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 of the Kyropoulos process 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.
The process (often referred to as the KY process) was proposed in 1926 as continuation of the Czochralski and Verneuil methods due to their basic dimensional limitations of grown crystals. Initially it was used to grow mono-crystals of alkali metal halides. The process of direct crystallization of melted material differed by decreasing the boule's temperature while still in the crucible. Compared to others the technique allowed to produce very large single crystals that were free of cracks and damage due to restricted containment.
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 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.
The sizes of sapphire crystals grown by the Kyropoulos process have increased dramatically since the 1980s. In the mid-2000s sapphire crystals up to 30 kg were developed which could yield 150 mm diameter substrates. By 2017, the largest reported sapphire grown by the Kyropoulos method was 350 kg, and could produce 300 mm diameter substrates. Because of sapphire's anisotropic crystal structure, the orientation of the cylindrical axis of the boules grown by the Kyropoulos process is perpendicular to the orientation required for deposition of GaN on the LED substrates. This means that cores must be drilled through the sides of the boule before being sliced into wafers. This means the as-grown boules have a significantly larger diameter than the resulting wafers. As of 2017 the leading manufacturers of blue and white LEDs use 150 mm diameter sapphire substrates, with some manufacturers still using 100 mm, and 2 inch substrates.
The most significant disadvantage of the process is an unstable speed of growth which happens due to heat exchange changes incurred by a growing boule size and which are difficult to predict. Due to this problem the crystals are typically grown at very slow speed in order to avoid unnecessary internal defects.
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