Preparation, quality characterization, service performance evaluation and its modification of sapphire crystal for optical window and dome application

Sapphire (i.e., Al₂O₃ single crystal) window and dome are important sensor components. Large-sized sapphire crystal was prepared by an improved Kyropoulos method, and its machining technique of ultrasonic vibration was also reported. The quality and structure were evaluated by illumination of a He–Ne laser transmission electron microscopy (TEM), double crystal X-ray diffraction (DCXRD) and Raman spectroscopy. For practical window and dome application, its high-temperature mechanical strength and optical transmission were both measured, and a modification measure of sputtering Mg film on sapphire surface followed by annealing and subsequent quenching was demonstrated. Large-sized sapphire crystals with high quality and different profiles were successfully grown by adjustment of pulling velocity and crucible size, design of suitable temperature field. Moreover, a series of sapphire windows and domes were fabricated for different optical application. The biaxial flexure strength of sapphire crystal decreases with increasing temperature, and the infrared transmission is also slightly degraded at high temperature. In addition, it should be noted that there is sharp strength drop in the temperature range of 400° to 600 °C which is related with twin formation in sapphire crystal. Submicron MgAl₂O₄ spinel precipitation can effectively harden sapphire crystal and do not induce large optical-scattering loss, which is a potential modification method for window and dome application of sapphire crystal.