Investigation On The Growth Characteristics Of KDP Single Crystal And Thin Surface Layer

As one kind of peculiar crystal growth phenomenon,thin surface layer growth is different from the singular surface layer growth.It is well known that singular surface layer growth depends on the step’s advancing to adsorb solute molecules.However,the growth of the thin surface layer is like that the step moving hanging in the solution.Tt is surprising that thin surface layer growth increases the interface energy of crystal compared to layer growth obviously due to it grow from edges in crystallographic planes without pre-existing layers under them.In theory,the driving force required for thin surface layer growth is significantly higher than which required for the layer growth,but foreign scholars had found that there is no significant difference in the growth rate of both under the same supersaturation condition,which makes the thin surface layer growth mechanism more incredible.In addition to KDP(Potassium dihydrogen phosphate,KH2PO4)crystal,ADP,urea and other crystals also have thin surface layer growth phenomenon,showing that the growth of thin surface layer is a widespread phenomenon of crystal growth.However,the intrinsic mechanism of this phenomenon is not clear at present,so it is of great academic significance to study the growth mechanism of thin surface layer.As a typical nonlinear optical material,KDP crystal can achieve laser double frequency and triple frequency,it is the only one could be applicable to laser fusion ignition experiment currently.The rapid growth technology of KDP crystal seedling in the 1980 s has increased the growth rate by an order of magnitude on the basis of the original,and its’ growth mechanism is different from the traditional Z-cut plate growth mechanism.According to the results of crystal quality detection,it is found that increasing the growth rate does not necessarily lead to the decline of the optical quality of the crystal,which makes this technology have broad prospects,but the bottleneck of developping thi technology is how to control the produce of liquid inclusion under high saturation conditions.With the continuous research on the rapid growth of KDP crystals,more and more scholars believe that the production of liquid inclusions is more likely to be related to the growth of thin surface layer.In order to decrease the produce of liquid inclusions,it is necessary to suppress the surface of the crystal surface to produce thin surface layer.So the study of thin surface layer growth mechanism in the engineering field has important significance.In this paper,the growth of thin surface layer was studied by KDP crystal.The main contents include:(1)Grow KDP crystal seed with evaporation method,choose seed with regular shape,no visual defects.Then grow KDP crystals with high quality seeds by rotating-crytsal,and under different rotational speed,different temperature and different supersaturation conditions.It is found that the quality of the crystal does not necessarily decrease with the increase of the growth rate.The crystal’s movement can not only promote the solute transport to improve the crystal growth rate,but also improve the uniformity of the solute concentration to make the crystal quality better.(2)The growth experiment of thin surface layer of KDP crystal with non-habit surface was carried out to study the growth characteristics of thin surface layer.It is found that if the parallel KDP crystal polycrystalline system appears to be coplanar,the two habit surfaces will be merged into a whole and a thin surface layer will be formed and the polycrystalline system will be restored to single crystal.The edges that deviate from the normal crystallographic orientation inevitably lead to grow thin surface layer,and a simple and effective mapping method can be proposed to predict the growth of the thin surface layer according to the crystal equilibrium morphology theory.(3)The growth experiments of concave polyhedral thin surface layer surrounded by KDP crystal {100} and {101} faces ware carried out to study the "self-repair" mechanism of KDP crystal.In this paper,a new "minimum convex polyhedron" method is proposed to predict whether the thin surface layer can grow and the desired growth result.It is a small step forward on the basis of the "growth from the crystal edges".(4)Proved solution nearby edges with higher supersaturation than that of the crystal face center is the critical issue of thin surface layer growth,to a certain extent,explain the rationality of thin surface layer.(5)According to the principle of thermodynamics to derive the critical thickness of the thin surface layer formula,and calculate the value equivalent to the numbers of single step height under a known supersaturation conditions.