Aluminate Glass Suitable for Optical Uses
Anne L. Fischer
Aluminum oxide is the material of choice for use in demanding applications, such as in the protective tiles on space probes, the supports for catalytic reactor systems and gas turbines. Its benefits include fracture- and heat-resistant qualities and the ability to be formed into complex shapes, but working with it requires expensive machining operations.
This limitation potentially could be circumvented if glass-forming techniques could be employed, but aluminum oxide first must be made into an amorphous form. Previously, producing aluminate glass in bulk form was not possible because of its high melting temperature, its low viscosity upon melting and its tendency to crystallize.
Now scientists at 3M Co.'s Central Research Laboratory in St. Paul, Minn., have developed a method for creating bulk aluminate glasses that have potential optical applications. They use a flame technique to form micron-size beads of the glasses, then they sinter the beads into bulk glasses, which are reheated to create durable ceramics with nanocrystalline alumina distributed throughout.
The resulting ceramics produce more homogeneous microstructures than ceramics processed in other ways. The glasses also are transparent through the mid-IR and exhibit various colors and optical characteristics, including luminescence and a high refractive index (above 1.8). Such optical properties are imparted largely by optically active rare-earth ions that are essential ingredients in glassmaking. As a result, these glasses can solubilize large quantities of such useful fluorescent ions as erbium, praseodymium and thulium.
According to Anatoly Rosenflanz, this new method of preparing bulk forms of aluminate glasses and glass ceramics should allow near-net shaping -- fabrication with minimal waste -- of important alumina-based glasses and nanoceramics at temperatures below 1000 °C. These new materials may find application as lenses in optical elements such as ultrathin eyeglasses, IR-transparent windows, optical fibers and waveguides.
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