Leuco Sapphire

Leuco sapphire is mono-crystalline aluminum oxide (Al2O3), the transparent colorless type of sapphire. Sapphire is one of the hardest minerals, it is optically transparent, has a high melting temperature, is exceptionally resistant to fracture even in aggressive environments. It has a high thermal conductivity under low temperatures and has an extremely high specific resistance.

Leuco sapphire is produced in industrial quantities all around the globe. By now, the prime application of leuco sapphire is manufacturing wafers for light emitting diodes (LED) and silicon on sapphire (SoS) microchip integrated circuits.

The Kyropoulos method is usually employed when growing highly purified large sapphire monocrystals for wafers of various orientation and other articles with high optical properties. Powder or crystalline aluminum oxide is the stock for it. The method is based on growing crystals out of melt and relatively inexpensive. According to the Kyropoulos method, directed solidification is provided by changing the melt temperature in a stationary crucible. The gradual growth of the hemisphere-shaped crystal occurs on a seed via continuous heat dissipation through crystal with a water-cooled crystal holder. As the crystal grows, it is gradually pulled out of the melt rotating around its axis. Under this method, the growing crystal diameter is only limited by the size of the crucible and can reach 350 cm and more. To reduce residual tensions that lead to cracking and splitting, the grown crystals should undergo post-growth annealing in specially designed high-vacuum annealing furnaces.

The other methods of growing crystals out of melt (Czochralski, Musatov, Stepanov, Bridgeman-Stockburger) are based on moving the crystal or crucible filled with melt through the temperature gradient. For example, the Czochralski method involves vertically pulling the crystal seed out of melt in a hot crucible, most frequently of molybdenum. The method of horizontal-directed solidification (HDSM) and the Shubnikov-Obereimov method are based on creating a temperature gradient by moving the crystal seed horizontally or by cooling the bottom of the crucible through a gas blowout. The method of obtaining crystals from solution – hydrothermal and “out of solution in melt” – uses the fluoric, plumbic and water solutions of aluminum oxide.

Synthetic sapphire (Kyropoulos and Verneuil)

Table: Leuco Sapphire properties
Molecular Weight 101,94
Density, g/sm³     3,986 at 20 °С
Lattice Constants, А     а=4,75; с=12,97
Cleavage     at (1011)
Hardness, kg/mm², Moh’s scale      1370
Solubility, g/100g of water     0,000098 at 29 °С
Melting Point, °С 2030
Boiling Point, °С     3500
Specific Heat, T.U./g*grad 0,0249 (при 91°K)
0,1813 (at 291°K)
Thermal Conductivity, T.U./grad cm*sec 0,060 at 299 °К
0,041 at 343 °К
0,055 at 296 °К
0,040 at 350 °К
Young Modulus 4,7x106 kg/cm² (at 20 °С)
Specific Resistance 1020 perpendicular to С axis, 1018 parallel to С axis
Area of Transmittance 15000 – 60000А
Integral Transmittance 97-98%