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New opportunities for transparent ceramics alumina is in a phase that forms hexagonal close-packed structures in which aluminum ions can fill two-thirds of the octahedral sites because of their small ionic radii. GE’s research produced its best transparent alumina by firing the green body with a 0.0625–0.10-weight-percent doping of magnesia at 1,900°C in a hydrogen atmosphere.10 Although the diffused forward transmittance is high, the inline transmittance is relatively low, especially in the visible region (13 percent at 500 nanometers, 0.94 millimeters thick) because of the birefringence. Conventional sintering techniques already can make alumina ceramics with very low porosity. To minimize the birefringence, it would be better to either reduce the grain size to nanometers or align the grain orientation along the same optical axis. Along these lines, researchers employed pressure-assisted sintering to make submicrometer-sized grain transparent alumina ceramics. Using SPS, they fabricated alumina ceramics with a 270-nanometer average grain size that had a real inline transmittance (small angle aperture) of 47 percent at 640 nanometers (0.88 millimeters thick) that was close to the theoretical value of 68 percent. They also reported that a lower heating rate (8°C per minute) is much more advantageous than a higher heating rate (100°C per minute) for the optical and microstructure properties of the alumina ceramics. The researchers found that HIP could easily reduce porosity to less than 0.05 percent and the grain size could be suppressed to between 0.3–1.0 micrometers. Although the grain size was bigger (500 nanometers), the inline transmittance was higher (64 percent at 640 nanometers, 0.8 millimeters thick) than the SPS sample because of the lower porosity. Grain-orientation technologies, another strategy for anisotropic optical ceramics, include templated grain growth, screen printing, pulsed electric current, and magnetic field assistance. For example, with an applied magnetic field of 12 tesla, the c-axes of alumina particles align parallel to the direction of the magnetic field during slip casting. (Credit: Chen and Wu.) (a) (b) Figure 6. An (a) image of hexagonal Yb: Sr-FAP transparent disk and its (b) TEM-EDS characterization. After three hours sintering at 1,850°C in a hydrogen atmosphere, the transparent alumina ceramic, has approximately a 58 percent inline transmittance (at 640 nanometers, 0.8 millimeters thick). The XRD pattern shows that the only peak related to the crystal planes is perpendicular to the c-axis. Armed with the results of years of research, companies moved to use the high-temperature chemical resistance of alumina to produce transparent alumina ceramic bulbs for commercial lighting applications. These bulbs have been used for high-pressure sodium-vapor lamps, producing a yellow light, as well as for metal halide lamps, producing highintensity white light comparable to that of sunlight. These lamps possess a much higher luminous efficacy than mercuryvapor lamps and incandescent lamps. Thus, they have been used throughout the world, even in developing countries, in parking lots, sports arenas, factories, as well as residential security lighting and automotive lighting. Apatites: Fluorapatite materials, such as strontium phosphate fluoride (Sr5(PO4)3F, or Sr-FAP), have attracted considerable attention in recent years because of their promising application in high-power laser systems that can be operated continuously without cryogenic accessories (Figure 6). Sr5(PO4)3F crystallizes in the hexagonal system with refractive indexes (at 589 nanometers) ne of 1.62523 and no of 1.6314 (where ne and no represent the extraordinary and ordinary indices, respectively). Sr-FAP doped with ytterbium or neodymium ions has a large cross section and long fluorescence lifetime, properties that make it an excellent candidate for laser applications. Sr-FAP material is typically doped with rare-earth ions, and the spin orbit interaction of the 4f electrons of the ion strongly enhances the net magnetic anisotropy, resulting in the ability to generate large magnetic torque. This synthesis of highly oriented anisotropic ceramics with a magnetic field as low as 1.4 tesla compared with that of 12 tesla for alumina. The Ca5(PO4)3F green body formed with the assistance of a low magnetic field is sintered at 1,600°C for two hours in open air and then subjected to HIP under 190 megapascals at 1,600°C for one hour in an argon atmosphere. The sintered Ca5(PO4)3F laser ceramic exhibited inline transmittance of 82 percent at a wavelength of 1,064 nanometers with a thickness of 0.48 millimeters. However, a downfall of this technology is that for materials that have a magnetic anisotropy of the c-axis lower than that of the a-axis—such as ytterbium-doped Sr-FAP—fabrication requires a rotating magnetic field to achieve the crystal orientation. With wet-chemical-precipitated powder, highly transparent ytterbium-doped Sr-FAP can be prepared by SPS at only 1,050°C for eight minutes under a pressure of 100 megapascals. Researchers have measured the inline transmittance of the spark-plasma-sintered ytterbiumdoped Sr-FAP ceramic to be 78 percent at a wavelength of 1,064 nanometers (2 millimeters thick). Microstructure characterization showed that this sample has an average grain size of around 150 nanometers (with a size range of 40–200 nanometers). When the sintering temperature increased to 1,400°C, the resulting ceramic had an average grain size of 9.86 micrometers and the inline transmittance decreased to around 50 percent 36 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No.2


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