Ultrapure silica fibers

Silicate glass fibers, which command by far the largest sales volume in the market, are derived from the liquid phase, a viscous melt. Ultrapure silica fibers are either derived from a melt, which is downdrawn from a preform, or they are dry spun from a viscous solution. While a high melt or solution viscosity seems to be a generai prerequisite for fiber formation, it is possible to form glass fibers, such as YAG or aluminate glass fibers, from melts. 

Ultrapure silica fibers from viscous sol-gels... 

Figure 15. Strength retention of commercial silica yams at elevated temperatures. Sil and QSA fibers are derived from the liquid phase. Silfa Is a pure silica yam made from a viscous waterglass solution. Q A is an ultrapure silica fiber made from a high viscosity melt. Asahl, Nextel and Refrasil are derived from solid phase precursor fibers. Asahi is an ultrapure silica fiber, Nextel is an alumlnate fiber, and both are derived from a sol-gel precursor fiber. Refrasil is a high silica fiber It is derived from borosllicate E-glass by add leaching. Redrawn from product information supplied by Ametek Corporation in Wilmington, DE...

Ultrapure silica fibers from sol-gels Silica fibers by other processes... 

Ultrapure silica fibers from solid preforms... 

Only dry spinning facilitates the formation of amorphous silica glass fibers from viscous solutions. Two process variants are known. One uses a viscous water glass or sodium silicate solution, wherein water is the solvent, and produces a pure and relatively low cost fiber. The other uses a viscous tetraethylorthosilicate solution wherein alcohol is the solvent, and produces an ultrapure premium fiber. In either process, the viscous solution is spun from, or extruded through, multiple spinneret orifices into a hot column that removes the solvent. Before the as-spun fibers are wound onto a package, they are solidified in a final, high temperature curing step. 

Table XII. Properties of ultrapure silica glass fibers Astroquartz and Quartzel [101...

Ultrapure silica or quartz fibers are used in fabrics, yams, rovings and threads. Fabrics are used to reinforce radomes, antenna windows for missiles, high temperature circuit boards, and rocket nose cones. Braided yarns provide high temperature electrical insulation, e.g., for coaxial cables, thermocouple wires, and space separators. Rovings are used to reinforce polymer matrix composites for ablative and electrical uses, as well as high performance sporting goods, e.g., tennis racquets and skis, especially when hybridized with carbon fibers. Threads are used to stitch cable tray insulation for nuclear power plants. 

Ultrapure and pure silica fibers from solutions... 

Vitreous silica optical fibers for modern telecommunication systems must be made from ultrapure materials. Glasses produced from melts are incapable of reaching the quality levels required by these fibers. The glasses used in current fibers are produced in situ as the preform is formed by vapor deposition processes. Since the raw materials are liquids, purification by distillation can radically reduce impurity contents. Furthermore, since the glass never contacts crucible or refractory materials, the purity of the glass is maintained throughout the process. 

Two of the four known silica giass fiber processes are dry spinning processes. They are continuous processes, which afford either a continuous sliver or staple product, i.e., a cohesive array of discontinuous fibers, or a continuous multifilament yam, respectively. The other two processes are continuous processes yielding continuous fibers and multifilament yarns. Of these, the preform process (Chapters 4 and 6) relies on the use of silica preforms, and produces continuous, ultrapure siiica giass fibers, or quartz fibers. This process yields the strongest silica giass fibers on record. 

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