| Sapphire possesses high hardness, strength, corrosion-resistance and light-transmission. It is one of the firstly used crystals by humans. In1890, the France scientist Verneuil got artificial sapphire crystal for the first time. From then on, various techniques for producing sapphire crystals have been developed. The quality of synthetic sapphire crystals become better and better, and the applied fields of sapphire are broadened. Now sapphire is widely used for civil and military purposes. In the civil field, sapphire is used as wear-resistant component, medical material, high temperature window, substrate for microelectronics, matrix for laser, optical prism, telephone window and so on. In the military field, sapphire is used as wave-transmission window for high speed plane and guided missile, optical-electronic nacelle, optical-electronic mast for submarine. The broadening applied fields have new requirements for sapphire. Besides high hardness, strength, and corrosion-resistance, sapphire should own low inner stress, high optical uniformity, high crystalline quality, low dislocation density, large diameter, et at. So, the developing trend for sapphire is to grow large size, high quality crystal with low cost.There are plenty of methods used for sapphire crystal growth, like the Czochralski(Cz) method, Heat exchanger method(HEM), Edge defined film fed growth(EFG), Bridgeman method, Kyropoulos(Ky) method and so on. Kyropoulos(Ky) method is the most widely used method which produces the crystal of the biggest size and the highest quality. But Kyropoulos(Ky) method still exists some drawbacks, the crystal growth process is controlled mainly by human which is not convenient for automatic control. The defects as bubbles and grain boundary are often seen. Nowadays, a lot of equipments and technologies for sapphire crystal growth using the Ky method are introduced to our country, but the realization on equipments, processes and the material is limited. Comparing to small crystal growth process, a lot of new problems will acre with the development of large crystals. The article is done based on the fact listed above. In this dissertation, we have grown large sapphire crystal of60Kg and studied the thermal field for the sapphire crystal growth by the Kyropoulos(Ky) method and learned the change of the heat shield system between different runs. The model for sapphire crystal growth was proposed and the relation between the crystal weight and time were given. We presented the bubbles nucleation theory and how the bubbles were trapped in the crystal. Also, the bubbles distribution in the crystal and the measures to eliminate them were raised. The images of the dislocations and low angle grain boundaries in sapphire were characterized and the orientation of the grain boundaries was detected by the synchrotron white-beam X-ray topography. In addition, the attachment to the crucible, the inclusion and the crack in the crystal and their relations were also studied.1. The thermal field for sapphire crystal growth by the Ky method and crystal growing modelSapphire crystal growth process was the solid-liquid interface moving process determined by the thermal and mass transportation. And the key factor for crystal growth was the thermal field. We grew large sapphire crystals of60Kg and recorded the parameters during several growth run, and found that the temperature decreased linearly. Comparing the temperature conditions among different runs, we found that as the growth equipments run, the thermal insulation became better, and the heating efficiency became higher.The model for the crystalline surface moving was proposed, and the weight was proportional to t3. Considering the weight of different runs, we found that the crystal growth velocity increased as the equipments run which was caused by the decreasing of the temperature gradient. Lowering the voltage decreasing speed was useful for reducing the growth velocity.2. The bubbles in sapphire crystal grown by the Ky methodBubbles were the most common defect in sapphire crystal grown by the Ky method. We observed and marked the bubbles in the crystal by halogen lamp and green laser, and the bubbles distribution was investigated. The composition of the bubbles was considered to be complex gaseous oxidations formed by the decomposer of molten alumina and tungsten and molybdenum heat shields as well as the impurities in the ceramic heat shields. The formation mechanism of bubbles in sapphire grown by the Ky method was investigated. The bubbles were nucleated in the solute boundary layers, and were trapped in the crystal if the diffusion and transportation speed were lower than that of the solid-liquid interface movement. The bubbles distribution was studied, and the appearances and gathering of bubbles in different zone of the crystal were investigated, and the specific formation mechanism of bubble was given. The crystal with few bubbles was grown after growth parameters were optimized.3. The low angle grain boundary in sapphire grown by Ky methodLow angle grain boundary was another typical defect in sapphire crystal; however, the research on low angle grain boundary was few. Low angle grain boundary was closely related to dislocation. In this dissertation, the sapphire was etched by the molten alkali and the topography and etch pits density were investigated. The low angle grain boundary was observed using the polariscope. The samples with low angle grain boundary were etched by the molten alkali, and the grain boundary was observed by the microscope. The orientation of the grain boundaries was detected by the synchrotron white-beam X-ray topography.The grain boundary in sapphire grown by the Ky method was introduced by the crystallization layer mismatch caused by the instability of the seeding stage or the impurities on the shouldering surface; in addition, grain boundary may occur spontaneously on condition of growing too fast. Sharp boundary between the grain boundary area and non-grain boundary area can be seen through polariscope and microscope because the refractive index and the polishing velocity were different between the grain boundary area and non-grain boundary area. The samples with grain boundary were etched in molten alkali. In the grain boundary area or near the grain boundary area, the etching pits density was much higher, and the etching pit arranged regularly.The grain boundary was character by the synchrotron white-beam X-ray topography. The diffraction spots were split; the disorientation angle of the grain boundary was2-3degrees, and the grain boundary was (1010) direction distributed.4. Other defects in sapphire grown by the Ky methodThe attachment to the crucible, the inclusion and the crack in the crystal and their relations were studied. The attachment to the crucible started from the extraordinarily large radial growth velocity caused by the extraordinarily small radial temperature gradient. The inclusions in the crystal were complex oxidations formed by the decomposer of molten alumina and tungsten-molybdenum heat shields. The oxidations adhered to the top heat shield, and dropped into the crucible forming the inclusions at the bottom of the crystal. The cracks in the crystal could be divided into two kinds:spontaneous cracks and induced cracks. Crystal cracked spontaneously because of the especially large inner stress caused by the large temperature decreasing speed. And the induced cracks started from the stress concentrating area. The crucible attaching zone and the inclusion were usually the stress concentrating area and induced cracks... |
PDF Full Text Request