| The potassium dihydrogen thophosphate (KH2PO4, KDP) crystal has been investigated since the 30's of the twentieth century. As one of the non-linear optical materials and electro optics, it has been broadly used for many high technology fields such as electro optics modulated, laser frequency conversion, ray celerity switch and so on. It is characterized by its big non-linear optical coefficient, electro optics and high damage threshold. The most important is that KDP is the only nonlinear optical crystal that can be grown to large sizes (50～80cm) used for the ICF.With the development of ICF technology, more high-quality and large-sized KDP crystals are needed urgently. Recently, the investigations on KDP crystal focus on improving its growth speed and improving the quality of crystal. In order to get high quality KDP crystal, it is needed to understand the microscopic processes which happen on the growing crystal surface and know the growth mechanism that controls the crystal growth. The atomic force microscope (AFM) is a powerful instrument that can help people to investigate the microscopic growth of crystal. In addition, adjusting the pH value of KDP solution can change the nature of the solution, thus the crystal growth habit of KDP can be changed. However, there are still lots of controversy on how pH value affects the growth of KDP crystals, and is not a uniform conclusion. Therefore, as an important parameter in the crystal growth, the effects brought by changing pH value need to be studied in depth.In this paper, the impacts of pH value of solution were studied through investigating on the growth of KDP crystal under different conditions with the aid of basic theory of thermal science. The main contents are as follows:①The solubility curves and the metastable zone width of KDP solution were measured with different pH values. The results show that the solubilities of KDP crystal are increased with the changes of pH value. The widths of metastable zone become wider and the stabilities of solution are improved. The influence of pH value on KDP crystal growth is discussed associated with KDP crystal growth experiments. It is found that the higher pH value of KDP solution is more suitable for the high quality and large-sized crystal growth. The pH values of KDP solution which are in the range of 4.5～5.0 are more proper for KDP crystal growth.②The effects of different pH values on the nucleation of KDP supersaturated solution were studied under different temperatures. The induction periods with different growth conditions were measured by the method of photography. The results indicate that when the supersaturation ratio of KDP soulution S >1.3, the homogeneous nucleation playes a dominating role in the nucleation; when S <1.2, the primary nucleation is heterogeneous. According to the classical homogeneous nucleation theory, nucleation parameters are calculated by using the experiment results. By the calculation of the surface entropy factor, the crystal surface growth model of KDP in these experiments is confirmed, and it is continuous growth.③Though investigating on the growth of (100) face of KDP crystal by ex situ AFM under different pH values at different supersaturations at 40℃, it is found that the growth of crystal is controlled by step flow at lower supersaturations, and the morphologies of steps are different under different growth conditions. In addition, at the higher supersaturations, 2D nucleation mechanism controls the growth. When the supersaturations are lower, the dislocation mechanism controls the growth of crystal, and whenσ≥0.05, 2D nucleation mechanism plays a dominant role in the growth of the (100) face for pH=4.2 and pH=2.5. However, for pH=5.0, the dislocation mechanism also dominates the growth of crystal when the supersaturations are lower, but whenσ≥0.03, the crystal growth is controlled by 2D nucleation mechanism. By using in situ AFM, we find that with the reduction of the supersaturation of the solution, the density of steps also decreases, the width of steps becomes larger. Finally, a phenomenon which is the obvious anisotropic growth of steps in the step flow was observed by in situ AFM atσ= 0.025 at pH=5.0.④Though investigating on the kinetics of step flow of the (100) face of KDP crystal by using in situ AFM with different pH values at lower supersaturations at 25℃, the horizontal direction growth velocities and normal growth velocities of the (100) face were estimated at different growth conditions by in situ AFM. It can be found that when the pH value is 5.0, the normal growth rate is the fastest at the same supersaturation comparing to the other pH values, and the screw dislocation mechanism controls the crystal growth. The step edge free energies are calculated under different pH values, we find that when the pH value is changed, the edge free energy becomes smaller, and the growth of step becomes much easier and faster.⑤By in situ AFM measurement, the morphology changes of the (100) face of KDP under different pH values were observed in the process of crystal growth. The real step flowing rates of KDP are also calculated when the supersaturations are between 0.01～ 0.025 and the temperature is 25℃. Using the results of step flowing rates, we get the kinetic coefficients under different pH values. Through contrasting the kinetic coefficients between experiment values and theoretical values, we find that the mass transfer is controlled by bulk diffusion under different pH values in the growth of steps. And with changes of pH value, the kinetic coefficients become larger, the diffusion rates of growth unites are also faster, so the growth rates of (100) face of KDP increase.⑥Various images of 2D nucleation under different growth conditions were presented by using in situ AFM. It is found that the growth of KDP is controlled by polynuclear nucleation mechanism at the high supersaturation. With reduction of the supersaturation, the growth velocity of 2D nuclei becomes very slow and shows typical anisotropy. It is found that the process of coalescence of 2D nuclei does not lead to defect. The experiments show that the growth mechanism for KDP at 25℃changes between step flow and 2D nucleation in the supersaturation range of 4.5～5%. The triangular nuclei which are close to equilateral triangle were observed in the experiment at the supersaturationσ=6% for the first time, showing typical anisotropic growth. Through observing the dissolution process of 2D nuclei, it can be regarded as the reverse process of growth. Finally, we find that the microcrystals landing on the surface atσ=9% will grow and coalesce with each other and there is no observable defect in the coalescence. |
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