Flame Fusion Method

Crystal Growth from Melts and Solutions 17.2.4.1. Growth from Melts 17.2.4.1.5. Flame Fusion Method. 

The flame fusion method is used to grow sapphire crystals for bearings and similar applications, and tons of AI2O3 are crystallized annually, particularly in factories in Switzerland, France, Czechoslovakia, China, Japan, and Russia. Ruby, sapphire, and... 

Figure 1. (a) Schematic diagram of crystal growth by the flame fusion method, (b) Stages of development of undoped AI2O3 and Cr-doped (ruby) crystals, (c) Furnace room used for AI2O3 production by H. Djevahirdjian Company at Monthey, Switzerland. (Courtesy of V. Djevahirdjian.)... 

The highest melting material that can be grown by the flame fusion method is determined by the maximum temperature of the flame. A plasma torch may be used as an alternative heat source for materials that decompose in a hydrogen-oxygen or coal gas-oxygen flame. 

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. 

Miyake, S. Kinomura, N. Suzuki, T Suwa, T. Fabrication of spherical magnetite particles by the flame fusion method. J. Mater. Sci. 1999, 34, 2921-2928. 

In the flame fusion method (Verneuil), the powder falls onto an O2-H2 flame and the melt drops on a seed crystal, which is slowly lowered as the crystal grows. The method has been applied to grow high-melting-point oxides (ruby and sapphire). The starting powder is usually placed on a gently hopped mesh to get a continuous flow of solid particles of the same size. 

Most SrTi03 crystals have been grown by flame-fusion methods where initially the overflow problem was observed after growth of the initial crystal cone, even when excess SrO was added to the starting powder. This phenomenon is explained by the SrO - Ti02 phase diagram shown in Fig. 16.19 overflow is expected both for shortage and for excess of SrO. 

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