Processing sodium tellurite melts in low gravity drop shaft: Part II Melt solidification and glass formation

The effect of gravity on glass formation and crystallization of the Na₂Ocdot;8TeO₂ (NT₈) and Na₂O4TeO₂ (NT₄) melts were investigated using the low gravity drop shaft at the Japan Microgravity Center (JAMIC). This drop shaft produces a low gravity of <10^–3 g for 10 s during free-fall and about 8 to 10 g for 5 s during deceleration of the capsule. The glass initially adhered to a small platinum heating coil was re-melted in low gravity. The melt detached from the heating coil during the high-g period and solidified after being splattered on a plate (substrate) located 4 cm below the heating coil. The parameters that were varied for the drop shaft experiments were the melt temperature and the substrate material on which the melt splattered. Like what was observed at 1-g (ground), the NT₈ splatters from the drop shaft experiments always formed glass, being independent of the melt temperature and the substrate material used. The splatters from the NT₄ melts partially crystallized in all the drop shaft experiments, even though this melt is an excellent glass former at 1-g. The splatter on a substrate of higher cooling ability such as copper had a smaller amount of crystals than the splatter on a substrate of smaller cooling ability such as glass or alumina. The glass transition temperature, heat capacity in the glass transition region, activation energy for crystallization and the infrared (IR) spectra for the drop shaft splatters were not significantly different from those for the similar splatters prepared at 1-g. However, the crystallization temperature of all the drop shaft splatters was 5 to 10°C lower than that of their 1-g counterparts. This result suggests that the NT₈ and NT₄ melts solidified under drop shaft conditions are less resistant to crystallization than the similar melts solidified at 1-g.