Impurities in glasses

Iron oxide is always present as an impurity in glass. It is introduced through the natural raw materials such as sand and limestone. Another source is from trap iron mixed in the cullet and abraded metal from the handling of batch. All of this adds up to several hundred parts per million which causes light absorption at the ends of the spectrum rather than the middle and causes a yellow-green color in the glass. This can be overcome by a process known as decolorization. There are two types chemical and physical decolorizing. )... 

Because of the difficulty in controlling the impurities in glass, the development of a single glass filter with closely defined optical characteristics may be economically prohibitive. When one also considers the positive and negative effects of dopants on solarization and the desire by companies to differentiate products, progress toward this goal on an international scale may be very slow. 

The main peculiarity ofXAS is that it can provide the local atomic order around chosen chemical species. No long-range order is required and this makes XAS an ideal technique for the determination of sites ofimpurities (10 -10 Ppm) in different matrices. Atypical issue discussed in the literature is on the local order aroimd luminescent impurities in glasses for optoelectronic applications. Indeed, a typical aU optical device should consist of optical guides and active components (amplifiers, switches,...) of minimum size. The sol-gel route, permitting the deposition of a thin film on a substrate, appears to be an ideal technique for the fabrication of such devices (Orignac, 1999 Stone, 1996). 

This process yields satisfactory monomer, either as crystals or in solution, but it also produces unwanted sulfates and waste streams. The reaction was usually mn in glass-lined equipment at 90—100°C with a residence time of 1 h. Long residence time and high reaction temperatures increase the selectivity to impurities, especially polymers and acrylic acid, which controls the properties of subsequent polymer products. 

There ai e noted the most convenient, simple and chip methods, which ensure the high quality of specimens and can be easily combined with different techniques for analytical pre-concentration of impurities. In particulaidy, it is proposed to make specimens in the form of gel, film or glass in the case of XRF analysis of concentrates obtained by low-temperature crystallization of aqueous solutions. One can prepai e film or organogel specimens from organic concentrates obtained by means of extraction of impurities by organic solvent. Techniques for XRF analysis of drinking, natural and wastewater using considered specimens ai e adduced. 

Trimethylsilyloxy-3-penten-2-one cis) (acetylacetone enol trimethylsilyl ether) [13257-81-3 M 172.3, b 66-68"/4mm, 61-63"/5mm, d4 0.917, Up 1.452. Fractionally distilled and stored in glass ampoules which are sealed under N2. It hydrolyses readily in contact with moisture giving, as likely impurities, hexamethyldisiloxane and 2,4-pentanedione. [J Am Chem Soc 80 3246 795S.]... 

Attempts by Kao and others to enhance transparency by chemically removing impurities from glass met with little success the level of purity required was indeed comparable with that needed in silicon for integrated circuits. In the event, the required purification was achieved in the same way in which semiconductor-grade silicon is now manufactured, by going through the gas phase (silicon tetrachloride), which can be separated from the halides of impurity species because of dilTerences in vapour pressures. This breakthrough was achieved by R.D. Maurer and his... 

Storage stability Extremely unstable in presence of trace metals or other impurities. Traces of iron chloride may cause explosive decomposition. Pure material is stable for only 1 or 2 months. It may be stabilized by nitromethane, chloropicrin, glycine, ethyl acetate, or ether - but only in glass vessels below 20°C. Apparently, it is most stable in aromatic solvents. 

The polymerization reaction (Figure I) is markedly influenced by the presence of trace impurities which was one of the difficulties encountered in earlier investigations. The conventional route is a melt polymerization of highly purified trimer (NPC1 ), or a mixture of trimer and a small amount of tetramer (NFCl.), sealed under vacuum in glass ampoules, at approximately 250°C. Proper selection of time and temperature is necessary to obtain II and avoid the formation of cross-linked matrix (III). 

Domestic consumption of selenium in 1981 exceeded 453,000 kg. About 50% was used in electronic and copier components, 22% in glass manufacturing, 20% in chemicals and pigments, and 8% miscellaneous (Cleveland et al. 1993). In 1987, world production of selenium was about 1.4 million kg (USPHS 1996). In 1986, 46% of the global selenium produced was used in the semiconductor and photoelectric industries 27% in the glass industry to counter coloration impurities from iron 14% in pigments and 13% in medicine, in antidandruff shampoos, as catalysts in... 

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