Research On The Marangoni-thermocapillary Convection In A Czochralski Configuration Under Axial Magnetic Field

The Czochralski method as an effective methods of crystal growth technologies is widely adopted to produce single crystals.During the process of producing crystal materials under microgravity conditions,the flow of the melt caused by the surface tension gradient will affect the growth quality of the crystal material directly.When the imposed temperature gradient is perpendicular or parallel to the free surface,the flow instability mechanism and flow structure after destabilization are different,and the corresponding flow is named Marangoni convection and thermocapillary convection,respectively.At present,most of the scholars focus on the flow that is driven only by the radical temperature gradient or vertical temperature gradient,but there are little investigations on the Marangoni-thermocapillary convection that is induced by the bidirectional temperature gradients.In the industrial process of the Czochralski crystal growth method,the convection of the melt could lead to the segregation effect,and the application of the axial magnetic field has a good effect in suppressing the convection of the melt and improving the quality of crystal growth.Therefore,this paper will use three-dimensional numerical method to explore the Marangoni-thermocapillary convection under the bidirectional temperature gradient,and discuss the influence of different magnetic field strength,liquid depths and the gravity.The results could enrich the investigations of Marangoni-thermocapillary convection,and provide practical guidance for the growth of silicon crystal materials.The major works and conclusions are as follows:Firstly,the effect of the axial magnetic field on the steady-state flow dominated by the radical temperature difference and the bottom heat flux density was studied.It was found that the axial magnetic field has an inhibitory effect on the two types of steady-state flows.As the strength of the magnetic field increases,the amplitude of the temperature fluctuation on the free surface is weakened,but the melt flow structure and the isotherm distribution of the R-Z surface almost remain unchanged.Secondly,the influence of the axial magnetic field on the unsteady flow dominated by the radical temperature difference and the bottom heat flux density was analyzed.As the strength of the magnetic field increases,the three-dimensional oscillatory convention gradually transits to the three-dimensional steady flow,but the unsteady flow caused by different temperature gradient will undergo a different transition route.Thirdly,the critical horizontal temperature gradient Ma_cri and critical bottom heat flux Q_cri of the flow under different axial magnetic field strengths was obtained by linear fitting method.As the axial magnetic field strength increases,the critical value increases monotonously.Namely,the imposed axial magnetic field could suppress the flow instability.Finally,the effects of different magnetic field strength,depth of the pool and gravity on Marangoni-thermocapillary-buoyancy convection was explored in a Czochralski configuration with the liquid depth of 6mm.It was found that the buoyancy effect can not be ignored when considering both the liquid depth and gravity.It was certified that the buoyancy effect could promote instability of melt flow significantly.However,since the thermocapillary force and buoyancy play relatively dominant roles in the flow of silicon melt,the effect of the magnetic field on the melt is not significant,and the flow remains a three-dimensional unsteady state.