| Potassium dihydrogen phosphate?KH2PO4,KDP?is an important member of the KDP-type crystal family.This crystal is extensively used in the fields of optical modulation,frequency converters,piezoelectric transducers,etc.Particularly,in powerful laser systems,large-aperture KDP crystals with high optical quality are required for satisfying the application demands of single crystal devices?i.e.,Pockels cell and harmonic generator that are generally used in inertial confinement fusion?.The demand for large-aperture KDP crystals has significantly increased.Therefore,KDP has also attracted extensive attention in the investigation of how to grow high-quality crystal with high rate.In order to shorten the period of crystal growth,the common measure is to enhance the convection and improve mass transfer around the crystal,which significantly accelerates the crystal growth rate.Unfortunately,the increase of growth rate is often accompanied by the deterioration of crystal surface morphology and the formation of inclusions,thus leading to a decrease in crystal quality.Morphological instability of the crystal surface is considered to be a crucial reason of inclusions formation,and it is closely related to inadequate convection conditions.Considering the present method of crystal growth,it is far from enough to solve the above problems only by ameliorating the macroscopic operating conditions?solution temperature,supersaturation,doping and rotating-crystal mode?of crystal growth.So it is needed to search for improving conditions of the convective mass transfer above the growth interface and develop new methods for rapid growth of high-quality KDP crystals on the basis of in-depth analysis of how convective mass transfer near the phase interface affects the surface morphology and crystal quality.In the present paper,according to basic theories of fluid dynamics,heat and mass transfer,some new crystal growth techniques were proposed for the problems existing the process of rotating-crystal method grown KDP crystal.Meanwhile,the numerical simulation and growth experiment of the rapid growth of KDP crystals by these new crystal growth methods have been carried out.The main contents are as follows:?1?Based on summarizing the adverse effects of hydrodynamic condition caused by the traditional rotating-crystal method,a novel technique of growing crystals named solution-jet method was proposed.The fluid flow and surface supersaturation distribution of crystals grown under different conditions were computed using numerical simulation.Results indicate that the time-averaged supersaturation of the pyramidal face in the proposed method significantly increases and the supersaturation difference from the crystal center to edge clearly decreases compared with the rotating-crystal method.As jet velocity increases,supersaturation on the pyramidal face steadily increases.Rotation rate considerably affects the magnitude and distribution of the prismatic surface supersaturation.With increased crystal size,the mean value of surface supersaturation averaged over the pyramid gradually decreases;conversely,standard deviation increases,which is detrimental to crystal growth.Moreover,the significant roles played by natural and forced convection in the process of mass transport are discussed.Results show that further increased jet velocity to 0.6 m/s renders negligible the effects of natural convection around the pyramid.The simulation for step propagation indicates that solution-jet method can promote a steady step migration and enhance surface morphology stability,which can improve the crystal quality.?2?For the purpose of making full use of the advantages of convection,the periodic translational motion growth method was proposed.At the same time,a new driving mechanism was designed and built.The hardware architecture and selection of this mechanism were introduced in detail.Taking the MPC08SP motion control card as its core control unit,the motion control system under Windows environment was developed with the aid of Visual Basic.In addition,the operation interface and some codes of motion control software were also given.The test results show that the driving mechanism meets the requirements of design,which provides conditions for crystal growth.?3?The solubility curve and metastable zone width of KDP solution were measured.The results show that the metastable zone width is increased as the solution temperature decreases.Besides,the width of metastable zone becomes wider with the increase of cooling rate.A series of KDP crystals that are grown from different translational velocities and orientation configurations were obtained by 2D translational motion method.The grown crystals were investigated by X-ray diffraction,microhardness,dielectric,chemical etching,thermal analysis and laser damage threshold studies.The results shows that the change of motion mode does not affect the crystal structure.A higher hardness value is observed in crystals grown by 2D translational motion method than that grown by the rotating-crystal method.Low dielectric loss and less dislocation density show that 2D translational motion method grown crystals contain minimum defects.The thermal stability and damage threshold of the rotating-crystal method grown crystals are lower than those of the crystals grown using 2D translational motion method.Furthermore,the variation in translational motion velocity results in a distinct variation in crystal quality.The grown crystal quality is the best as the translational motion velocity is 0.04 m/s.Scanning by optical microscopy reveals that step migration on the surface of the crystal grown by 2D translational motion method is more stable than that of the traditional method grown crystal,which is beneficial to reduce the probability of defects.?4?The growth of KDP crystals was carried out by 3D translational motion method and rotating-crystal method,respectively.The grown KDP crystals were characterized by Raman spectroscopy,transmission spectra,conoscopic interference,extinction ratio,scattering particles,chemical etching,and laser damage threshold.Raman spectrum shows that 3D translational motion method does not cause any deformation or distortion of crystal structure.The transmittance of the crystal obtained by 3D translational motion method is obviously higher than that of rotating-crystal method grown crystal.The transmittance at fundamental wavelength and higher harmonics is mostly above 84%.From the results of measurements of conoscopic interference and extinction ratio,the optical homogeneity of the crystal using 3D translational motion method is better in comparison with that of the traditional method grown crystal.Less scattering particles and dislocation density of 3D translational motion method grown crystal are attributed to fewer defects within the crystal,indicating that it has better quality.The damage threshold of the crystal adopted by 3D translational motion method is about 37%higher than that of conventional crystal.It suggests that 3D translational motion method is an efficient way to improve the damage resistance.?5?The continuous filtration growth system based on translational motion in the present paper was designed and built in a way that eliminated impurities,microbes and nucleation produced in the solution.As a consequence,the stability of the growth solution is improved.The crystallizer and platform holding a growing crystal have been redesigned.The relationship between growth temperature and superheater temperature is determined.The range of temperature drop required for crystal growth is obtained by measuring the crystal size.Through the experimental verification,it can be inferred that the design can maintain the stability of the solution that helps the crystals grow. |
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