Point-seed-oriented and partial shape-restricted growth of KDP crystals: Numerical simulation and experimental study

Abstract

Numerical simulations are performed to understand the mass transfer in point-seed-oriented and partial shape-restricted growth of potassium dihydrogen phosphate (KDP) crystals. A type-II cut point-seed is positioned in between two parallel and rotating platforms separated by a fixed distance. Numerical simulations show that the periodic rotational motion of the two adjacent parallel platforms can lead to significant differences in surface supersaturation across the same prismatic face. The non-steady and non-laminate flow result in defects such as growth avoids and inclusions that seriously affect the optical properties of the crystal. In some cases, it results in the failure of the growth (i.e., significant cracks). Lack of surface supersaturation homogeneity results in significant difference in step-motion rates and in the step-slope, which further cause step bending and even the formation of macroscopic steps. Inclusions and avoids are often filled with solution trapped in gaps between macroscopic steps. We have found both experimentally and numerically that increasing the separation between the two parallel platforms to above 30 mm can significantly mitigate the supersaturation gradient on the crystal surface and drastically reduce the number of inclusion defects.