Research On Several Key Problems During The Rapid Growth Of Large-diameter KDP Crystals

The development and utilization of new energy has greatly promoted the changes in the development of the world economy and human lifestyles.Nuclear fusion is an ideal clean energy source and rich in resources.Inertial confinement fusion(ICF)is a development direction in controllable nuclear fusion technology,with the upgrade of the device,more and bigger optical components are needed to achieve high-density and high-power laser output.Potassium dihydrogen phosphate(KDP)crystal is a classic electro-optical,piezoelectric and nonlinear optical crystal,large-type single crystal can be obtained by the aqueous solution cooling method,which can meet the required dimensions and used as optical components.At present,domestic research mainly focuses on optimizing the growth process of large-size KDP crystals and increasing the damage threshold of potassium dideuterium phosphate(DKDP)crystals.Although the"point seed" rapid growth method can achieve rapid growth of large-type KDP crystals,there are problems such as poor solution stability,crystal inclusion and impurities effect during the crystal growth process which can result in low success rate,utilization and crystal quality.This paper focuses on optimizing the growth process of large-type KDP crystals,the atomic force microscope(AFM)was used to study the microscopic growth mechanism of different crystal faces.Systematic studies have been carried out by changing the rotational speed,supersaturation,growth orientation and impurity content in macroscopic conditions.The growth habits of crystals were gradually understood and the standardized growth of high-quality large-type KDP crystal was achieved successfully.The effects of crystal growth method and the seed direction on crystal growth was also studied.Series of KDP crystals were grown by changing the growth frame,seed direction and the control of the growth parameters in the later stage,which not only reduced the growth period,but also improved the quality of the crystal,which will provide a reference for the growth of higher quality crystals in the future.The main research contents of this thesis are as follows:Microscopic morphologies of KDP crystals grown at different supersaturation was measured by AFM to study the microscopic growth mechanism.It is found that the height and width of the macrosteps increase with the increase of supersaturation.Growth hillock with a hole in the center was observed under a supersaturation of 0.77%,it has a strong adsorption force and can preferentially absorb the adjacent solute to achieve layered growth around the hole.With the increase of supersaturation,the 2D nucleuses and 2D platforms mainly appear on the macrostep.When the supersaturation is 8.09%,the crystal surface is composed of macrosteps with large height,and the surface of the macrostep presents an irregular pattern.It is found that the formation of this unstable structure is related to the degree of supersaturation,uneven distribution and insufficient flow rate.To further understand the influence of the interaction between the seed direction and the liquid flow on the growth mechanism,the prismatic face microscopic morphologies of the KDP crystal grown in type-?,type-?,45° and[101]directions were measured.It is found that the morphology of 45° exhibits a 2D nucleuses platform and 2D nucleuses distributed over the wide step surface when the supersaturation is 1.87%,the shape of macrosteps of[101]direction crystal presents a unique zigzag parallel arrangement.With increasing supersaturation,two 2D step platforms are generated on the prismatic face of type-? crystal,the 2D step platform and the hole growth hillock coexist on the surface of type-? crystal and the height and width of zigzag macrosteps on the[101]direction crystal increase.When the supersaturation continued to increase,large macrosteps and 2D nucleuses appear on the type-? crystal.Macrosteps become irregular and the 2D nucleuses are densely distributed on the surface of type-? crystal.Large step growth hillock appear on the surface of 45° crystal and 2D nucleuses distribute on the top.The macrosteps on the surface of[101]direction crystal become irregular,small steps with different heights are piled up at the top of the 2D step platform.As the supersaturation continues to increase,macrosteps become larger and wider,the patterns on the macrostep surface become irregular.Due to the uneven supersaturation,the growth rate of the step is different,the pore structure is easy to form.The elementary steps on the(101)faces are evenly distributed under low supersaturation without the tendency of coalescence.The aggregation was only generated in triangular shape pit,dislocation outcrops causes nearby elementary steps convergence and bending at the edge and interior of the triangular pit.High supersaturation and insufficient flow rates cause the elementary steps to converge,resulting in irregular structures,these unstable structures have a large slope and are mainly composed of elementary steps,2D steps growth hillocks and 2D nucleuses.The continuous evolution of these unstable structures will reduce the stability of the surface and cause defects in the crystal,so appropriate supersaturation should be maintained.By changing the rotational speed of KDP crystal,the growth habit and performance of the crystal grown from low supersaturation to high supersaturation at 15 rpm-77 rpm rotation rate were systematically studied.Experiments show that the KDP growth solution has high solution stability.Under low rotation conditions,the crystal is prone to form inclusion.As the rotation speed increases,the probability of inclusion formation decreases gradually,but excessive supersaturation also promotes inclusion generation.The inclusion can be effectively reduced by lowering the temperature range of growth or changing the growth direction of seeds at low rotational speeds.Analysis of the morphology and composition of the inclusion revealed the element in inclusion was mainly KDP solution.With the increase of the rotational speed,the aspect ratio of the crystal shows an increasing trend and the aspect ratio increases first and then decreases as the growth supersaturation transitions from low to high.Studies on the growth of[100]and 45° direction KDP crystals by changing supersaturation show that the growth rate of crystals is related to the strength of the liquid flow interacting with crystal surface.The performance test of KDP crystal grown under different parameters shows that the crystal has high transmittance and uniform absorption coefficient distribution.lower speed will cause the R-on-1 damage threshold to be lower.Increasing supersaturation under the 77rpm rotation speed has little effect on the structural stability of the Z-direction crystal.By regulating the rotation speed and supersaturation during the growth of KDP crystal,the formation of inclusion can be avoided without affecting the crystal performance.When the rotation speed is 3 0rpm,the growth of inclusion-free crystals were realized by changing the seed direction of crystals and crystals have good structural stability and laser damage resistance.The results of X direction and 450 KDP crystal performance test at different supersaturations show that they have good structural stability and damage threshold.By improving the growth process of large-type KDP crystals,the stability of the solution was improved.The KDP crystal of 600mm aperture with no macroscopic defects was successfully grown and the uniformity of optical properties was tested.The crystal has a high homogeneity of transmittance.The absorption coefficient test showed that the crystals of Z-direction,?-type and ?-type slices have large differences,the overall performance is Z-direction>?-type>?-type and the same direction is slightly different.The crystal has good crystalline perfection,optical uniformity and laser damage threshold,the LDT of pyramidal area is lower than the prismatic area.To achieve shape controlled growth of crystals,EDTA-2K was used in KDP solution,the inhibition effect of Fe3+ ions on the prismatic face can be effectively alleviated and the formation of inclusions in the crystal can be reduced,it has no obvious effect on the properties of crystal.The effect of Zn2+on the growth habit and performance of KDP crystal was studied.The introduction of Zn2+can hinder the movement of prismatic face step.When the concentration is lower than 500 ppm,it can increase the aspect ratio of the crystals and has little effect on the crystal performance.When the concentration is more than 1000 ppm,the wedge growth of conventional KDP is more severe and the rapid growth KDP crystals begin to show a slender shape.Excessive Zn2+could cause the inclusion formation and cracking.Growth of the conical seed by conventional method can reduce the process and time of recovery.The Z-direction wafer was grown by rapid growth device,the crystal can only grow in the[001]direction,the crystal uniformity is improved.By using the principle of "necking down" in the pulling method by improving the growth platform to reduce the number of dislocations in the crystal.The grown KDP crystals above have high transmittance and good structural stability and high laser damage threshold.