Melt spinning processes generic

The double crucible apparatus, which yields concentric bicomponent fluoride fibers, is a generic melt spinning process except that the dual melt is separately maintained under carefully controlled conditions in a supercooled state well below the llquidus temperature of the fluoride glasses [6-7]. The inner and outer crucibles form the concentric tip system through which the glass melts flow. 

Melts of metals as well as crystalline ceramic oxides have low viscosities. All solidify at a sharp melting point and their viscosity above the melting point increases rapidly and then reaches a viscosity comparable to that of motor oil at room temperature, i.e., log <0.2 poise. Yet, fiber formation from the liquid phase requires a viscosity of log 2.5 to log 3.0 poise. Only liquid droplets are formed if a low viscosity liquid is extruded at a normal quench rate of -10" K/s through an orifice, spinneret hole or bushing tip. To facilitate fiber formation, the viscosity must be raised from log <0.2 poise to log 2.5 to log 3.0 poise. This can be achieved by one of two generic routes, i.e., by a rapid solidification (RS) or an inviscid melt spinning (IMS) process. 

A liquid phase, as opposed to a vapor or solid phase, includes dispersions, solutions and melts. Several processes, which yield continuous inorganic fibers directly from the melt, have been discussed in Chapter 4. Only one generic process, dry spinning, is known to yield one specific amorphous oxide fiber directly from a liquid phase other than that of a melt. All other processes which start with a liquid phase (see Chapters 8-12) yield first a solid, non-functional precursor or green fiber, and then a functional, nano- or polycrystailine ceramic fiber. Such refractory ceramic fibers are therefore directly derived from a solid phase, a precursor or a green fiber, and only indirectly from a liquid phase. 

The same generic dry spinning process can be used to fabricate the precursor fiber for a carbon fiber from an infusible polymer, such as polyacrylonitrile, for a polycrystalline aluminate fiber from a sol-gel or for a polycrystalline alumina fiber from a slurry. Again, a high temperature curing step is required to convert the as-spun, amorphous precursor fiber into the final functional fiber. In these cases, however, an amorphous polycrylonitrile precursor fiber changes into a carbon fiber, and an amorphous aluminate precursor fiber into a crystalline aluminate fiber. The final functional fiber is therefore directly derived from a solid and amorphous precursor fiber and only indirectly from a liquid phase, i.e., a melt or sol-gel, respectively. 

The one step process for the fabrication of pure silica sliver from sodium silicate [1-3] is an adaptation oil the generic dry spinning process that has been practiced for over 50 years in the fabrication of polymer organic textile fibers. Acrylic polymers, unlike polyesters or nylons, are infusible and cannot be melt spun. They can be dry or wet spun from viscous solutions. The model for inorganic dry spinning processes is the dry spinning process by means of which acrylic fibers such as Orion, are spun using dimethyl formamide as the solvent (Table I). 

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