Thermal Field Analysis Of Large Sapphire Crystal Growth Based On Paulson Method

Sapphire,chemical name α-Al2O3,is an anisotropic material,high purity sapphire,transparent,brittle,electrical insulation,does not react with other substances at room temperature,only at high temperature,can be corroded by weak acid and weak alkali.The unit cells are combined in the form of covalent bonds,forming hexagons,which are not easy to be destroyed.In this paper,we will discuss the growth of sapphire crystal by the bubble generation method from the following aspects:Firstly,the physical,chemical,mechanical and electrical properties of sapphire are listed in detail,and the mainstream applications of sapphire chips in the market are summarized.The advantages and disadvantages of the existing methods for preparing large crystals are summarized,and the most widely used bubble growth method is studied in detail.From the macro and micro point of view,the growth principle,crystal defects and technological process of sapphire crystal grown by bubble method are described.According to the anisotropy of sapphire cells and the packing characteristics of "basic elements",the most suitable growth direction is concluded;Combined with the experience of the workers and a large number of literature,the growth characteristics of different stages and the crystal defects easily occurred due to improper operation in each stage were summarized,and the representative solutions for crystal defects were summarized.According to LJ-B200 crystal furnace studied in this paper,the technical parameters of different parts of the equipment are listed,including vacuum system,water cooling system,long crystal chamber,power cabinet and lifting weighing system.The furnace model and boundary conditions of temperature field are described mathematically.According to the three laws of energy conservation,momentum conservation and mass conservation of heat field,the mathematical model of heat field of bubbling furnace is deduced,and the heat boundary equation is given according to the specific boundary arrangement;Based on the material parameters adopted by Brandon et al.In the research area,the thermo-physical parameters of the main materials are listed.On the premise of not affecting the main calculation results,the furnace model is simplified by neglecting the vacuum,water cooling,electric control and lifting system,and the heat transfer mode between the components in the room is introduced from the perspective of heat transfer;According to the two-dimensional model given by AutoCAD,Solidworks software is used to build three-dimensional furnace model.Based on the ANSYS Workbench software,the boundary conditions of the long crystal chamber model were loaded,and the temperature,flow field and melting solidification cloud images of the initial stage of crystal initiation and equal diameter were given.The simulation results show that in any growth stage,when the temperature field meets the characteristics of low temperature in the center of the crucible,high temperature in the outer wall,low temperature in the upper side and high temperature in the lower side,and the crucible diameter is larger than the height of the molten liquid,the vortex ring with opposite flow direction from bottom to top along the center of the crucible can be formed in any two-dimensional horizontal plane.Aiming at the main use area of finished products,i.e.long crystal section in equal diameter stage,marking the vertical and horizontal section near the solid-liquid interface at the initial stage of equal diameter,the thickness of the upper,lower and lateral insulation layer,the ambient temperature of the outer wall of the crucible,the working fluid temperature and flow rate of the heat exchanger are changed successively from the angle of structural dimension adjustment and process parameter adjustment The thermal conductivity of the melt is used to analyze the effect of the long crystal at the solid-liquid interface.It is found that the change of the thickness of the insulation layer in different positions will affect the temperature distribution in the furnace,among which the change of the thickness of the upper insulation layer is more obvious than that of the lower side,and the influence of the thickness of the side insulation layer is the most significant;With the increase of the temperature of the outer wall of the crucible,the temperature in the crucible increases obviously.The eddy current inside the molten liquid is disordered and the static and immobile proportion of the eddy current center increases.If the temperature is too large,the crystal will melt easily,and the long crystal will stop even re-melting;The temperature of working fluid has more significant influence on the heat exchanger than that of heat exchanger;When the thermal conductivity of solid-liquid phase is larger than that of liquid phase,it is more favorable for crystal growth and maintains solid liquid equilibrium.This paper can provide theoretical data for thermal field analysis of large-scale crystal growth,and also provide theoretical reference for crystal growth of other materials.