A photomicrographic and electron microscopy study of nucleation in ammonium perchlorate

Abstract

The nucleation processes associated with the formation of thermal decomposition sites in single crystals of ammonium perchlorate have been studied by optical and electron microscopy. In crystals subjected only to heating the nucleation sites appear to be randomly distributed on the c face ; however, on the m face some are randomly distributed but others lie along lines in specific crystallographic directions. When crystals are strained, in any of several different directions, before thermal decomposition the nuclei are strongly alined on both faces. These alined nuclei lie, with few exceptions, in the same crystallographic directions as similarly alined p its formed by chemical etching. Consequently, it is concluded that the decomposition nuclei preferentially form where dislocations intersect the crystal surface. During heating at 170 to 210 °C the first-formed nuclei are widely distributed and appear to be on the surface. These and subsequently formed nuclei appear to cease growing when they reach a critical size. Additional nuclei continually appear between previously formed ones until a completely reacted layer is formed. (At these low temperatures complete reaction corresponds to the decomposition of only 30 % of the original material.) Further decomposition occurs in two ways. In one, the reaction progresses inward and roughly parallel to the surface as successively deeper layers of nuclei are formed. In the other, the reaction produces bands, possibly at an accelerated rate and with the formation of additional internal nuclei, along (001) planes in the [010] and [510] directions. Scanning electron microscope examination shows that these bands, as well as other parts of the reacted surface, consist of porous residue of ammonium perchlorate.