Feature Article - New opportunities for transparent ceramics

mar13_2013

New opportunities for transparent ceramics cubic-phase materials with different refractive indexes along different optical axes), birefringence is another important factor that prevents light from propagating without extinction. Thus, to obtain the perfect microstructure with a minimum level of pores and submicrometer (or smaller) grain size for optical anisotropic materials, the use of appropriate powder processing and consolidation technology is indispensable. Although researchers have studied the technology of transparent ceramic fabrication for several decades (Figure 1), the achievement of highly efficient ceramic laser oscillation within the past decade sparked a burning curiosity about these materials, and some consistent and proven processing methods steadily have been established. For example, powder processing, with either commercial- or lab-synthesized nanosized powder, has become a standard procedure in avoiding large pores. Typical green body forming methods include dry pressing, slip casting, gel casting, and tape casting. However, approaches to sintering technology vary dramatically depending on the crystal, chemical structure, and consolidation mechanism of the particular material. High-vacuum sintering is the most common technique for the fabrication of transparent cubic-phase oxide ceramics. Other methods, such as hot pressing and spark plasma sintering (SPS), apply pressure By Shi Chen and Yiquan Wu An overview of how cubic-phase oxide, sulfide, and mixed anionic systems, and non-cubic-phase oxide and apatite systems—subjected to various sintering process—are leading to new applications and new frontiers for transparent ceramics Images of transparent ceramics on the macro and nanoscales. T ransparent ceramics are emerging as a highly promising alternative to current glass and single-crystal technologies in a number of diverse application fields that include armor, optical fiber, lasers, infrared domes, and high-energy radiation detection. In this context, the term “transparent” refers to a ceramic material with at least 90 percent of the theoretical transmission over the wavelengths of interest. With this property, transparent polycrystalline ceramics can provide unique and versatile optical materials highly suitable for scintillators, optical components, solid-state lasers, and nonlinear optics. In general, polycrystalline ceramic materials, which normally contain a high content of optical scattering sources, can be engineered to be transparent by eliminating defects. Grain boundaries, residual pores, secondary phases, double refraction, inclusions, and surface roughness can, more or less, act as elastic scattering sites that prevent incoming light from passing straight through conventional ceramics, which are composed of randomly oriented microcrystallites.1 When the size of the governing scattering sites, known as residual pores, become larger than approximately 10 percent of the wavelength of the incident light, engineers can use a Mie scattering model to demonstrate the relationships between pore size, porosity, and scattering losses, and obtain a general idea about the transmission properties of a ceramic material.2 For optical anisotropic materials (most of which are non- (Credit: Chen and Wu.) (Credit: Chen and Wu.) Figure 1. A timeline of some major accomplishments for the transparent ceramics. 32 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No.2


mar13_2013
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