Boundary Layer Effect in Crystal Growth-Comments on the Optical in Situ Observation Technique for Melt Crystal Growth

The model of boundary layer in crystal growth stems from the term boundary layer in fluiddynamics. But they are not the same. As crystal growers we are actually concerned with two(coupled) aspects of the melt-to-crystal transport: (1) with the mass flux across an interface,which we will call the interfacial flux and which determines the crystal growth rate; and (2) withthe concentration profile of growth species in the melt adjacent to the crystal, which is an essentialparameter in morphological stability.Characteristic diffusion distance is an important factor to discuss the mass flux normal tothe interface. For melt growth, the theoretical values range from 0.04~0.4cm. The optical in-situobservation method has been newly developed. The method aims at observing and recording thewhole process of growing and. melting of chide crystals in high temperature up to 1400℃, withvery high visibility of surface kinetic processes and mass flow behaviors near the growing interface.It is measured that during KNbO3 melt growth the values of characteristic diffusion distances arebetween 0.01~0.1 cm of different KNbO3 crystal growth rates. The formation and development ofcellular interface structure are also visualized by means of this method. The result shows that themicro concentration flow near the interface is an essential parameter in morphological stability.