Pulled Czochralski Method

Blue Sapphire Rough

In 1916, Jan Czochralski (1885 -1953), a Polish chemist, developed a new method for synthesizing rubies. Known as crystal pulling, this is a fast and effective method of producing nearly flawless stones from a melt. When cut as gems the stones are so clear that they look like glass imitations.

Czochralski discovered his method when he accidentally dipped his pen into a crucible of molten tin rather than his inkwell. He pulled his pen out to discover that a thin thread of solidified metal was hanging from the nib. He replaced the nib by a capillary, and verified that the crystallised metal was a single crystal. His experiments produced single crystals of a millimetre in diameter and up to 150 centimetres long. Czochralski published a paper on his discovery in 1918. In 1950, G.K. Teal and J.B. Little from Bell Labs in the USA used this method to grow single germanium crystals which later led to their large scale production for semi conductor applications.

Today the Czochralski pulled-growth method is used to make ruby, sapphire, spinel, yttrium-aluminum-garnet (YAG), gadolinium-gallium-garnet (GGG), and alexandrite. The ingredient powders are melted in a platinum, iridium, graphite, or ceramic crucible. A seed crystal is attached to one end of a rotating rod and the rod is lowered into the crucible until the seed just touches the melt. The rod is then slowly withdrawn. The crystal grows as the seed pulls materials from the melt, and the material cools and solidifies. Owing to the surface tension of the melt, the crystal stays in contact with the molten material and continues to grow until the melt is depleted. Crystals formed with this method can be very large, -- more than 50 millimetres in diameter and one metre in length with a high degree of purity. Although Czochralski pulled boules are much more expensive than flame fusion boules to produce, their cost is still a fraction of the natural materials that they imitate.

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