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(Credit: Chen and Wu.) Highly transparent ceramics with designed applications. along with thermal treatment as a driving force for densification. Hot isostatic pressing (HIP) frequently is used after the pores in the ceramics break up and form closed spherical bubbles during the third stage of sintering. Another method commonly used to make transparent ceramics is doping with a sintering aid, because the aid does not affect the physical properties of the final product. The sintering aid can be either a higher-melting-point material— used for defect-induced diffusion from charge compensation—or a lowermelting point material—used for liquidphase enhanced diffusion. For non-cubic-phase materials, some grain orientation technologies, such as magnetic field, pulsed electric current, and screen-printing, can align the optic axes for transparency. Based on their crystal structure, the transparent ceramics reviewed below focus on the fabrication technology and product application, which can be divided into two general categories: cubic-phase and non-cubic phase. Cubic-phase transparent ceramics Oxides: General Electric developed transparent yttrium oxide (Y2O3) ceramics in the 1970s, with a 10-percent thorium oxide doping. GE believed this material had a good potential for lighting and laser applications, but these ceramics required sintering temperatures of 2,000–2,200°C. Concurrently, GE prepared pure yttria ceramics through hot-pressing at much lower temperatures, usually between 1,300–1,500°C, which produced a material transparent in the IR range and translucent in the visible range. As the first oxide ceramic laser, this material had a slope efficiency of only around 0.1 percent. Decades later, Raytheon and GTE succeeded in developing highly transparent yttria ceramics by means of applying HIP after regular sintering (where a sintering additive aided the technologies). These two companies were able to manufacture dome- and flat-shaped, high-quality transparent ceramic yttria missile windows that exhibited flexural strengths at room temperature of 74 and 98 megapascals, and Weibull moduli of 7.6 and 4.3, respectively. The mechanical data revealed that the yttria-composed windows prevented high-tensile-stress failure generated by thermal shock when exposed to an air stream. Engineers predicted the survival altitude to be above sea level for Mach 4, above 23,000 feet for Mach 5, and above 36,000 feet for Mach 6.3 Today, transparent yttria ceramics can be fabricated readily by vacuum sintering at around 1,700°C. A measured slope efficiency of 2-percent Yb3+:Y2O3 transparent ceramic lasing at 1076 nanometers can reach 44.6 percent using a 5-percent output coupler.4 Engineers also employ polycrystalline ceramics for up-conversion (a nonlinear optical process to convert long-wavelength excitation radiation into shorter-wavelength output radiation) and down-conversion of transparent coatings that they use to develop full-spectrum solar cells by converting IR or an ultraviolet (UV) radiation into absorbable visible spectrum (Figure 2). Rare-earth-doped yttria transparent ceramics can be tailored to have up-conversion and down-conversion functions. Yttria is not the only oxide to receive attention as a laser host material. Japanese researchers demonstrated that yttrium aluminum garnet (Y3Al5O12, or YAG) transparent ceramics could be a successful laser oscillation host material. In the early 1990s, they fabricated YAG transparent ceramics by solid-state reactive sintering under vacuum at 1,750°C with silica as a sintering aid. Within a few years, several companies were providing commercial products with large, varying sizes and complex shapes. Engineers have measured a slope Figure 2. Principles of (a) up-conversion and (b) down-conversion process of rare-earthdoped Y2O3 transparent (c) ceramics and (d) an application for developing a full-spectrum solar cell. American Ceramic Society Bulletin, Vol. 92, No. 2 | www.ceramics.org 33 (Credit: Chen and Wu.) (a) (b) (c) (d)


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