Verneuil technique

Fig. 1. The Verneuil technique, or flame-fusion growth, as used for synthetic mby and sapphire.

The Verneuil Technique. The Vemeuil technique is also known as flame-fusion. A very pure feed powder is first made by chemical decomposition. For the growth of corundum, ammonium alum [7785-25-0], NH4A1/(S04)2 12H20, is recrystallized from water containing added... 

Because of the small volume of liquid phase, the Verneuil process is predestined to the growth of graded materials. The development of the flux-Verneuil technique [4] enables the variation of the material support to the growing crystal. In this way, a specific influence on the stoichiometry of the liquid phase is possible. 

The Verneuil technique is related to the growth of stalagmites in nature. Liquid drips (from the stalactite) onto the stalagmite, but instead of settling there and freezing (like a stalagmite icicle would) some of the mineral content is deposited as the water evaporates. 

The Growth of Functionally Graded Crystals by Verneuil s Technique... 

Further purification is possible by the zone meltii technique of Pfann. In order to avoid contamination from the walls of the crucible during the melting, a technique similar to that of Verneuil-Miethes is recommended. 

The Verneuil, or flame-fusion, method is illustrated in Figure 29.1. It is a well-established technique for growing single crystals of oxides that have high melting temperatures. The largest application of the Verneuil method is for the growth of sapphire and ruby. 

TABLE 29.3 Crystals Grown by Verneuil and Arc-Image Techniques ... 

The flame fusion technique (see Figure 7.20b) was originally devised in 1904 by Verneuil for the manufacture of artificial gemstones, such as corundum (white sapphire) and ruby. This method is now used for the mass production of jewels for watches and scientific instruments. A trickle of fine alumina powder plus traces of colouring oxides is fed at a controlled rate into an oxyhydrogen flame. Fusion occurs and the molten droplets fall on to a ceramic collecting rod. A seed crystal cemented to the rod is fused in the flame and the rod is lowered at a rate that allows the top of the growing crystal (known as a boule) to remain just molten. Renewed interest has recently been shown in this method for the production of rubies for lasers. 

Figure 7.20. a) The Bridgman-Stockbarger technique , (b) the Verneuil boule furnace... 

Single crystals of pure and doped alumina can be grown using well-established techniques such as flame fusion (Verneuil process), Czochralski crystal puUing, and top-seeded solution growth (TSSG), as well as under hydrothermal conditions and from the vapor phase (Franke et al., 1968). Some of these applications will be... 

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