Impact of graphite and alumina ceramic thermal insulation on the growth of undoped sapphire plates by micro-pulling down

• Undoped sapphire plates has been successfully grown by the micro-pulling down technique from Mo crucibles. • The effect of graphite and alumina ceramics as thermal insulation was compared. • The alumina ceramics allows for a small oxygen pressure in growth chamber and Mo particles are resulting from the degradation of the crucible, which incorporate into the crystal. • Transmission at 532 nm was for the plates grown from Mo crucibles in graphite thermal isolation found to be around 85%. Undoped sapphire plates were grown by the micro-pulling down technique from Mo crucibles, and the use of graphite and alumina ceramics thermal insulation was evaluated . Through the use of these two thermal insulations, the bubbles encapsulated in the plates were analyzed both qualitatively and in terms of their distribution. By comparing with plates grown from Ir crucibles, it is shown that growing sapphire plates from Mo crucibles in graphite thermal insulation results in the best quality, including a more favorable bubble distribution and fewer particle inclusions. It is evaluated that the critical factor is to avoid any source of oxygen in the chamber, as this highly influences the state of the Mo crucible and thereby the sapphire crystals.