Silica fibres

Figure C2.15.16. Wavelengtli dependent loss (upper) and dispersion (lower) in a silica fibre.

Of the phosphorus-containing polymers the polyphosphates have been known for many years. Aluminium phosphate had been used in the manufacture of heat-resistant silica-fibre-reinforced laminates. 

FIBREGLASS - SILICA FIBRE-EPOXY CARBON FIBREl... 

Many other opportunities exist due to the enormous flexibility of the preparative method, and the ability to incorporate many different species. Very recently, a great deal of work has been published concerning methods of producing these materials with specific physical forms, such as spheres, discs and fibres. Such possibilities will pave the way to new application areas such as molecular wires, where the silica fibre acts as an insulator, and the inside of the pore is filled with a metal or indeed a conducting polymer, such that nanoscale wires and electronic devices can be fabricated. Initial work on the production of highly porous electrodes has already been successfully carried out, and the extension to uni-directional bundles of wires will no doubt soon follow. 

Figure 85. The production of a silica fibre by drawing out a fused rod with a weight falling under gravity...

The extension of an amorphous material under a tensile force can be resolved into three parts first, an immediate elastic extension. Which is immediately recoverable on removing the tensile force Mcondly, a delayed elastic extension which is recoverable slowly and thirdly, a plastic extension, viscous flow, or creep, which cannot be glteovered. With glass at ordinary temperatures, this plastic exten- ion is practically absent. A very slow delayed elastic extension OOCUrs. This effect can be troublesome in work with torsion fibres. The delayed elastic effect in vitreous silica fibres is 100 times less than in other glass fibres, and viscous flow of silica is negligible below OO C (N. J. Tighe, 1956). For exact work vitreous sihea torsion flbres are therefore used. 

P. L. Kirk and R. Craig (1948) have discussed the construction of fibre devices L. Walden (1937) has given a practical account of several uses of fibres as instrument suspensions D. R. Barber(1930) has described a device for mounting fibres with a degree of tension and H. V. Neher (1940) has given a very practical account of the manipulation of silica fibres. 

Advanced computerisation and sensorisation and developments in the field of multielement optical detectors (CCD and PDA) and fibre optic remote spectroscopy have added modularity and flexibility. Silica-silica fibres used for spectroscopy applications are multimode with core diameters from 50 to 1000 p,m. The application of new technologies to optical instrumentation (e.g. improved gratings in spectrographs, the use of... 

Kleitz, F., Marlow, F., Stucky, G.D. and Schuth, F. (2001) Mesoporous silica fibres synthesis, internal structure and growth kinetics. Chemistry of Materials, 13, 3587-3595. 

Close to silica fibres are silicate fibres drawn from optical glasses. Silicate fibres are typically applicable in the visible spectral region. Their optical losses in the visible region usually reach much higher values than silica fibres - at least 102dB/km. On the other hand, the refractive index can be tailored in a large interval (from 1.5 for the BK-class to 1.95 for the... 

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. 

When the core diameter is inside several microns, only one (or several) modes can be guided in the fibre core and so these structures are called single-mode or few-mode ones. Single-mode (SM) silica fibres have a typical outer diameter of 125 pm with a 5-8-pm diameter of the core and a... 

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. 

The SPME process, adapted for solid or viscous matrix, is shown in Figure 10.1. A fused silica fibre, coated with a polymer, is installed inside a stainless steel hollow needle. In the first step, the needle is introduced in the sample vial through the septum. The fibre is then exposed to the headspace above the sample and the organic analytes adsorb to the coating of the fibre. After a variable sampling time, the fibre is drawn into the needle and the needle is withdrawn from the sample vial. Finally, in the same way, the fibre is introduced into the chromatograph injector where the analytes are thermally desorbed. 

Solid-phase microextraction has also been used for to determine volatile organic compounds in soil [26]. Target compounds were adsorbed directly from a head-space sample above a soil layer onto a fused-silica fibre. Vacuum distillation coupled with gas chromatography-mass spectrometry [27], head-... 

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. 

Optical fibres need to be free of impurities such as transition metal ions (see Chapter 8) and conventional methods of preparing silica glasses are inadequate. The sol-gel process is one way of forming fibres of sufficient purity (chemical vapour deposition. Section 3.7, is another). These processes use volatile compounds of silicon which are more easily purified, for example by fractional distillation, than silica. It is possible to produce silica fibres using a method similar to that studied in the nineteenth century, but with the geldrying time reduced from a year to a few days. Liquid silicon alkoxide (Si(OR)4), where R is methyl, ethyl, or propyl, is hydrolysed by mixing with water. 

An SPME unit consists of a piece of fused-silica fibre coated with a layer of a stationary phase such as non-polar poly(dimethylsiloxane) or polar polyacrylate or divinylbenzene/Carboxen/poly(dimethylsiloxane). The latter, for example, was suitable to trap the odorants (including sotolon) of soy sauce [19]. In the analytical procedure the fibre is exposed to the headspace of a food sample for 10-15 min. Then, the fibre is inserted into the injection port of a GC-mass spectrometry (MS) system. After desorption, the odorants are analysed. To improve the yields of the odorants, the fibre is placed in the effluent of a food sample purged with nitrogen [20]. 

Typical values for low-hydroxyl silica fibres are in the range of 0.22-0.3. 

Flow diagram of the Sumitomo process for making a mixture of alumina and silica fibre (reproduced by permission of Chapman Hall)56. 

As pointed out before, one must strictly distinguish between distribution or accumulation of chemical elements in biomasses and their functions for the corresponding organism the action need not be where the highest concentrations are but elsewhere, even disregarding inactive top concentrations in biogenic mineral phases such as bones or silica fibres fortifying plant shoots. Distribution and accumulation are related to the stability and variety of metal-biomass complexes (e.g. Wiinschmann et al. 2004) whilst function ... 

Hodson M. J., Smith R. J., Van Blaaderen A., Crafton T., and O Neill C. H. (1994) Detecting plant silica fibres in animal tissue by confocal fluorescence microscopy. Ann. Occupat Hyg. 38, 149-160. 

A coated fused silica fibre is directly introduced in the liquid sample or in the headspace above the sample. Respective ingredients from the sample material are absorbed onto the fibre material until equilibrium is reached. The fibre is removed from the sample and directly applied into the injection system of the GC. The absorbed compounds are thermally desorbed into the GC column for analysis. The method is solvent-free and requires no special additional equipment. It is used for the analysis of special groups of compounds, depending on the enrichment based on the type of fibre used/52-567. 

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