Experimental And Numerical Investigation On The Thin Surface Layer Growth Of Incomplete KDP Crystal

Energy has always been what human beings long for,since the 1960 s scientists have tried to harness the enormous energy generated by inertial confinement fusion.KDP family crystals are currently the only available candidate for optical switching and frequency doubling in the inertial confinement fusion due to their excellent nonlinear optical and electro-optical properties.The research of KDP crystal growth began from the 1930 s and peaked in the late 1990 s.In the beginning people devoted to grow large(>40 cm)crystals,later they turned to focus on crystal quality and production cost.However,some basic questions in the KDP crystal growth,such as the unsolved mystery of the thin surface layer growth,are rarely studied.What this reflects is not only the KDP crystal growth,but also the whole crystal growth theory needs to be improved.Thin surface layer growth is a phenomenon in the regeneration of incomplete crystal: The incomplete crystal repairs itself by thin surface layers overhanging into solution,instead of normal layer-by-layer growth attaching to an existing crystal surface,and the repaired part is filled with growth solution.The question is,the thin surface layer growth creates approximately twice the surface area(i.e.,surface energy,growth barrier)than the layer-by-layer growth,why do incomplete crystal choose to self-repair through a way of greater resistance? This regeneration seems to be incompatible with the existing crystal growth theory and the basic principles of thermodynamics,so the research toward it has far-reaching meanings in the field of crystal growth.Based on the knowledge of engineering thermophysics,chemistry and crystal materials,using experimental and numerical simulation methods,this paper builds crystal growth system and grows small-size crystals,then conducts experiments and numerical simulations on the thin surface layer growth.The main contents and results are:(1)Successfully built an aqueous solution crystal growth system,and produced enough small-size KDP crystals.Inspired by crystal growth experience,we also modified the growth platform by adding a frustum to it.Simulation results show that the frustum eliminates the vortexes that station at the bottom of prismatic faces,and as the crystal grows,the concentration field improvement becomes more obvious.With appropriate high rotation rate,the frustum platform can improve the supersaturation homogeneity of both pyramidal and prismatic faces.The optical transmission and laser damage threshold tests of crystals grown with ordinary platform and frustum platform indicate that,even at a small size level,the frustum platform already shows the advantage of improving the crystal quality.(2)Micro-observation finds that the Z-cut plate turns the non-singular(001)surface into singular {101} surfaces through rough interface growth,then the small pyramids at the center and the edge,whose pyramidal faces happen to in one plane,begin to merge into a bigger one.And finally,the pyramids near the four edges merge a big hollow pyramid corresponding to the entire Z-cut plate.The results of dynamic experiments and simulations show that the forming of thin surface layer from one place of the crystal,is not because the supersaturation there is higher than other places;the supersaturation just affects its subsequent growth rate.(3)Designed,cut and obtained experiment crystals of all surfaces being singular face,these crystals still regenerated in the way of thin surface layer growth.Experimental phenomenon clearly shows that the thin surface layer is not formed by the crystal surface steps spreading out from the edge;the surface energy calculation shows that the growth of thin surface layer encounters greater resistance than the layer-by-layer growth;and combined with the conclusion that supersaturation is not the essential reason for the formation of thin surface layer,we surmise that the incomplete crystal in supersaturated solution possesses “potential of morphology repair”,which is the drive force of thin surface layer growth and drives the growth of thin surface layers to quickly repair itself into a complete convex polyhedron.The thin surface layer fragments experiment shows that the tangential growth rate of thin surface layer in the incomplete crystal is one order faster than that apart from the incomplete crystal.(4)Repeated experiments of sofa-shaped crystals find that the repair final morphology always presents a specific type of convex polyhedron.Based on the surmise of “potential of morphology repair”,the mathematical derivation proves that the convex polyhedron is a better final morphology compared to the concave polyhedron.The actual repaired crystals all correspond to the situation of maximum cutting,which reveals three distinct morphological characteristics in the repaired incomplete crystal: singular surface enclosing,convex polyhedron and the smallest volume.Numbers of regeneration details suggest that the incomplete crystal achieves the self-repairing by morphology prejudging.(5)Numerical simulation of the capping of large size Z-cut KDP plates finds that keeping still is very unfavorable for the growth of thin layer,a certain rotation rate should be applied to the Z-cut plate to increase the concentration gradient across the(001)face,thus making sure that thin layers can grow from the edges while small pyramids barely grow from the face center.Given the result that end faces of thin layers nearly bear a bulk supersaturation,a relative low bulk supersaturation will be better to ensure both the growth of thin layers and the good surface uniformity brought by low supersaturation.Moreover,as thin layers grow,rotation rate should be increased to make up the insufficient solute supply resulting from the decreasing linear speed.The optimal parameters for rapid and high-quality capping of large size Z-cut plates are suggested,the relevant simulation results are also presented.