Processing sodium tellurite melts in low gravity drop shaft. Part I. Melt evaporation and formation of solid particles from vapor

Glasses of Na₂O · 8TeO₂ and Na₂O · 4TeO₂ compositions adhered to a small platinum heating coil (2 to 3 mm ID, 5 to 6 mm long) were melted and evaporated in low gravity using the drop shaft at the Japan Microgravity Center (JAMIC). The gravity level attained during the 10 s free fall was in the order of 10^-3 g. The species evaporated from the melt in low gravity generally formed a spherical smoke cloud surrounding the melt, whose size depended on the melt temperature and also on the time the melt evaporated in low gravity. The shape of the cloud was found to depend on several other factors, namely, the uniformity of heating, amount of melt, and the presence of gas bubbles in the melt. The evaporating species formed nearly perfect spheres of pure TeO₂ whose diameter ranged from 0.05 to 20 μm. The size of TeO₂ microspheres increased with increasing melt temperature and time in low gravity, and was 5 to 10 times larger than that of similar particles prepared at 1-g.