Investigation Of Transport Characteristics In The Capping Process And The New Rapid Growth System For KDP Crystal

Potassium dihydrogen phosphate?KH₂PO₄,KDP?crystal is an excellent non-linear optical material developed since the 1930s.Due to the large non-linear optical coefficient,high laser damage threshold,and good transparency,it has been widely applied in the field of laser frequency conversion.In particular,KDP crystal is the preferred material used in the Inertial Confinement Fusion?ICF?laser driver systems at present,because of the easy to prepare and grow up to large sizes.Two methods are commonly used to grow KDP crystals in industrial production:traditional slow growth with a Z-plate seed and rapid growth with a point seed.Regardless of the method applied,seeds in solutions should be prepared in advance,and their quality remarkably influences the growth process of large-scale crystals.Therefore,high-quality seeds should be selected for the successful growth of large-scale crystals.Seeds are usually obtained by evaporating a solvent.If KDP crystals have grown,a slice can be cut from them and used as a seed.Among different cuts of seeds,Z-cut seeds are the most widely used,and they appear to have an incomplete crystallographic shape in the absence of pyramids.Before the process of normal crystal growth for Z-cut seeds,there is a regeneration process of pyramidal faces,known as the capping of KDP crystal,which has a significantly influence on the following normal growth process.The regeneration process is characterized by the growth mechanism of thin surface layers,which only appears in the growth of crystal with an incomplete crystallographic shape.However,their growth mechanism has yet to be investigated in detail,and related theories describing the formation of thin surface layers have yet to be clarified.At the beginning of a normal crystal growth,the flow and solute concentration fields in solutions are key factors that determine the quality of grown crystals.Current methods for KDP crystal growth involve a periodically-reversing crystal rotation schedule to improve the nutrient supply and increase the crystal growth rate.With crystal rotation,flow and concentration fields become complicated and present a typical three-dimensional unsteady characteristic.Consequently,related investigations are difficult to perform by using experimental measures.However,they are easy to achieve by a numerical simulation,which can provide a numerical basis for the process improvement of crystal growth.With the advancements in computer technology,the numerical simulation has been developed into a necessary tool in the research of crystal growth technique.In this paper,a series of studies on the flow and mass transport phenomenon for KDP crystal growth was conducted according to basic theories of thermodynamics,fluid dynamics,and mass transfer.The main contents were as follows:?1?The growth mechanism of thin surface layers was studied on the basis of crystal morphology by observing the regeneration of KDP crystals with different incomplete crystallographic shapes.A principle of crystallographic shape recovery,?the smallest convex polyhedron principle?,was proposed and explained according to fundamental thermodynamics theory.The advancement velocities of thin surface layers were measured with an optical microscope.Experimental results showed that these velocities are related to the bulk supersaturation of the solution,the thickness of a thin surface layer,and the crystallographic orientation of abnormal edges on the growth front.A growth kinetics model of the thin surface layer was then established under the bulk diffusion mechanism.The kinetics of crystal growth confirmed that the growth of thin surface layers is necessary to expand these abnormal edges along the tangential direction of the layer.And such expansion is terminated at the normal edges of the corresponding singular face.?2?The flow and mass transfer during the regeneration process of KDP Z-plate seeds were numerical simulated.The simulation results showed that when the surface supersaturation is high in the edges and low in the center areas of the?001?faces,the thin{101}surface grows rapidly,and the regeneration process of the Z-plate seeds ends within a short period.To ensure the quality of regeneration,we should achieve a homogeneous distribution of surface supersaturation on the outward pyramidal faces of thin surface layers.In the initial stage of regeneration,the Z-plate seeds should remain still or rotate at low speeds.In the intermediate stage,the rotation speed of these seeds should be gradually increased to the required value for the normal growth of KDP crystals.In later stages,the rotation schedule should be stabilized.?3?The flow and mass transfer involved in the preparation of KDP seed crystals with crystallographic shapes through evaporation were investigated via numerical simulations.The analysis focused on the changes in the shape of the seed crystals and their surface supersaturation distribution.The results showed that the supersaturation nonuniformity on crystal faces is directly responsible for step bunching and inclusion formation.When the KDP seeds grow in a stagnant solution,the maximum gradient of surface supersaturation appears near the diagonal lines on the top face of the seeds.Reducing the supersaturation degree of the solution or tilting the growth vessel can improve the homogeneity of surface supersaturation on the top face and enhance the quality of the growing seeds.?4?A computational analysis of solution hydrodynamics and mass transfer during the growth of KDP crystal in the traditional self-rotation system was conducted.The results showed that during the phases of crystal spin down,stalling and spin up,the solution flow around the crystal will be weakened seriously,and the crystal surface supersaturation will fluctuate sharply.In order to overcome this drawback,a new growth system for KDP crystal was proposed.In this system,an additional orbital revolution with a constant revolution rate is imposed in the crystal so that it will execute a planetary motion in the solution.The numerical simulations of the flow and mass transfer had been performed,and the focus was on the effect of the operating conditions,such as the self-rotation rate,revolution rate and orbital radius on the surface supersaturation of KDP crystal.The simulations indicated that with the increase of the revolution rate,orbital radius or bulk supersaturation,both the time-averaged supersaturation of crystal faces and the crystal growth rate will be improved effectively.?5?A further improvement of the crystal motion model was performed by introducing a reciprocating motion in the vertical direction when the crystal executed the circular motion on the horizontal plane?i.e.the three-dimensional spiral motion?,with the aim of agitating the solution vigorously and ensuring uniform nutrition of all the crystal faces.The numerical studies indicated that this crystal motion model can obtain more homogeneous surface supersaturation.And the results showed that the mass transfer characteristic on the crystal faces is mainly affected by the line speed V₀ of crystal in the horizontal plane rather than the reciprocating motion speed V_z.The increase of line speed V₀ will decrease the thickness of the diffusional boundary layer,reduce the resistance of the solute transport and increase the time-averaged supersaturation on the crystal faces.At the same time,the increase of line speed V₀ will improve the homogeneity of the surface supersaturation too,which would benefit the morphological stability of the crystal surfaces and decrease the inclusion formation.However,when the crystal size increases,the time-averaged supersaturation on the crystal faces will decrease and its homogeneity will deteriorate.Therefore,it is necessary to appropriately increase the orbital radius and reduce the period of the crystal circular motion?i.e.increase the line speed V₀?with the KDP crystal growth in the 3D spiral motion system.