Chemical and crystallographic investigations on the cracking of blue corundum, Al₂O₃, produced by the Verneuil technique

Corundum (Al₂O₃) boules grown by the Verneuil process generally crack along an irregular surface parallel to the elongation axis. In order to investigate the reasons for such cracking, a half boule of blue corundum, industrially produced following the Verneuil technique, was cut into slices (about 1 mm thick) parallel or normal to the elongation axis of the sample. All the slices were analysed by optical microscopy, energy dispersive spectrometry (EDS), flame atomic absorption spectrometry (FAAS), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and by X-ray transmission topography, using synchrotron radiation. The two most interesting phenomena observed were: (a) a blue colour, the location of which was not homogeneous in the boule, but concentrated in a narrow (∼2 mm) ring in the outer region of the sample; (b) the development of a single crystal during the first stages of the growth and the formation of a macromosaic crystal, with domains that were slightly misoriented and subparallel to one another, in the middle part and at the top of the boule.