Evolution of SiO_x Shell Layers on SiC-SiO_x Core-Shell Nanowires

Abstract

Composite core-shell SiC-SiO x nanowires can be produced by heating quartz and SiC powders, with addition of Ar(g) or He(g). The two powders are mixed to create pellets, which will react to SiO(g) and CO(g) at elevated temperatures. The two gases will react on a colder surface, producing a web of SiC-SiO x nanowires. The product serves as a precursor for SiC nanowires production. During the process, silicon and oxygen accumulate at high energy points, forming SiO x nodules. Nodules can either generate in proximity of stacking faults, or where two or more nanowires are close to each other. The present work investigates the role of crystal defects in the wettability between silica and silicon carbide. Samples were collected and analyzed under Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The results show that β-SiC grows mainly in the [111] direction. Crystal defects are located in the SiC core-phase. SiO x initially develops a uniform layer as thick as the core-phase itself. SiO x nodules would first form where the defects are present, by accumulating at high energy sites. Droplets on a flat surface imply poor wettability. The mechanism of formation of the nodules is compared to two earlier proposed theories. In conclusion, the wettability of SiO x and SiC at nanoscale is controlled by the presence of crystallographic defects. Continuous SiO x layers and bead-like structures can be found in the same temperature interval. The microstructural changes depend on the local energy balance.