Inviscid melt process

Whether the process for forming solid fibers (or ribbons) from inviscid melts is fast or slow, i.e., whether amorphous or single crystal fibers are formed, it proceeds through a transient viscous range. The inviscid melt will solidify as a glass fiber when a transient viscosity of log 2.5-log 3.0 is reached as required for formation of any fiber that is obtained from the liquid phase or melt [12]. 

Continuous glassy metal ribbons can be formed with high quench rates from their inviscid melts by a rapid solidification process [60] that is akin to a generic bushing process (Figure 3, top left), except that the extruded ribbon must be rapidly cooled on the surface of a cold quench wheel. Continuous aluminate glass fibers and metal wires [10-12] and continuous amorphous YAG fibers [73] can be melt spun from inviscid melts by increasing the jet lifetime... 

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. 

One inviscid melt spinning process, the containerless laser heated melt process (Chapter 4.4.4) is believed to facilitate the formation of fibers by increasing the viscosity of the inviscid melt (and jet lifetime) at a normal quench rate of IC K/s, i.e., without increasing the quench rate to -10 K/s. 

Commercial wire drawing processes produce metal wires with round cross sections but they are highly energy and labor intensive. Wire drawing falls outside the scope of this book. Commercial rapid solidification processes yield amorphous metallic ribbons. Inviscid melt spinning yields metal fibers by a chemically assisted jet stabilization process. 

In the inviscid melt spinning process [10] [51], steel wires are formed by the same mechanism as glass fibers. In this case, the process shown in Figure 14 depends on the presence of silicon in the steel formulation and on the presence of carbon dioxide in the process environment. 

Figure 14. Inviscid melt spinning process (schematic drawing). Redrawn from F. T. Wallenbeiger, N. E. Weston and S. A. Dunn, Inviscid melt spinning as-spun amorphous alumina fibers. Materials Letters, 2 [4] 121-127 (1990).

Inviscid melt spinning is considered to be a potentially viable alternative to wire drawing [51] for making steel wires for radial automobile tires, but a prior product development did not reach beyond the pilot plant level. Using silica steels, the complex chemistry (Equations 4-6) produce also minute amounts of iron oxides which were detected by ESCA [51], and are a potentially undesirable trace byproduct. The challenge [4] remains to fine tune the chemistry of this process before commercial development. 

Continuous binary calcium aluminate glass fibers can also be formed by inviscid melt spinning. In this case, carbon particles which are formed by the decomposition of propane enter into the surface of the molten jet and raise its surface viscosity, a process that lengthens the lifetime of the jet and prevents its breakup. 

A viable process for the formation of continuous, self-supporting fibers such as hydrogen from liquefied gases has emerged over the past two decades [74]. Like all prior process iterations [74], it appears to be an inviscid melt spinning process (IMS) and not a rapid solidification (RS) process. 

Continuous single crystal fibers can be grown from inviscid melts by two relatively slow processes the edge defined film fed growth (EFG) process [13] and the laser heated float zone (LHFZ) or laser heated pedestal growth (LHPG) process [14]. Both offer growth rates of toO.3-0.7 mm/s [13-14]. 

Inviscid melt spinning A process which allows low viscosity molten material to be spun into fibers. The low viscosity jet is chemically stabilized rather than rapidly solidified. 

The behavior of fiber forming inorganic melts is well understood [1-2], They are viscous or inviscid [3-4], i.e., have high or low viscosities, and fiber forming processes are either very fast (>1000 m/min,) when continuous amorphous glass fibers are desired, or slow (<0.1 m/min) when continuous single crystal fibers are desired. 

Continuous fibers can be formed from viscous (high viscosity) meits [2] [17], and from inviscid (low viscosity) melts [11] [19]. The design of a viable fiberizing process from either meit depends primarily on three important factors (1) the relationship between melt viscosity and temperature, above and below the fiber forming temperature, (2) the liquidus temperature, the highest temperature at which crystals can form, (3) the nature of the crystalline phase at and below the liquidus and the crystal growth rate. 

In processes with conventional quench rates of 10 K/s, the inviscid liquid (melt) or resulting jet is carefully under- or supercooled to its narrow fiber forming range (log 2.5 to log 3.0 poise). Three types of fibers have been made by variants of this process continuous optical yttrium aluminum garnet (YAG) glass fibers, continuous aluminate glass fibers and steel fibers. 

Similar to class I, class II systems also typically require high processing temperatures and long residence times to fully melt and disperse the API. Thermal degradation of the polymer and/or the API is again an issue. The inviscid polymers may possibly be prone to thermal degradation at these temperatures, but the lower viscosity should... 

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