The Calculation And Experimental Study Of The Transport Characteristics For Different Growth Systems Of KDP-type Crystals Under Modified Rotating Crystal Method

KDP-type crystals usually refered to ammonium dihydrogen phosphate(NH₄H₂PO₄,ADP)and potassium dihydrogen phosphate(KH₂PO₄,KDP)crystals,which have the effects of the second,third and fourth frequency conversion on laser of1064 nm due to excellent performances of electrooptic and frequency convert,are widely used in harmonic-generation,electrooptical modulator and Q switcher of laser medical and inertial confinement fusion(ICF)devices.Around the growth of KDP-type crystals,many solution growth systems are proposed,such as traditional temperature-reduction system,solution circulating system,rapid growth system based on a point seed and restrictive rapid growth system.For solution crystal growth,solute near the crystal surface is absorbed continuously on the steps and kinks,which forms a solute boundary layer of low concentration.Consequently,solute is required to transfer from the bulk of the solution to the crystal surface to maintain crystal growth.Normally,solute can be transfered via diffusion and convection.High quality crystals with uniform composition can be obtained if crystal grows under the condition of pure diffusion,such as Sankaranarayanan-Ramasamy(SR)method.However,the growth process is highly uneconomical because the pure diffusion is very slow.Therefore,in different growth systems,rotating crystal method(RCM)is usually applied to introduce convection to reduce the resistance of solute transfer and accelerate the growth rate.Unfortunately,while RCM increases the growth rate,it also deteriorates the stability of morphological on crystal faces,leading to the formation of liquid inclusions.In terms of the current different growth systems,it is insufficient to improve crystal quality only through the integration of growth system,including the integration of pipe system of growth device,the upgrading of continuous filtration system,the development of real-time monitoring system and the high-precision controlled technology of thermal annealing.In fact,convection plays an important role during the solution crystal growth,unfavorable convective conditions is the primary factor inducing morphology instability and formation of liquid inclusions.Consequently,it is crucial to the growth of crystal with high quality by optimizing the flow around crystal faces.Since the flow inside the growth system is dominated by three-dimensional unsteady turbulence,which is difficult to study through experiments.Accordingly,in order to describe the convection in the growth system and its mechanism of the influence on solute transport more accurately and quantitatively,numerical calculation is needed.Based on the above background,the present paper uses the basic knowledge of thermal science(fluid dynamic,heat transfer and mass transfer)to analyze and optimize the flow,heat and mass transfer inside the different growth systems of KDP-type crystals under the RCM through the numerical and experimental study,the main contents are summarized as follows:(1) The numerical simulation of small scale ADP crystal growth under incline mounting mode indicates that this mode can eliminate the vortexes and flow stagnation regions previously existing near the prismatic faces and improve the magnitudes and homogeneity of the surface supersaturation.The higher rotation rate is,the more obvious improvement of the magnitudes and homogeneity of the surface supersaturation is.The increase of the crystal size decreases the magnitudes and the unifomity of the surface supersaturation,which is unfavorable for the crystal growth.Therefore,as the crystal growing,an appropriate higher rotation rate should be adopted.Finally,the comparison of surface morphology,dielectric loss and etching test of the ADP crystals grown by the horizontal and incline mounting modes reveals that the ADP crystals with higher quality can be obtained by the incline mounting mode.(2) The numerical simulation of the flow and mass transfer during the large scale ADP crystal growth under the RCM with the additional stirrer are performed.The results show that the addition of stirrer can produce stronger axial flow and promote the dominant effects of forced convection on mass transfer comparing with the RCM,accordingly,improving the magnitudes and homogeneity of the surface supersaturation evidently and keeping the stability of morphology on crystal surfaces.The rise of stirrer rotation rate enhances the shear stress on crystal surface,resulting in a reduction of the thickness of the solute boundary layer and an improvement of the values and uniformity of the supersaturation.With the crystal rotation rate increasing,the distribution of the supersaturation on the prismatic surface becomes uniform.However,as the crystal sizes gets large,there is a little reduction in the supersaturation on the prismatic faces,but there is an improvement in the uniformity of supersaturation.(3) Considering the effect of temperature field on crystal growth,a comprehensive mathematical model is established to describe the interaction of heat,momentum and mass transfer under solution circulating system.The variation of the time-averaged supersaturation on crystal surface under different operation conditions is investigated by numerical simulation,focusing on the analysis of the relationship of flow,temperature and mass transfer.The effects of the secondary vortex adjacent to the prismatic faces on mass transport is discussed,and the role of the crystal size on the crystal cracking is analyzed.The conclusions are as follows:the raise of the crystal rotation rate weakens the blocking effect of secondary vortex on mass transfer and promotes the solute supplement of the fresh solution to the crystal faces,as a result,the values and uniformity of the supersaturation are ameliorated,and the temperature field of the growth system enjoys a better uniformity.The inlet velocity and the inlet temperature affect the supersaturation on the crystal faces through mainly changing the temperature field.As the inlet velocity and the inlet temperature gets high,a considerable increase can be seen in the temperature inside the growth system,leading to the raise of equilibrium concentration of the solution remarkably,so the supersaturation on the crystal faces decreases.With the crystal size getting large,there is an increase in thermal stress on the pyramidal faces;meanwhile,there is a reduction in the homogeneity of supersaturation on pyramidal faces,promoting the probability of the formation of inclusions,reducing the critical value of cracking stress at the pyramidal region of crystal.Hence,larger crystal size cracks more easily.(4) A new system named"jet-rotating crystal method(J-RCM)"is proposed applying the effect of forced convection of the solution jet flow.The advantage of the new system is evaluated by numerical method.The results indcate that,unlike the RCM,the J-RCM increases the supersaturation on the prismatic faces obviously,especially,the characteristic of surface supersaturation distribution is changed completely,which suppresses the generation of inclusions.With the solution jet velocity increasing,there are improvement in the magnitudes and homogeneity of the supersaturation on the prismatic faces.And the increase of the crystal size improves the magnitudes and homogeneity of the surface supersaturation slightly.The design of period swing jet flow can eliminate the influence of the secondary vortex adjoining the prismatic faces and further improve the mass transfer.When the solution jet velocity is equal to and more than 0.6 m/s,the forced convection is dominant in mass transfer.Besides,the forced effect of the solution jet flow promotes the characteristic of the convective mass transfer near the crystal faces,so the Damkohler number decreases evidently.