Fluorozirconate glass
Fluoride glass is a class of non-oxide optical glasses composed of fluorides of various metals. Due to their low viscosity, it is very difficult to completely avoid the occurrence of any crystallization while processing it through the glass transition (or drawing the fiber from the melt).
Thus, although heavy metal fluoride glasses (HMFG) exhibit very low optical attenuation, they are not only difficult to manufacture, but are quite fragile, and have poor resistance to moisture and other environmental attacks.
Fluoride glasses' best attribute is that they lack the absorption band associated with the hydroxyl (OH) group (3200 – 3600 cm−1) which is present in nearly all oxide-based glasses.
HMFGs were initially slated for optical fiber applications, because the intrinsic losses of a mid-IR fiber could, in principle, be lower than those of silica fibers, which are only transparent to wavelengths shorter than ~2 μm.
Such low losses were never realized in practice, and the fragility and high cost of fluoride fibers made them less than ideal as primary candidates. Later, the utility of fluoride fibers for various other applications was discovered.
These applications include mid-IR spectroscopy, fiber-optic sensors, thermometry, and imaging. Also, fluoride fibers can be used for guided lightwave transmission in media such as YAG (Yttrium aluminium garnet) lasers at 2.9 μm, as required for medical applications (e.g. ophthalmology and dentistry).
An example of a heavy metal fluoride glass is the ZBLAN glass group, composed of zirconium, barium, lanthanum, aluminum, and sodium fluorides. These materials' main technological application is as optical waveguides in planar and fibre form. They are advantageous especially in mid infrared (2000-5000 nm) range.
Some fluoride glasses are difficult to produce on Earth due to their rapid crystallization. The crystallization is slower in microgravity conditions due to reduced convection effects.
Fluoride glass may also mean an oxide optical glass doped with zirconium tetrafluoride.
A mix of fluoride glass and phosphate glass is fluorophosphate glass.
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