Alumina-silica fibres

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... 

C. Lesniewski, C. Aubin and A. R. Bunsell, Property-structure-characterization of a continuous fine alumina-silica fibre. Composites Science Technology, 37,63-78 (1990). 

Alumina-silica fibres were the first ceramic fibres produced in the early 1970s, for thermal insulation applications. Small amounts of silica, %3 wt% in the Saffil short... 

Fibrekote. A sprayed refractory lining, comprising a mixture of alumina-silica fibres with an inorganic binder, stable to 1600°C. 

The resistance of PTFE to creep can be improved by blending in up to 25% of glass or asbestos fibre using PTFE dispersions as mentioned in the previous section. By the same technique alumina, silica and lithia may be incorporated to... 

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

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. 

Wu J, Chen M, Jones FR, James PF, Characterisation of sol-gel derived alumina-silica matrices for continuous fibre reinforced composites, J Eur Ceram Soc, 16(6), 619-626, 1996. 

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

The complex crystal structure of mullite provides creep resistance materials if a sintered microstructure can be obtained without the help of an excess of silica. 3M produced a dense mullite-a-alumina fibre, called Nextel 720 (Wilson et al., 1995), by using an aqueous sol, composed of intimately mixed silica and alumina precursors, with iron compounds used as the seeds for a-alumina. The fibre shown in Figs, 16 and 17 is composed of a continuum of mullite mosaic grains of about 0.5 p-m with wavy contours, with no silicate intergranular phase (Deleglise et al., 2001). 

Thermally insulative undergarments from blends of metal containing spandex and other fibres have been produced. A blend containing 15% spandex fibres having alumina, silica, titania and Pt at 10 82 3 5 wt. ratio, and 85% cotton showed heat conductivity 4.906 x 10 " W/cm°C and heat retention 28.6%. A ladies stocking of a blend " comprising 6.4% spandex fibres containing alumina, silica, titania and Pt at 10 82 3 5 wt. ratio, 24.1 % acrylic fibres and 13.5% nylon fibres also showed heat conductivity 3.525 X lO W/cm C and heat retention 42.3%. 

Chemical Engineering Co) alumina boria silica fibre ... 

In some specialised cases the pore structures are known to be anisotropic. This occurs in porous fibres and can be studied by measurements in which the sample orientation is varied to give conditions in which die Q-vector is either parallel or perpendicular to the fibre axis. An example is given by Stacey [12] for SANS studies of alumina and Fig.5a shows a similar measurement made widi X-rays Hg.5b shows an example for a naturally-occurring silica fibre taken from the skeletal structure of a sea animal [11] where the anisotropy is more pronounced. It can be seen that the scattering intensity is dramatically altered by changes in the sample orientation. Suitable analytic methods are now being developed to extract information on the alignment properties of the pores based on partially-oriented cylindrical voids. 

Identification of dyes on dyed textiles is traditionally carried out by destructive techniques [493], TLC is an outstanding technique for identification of extracted dyestuffs and examination of inks. Figure 4.9 shows HPTLC/SERRS analysis of acridine orange [492], Wright et al. [494] have described a simple and rapid TLC-videodensitometric method for in situ quantification of lower halogenated subsidiary colours (LHSC) in multiple dye samples. The results obtained by this method were compared with those obtained by an indirect TLC-spectrophotometric method and those from HPLC. The total time for the TLC-videodensitometric assay of five standards and four samples applied to each plate was less than 45 min. The method is applicable for use in routine batch-certification analysis. Loger et al. [495,496] have chromatographed 19 basic dyes for PAN fibres on alumina on thin-layer with ethanol-water (5 2) and another 11 dyes on silica gel G with pyridine-water... 

Mixtures of silica gel and the liquid difluoride sealed in tubes at 334 mbar exploded above -196°C, presence of moisture rendering the mixture shock-sensitive at this temperature [1], Reaction of oxygen difluoride with silica, alumina, molecular sieve or similar surface-active solids is exothermic, and under appropriate conditions may be explosive [2], A quartz fibre can be ignited in the difluoride [3],... 

Sumitomo Chemical Co. produces a fibre that is a mixture of alumina (85%) and silica (15%). The fibre structure consists of fine crystallites of spinel. Si02 serves to stabilize the spinel structure and prevents it from transforming to a-alumina [14], The flow diagram of this process is shown in Fig. 3.2. 

ICI Co. uses a sol-gel method to produce silica-stabilized alumina (Saffil) and calcia-stabilized zirconia fibre [15], The saffil fibre is a 8-alumina short staple fibre that has about 4% Si02 and a very fine diameter (3 pm). 

Janes, Neumann and Sethna ° reviewed the general subject of solid lubricant composites in polymers and metals. They pointed out that the reduction in mechanical properties with higher concentrations of solid lubricant can be offset by the use of fibre reinforcement. Glass fibre is probably the most commonly used reinforcing fibre, with carbon fibre as a second choice. Metal and ceramic fibres have been used experimentally to reinforce polymers, but have not apparently been used commercially. To some extent powders such as bronze, lead, silica, alumina, titanium oxide or calcium carbonate can be used to improve compressive modulus, hardness and wear rate. 

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]. 

Practically all types of commercially available thin-layer plates and precoated sheets can be used silica gel, cellulose, kieselguhr, alumina and glass fibres. It is, perhaps, not necessary to emphasize that both one- and two-dimensional arrangements can be materialized as well as different combinations with chromatography and multidimensional procedures [65]. 

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