Sapphire (blue gemstone) is a mineral, a variety of corundum α-Al₂O₃, containing titanium and iron impurities. Naturally occurs in rocks and rubble. Sapphire deposits are found in Thailand, Sri Lanka, India, the Urals, usually, together with rubies.
A synthetic sapphire was first produced in 1910. Pure and colourless corundum is called a “white sapphire”. It is resistant to fire, acids, and alkalis. The hardness of a sapphire is right behind diamond (on the Mohs scale: diamond – 10, sapphire – 9). Single crystals of an artificial sapphire are grown from the aluminium oxide Al₂O₃. It should be noted that there are currently several common methods for growing sapphires artificially:
Verneuil’s method. It is one of the oldest methods discovered in the early 20th century. Later, it was improved by the Russian scientist Popov. By using this technology, a sapphire is grown within two hours; it has a shape of a cylinder: diameter is 6 mm, height – 25 mm. This method is still very popular, as it is one of the cheapest ways to grow a sapphire and/or a ruby. However, parameters or characteristics of a sapphire grown by using this method, do not meet requirements of the high-tech sector.
Czochralski (CZ) method. This method was also discovered in the early 20th century but was forgotten for a little while after the World War II. However, since the industry of semiconductors was growing, American scientists improved this method and adapted it for the production of large-size industrial sapphire in 1950. Sapphire boules, produced by this method, have a diameter of up to 6 inches (150 mm), and their height is 10 inches (250 mm). Maximum weight is usually 20 kg. A cycle of producing a single-crystal boule is 8-10 weeks. The crystals, grown by means of this technology, are valued for their optical properties; therefore, they is mainly used in optics and laser industry.
Kyropoulos (KY) method. This method was also discovered in the 20th century. Currently, Kyropoulos method is one of the most effective and sophisticated processes to grow large sapphire boules. Currently, the market offers boules weighing 60-100 kg. A growth cycle is 12-18 days. Such sapphire boules can be cut at any orientation and/or axis; therefore, crystals grown by means of this technology are suitable not only for the optics industry, but also can be effectively used in the industries of light emitting diodes and microelectronics.
EFG/Stepanov’s method. This method was discovered in the second half of the 20th century. A sapphire grown by this method has poorer optical properties (lower than average), but can be grown in different axes (A, C, or optional), that allows reducing the processing costs and increasing the yield. A sapphire grown based on this method is attractive for enabling to obtain sapphire products of larger dimensions and a specific shape (strips, rods, tubes). A sapphire grown based on this method is used in mechanics and industry, where optical properties are not so important. In addition, it may be used in the production of certain semiconductors.