Verneuil growth

For the Verneuil growth of mtile and strontium titanate it is necessary to maintain strongly oxidizing conditions to prevent excessive reduction of TF+ to Tk+. This is achieved by adding a third outer tube carrying extra oxygen to the Verneuil torch (Fig. 1) in the tricone modification. Annealing in O2 at about 1100°C is subsequently used to achieve full oxidation. 

The Verneuil Techni(jue. The VemeuH technique is also known as flame-fusion. A very pure feed powder is first made by chemical decomposition. For the growth of comndum, ammonium alum [7785-25-0] NH Al/(S0 2 12H20, is recrystallized from water containing added... 

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

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

The main part of a conventional Verneuil apparatus is a vertical inverted oxyhydrogen burner. The burner consists of several concentric tubes [1, 2]. The inner tube supplies oxygen and crystal powder to the flame, which burns into a ceramic muffle. A seed crystal is positioned in such a manner that only a thin film of liquid phase is on the top surface of the crystal. If the powder supply, i.e. the feeding of molten droplets into the molten layer, and the crystal growth rate are weU balanced, the crystal will grow under constant conditions. The principal design and operation of a Verneuil flame fusion apparatus was described earlier [3]. 

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. 

Figure 36 Schematic of the Bridgeman crystal growth method (a). Verneuil crystal growth method (b)...

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. 

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. 

Briefly describe how ruby could be produced by the Czochralski (Cz) process. Under what situations, if any, would it be preferable to use the Cz process rather than the Verneuil process for ruby growth ... 

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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