Flame-fusion technique

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. 

Rutile (TiOj) can be prepared in the form of single crystals by the Vemeuil (flame fusion) technique. Slightly reduced rutile is an n-type semiconductor with an energy gap of 3.05 eV and electron mobilities of -1.0 cmWs at room temperature. TITANIUM DIOXIDE SUPPLIERS ALFA AESAR, A JOHNSON M AITHEY COMPANY 30 Bond St. 

For those substances which melt congruently and do not suffer reconstructive phase transformations between the melting point and the temperature of interest the flame fusion technique has been successfully applied. The H2 + O2 flame originally used is sometimes replaced by an argon plasma made oxidizing or... 

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 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. 

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... 

Oxygen and nitrogen also are deterrnined by conductivity or chromatographic techniques following a hot vacuum extraction or inert-gas fusion of hafnium with a noble metal (25,26). Nitrogen also may be deterrnined by the Kjeldahl technique (19). Phosphoms is determined by phosphine evolution and flame-emission detection. Chloride is determined indirecdy by atomic absorption or x-ray spectroscopy, or at higher levels by a selective-ion electrode. Fluoride can be determined similarly (27,28). Uranium and U-235 have been determined by inductively coupled plasma mass spectroscopy (29). 

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. 

Silicon may be present in various forms in some preparations, e.g. as tri-silicate in antacids, polydimethyl-siloxane or silicone oil (see also section VII, ref. 77). The sample may be evaporated to dryness and the residue fused with sodium bicarbonate or another fusion agent, or taken up in hydrofluoric acid provided strong heating and loss of silicon is avoided. A nitrous oxide/acetylene flame must be used. Chromium may be found in disinfectants and antiseptics. Unless dilution of the sample is possible the use of the injection-cup technique (see section II.A) may be preferable as otherwise large amounts of corrosive salts such as sodium hypochlorite will be aspirated. If iron is also present it may be necessary to use a nitrous oxide/ acetylene flame. Arsenic in arsenamide and lead arsenate preparations can be determined by boiling the sample in 5% nitric acid and aspiration of the sample [111]. Better sensitivity would be obtained using hydride generation (see section II.A). 

A plasma is a partially ionized gas composed of ions, electrons and neutral species. It is a state of matter that can be created by such diverse techniques as flames, electrical discharges, electron beams, lasers or nuclear fusion. The technique of most interest to plasma polymerization is the glow discharge, in which free electrons gain energy from an imposed electrical field, and subsequently loses it through collisions with neutral molecules in the gas. The transfer of energy to gas molecules leads to the formation of a host of chemically reactive species, some of which become precursors to the plasma polymerization reaction. 

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