The analysis of CdTe solidification in absence of thermal convection via short-duration microgravity

摘要

The solidification of CdTe in the absence thermal convection was investigated via short-duration microgravity. The National Institute of Advanced Industrial Science and Technology (AIST) drop tower facilitated the microgravity synthesis, where the duration of microgravity is 1.4 s with a 10(-3) G gravity level. Synthesis of CdTe was conducted in a unidirectional cooling apparatus with rapid gas cooling and thermal control capabilities. The starting sample consisted of a single phase CdTe compound. The sample was then heated and cooled in microgravity and terrestrial conditions. It was observed in microgravity conditions that the cooling rate decreased by 10%, undercooling was observed, and the products maintained stoichiometry during solidification. The samples synthesized in microgravity conditions had somewhat rectangular shaped grains on the order of 4.3 x 3.0 mm. This was observed to be a significant increase in grain size compared to the grains synthesized in terrestrial conditions given that the growth was achieved within 0.8 s. Fe-doped CdTe experiments were also conducted in microgravity. The Fe ion dopant promotes heterogeneous nucleation. The solidification initiated in many areas near the cooling target. The grains were observed to be needle-like structures with an ordered growth direction. Rectangular-like structures were not observed in these samples. Grain sizes of CdTe are significantly effected by the starting temperature of the molten precursor. Samples synthesized with a temperature 3 degrees C above the solidification temperature exhibited square shaped grain structures with sides on the order of 1-2 mm. Samples synthesized with a temperature 10 degrees C above the solidification temperature consisted of needle shaped grains or fine grains on the order of 1 mm in length. (c) 2006 Elsevier B.V. All rights reserved.