Fabrication of hollow silicon millispheres through a coaxial nozzle, with observation of a novel horn structure

Abstract

Hollow silicon millispheres have been fabricated using a hollow coaxial jet for the first time. Silicon was melted in a graphite crucible at 1823 K, then ejected from the annulus of a coaxial nozzle as argon gas was injected from an inner nozzle whose inner and outer diameters are 0.4 mm and 1 mm respectively. Sphere diameters were 1–2 mm with spherical outer surfaces, and the frequency of formation was approximately 145 Hz. The wall thicknesses varied gradually between antipodes. The interior surfaces were somewhat non-spherical, with horn-like structures where the wall was thicker, most likely owing to silicon expansion as solidification finishes. This is the first report of such a “horn expansion structure” observed on a concave substrate. EDS analysis of the inner, outer, bulk and horn regions revealed variations in carbon concentration, providing clues that inform hypotheses for the order of solidification. Finally, previously produced hollow spheres of tin-lead solder, which shrinks upon solidification, have been re-examined on their inner surfaces. We discovered craters, also where the walls are thicker, whose formation mechanism appears to mirror that of the horns observed in silicon.