Feedstock for growth

The primary stock for leuco sapphire growth is aluminum oxide (of purity no less than 99.99%) that may come as a small-crystal corundum or aluminum oxide powder. Powder can be granulated or have particles of different shape and size. When melting, the aluminum oxide batch shrinks significantly, so for efficiently growing a crucible volume, smaller leuco sapphire monocrystals made with the Verneuil process or compacted aluminum oxide powder ingots are to be used.

Our company offers these two raw products as a feedstock for manufacturing larger crystals through the Kyropoulos, HDSM and other methods.

Leuco Sapphire Boules manufactured by the Verneuil Process

The Verneuil process is a common method of growing leuco sapphire monocrystals of smaller diameter and is based on melting powder aluminum oxide in a hydrogen fire created by vertical burners. A batch comes on a thin crystalline rod of sapphire (seed) from above in small portions administered by a special device. On a crystal seed, melted batch particles create a thin film that solidifies as it descends. The synchronization of batch, hydrogen and oxygen consumption with the rate of seed lowering provides the required level of crystallization. The absence of a container for melt simplifies the whole process by eliminating the problem of interaction between the container and the melt, though the purity of crystals grown this way relies on the presence of reliable control over impurities coming from the atmosphere and furnace ceramics.
RHT offers mono-crystalline feed grown by the Verneuil method for manufacturing larger crystals. The quality of our mono-crystals is no worse than that of well-known Czech crystalline feedstock grown using the same method. The content of Al2O3 is no less than 99.998%, and it contains impurities as follows:

  • Si –    <  4 ppm
  • Fe –   < 3 ppm
  • Cu –   < 1 ppm
  • Ca –   < 1 ppm
  • Ti  –   < 1 ppm
  • Mg –  < 1 ppm
  • Na –   < 1 ppm
  • K  –   < 1 ppm

Synthetic sapphire (Kyropoulos and Verneuil)

Melted Al2O3 ingots

RHT offers alternatives to the traditional crystal feedstock obtained through the Verneuil process - the so-called “compacting” method. The method is based on high-frequency aluminum oxide melting in a cool container. The thin layer of initial powder (lining, or garnissage) stays between the water-cooled wall of the container and melted aluminum oxide, thus preventing any possibility of polluting the melt. The technology of aluminum oxide compacting gives a high-purity poly-crystal feedstock, a cylindrical ingot that fills the volume of the growing furnace wolfram crucible in the most efficient way. By changing container geometry, it is possible to obtain ingots of more complicated shapes, such as prisms for HDSM.

The advantages of the compacting method are:
  • High purity of ingots obtained (up to 99.998%).
  • High capability of synthesis results (chemical composition of leuco sapphire manufactured) to be reproduced.
  • High productivity. One compacting furnace outputs about 10 kg of compacted powder per hour (with density ~ 3,3 g/sm3).
  • The opportunity to choose the geometry and size of ingot, which is important for certain customers.
  • The simplicity and low cost of the process compared to Verneuil production method (energy consumption for 30 kg ingot is 80 kW per hour).

Not only do we offer compacted aluminum oxide ingots themselves, but also the equipment and technology required for their manufacturing.

Compacted Aluminum Oxide Ingot