page_title: Kyropoulos method
section_title: The method
hierarchical_section_title: Kyropoulos method / The method
caption_attribution_description: English: Single crystal sapphire boule grown by Kyropoulos method. Approximately 200 mm diameter and approximately 30 kg. (A second boule is visible in the background.)
context_page_description: The Kyropoulos method, or Kyropoulos technique, is a method of bulk crystal growth used to obtain single crystals. It 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 of the Kyropoulos method is to grow large boules of single crystal sapphire used to produce substrates for the manufacture gallium nitride-based LEDs, and as a durable optical material.
context_section_description: High purity aluminum oxide (only a few parts per million of impurities) is melted in a crucible at over 2100 °C. 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 method, the Kyropoulos technique 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.
caption: Single crystal sapphire boule grown by Kyropoulos method. Approximately 200 mm diameter and approximately 30 kg. (A second boule is visible in the background.)
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