Kyropoulos process
The Kyropoulos process is a method of bulk crystal growth used to obtain single crystals. The process is named for Spyro Kyropoulos, who proposed the technique in 1926 as a method to grow brittle alkali halide and alkali earth metal crystals for precision optics.
The largest application is the growth of large boules of single crystal sapphire used in the electronics industry as a substrate used to manufacture gallium nitride-based LEDs, and in other industries as a durable optical material.
The Kyropoulos process (often referred to as the KY process) for sapphire crystal growth was developed in the 1970s in the Soviet Union. It is currently used by several companies around the world to produce sapphire for the electronics and optics industries.
High-purity, aluminum oxide (only a few parts per million of impurities) is melted in a crucible at over 2100 degrees Celsius. Typically the crucible is made of tungsten or molybdenum. A precisely oriented seed crystal is dipped into the molten alumina. The seed crystal is slowly pulled upwards and may be rotated simultaneously. By precisely controlling the temperature gradients, rate of pulling and rate of temperature decrease, it is possible to produce a large, single-crystal, roughly cylindrical ingot from the melt. In contrast with the Czochralski process, the Kyropoulos process crystallizes the entire feedstock volume into the boule. The size and aspect ratio of the crucible is close to that of the final crystal, and the crystal grows downward into the crucible, rather than being pulled up and out of the crucible as in the Czochralski method. The upward pulling of the seed is at a much slower rate than the downward growth of the crystal, and serves primarily to shape the meniscus of the solid-liquid interface via surface tension. The growth rate is controlled by slowly decreasing the temperature of the furnace until the entire melt has solidified. Hanging the seed from a weight sensor can provide feedback to determine the growth rate, although precise measurements are complicated by the changing and imperfect shape of the crystal diameter, the unknown convex shape of the solid-liquid interface, and these features' interaction with buoyant forces and convection within the melt. The Kyropoulos method is characterized by smaller temperature gradients at the crystallization front than the Czochralski method. Like the Czochralski method, the crystal grows free of any external mechanical shaping forces, and thus has few lattice defects and low internal stress. This process can be performed in an inert atmosphere, such as argon, or under high vacuum.
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