| Sapphire(Al2O3) has stable physical and chemical properties,as well as excellent mechanical behavior due to its distinctive crystal structure, which plays an irreplaceable role in both defense industry and civilian field. However, because of the high temperature for crystal growth, sapphire prepared is inevitable to be found a certain amount of macro and micro defects such as bubbles, inclusions, dislocations and residual stress. The Edge-defined Film-fed Growth method(EFG) is an advanced molding process, which overcomes the disadvantages of low material utilization, large amount of machine and high production costs in traditional artificial growth methods, and produces high-quality sapphire crystal with low defects. To reduce the sapphire defects grown by EFG method, it is essential to control the growth environment, especially the stable temperature field. Simultaneously, the structural characteristics of sapphire crystal like growth direction also have a great impact on the crystal quality and performance. To further improve the morphology and optical properties of the crystal after growing and to eliminate the residual stress, it is critical to optimize the annealing process.This paper considers the influences of three factors on the distribution of the temperature field, namely the relative position between induction coil and crucible, the angle on the top of mold, as well as the thickness of the insulation cover. The results shows that there are no bubbles and inclusions in the sapphire when the crucible is in the middle of the coil, and sapphires prepared in the part has an optical transmittance of more than 82% between the wavelength of 400~900nm, performing the best crystal quality. Sapphire grown with an mold angle of 45° is free of cracks and inclusions, the value of FWHM(full width at half maximum)is 29.13 radsec, shows better structure integrity than that grown with an mold angle of 90°. Using an zirconia fiber brick as the insulation cover with a thickness of 45 mm, there are no cracks in high temperature environment. After three-points bending mechanical test, the crystal breaks along the slip system with the bending strength of 578 MPa, much higher than those with the insulation covers of 35 mm and 55 mm. The temperature field for EFG method is determined: The molybdenum crucible with the mold of 45°is set in the middle of the induction coil, surrounded by the zirconia fiber brick with the thickness of 45 mm, and the multi-layer graphite felts are placed above the crucible as a post-heater and insulation layer.This topic prepares the sapphire of(0001) plane with [0001]-axis and 0]2[11-axis respectively, and compares the differences of resistance. After corroding for 15 min under 390℃ by molten KOH, the dislocation density of a-axis sapphire is 2.2ⅹ103cm-2, which is half of that of c-axis sapphire, and the dislocation etch pits distributes more evenly. After corroding for 6 times in 300℃ by concentrated phosphoric, the sapphire of a-axis exhibits better corrosion resistance with thinner corrosion layer of 0.75μm. Therefore, the 0]2[11-axis alumina single crystal is used as the seed to induce the(0001) plane sapphire crystal.The sapphire crystals are annealed in the air and hydrogen respectively, it is found that both oxidizing and reducing atmosphere can eliminate black inclusions well. However, hydrogen annealing is the more effective way to remove the internal color centers and absorption shoulder generated by cationic of the sapphire crystal grown by EFG method. This paper studies the effects of annealing temperature, holding time and cooling velocity on crystal quality by orthogonal experiment, and explores the optimal annealing process ultimately: Crystals are heated at 1900℃, hold for more than 20 h and cool down at a rate of less than 20℃/h. The sapphire crystal after annealing performs pure and transparent, without black inclusions and other defects, both internal residual stress and dislocation density is greatly reduced. |
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