Numerical Simulation, Growth And Properties Investigation Of Several Oxide Crystals

Functional crystal matetials have been widely used in the fields of microelectronics, communication, medical treatment, military, exploration, science reaserch and education for its unique optical properties, electronic properties, thermal properties and magnetic properties With the development of such high technology as lasers, energy, communication and spaceflight, furthur demands on crystal materials have been put forward, making them the preceding domain and research hot point both in the fields of material science and engineering development.The Czochraski method is a preferred one among all the crystal growth methods for its short growth period, convenient observation and controllability. Crystal growth is a process of transformation from liquid sphase to solid sphase and achieved by the regularly solidating of grow species at the growth interface whose profile has much influence on the shape and quality of the as-grown crystal. Release of latent heat occurs during crystal growth and heat exchanges exist between different components of growth system by conduction, convection and radiation, and various type of melt convections, which influence the transportation of mass and heat, can be induced under the influence of buoyancy, pressure, magnetic field , crystal and crucible rotation. The above mentioned phenomena of heat transfer and mass transfer are the key factors that determine both the process of crystal growth and crystal quality. Transparent liquids simulation and numerical simulation are usually applied to the study of heat transfer and mass transfer during crystal growth. But, for some shortages of transparent fluids simulation, people still can not know much about the real process of crystal growth. While as long as appropriate model and parameters were adopted, numerical simulation can provide detail description on the thermal field, velocity field and concentration field of the melt; accurate prediction for the shape of growth interface and reasonable explaination on the formation and distribution of defects, which can be very helpful in understanding the mechanism of heat transfer and mass transfer, as well as the influence of various growth paramaters on the crystal growth and quality with less time and economical cost. As a result, numerical simulation has become key tools in the research and improving of crystal growth, and play an important role in commercial crystal production. In this work, using a numerical simulation software of CGSim, the heat transfer, mass transfer and influence of growth parameters on crystal growth were investigated. LGS, Nd:LGS and Nd:CNGG crystals were grown by Czochralski method, problems occurred during crystal growth were numerically simulated and corresponding resolvents were proposed. Characterization on crystal quality, thermal properties, optical properties and laser performance were also carried out. The outline is shown as follows:1. Numerical simulationThe constituent modules, governing equations and boundary conditions of the CGSim software were introduced. Various physical effect on the mass and heat transfer, including influence of crystal rotation rate on the growth interface, crystal and crucible size on the behavior of melt flow, role of cucible rotation and Marangoni flow on crystal growth, were systematically simulated.2. Crystal growth2.1 Growth of LGS crystal with optical quality and Nd³⁺-doped LGS laser crystalPolycrystalline materials of LGS and Nd:LGS was synthesized by solid-phase reaction with 99.99% purified starting reagents of La₂O₃,Ga₂O₃,SiO₂. and La₂O₃, Ga₂O₃, SiO₂, Nd₂O₃, respectively. Optical quality LGS electro-optical crystal and Nd:LGS laser crystal were grown by Czoahralski method with auto diameter controll (ADC) technique. Influence of thermal field, starting composition of raw materials, solid/liquid interface and afterheater on crystal growth was discussed.Defects listed below occurred in the crystal growth were analyzed with the assistance of numerical simulation and approaches for problems resolving were proposed:Core and the growth section boundaries: A core defect easily occurres in the middle part of as-grown crystal by the formation of some facets, which can be avoided by crystal growth with flat interface.Scattering particles: Scattering particles are usually formed before boule stage of crystal growth for that such particles can not be efficiently expelled from the region under crystal. Such scattering can be reduced by the crucible rotation or the increase of crystal rotation rate.Growth striation: Growth striations can be introduced by the variation of growth speed which induced by the fluctuation of power input, so to ensure a stable growth environment is beneficial for striation reducing. Shoulder crack: During the period of crystal growth with flat shouldering, the shouder part of crystal was exposed directly upward to the coolest portion of growth enclosure, this increases the heat loss throgh crystal, which results in large thermal gradient in the shoulder part of the crystal, corresponding the crack. Increasing the height of afterheater can decrease the thermal gradient in the shoulder part of as-grown crystal, such crack can be avoided dramaticallly..2.2 Growth of Nd:CNGG crystalPolycrystalline material for Nd:CNGG single crystal growth was aslo synthesized by solid-phase reaction with 99.99% purified starting reagents of CaCO₃,Ga₂O₃,Nb₂O₅,Nd₂O₃. On the basis of numerical simulation, Disordered Nd:CNGGcrystal with Nd³⁺-doping concentration of 0.5at% was grown by Czoahralski method from half-loaded melt.3. Investigation on the crystal quality of LGS and thermal and laser properties of Nd:LGS and Nd:CNGG crystals3.1 Determination of crystal quality of LGSLGS crystal with high optical quality is necessary for applications of electro-optical Q-switch.. Optical homogeneity was examined by measurement of static extinction ratio, and that of composition was investigated by X-ray fluorescence experiment and variation in cell parameters with XRD powder experiments. All the results show that the as-growgn LGS crystal have both high optical quality and composition uniformity, which is qualified for eletro-optical Q-switch applications.3.2 Study on properties of Nd:LGS crystal3.2.1 Segregation measurementThe segregation coefficient of Nd³⁺ in Nd:LGS crystal was determined to be 0.87 by X-ray fluorescence analysis, and hence, LGS crystal can be highly Nd³⁺-doped.3.2.2 Thermal propertiesImportant properties of Nd:LGS crystal such as the specific heat, thermal expansion, thermal diffusion and thermal conductivity have been systematically determined. The specific heat was measured to be 0.376 J·g^-1·K^-1 at room temperature by a Differential Scanning Calorometer(DSC). Thermal expansion coefficients were measured to beα₁₁=6.05×10^-6/K andα₃₃=4.24×10^-6/K by thermal-mechanical analyzer at a temperature range of 30-500℃. Thermal diffusion coefficient was measured by laser flash method, and thermal conductivity was calculated to be k₁₁=1.42 W/m·K and k₃₃=1.75W/m·K with the measured data of specific heat, thermal diffusion coefficient and density, which was higher than that of glass, making Nd:LGS crystal suitable for such applications in medium and high laser system.3.2.3 Optical propertiesThe optical properties of a laser crystal determine its application regimes and laser performznce. The absorption and emission spectra of Nd:LGS have been measure. From the absorption spectra, it has been found that theσpolarized absorption is stronger than that ofÏ€polarized and the strongest one is centered at 588 ran. The full width at half maximum at 808 nm is about 20 nm wider which is over 10 times that of Nd doped vanadate and YAG crystals. From the emission spectra, we found that there is a strong emission peak centered at 1065.5 nm. The crystal has a wider emission band from 1020 nm to 1120 nm, which is comparable with that of Nd:Glass. Beside this emission peak, there are also two peaks centered at 904 nm and 1342 nm, respectively.3.2.4 Laser propertiesFor the first time to our knowledge, the high-power LD pumped laser performance of Nd:LGS has been studied. The highest cw laser power was obtained to be 2.25 W with the slope efficiency of 28.7%, and the laser beam has been characterized. With a Cr:YAG as the saturable absorber, the passive Q-switched Nd:LGS laser was achieved for the first time to our knowledge. The maximum average output power, largest pulse energy, shortest pulse width and highest peak power were 0.54 W, 175μJ, 23.4 ns and5.02 kW, respectively. The LGS crystal have been identified to be a excellent electro-optical crystal, we believed that the Nd:LGS should be a self-Q-switched laser crystal, and the experiments are being proceeded.3.3 Properties of Nd:CNGG laser crystal3.3.1 Thermal propertiesThe specific heat was measured to be 0.36 J·g^-1·K^-1 at room temperature by a Differential Scanning Calorometer(DSC), and themal expansion coefficients was measured to be 7.88×10^-6/K by thermal-mechanical analyzer at a temperature range of 30-500℃. Thermal diffusion coefficient was obtained by laser flash method from 29 -300℃and has a value of 1.223 mm²/s at room temperature. Thermal conductivity was also calculated to be 3.43 W.m^-1.K^-1 at room temperature. 3.3.2.Thermal-optical coefficient of Nd:CNGG crystalThe effect of thermal-focusing lense must be considered for high power laser systems. Based on a flat-flat cavity, the distance of thermal-focusing lense was measured along a <111> derection oriented Nd:CNGG sample using a LD centered at 808 nm as pump source. Through functional fitting on the distance of thermal-focusing lense vs incident power curve, the thermal-optical coefficient was obtained to be 9.2×10^-6K^-1, which is almost three times that of Nd:GdVO₄ (2.7×10^-6K^-1).3.3.3 Laser propertiesLaser properties of Nd:CNGG crystal was investigated based on a flat-concave cavity using a LD pump source centered at 808 nm. The continous-wave output power, slope efficiency and optical-optical conversion efficiency were obtained to be 1.91 W, 28% and 18.6%, respectively. We believed that laser performance can be further improved with modified cavity based on the fact that the output power was still not saturated.