Flame-fusion process

The flame fusion process was the first method to give crystals of acceptable size for commercial applications from the melt, namely ruby crystals for use as gemstones. 

Table 1. Materials Grown by the Flame Fusion Process...

Thus, the flame-fusion process is a crucible-free crystal growth process for high-melting compounds, an economic process that allows control of nucleation and crystal diameter within a relatively simple apparatus, and was a signiflcant breakthrough achieved by Auguste Verneuil around 1900. 

The flame-fusion process allows the dopant concentration of the growing crystals to be changed during growth by means of a double-feeder system shown in Fig. 16.13. This allows growth of, for example, ruby laser crystals with sapphire ends for high-power laser crystals with reduced surface damage. 

When the oxidizer and fuel components are physically separated and allowed to diffuse into each other in the combustion zone, a diffusion flame is formed. Since the molecular distributions of the oxidizer and fuel components are not uniform, the temperature and combustion products are also not uniformly distributed in the combustion zone. Thus, the rate of the reaction generating the combustion products is low when compared to that in a premixed flame because an additional dif-fusional process is needed to form the diffusion flame. 

II Quartz powder Flame fusion (Vemeuille process) = Type I 100-400 Herasil Homosil Optosil O.G. O.H. Vitreosil ... 

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... 

Flame Fusion (Type ii) This also is a boule process. The fine grains of crystalline quartz are fed into the flame of a hydrogen-oxygen burner to soften and drop onto the surface of a rotating rod or boule, where they flow and merge. This approach was developed by Heraeus and is presently used by that company. 

A plasma is an appreciably ionized gas(about 1% or more) having no net charge, and may have a wide range of densities. Plasmas are of particular interest because of the possibility of initiating nuclear fusion in them, but they also appear in such phenomena as a neon sign, a lightning stroke, the ionosphere about the earth, shock waves, and the compressed layer of hot gas about an object entering the earth s atmosphere They appear in flames and detonation waves. It seems well established that free radicals and ions are present at well over equilibrium concentrations in flames (Ref 1). The ions appear to be produced not by thermal processes but by chemical factors which cause abnormal electronic excitation... 

Modern Manufacturing Techniques. Manufacturing techniques for making bulk vitreous silica are for the most part improved variations of the historical processes. The main exception is the sol—gel process (see Sol-gel technology). All processes involve the fusion or viscous sintering of silica particles. The particles can be in the form of a loose powder or a porous preform. The powders can be made from natural quartz or from the decomposition of chemical precursors, such as silicon tetrachloride, and tetraethylorthosilicate (1 EOS). In some approaches, such as flame hydrolysis, the powder is produced and fused in a single step. The improvements made to these techniques deal mainly with the procedures used to prepare the powders, that is, to control purity and particle size, and the specific conditions under which the powders are consolidated. 

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