Fiber, formation from sols

Silica in the form of thin films as well as oxide monoliths, fibers, and powders can be prepared from sol-gel method. In contrast with the fabrication of conventional inorganic glasses at much higher melting temperature, sol-gel processing is performed at low temperatures to produce oxide materials with desirable hardness, optical transparency, chemical durability, tailored porosity, and thermal resistance. The sol-gel method involves formation of a colloidal suspension (sol) and gelation to form a network in a continuous liquid phase (gel). One starts with an aqueous solution containing oxides or alkoxides, mutual solvent, and catalyst. Usually an external catalyst is added like mineral acids and ammonia as well as acetic acid, KOH, amines, KF, and HF for rapid and... 

One favorable aspect of these layered compounds is their ability to form a layered structure from sol-gel precursors at temperatures as low as 600°C (1,112°F). This ability is critical, especially with complex chemistries, because a lower formation temperature can help avoid reactions between the fibers and intermediate coating phases. Additional work will be required to determine if these classes of compounds are stable with potential and commercially available fiber reinforcements. 

Another important problem to be solved for the commercial scale sol-gel fiber formation may be the cost-performance, since metal alkoxides are, in general, not so common in the market and accordingly expensive. From this point of view, the substitution of metal chlorides (or oxychlorides), acetates and sometimes metal powders for metal alkoxides have been examined to make the sol-gel process cost-effective (Khavari et al., 1988 Nishio and Fujiki, 1990). 

Fully dense fibers consisting of pure YAG phase were obtained by the 1600°C-heat-treatment of the gel fibers drawn from the solution of yttrium acetate hydrate/aluminum formate hydrate/H20/formic acid/ethylene glycol. The bending strength of the fibers was 1.7 0.2 GPa (Liu et al, 1998). Pullar et al. (1998a, 1999) also fabricated pure YAG fibers from aqueous sol-gel precursors. They blow-span the stoichiometric mixture of the alumina sol and silica sol by using PEO as a spinning aid. The HCl added to the sol as a peptizer was found to stabilize the sol, and to play important roles for the appearance of spinnability and also to improve the sinterability of the fibers. Fully dense YAG fibers obtained by them at 1600°C exhibited improved friability. 

For possible fiber formation through drawing from a sol, this has to satisfy the following two conditions ... 

The continuous mullite fibers were first made by heat-treating the gel fibers spun from the sol prepared by hydrolyzing the stochiometric mixture of TEOS and acac-modified AIP (Tucker, 1990). The amorphous gel fibers were crystallized to mullite fibers above 1000°C. Thereafter, several cost-effective sol-gel methods of the mullite fiber formation have been developed. In one method, the mixture of silica sol and alumina sol prepared from aluminum salts were spun with the aid of polyethylene oxide (PEO) (Bhattacharya, 1996a). In others the stoichiometric mixture of inexpensive alumina sources (nitrate, acetates. 

One of the first examples of mesoscopic-macroscopic two-dimensional ordering within a structure involved a bacterial superstructure formed from the co-aligned multicellular filaments of Bacillus subtilis that was used to template macroporous fibers of either amorphous or ordered mesoporous silica [82], The interfilament space was mineralized with mesoporous silica and, following removal of the organic, a macroporous framework with 0.5 pm wide channels remained. Mesoporous silica channel walls in this hierarchical structure were curved and approximately 100 nm in thickness. Dense, amorphous walls were obtained by replacing the surfactant-silicate synthesis mixture with a silica sol solution. The difference in the mode of formation between porous and non-porous wall structures was explained in terms of assembly from close-packed mesoporous silica coated bacterial filaments in the former compared to consolidation of silica nanoparticles within interfilament voids in the latter. 

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