Influence Of Medium Environment On Micro Morphology And Growth Rate Of KDP Crystals

With the development of Inertial Confinement Fusion（ICF）engineering,the high requirements for the demand,quality and growth rate of KDP crystals have been proposed,which can be summarized as "rapid,large size and high quality".The aim of rapid growth is to provide the numbers of KDP chips and reduce the production cost.The large size can satisfy the need of the ICF system.The high quality means that the crystals meet the performance requirements when they are applied in ICF devices.The growth technology,optical quality and structure of KDP crystals have been studies in recent years.We believe that the study of micro morphology and growth mechanism can be help to improve the growth rate and performance of KDP crystals fundamentally.As the classical aqueous solution growth crystal,the growth habit and optical quality of KDP are sensitive to the medium environment,for example saturation temperature,supersaturation,impurity and pH value.Therefore,the micro morphology and growth rate of steps on crystal surfaces grown at different growth conditions have been investigated in this paper.The laser induced damage threshold of KDP crystals were measured,and the influence mechanism has been discussed.The main contents of this paper are as follow:1.A series of KDP crystals were grown at different saturation temperature and supersaturation,the laser damage threshold of crystals were tested.The surface micro morphology of the growth steps were’ comparatively studied using an atomic force microscope systematically.The growth rates of the KDP crystals were measured using a laser polarization interference system.The results shown that the step bunching increased at the supersaturation of σ=0.01,0.05 and 0.08 at the low temperature around 35℃.The average elementary step numbers contained in macrosteps were 21,22 and 59,respectively.The corresponding step slope were 9.26×10-3,9.64×10-3 and 8.47×10-3,respectively.At high supersaturation of a=0.08,dendritic steps appeared on the crystal growth,we thought that it might be related with the Ehrlich-Echwoebel（E-S）barrier on the steps.When the saturation temperature were around 45℃ and 55℃,the step bunching both first increased then reduced with the rise of supersaturation when the supersaturation below 0.06,a peak appeared at the supersaturation of ca=0.04,at this point,the height of steps grown around 45℃ was higher than the situation of 55℃.The step slope had a peak at σ=0.06 when the saturation temperature was around 45℃,and a peak at σ=0.04 around 55℃.At high supersaturation,the step bunching and slope both increased rapidly,which is more pronounced at saturation temperature of 55℃.At high saturation temperature of 65℃,the step bunching and slope both first increased then reduced with the rise of supersaturation,a peak appeared at a=0.03,the average elementary step numbers contained in macrosteps was about 48,the step slope was about 8.8×10-3.At high supersaturation of σ=0.08 and σ=0.09,there were some "pits" and "holes" on the crystal surface.These holes were surrounded by protuberances,we thought that these protuberances were formed by the development of 2D nuclei.In different saturation temperature solutions,the curve of growth rate of KDP crystals versus supersaturation could be divided into three region.The supersaturation dead zone σd,the linear supersaturation σ*and the critical supersaturation σ’ all decreased with the rise of temperature.In addition,the rate increased with the rise of temperature at the same supersaturation.The laser damage threshold first reduced then increased with the supersaturation,the trend were liked "V" shape.2.The influence of Fe³⁺ on the step micro morphology and growth rate of KDP crystals grown at different growth conditions using atomic force microscope and laser polarization interference,respectively.The results shown that the step bunching and slope both increased with the rise of Fe³⁺ concentration for the KDP crystals grown by traditional cooling temperature,the uniformity of steps distribution fall down,the dead zone became large,the growth rate decreased,especially for the concentration of Fe³⁺ exceeded 20 ppm.When the concentration of Fe³⁺ was 30 ppm,the growth rate increased with the rise of temperature.It was interesting that the growth rate of KDP crystal in doped solution were higher than in undoped solution at high temperature of 65℃ and 75℃.When the saturation temperature was about 55℃,the concentration of Fe³⁺ was 5 ppm,the step bunching was larger than the undoped KDP only at the supersaturation of σ=0.02.When the concentration of Fe³⁺ was 30 ppm,the step bunching increased with the the rise of supersaturation.In addition,the bunching is larger than the situation of undoped crystals at the same supersaturation.When the concentration of Fe³⁺ was 50 ppm,the bunching was very high at low supersaturation,the kink became more and large at high supersaturation.3.A series of KDP crystals were grown from aqueous solution added with different concentration of CDTA by "point seed" rapid growth technique.Effect of CDTA on the surface micro morphology of KDP crystals were researched by atomic force microscope systematically.The prismatic face growth rate of KDP crystal grown from various solutions were measured by laser polarization interference system.The results shown that the CDTA did not enter into the KDP crystal,the additive of CDTA played a role in the process of growth units diffused on the crystal surface.The CDTA could form coordinate bonds with the metal ions and reduce their chemical activities.Thus,the growth surface was "clean".The step bunching on（100）face reduced dramatically and the step velocity increased obviously due to the complexation effect.When the concentration of CDTA was 500 ppm,after chelating with metal ions impurities,the excess of CDTA could be adsorbed on the crystal surface due to the hydrogen bonding.The force of hydrogen bonding was so weak that CDTA was in the process of adsorption and desorption.These macro molecules were like little"mounds" when they land on the steps terrace.These "mounds" were easily to attract the growth units of K+ and H2PO4-by electrostatics and hydrogen bonding which lead the diffusion of units difficult on the crystal surface.They also hampered steps ahead in the process of evolution.The steps bunching increased and the step velocity reduced compared with the situation of 100 ppm.In addition,the average linear thermal expansion coefficient of KDP crystals along X and Z direction both reduced4.A series of KDP crystals were grown from different pH solutions by traditional cooling method and "point seed" rapid growth method,respectively.The laser damage threshold of crystals grown at different growth condition were tested.The surface micro morphology of the growth steps were comparatively studied using an atomic force microscope systematically.The growth rates of the KDP crystals were measured using a laser polarization interference system.The results shown that the step bunching decreased with the rise of pH value at the low ’supersaturation of σ=0.01,especially for the pH=3.5,the growth steps were mainly elementary steps.Whether the pH value was higher or lower than the normal pH value（4.2）,the laser damage threshold were all increased.When the solution pH value was 5.2,some protuberances appeared at the leading edge of step terrace.These protuberances became large with the rise of supersaturation.The laser damage threshold first increase then reduced and then remained unchanged with the rise of supersaturation.The value were maximum at σ=0.04,it was about 21J/cm2.Adjusted the pH value to 3.5,with the rise of supersaturation,the growth way transformed from screw dislocation steps to 2D uncle development gradually.The 2D islands were continuous stacking.The laser damage threshold had a maximum and minimum value at σ=0.04 and σ=0.06,the values were 27J/cm2 and 18.3J/cm2,respectively.When the solution pH value was 2.5,the step bunching and terrace width both first increased then reduced with the rise of supersaturation.The step slope first reduced and then increased.The laser damage threshold reduced generally,a small fluctuation appeared at σ=0.08,the value was 22J/cm2.In addition,the step growth rate increased no matter up or down the pH value.