Bubbles defects have been always observed in sapphire
crystals. Their distribution and size are strongly dependent on the
growth conditions. We have studied the effect of the pulling rate
on the bubbles' size and their distribution in sapphire rods grown
by the micropulling down (μ-PD) technique. Using a central circular
capillary die of shape factor 0.33, the bubbles' diameter decreased
as a function of the pulling rate. The transmission decreased at a
pulling rate greater than 1 mm/min.