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New opportunities for transparent ceramics Figure 3. An (a) XRD pattern and (b) transmittance spectrum of Nd-doped YAG transparent ceramics, and (c) an image of the formed material. efficiency of 85 percent for 10-percent- Yb3+:YAG transparent ceramics. In recent years, they also have demonstrated rapid progress in output power for Nd3+:YAG, starting with an output power of 72 watts in 2001, 1.46 kilowatts in 2002, 67 kilowatts in 2006, and 100 kilowatts in 2010.5 Companies are already supplying low-power Er3+- or Nd3+-doped YAG transparent ceramics, such as in medical applications, as lowcost laser grain media alternatives to single crystals (Figure 3). In regard to mechanical properties (from the Weibull parameters of the characterized transparent ceramics), YAG was found to have superior flexural strength compared to spinel and yttria. This strength makes it an attractive candidate for IR transparent missile domes and reconnaissance windows.6 Using vacuum sintering, researchers subsequently developed similarly structured terbium aluminum garnet (Tb3Al5O12, or TbAG) transparent ceramics for magneto-optical applications; lutetium aluminum garnet (Lu3Al5O12, or LuAG) doped with cerium for high-energy radiation scintillating; and transparent thulium aluminum garnet (Tm3Al5O12, or TmAG) for use in lasers. Researchers also have prepared highly transparent lutetium oxide (Lu2O3, Figure 4) via the same vacuum sintering process, beginning with nanosized powders synthesized by coprecipitation or combustion. Lu2O3 is a potential candidate for high-power laser grain media because it has twice the amount of thermal conductivity as YAG with Yb3+ doping higher than 20 percent.5 They have applied similar procedures to fabricate transparent ceramics from scandium oxide (Sc2O3) for laser applications. Vacuum sintering also can be a very useful tool for densifying other cubicphase oxide transparent ceramics, such as A2B2O7-structured materials, including La2Hf2O7, LaGdHf2O7, Y2Zr2O7, and Nd2Zr2O7. Scientists that have characterized these materials have found that some have excellent optical properties as laser host materials. For example, europium-doped lutetium transparent ceramics—that require pressure-assisted sintering to achieve high densification— have excellent scintillating properties and may be of great interest for X-ray imaging because of their high density. Other oxides are of interest, too. For example, highly transparent armor material, magnesium aluminum spinel (MgAl2O4) prepared by presintering plus a pressurized sintering stage with or without a lithium fluoride sintering aid. Additionally, with the assistance of pressure, 8-percent-yttria-stabilized cubicphase zirconium oxide (ZrO2) can be prepared as a transparent material for use as fuel cell and oxygen sensor applications. Furthermore, unlike the conventional technologies mentioned above, researchers recently have learned how to transform amorphous glass oxides into transparent ceramics through full crystallization. For example, Allix, et. al crystallized europium-ion-doped barium aluminate-based glass into a highly transparent polycrystalline BaAl4O7:Eu ceramic by simple annealing. This material has potential for phosphorescent optical applications.7 Fluorides: These materials have a wider transparency window than most oxides. Their optical and thermomechanical properties also make them attractive for multispectral imaging as well as UV and midwave IR windows and laser grain applications. In the mid-1960s, Kodak developed the first fluoride ceramic laser (CaF2:Dy3+). It had a visible range transmittance close to a single crystal, but demonstrated poor and limited laser performance.5 The same group of researchers developed magnesium fluoride (MgF2) for potential application in IR missile domes. This material essentially has the transmittance of a single crystal in the wavelength range between 2 and 6 micrometers, but the transmittance drops dramatically as the wavelength moves from IR to visible ranges. Hot pressing played a role in Figure 4. Images of (a) transparent Pr-doped Lu2O3 ceramic disk and (b) its illumination under a 245-nanometer UV irradiation. (Credit: Chen and Wu.) 34 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No.2 (Credit: Chen and Wu.) (c) Transmittance (%) 2θ (degree) Wavelength (nm) Intensity (a.u.) (a) (b)


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