation, we prove that the orientation relationship between the Au catalyst and the FTO substrate play a vital role in inducing the selective growth of ⟨100⟩ nanowires, which opens a new pathway for controlling the growth directions of nanowires via the elaborate selection of the catalyst and substrate couples during the vapor-solid-liquid (VLS) growth process. The ZnSe nanowires grown on the FTO substrate are further applied as a photoanode in photoelectrochemical (PEC) water splitting. It exhibits a higher photocurrent than the ZnSe nanowires do without preferential orientations on a Sn-doped In₂O₃ (ITO) glass substrate, which we believe to be correlated with the smooth transport of charge carriers in ZnSe ⟨100⟩ nanowires with pure zinc-blende structures, in distinct contrast with the severe electron scattering happened at the stacking faults in ZnSe nanowires on the ITO substrate, as well as the efficient charge transfer across the intensively interacting nanowire-substrate interfaces.