Experimental Investigation On The Formation And Growth Characteristics Of Thin Surface Layer At Z-cut KDP Plate By "Coating Method"

The potassium dihydrogen phosphate(KH2PO4, KDP) crystal was developed in early 1940 s. It is a very excellent electro-optical nonlinear optical material, because the laser damage threshold of large electro-optic coefficient and high nonlinearity, which is widely used in laser frequency conversion, electro optics modulated and optical switching and other high technology fields, and is the preferred material for high power laser system. Large-sized KDP crystal is the only nonlinear optical material which can be used in ICF(inertial confinement fusion) up till now.In the growth technique of KDP crystals by solution method,no matter the traditional Z-cut plate seed regeneration process, or rapid growth technology of the point seed crystal regeneration process, there is a peculiar phenomenon that has not yet been fully understood, that is the growth of thin surface layers. When the KDP type crystals in sabotage or natural growth deviated from the crystallographic shape, the crystal will grow with the thin surface layer to recovery the ideal shape of crystal. In order to explore the inner mechanism of this peculiar phenomenon, in this paper a “coating method” applied to experimentally investigatethe formation and growth of thin surface layer was proposed. By using this method, the formation and growth characteristics of thin surface layer were analyzed. To explore the role of edge in the formation and growth of thin surface layer,also to explore the relationship between edge and step. The growth kinetic parameters of thin surface layer under different coating conditions were measured. The main research contents and conclusions were as follows:(1) Searching for a coating material which does not pollute the growth solution.Decorating it on certain crystal surface, in which crystal does not grow due to the separation between its surface and the growing solution while, no coating part could grow normally. After repeated trials, we selected 1891 types of silicone Osborne, for subsequent coating experiments.(2) The effects of(001) face, edge and {100} faces on the formation and growth process of thin surface layers were analyzed. It showed that edge is a prerequisite for the formation of thin surface layers. New edge can form by other means when the normal edge loss due to the coating. The grow of the prismatic faces and combination of small crystalline cones on the(001) surface can form a new edge. There are differences in the time required for the formation of new edges.(3) Thin surface layer formation process seems to against thermodynamic principles, doubling the area of the thin surface layer growth mode significantly increases the Gibbs free energy of the system. It is difficult to give a reasonable explanation for this phenomenon. Under the support of the previous studies, we proposed that the formation and growth of the thin surface layer of the crystal have a "memory function". Based on this “memory function” of the crystal, we propose a hypothesis for thin surface layer growth:“Crystal has “memory” about its crystallographic shape. Once crystal deviates from it, the “recovery potential energy” it stored will help overcome the energy barrier and reconstruct its incomplete shape by forming thin surface layers as soon as possible”. Based on this hypothesis, we designed the experiment to prove it. The experimental results show that the hypothesis can well explain the growth characteristics of the crystal regeneration process.(4) In-situ observation of the growth of coated Z-cut plate by the aid of optical microscopy was also conducted. It was found that edge plays a key role in the formation of thin surface layers, moreover, prismatic faces plays an important role by providing growth step for the growth of thin surface layers. Meanwhile, the relationship between the corresponding thin surface tangential growth speed and kinetic coefficient is obtained.