Crystalline IR fibers are an attractive alternative to glass IR fibers, because most nonoxide crystalline materials can transmit longer wavelength radiation than IR glasses; in the case of sapphire, they exhibit some superior physical properties as well. One disadvantage over glass fibers is that crystalline fibers are somewhat difficult to fabricate. This is due in part to an important distinction between crystals and glasses: glasses are mostly covalently bonded, amorphous materials lacking long-range order, whereas crystals are ionically bonded materials possessing long-range order. Furthermore, glasses have a large glassy region for which the glass is in a softened or supercooled state. This temperature region is between the glass transition temperature, Tg, and Tx, the crystallization temperature. It is within this temperature region that fiber drawing occurs. Crystals, however, possess no softening region or Tg under normal conditions. Therefore, crystals cannot be drawn like fibers because they do not have a temperature that would give an appropriate viscosity for fiber drawing. In short, crystalline fibers must be fabricated either using modified crystal-growth techniques in which a fiber is pulled from the melt, or by heating the crystal to temperatures below the melting point and then applying significant pressure to force the material through a die.
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