No matter grown along A[11–20] orientation or R[10–12] orientation,
low growth velocity (1.5 mm/h, 4 rpm) or high growth
velocity (5 mm/h, 8 rpm), all the grown crystals were colorless,
transparent and exempt of visible macroscopic defects, bubbles
have been observed and distribute randomly in the cross section
as illustrated in Fig. 2. Crystals grown at high velocity contains
more bubbles (Fig. 2b) than the ones grown at low velocity
(Fig. 2a) in good agreement with Ghezzal et al. [17,18]. Besides,
high growth velocity has more possibilities to cause the melt flow
too rapidly thus more possibilities of bubbles to be captured by
solid–liquid interface and distribute in a line. This kind of linear
arrangement bubbles has no special orientation, no fixed length
and location. Linear bubbles and random arrangement bubbles
were mixed in crystals. Figs. 3 and 4 present irregular and linear
bubbles arrangement under reflection light and transmission light
respectively of sapphire wafers in thickness 0.5 mm. From Figs. 3a
and 4a we could observe that the size and figure of different bubbles
under reflection light are inequable, whereas Figs. 3b and 4b
indicate that bubbles under transmission light at the same region
show nearly uniform size and spherical shape.