Feature Article - Transparent polycrystalline cubic spinels protect and defend

mar13_2013

Transparent polycrystalline cubic spinels protect and defend Left: Spinel crystal structure. Right: Transparent aluminum oxynitride window manufactured by Surmet Corp. (Burlington, Mass.). By Mohan Ramisetty, Suri Sastri, Uday Kashalikar, Lee M. Goldman, and Nagrendra Nag T he first reported work on transparent polycrystalline ceramics goes back to the 1950s. For example, some of the literature on materials discussed in this article, magnesium aluminate spinel and aluminum oxynitride spinels, traces back to late 1950s and early 1960s.1–3 The advantages of these materials over current state-of-the-art materials include • Ease of manufacturing; • Superior mechanical properties, such as modulus, hard- ness, and strength; • Performance at high-temperature environments; and • Chemical durability. Candidate polycrystalline transparent ceramic compositions include yttrium aluminum garnet and yttria, but aluminum oxynitride (γ-AlON )and magnesium aluminate spinels seem to have established themselves as leading candidates in multiple market segments, such as the military, aerospace, and lasers, mainly because of their durability, availability in large sizes, and cost. Today, γ-AlON and magnesium-spinel are manufactured in large sizes and in large volumes. However, high cost remains a barrier to their deployment as replacements for glasses and some opaque ceramics. Growing demand in current and emerging markets position these materials at the cusp of new commercialization opportunities in terms of volumes and costs. This article reviews the unique properties of these materials that make solving the production challenges a worthwhile endeavor. Optical properties Properties drive applications, and, obviously, optical properties are among the most important for transparent polycrystalline ceramics. However, the combination of mechanical properties (and, for some applications,4 other properties, too) makes these spinel materials uniquely suitable for a range of applications in defense and aerospace systems. Many defense and aerospace applications require materials that are transparent in the ultraviolet, visible, and through the mid-infrared wavelength ranges. High transparency means low scattering losses, low reflectance, and low absorption. For cubic spinel polycrystalline materials, several factors determine optical quality. Cubic materials are isotropic, so they have no inherent birefringence. However, secondary phases, such as pores, impurities, and inclusions, typically lead to low transmittance. Controlling material purity and processing conditions minimizes defects, and increases transparency up to theoretical limits. In the absence of absorption and scatter, reflection losses from the material's inherent refractive index determine transmittance.Consequently, transmittance can be increased significantly via antireflection coatings. Figure 1 shows transmittance of γ-AlON and magnesiumspinel optical ceramics. Even though it is polycrystalline, γ-AlON is one of the best currently available materials in terms of optical quality. The transmittance of γ-AlON approaches its theoretical values in the near-UV, visible through mid-IR wavelengths, but starts dropping around 4.5 micrometers and cuts off at mid-IR range wavelengths of about 6 micrometers because of intrinsic (phonon) absorption. Additionally, it drops to zero at about 0.22 micrometers in the short wavelength range. In comparison, magnesiumspinel transmits further in the mid-IR range—transmittance starts dropping around 5 micrometers and stops at 6.5 micrometers wavelength. The transmittance of magnesiumspinel also drops to zero at about 0.2 micrometers. This is an With their manufacturing problems solved, the many desirable properties of the polycrystalline transparent ceramics γ-AlON and magnesium-spinel have led to many military and commercial applications. (Credit: Surmet.) 20 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 2


mar13_2013
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