Numerical Simulation Of Large Size CaF₂Crystal Growth Process And Its Experiment

CaF2crystal was a traditional crystal materials, it had a wide range of applications in the industrial and scientific research. Large-sized and high quality of CaF2crystals were grown by the vertical Bridgman Method. CaF2crystal has excellent optical properties, mechanical properties and chemical stability, it had good application prospect in the window materials, laser host crystal and ultraviolet carved. In this paper, φ200mm of CaF2crystals were grown by the vertical Bridgman method, and the crystal growth process were analyzed by numerical simulation. The composition of the crystal growing furnace and the process parameter of crystal growth were optimized. According to the results of numerical simulation analysis, crystal growth experiment was done. And the performance of CaF2crystals has been researched. The results of the experiment are as follows:1. With increase of crucible descent velocity, convection heat transfer effect was weakened, and the fluctuation of the melt flow increased near solid-liquid interface, the stability of the crystal growth were poor.With the increase of solidification fraction, melt convection flow was compressed, flow cell occur a phenomenon of division, and the fluctuation of the melt flow decreased near Solid-liquid interface. It had different temperature field distribution in system of crystal growth, the shape of the interface were changed. With the increase of solidification fraction, convex degree C value of the solid-liquid interface increased. Rate of crystallization and descent velocity of crucible were inconsistent.2. According to the results of the interface shape, temperature distribution and the flow function, it was concluded that temperature gradient of high temperature zone was3℃·cm-1, temperature gradient of gradient zone was25℃·cm-1, temperature gradient of cold temperature zone was5℃·cm-1and the length of gradient zone was5cm. From the melt flow analysis, the crucible descending speed of suitable crystal growth was1mm·h-1.3. Purification technology of raw material:PbF2addition was2%wt in the raw material. Vacuum degree was less than10-3pa in sintering furnace. Temperature control program include temperature preservation under850℃for2hours, temperature preservation under1000℃for1hours, temperature preservation under1200℃for1hours. The process of crystal growth:PbF2addition was2%wt in the purified material. Vacuum degree was less than10-3pa in furnace. Heating and cooling process used the method of control power. Crucible descending speed was1mm·h-1. Annealing cooling speed was20℃·h-1.4. The φ200mm of CaF2crystals were grown by the vertical Bridgman Method, it had complete crystal shape and good internal transparency. Samples (111) crystal plane had triangular dislocation etch pit. In different areas of the samples of CaF2crystals, dislocation etch pit distribution was inhomogeneity. The crystal had a good transmittance from250nm to8000nm(>85%).