Single crystal fibers melt processes

In summary, continuous sapphire fibers are commercially available, and new YAG fibers are readily achieved with the Saphikon process, or the LHPG process (see Chapter 6), or else by the new containerless laser melt process (Chapter 4). Currently however, there is only one route known, i.e., HP-LCVD, that might eventually be capable of yielding continuous, single crystal fibers such as SiC or titanium carbide fibers. A single crystal SiC fiber by LCVD has... 

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. 

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 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]. 

The strength level of fibers obtained from the vapor phase is generally higher than that of equivalent fiber made from the melt or precursor fibers, reflecting the fact that they are made directly from the vapor phase and/or by a containerless process. For example, single crystal CVD-SiC whiskers (7.5 GPa) are stronger than polycrystalline CVD-SiC fibers (7.5 vs. 3.5 GPa) and both are stronger than polycrystalline SiC fibers which are made from solid precursor fibers (1.1-3.0 GPa). 

The economics and scalability of the new process are not known. The materials cost and the cost of operating a laser process are probably about the same for an amorphous YAG sensor fiber made by the containerless laser heated melt process and a for single crystal YAG sensor fiber made by laser heated pedestal growth (Chapter 4.5.2). And both are containerless processes. However the higher process speed may favor the laser heated melt process (1.5 m/s) over the laser heated pedestal process (1 mm/s). 

Three processes are known to fabricate continuous yttrium aluminum garnet (YAG) fibers. Single crystal YAG fibers are obtained by the edge defined film fed growth process and by the laser heated float zone process (Chapter 4.5). Both are slow processes. Amorphous YAG glass fibers have recently been demonstrated by a containerless laser heated melt process (Chapter 4.4). Polycrystalllne YAG fibers can be obtained with sol-gel and related processes (this chapter). These are potentially fast processes. 

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