| Large-sized magneto-optical isolator is required by high power and high energy laser system.Terbium Gallium Garnet(Tb3GaxO12,TGG)magneto-optical crystal is considered as an important magneto-optical crystal in the visible and near-infrared wavelength region,for its high transmittance,large Verdet constant,low internal stress,good processing performance,low preparation cost,congruent melting property and suitability for large-sized crystal growth.A widespread attention has been paid on the growth technology of large size,high-quality TGG crystal.Meanwhile,TGG is also considered as a multifunctional application material with its potential of luminescent host and scintillation material.Due to serious Ga volatilization during crystal growth of TGG,it is very sensitive to the components and crystallization of start material for the crystal growth property of TGG.On the other hand,the spiral growth and concave interface growth often occur during the large-size crystal growth of TGG,which results in the bad quality and cannot be used.So it is important to design and optimize thermal insulator structure,pull and rotation speed through numerical simulation of crystal growth in order to overcome spiral and concave interface growth of large-size TGG.This work will give a systematic exploration on the above problem.Firstly,the phases transition,component variation and valence of Tb element during the solid state reaction of TGG phase under different calcination temperatures and initial molar ratios were investigated.Secondly,the key thermo-physical parameters required for TGG crystal growth simulation were obtained by comparing the interface shape of experimental crystal growth and quasi-static numerical simulation.Based on these parameters,the growth state of TGG crystal under different crystal rotation rate,growth rate,and insulation structure was obtained by simulation,and the process parameters were optimized.Finally,the magneto-optical property,extinction ratio,optical loss,electronic structure,luminescent property,optical dispersion,and optical band gap of TGG crystal were investigated.The major contents and results are as follows:1.The phase transition,valence variation of terbium ion with different components calcination process were investigated with XRD,XPS analysis and Rietveld refinement.The optimal condition for synthesizing pure TGG phase polycrystalline is:initial molar ratio of Tb:Ga=3:4.75～3:5.15,1200℃ and 48 hours.ΦThe tri-terbium gall ate dioxide(TGD)was found as a metastable phase which exists during the formation of TGG.According to XRF analysis,raw materials with initial molar ratios in the range of Tb:Ga=3:4.75-3:5.15 will transform into pure TGG phase polycrystalline with molar ratios of Tb:Ga=3:3.55～3:3.86 after calcination at 1200 0C for 48 hours.In the range of 1000～1200℃,the loss rate of Ga element compared to its initial molar quantity increases with temperature increasing,which stabilizes at around 23%above 1200℃.This indicates the necessity to add excessive Ga2O3 in the preparation of starting materials for TGG crystal growth.Combining the results of XPS,TG-DSC and XRD analysis,it was confirmed that there were no or little Tb4+ions existing in the samples with the initial molar ratio of Tb:Ga=3:5 and calcined at 900～1200℃ for 48 hours.2.The two TGG crystals of Φ30×80 mm and Φ60×63 mm were successfully grown with Czochralski method and their protrusion angles are around 30.50 and 28.2°,respectively.A series of numerical simulation models with different thermal conductivity and equivalent absorption coefficients were constructed according to the actual crystal growth systems,and their growth status were obtained by simulation with CGSim software package.The protrusion angle of the crystal tail in numerical simulation is closest to the actual situation with a crystal thermal conductivity of 3.5 W·m-1·K-1 and an equivalent absorption coefficient of 175 m-1,indicating that the simulation result under these physical parameters are close to the experimental results.3.The influence of rotation speed,growth rate and insulation structure on the growth of terbium gallium garnet crystals with different size were investigated with quasi-static numerical simulation.The rotation speed and growth rate were optimized,and the influence of insulatior structure on the crystal growth state was discovered.For the growth of Φ60 mm TGG crystal,4～7 r·min-1 is a relatively suitable rotation rate range,and 4 r·min-1 was determined as the optimal rotation rate according to the requirements of convex interface and stable growth.The protrusion of the crystal/melt interface decreases with the increasing of target growth rate.The optimal growth rate is 1 mm·h-1,when the growth time,crystal quality,cost and interface control were considered comprehensively.For Φ73 mm TGG crystal,4～6 r·min-1 is a relatively suitable rotation rate range,and 4 r·min-1 was found as the optimal rotation rate.The protrusion angle of crystal tail was found to decrease when the thickness of the upper insulator tube increases,according to the analysis of the models with different insulation structures;and increase with the increasing height of the upper insulator tube.This provides a improvement direction of thermal insulator structure.A Φ73 mm TGG crystal was actually grown with the optimized growth process parameters,its protrusion angle is close to the simulation results and has high crystal quality,high extinction ratio,and low optical loss.This indicates that the optimized growth parameters can be used for stable growth of large-sized and high-quality TGG crystal.4.TGG was found as a direct band gap material with DFT calculation.Part of the Tb-4f electronic states locate in the band gap,which couple with other electronic states,and form a narrow and strong localized state in the range of 2.15～3.27 eV.According to the photoluminescence spectroscopy analysis,the 7 major emission peaks correspond to the 5D4→7FJ(J=0～6)intra-band transitions were found under the excitation of 379 nm wavelength light source.It was found that the dispersion energy Ed is 24.682 eV,the single oscillator energy E0 is 9.012 eV,and the static refractive index no is 1.93,according to the analysis of dispersion properties of TGG crystal with refractive index spectroscopy and WDD model;The constant β was calculated as 0.26 eV,which proves that TGG is an ionic crystal.The optical band gap obtained from the WDD model and Tauc model are 3.76 and 3.85 eV,respectively.The optimal synthetic conditions for TGG polycrystalline,physical parameters for TGG crystal growth simulation were obtained.The optimized crystal growth technology parameters,thermal insulator,electronic structure and optical parameters were given.This work provides systematic theoretical foundation for the preparation and application of TGG crystal. |
