Bubble formation in Czochralski-grown lead molybdate crystals

Abstract

Lead molybdate crystals were grown by the Czochralski technique under different conditions from charges of equal molar fractions of PbO and MoO3 powders. Bubbles were found to be the main scattering sites in the crystals. In general, those grown at a pull rate of 3 mm/h are free of bubbles while those grown at higher pull rates may or may not contain bubbles depending on the rotation rate and the crystal boule length grown. At higher pull rates of 4 to 5 mm/h and a high rotation rate of 25 rpm, bubbles form early in the crystal growth process. At a low rotation rate of 2,5 rpm, bubbles will not form until the crystal-melt interface extends to the bottom of the crucible leading to a stagnant layer of melt in between them, The results are attributed to the high fluidity of lead molybdate melt and the efficiency of free convection in homogenizing the layer of melt adjacent to crystal-melt interface, thus alleviating the concentration build-up of gaseous constituents ejected from the growing crystal and preventing the formation of bubbles in the adjacent melt. The early formation of bubbles at high rotation rates is explained in terms of the negating effect of forced convection in countering the homogenizing effect of free convention, enabling the concentration of gaseous constituents in the melt adjacent to the growing crystal to build up sufficiently to effect bubble formation.