Silica-based fibres

The silica-based fibres are typified by Quartzel (Saint-Gobain, France), which while having slightly inferior fire and heat performance to alumina-based fibres (see Table 8.5), are available as continuous filament yams, rovings, and chopped strands (each comprising 9 or 14 pm diameter filaments) and sewing threads, filament-based nonwovens, and wetlaid papers. Continuous filament yams may be knitted and woven to yield fabrics with applications in furnace insulation, combustion chamber insulation in aircraft, ablative composites for military and other markets, and hot corrosive gas and liquid filtration. 

The construction of the optoelectronic interface can be based on a silicon photodiode since analytical and reference wavelengths are from the visible and the IR regions, respectively. The signals can be filtered out by optical filters (then two photodiodes are required) or one photodiode can be synchronised with modulation waves of the LEDs used. Finally, silica optical fibres can be used as light waveguides. The choice between single fibre or bundle is determined by the application of the sensor. 

In the near-IR, sensors almost exclusively rely on silica fibres (standard or low-OH) as they are accepted as industrially fully applicable32, 33 Silica-based glass fibres are chemically and mechanically robust, easy to handle, inexpensive, available with various core and outer diameters, a core-clad transfer fibres or bare sensing fibres, and have successfully been optimised to their theoretical attenuation limit.34. The spectral window allows application up to 2,5 pm. 

In this procedure the soil sample (spiked with isotopic marker compounds) is processed in a two-part enrichment procedure (Fig. 5.3). In part I, a mixture of the sample and sodium sulphate is subject to solvent extraction, and the extract is, in the same process, passed through a series of silica-based adsorbents and then through the carbon/glass fibre adsorbent. The extract passes through the adsorbents in the following order potassium silicate, silica gel, cesium or potassium silicate, silica gel and finally an activated-carbon... 

Silica-based glasses Si02 with 10% GeOj Optical fibres for communication... 

If one moves pn a step farther, binoxide of iln presente ltselfj a substance already more pronounced in its chemical character than charcoal. Still It is inclined to combine equally as well with bases as with acids, showing a transition character. Now, if pumice be soaked In a solution of bichloride of tin, and then dipped into a solution of carbonate of soda, the pumice acquires all the properties of animal charcoal. Very porous pumice, finely-divided silica, cotton fibre—all these show Bigne of chemical power depending on their extant of surface and it may well be supposed that this chemical power would he more prominent, could they be obtained in the fluid state. Silicic add, which is as inert as charcoal at the ordinary temperature, drives out the powerfal sulphuric acid from its combinations at a temperature nearer Its own melting point. 

It is ironic to consider the III-V nitrides, the premier materials for short wavelength blue and UV emitters, as sources of infrared light. However, Er-doped GaN is of interest for making electrically pumped, temperature insensitive, broad band and compact optical amplifiers or sources of 1.54 pm light. Applications include long-haul communication systems (amplifiers), local area networks (50/50 splitters) and sources (lasers) for transmission in silica-based optical fibres. 

The research has been carried out with financial support by the Italian Ministry of Education, University and Research (MIUR) no. 2003032158-002 project title Correlation between surface properties and cellular response of fibres substituting asbestos and other silica-based materials . 

Synthetic ion-exchange resins [213], silica-based exchangers [214] and activated alumina [209] have been most commonly used other materials used include cross-linked dextran gel (Sephadex) [215], fullerenes (especially Cis and Q,y)[216], polyurethane foam [217], PTFE turnings [218] or fibres [219], cigarette filters [220], functionalised cellulose [221], activated carbon [222], grape bagasse [223], rice husks [224] and bioorganisms (Saccharomyces cerevisiae) [225]. 

As with alumina-based fibres above, small amounts of silica of the order of 3.0 wt% permit the sintering of the transitional forms of alumina to delay nucleation and growth of a-alumina up to 1300°C. Varying the amount of silica and even adding small amounts of boron oxides lead to various forms of alumina-silica fibres with a... 

Fig. J. Evolution of elastic moduli of alumina-based fibres as a function of the silica content.

Hammond, C. R. (1978) Silica-based binary glass system wavelength dispersive properties and composition in optical fibres. Opt. Quant. Elect., 10, 163-70. 

Optical fibres composed of plastics are also transparent in the visible spectral region but optical losses reach 102 - 103 dB/km13. Their refractive index varies from 1.35 to 1.6 depending on the kind of polymer used (e.g. polymethymethacrylate PMMA -1.49). The chemical resistance is much worse than that of silica fibres and thermal stability is incomparable. On the other hand, low temperature processes of plastic fibre preparation allow us mix the starting polymer with organic dyes which enables the production of luminescent fibres suitable e.g. for fluorescence-based sensing13. 

Over the last several years, the number of studies on application of artificial neural network for solving modeling problems in analytical chemistry and especially in optical fibre chemical sensor technology, has increase substantially69. The constructed sensors (e.g. the optical fibre pH sensor based on bromophenol blue immobilized in silica sol-gel film) are evaluated with respect to prediction of error of the artificial neural network, reproducibility, repeatability, photostability, response time and effect of ionic strength of the buffer solution on the sensor response. 

The pH optical fiber sensor without any pH-sensitive dye was also described70. Porous silica layer made by the sol-gel method was cladded onto optical fibre core and was exploited as the optical transducer. Acid-base properties of silica surface caused that the surface charge of silica changed with pH of the solution. For example saturation of the sol-gel layer with cations leads to an increase of the electron density of the film, hence, the refractive index of the film. Since the surface charge of silica depends on pH, the refractive index of silica film varies also with pH. Thus, changes of... 

Resorcinol formaldehyde latex (RFL) cord dips have only a limited application within the general rubber goods industry and for adhesion to be achieved with synthetic fibres it is necessary to use the systems developed by Bayer and Degussa. These systems are based upon a combination of resorcinol, a formaldehyde donor and a hydrated silica filler (commonly called the RFK system). This system is incorporated as dry ingredients into the rubber compound and is activated by the application of heat. 

Impressive, highly ordered centimetre-sized fibres are obtained whose synergistic growth mechanism based on the kinetic cross-coupling of a dynamical supramolecular self-assembly and a stabilizing silica mineralization may well be the basis of the synthetic paths used by Nature to obtain its materials with well-defined multiscale architectures in biological systems. 

Materials based on amorphous silica fibres are of special interest these are manufactured in a variety of textile forms (cloth, tape, rope, etc.) which can be used for several applications (thermal, electric insulation) for service temperatures generally up to about 1000°C. Typically they contain 93-96% Si02 about 4% Al203 and small quantities of Ca or alkali oxides. 

The production of modern car tyres uses more than 100 raw materials, most of which are based on petroleum products. Tyres consist of natural and synthetic mbber, typically styrene-butadiene (SBR) reinforcing fillers (e.g., carbon black, silica, clay, calcium carbonate) reinforcing fibres... 

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