Numerical Simulation Investigation Of The Transport Characteristics In The New System For KDP Crystal Growth From Solution

Potassium dihydrogen phosphate (KDP) is one kind of nonlinear optical materials (NLO) with excellent performance which developed during the 30s and 40s of 20th century. Because of the high nonlinear optical quality and high laser-induced damage threshold, it is widely used in the fields of frequency conversion of laser, electro-optic modulation and optical switching, etc. The inertial confinement fusion (ICF) has been improved with the rapid development of laser technology. Research shows that the large-scale and high-quality KDP is the only nonlinear optical crystal which can be used for the ICF.In recent years,for the new demand on quality and size of KDP crystal in practical manufacture, the research is mainly focused on how to grow the large-scale and high-quality KDP crystal with high growth rate. The morphological stability of crystal surface is an important factor to determine the quality of crystal, which is well correlated with the characteristic of flow field and concentration field, even the step movement. Although a lot of good results have gained by experimental method, there still exist some limitations. Some key data in the flow field and concentration field, such as the surface shear stresses and surface supersaturation is very difficult to obtain in the experiment, but easy to obtain by numerical simulation. In this paper, the problem of transport and surface growth in the process of crystal growth has been studied with the aid of basic theory of hydrodynamics, heat and mass transfer. Some new growth systems are proposed for solving the problem meeting in the production and the flow and mass transfer are also discussed by means of numerical simulation. The main contents are as follows:(1) According to the in-situ AFM investigation of the KDP crystal growth from solution, the three-dimensional and steady-state mathematical model is established for solving the momentum and mass transport. The numerical simulation is based on the finite volume discretization with staggered grids. Results show that the pumping action at the upper surface of the crystal caused by the natural convection decreases with increasing the inlet velocity. The position of the minimum of the surface supersaturation has a positive shift along the direction of the main flow. It gets to the edge of crystal even disappear if the inlet velocity increases to a high level. The minimum value of surface shear stress appears at the area where the natural convection is most intense. But due to the comprehensive effects of both the natural convection and forced convection, the variation of the surface shear stress along the characteristic line is complex. The monotonicity can not be observed by increasing the inlet velocity. The thickness of the solute boundary layer on the center of the upper surface is closely related to the flow intensity and decreases by raising the inlet velocity. With the crystal size increasing, the distribution difference of the surface supersaturation is obvious and the average value of surface supersaturation decreases due to the enhancement of the solute consumption.(2) For the traditional method of KDP crystal growth, the main issue in the process of crystallization by rotating the growing crystal is that the temporal and spatial evolution of surface shear stress and surface supersaturation results in the formation of inclusions. Hence, the new growth system of direct injection is proposed. In this system, the statinary KDP crystal is employed and the solution is imported into the growing container facing to the crystal surface. The computational analysis is conducted and the results show that the inlet velocity is the key factor that has a significant effect on the surface supersaturation. As increasing the inlet velocity, the surface supersaturated increases, but for an excessive value, the region of high supersaturation in the vicinity of the interface between the prism and pyramid appears which is not beneficial for the crystal growth. The homogeneity of surface supersaturation can be improved by adjusting the value of Vpr and Vpy, and this will be helpful for eliminating the negative effect and reducing the occurrence of inclusions. As raising the inlet velocity, the thickness of solute boundary layer decreases linearly. In the meanwhile, the surface shear stress increases, but it is far less than the value in traditional growth system. The investigation on natural convection and forced convection shows that the characteristic of the flow field is not the same for different inlet velocities, which can be used for describing the variation of the distribution of surface supersaturation and homogeneity. In addition, the optimization for the configuration has also been performed, which is helpful for the surface homogeneity by adding some auxiliary pipes.(3) The further improvement is performed by introducing the rotary flow field with the aim of better solute mixing. The results show that the rotation rate of the disk has a significant effect on the surface supersaturation. For the high rotation rate (500~700rpm), the forced convection around the crystal surface is so intense that the surface supersaturation is high enough to ensure the crystal to grow rapidly and the homogeneity in most area of the surface is advantageous for the growth of high quality crystal. The surface supersaturation becomes small and the homogeneity gets unstable if the rotation rate reduces to a low level (100~400rpm). Hence, it is not appropriate for rapidly crystal growth. The inlet velocity has a tiny impact on the surface supersaturation on both prism and pyramid, but the surface homogeneity on prism gets worse with the increase of the inlet velocity. Though the surface shear stress increases with enhancing the rotation rate or the inlet velocity, it can always maintain in a low level. Hence, the morphological stability of crystal surface is guaranteed. The region of low supersaturation at the bottom of the prism nearly disappears by changing the height of the channel turning, and the bigger surface supersaturation and better surface homogeneity can also be realized.(4) The investigation on the transport characteristic for various bulk supersaturation indicates that the most direct and efficient way to enhance the surface supersaturation is increasing the bulk supersaturation. But the bigger bulk supersaturation seriously results in the worse surface homogeneity and quality of crystal. Hence, it is not a wise choice to improving the growth rate just by enhancing the bulk supersaturation in the production. In addition, the bulk supersaturation does not have obvious effect on the distribution of surface supersaturation, thickness of solute boundary layer and characteristics of flow field.