advantage for spinel applications that require high transmission in the 4.5–5 micrometer range. In addition to excellent transmittance, commercial γ-AlON products have exceptional optical clarity (exceeding 98 percent) and very low haze (less than 2 percent) in the visible wavelengths. (Optical clarity relates to the amount of light scattered at small angles. In contrast, haze relates to the amount of light scattered at large angles.) Pores, secondary phases, inclusions, defects, and inhomogeneous grain boundaries greatly affect the clarity and haze properties of transparent polycrystalline ceramic materials. High clarity and low haze requires nearly 100 percent density and no secondary phases, which are very challenging from a processing perspective. In addition, impurities can impart a tint to the final component, whereas less than full density causes haze. Thus, the production of high-quality transparent ceramics demands both careful powder synthesis and close control during densification. Another important optical property is the refractive index. In γ-AlON, it varies between 1.81 and 1.67 over the 0.4–5.0 micrometer wavelength range, with normal dispersion as shown in Figure 2. (Dispersion is the change in refractive index with wavelength for a material and is denoted by the dimensionless "Abbe number.") A typical Abbe value for γ-AlON and magnesium spinel is about 60, which means that the dispersion is much lower than some of the glasses with similar refractive indices, making the spinels strong candidate materials for lenses with low chromatic aberration. Also, the cubic spinel phase of aluminum oxynitride exists across a wide composition range. Therefore, properties, such as refractive index, can be tailored without losing transparency. Components like graded refractive index lenses (GRIN) are fabricated, for example, by controlling composition or by adding dopants. Unlike many glasses, which are transparent only in visible wavelengths, γ-AlON GRIN lenses are transparent in the visible through mid- IR range. Certain GRIN lenses can have flat surfaces, because the “lens curve” is built into the material via the refractive index gradient. Also, grading the composition can eliminate the aberration associated with spherical lenses. Additionally, GRIN lenses can reduce significantly the size, weight, and complexity of the optical Transmittance (percent) Figure 1. Calculated transmittance of γ-AlON and magnesiumspinel (at 2 millimeter thickness). The calculation includes Fresnel reflective losses that can be eliminated through use of an antireflection coating. train for defense applications, such as image systems for laser range finders, night vision goggles, and unmanned aerial vehicles. Night vision technologies improve visibility (transmission) in low light conditions. Most night vision devices (NVD) are for military applications, but some are used in the civilian arena. NVDs require transmission in the 0.4–0.92 micrometer range. In this range, γ-AlON and magnesiumspinel transmit better than glasses. As Figure 3 shows, γ-AlON-based armor offers significantly more night vision capability (about 40–50 percent more transmission) over glass laminates in low-light conditions. More transmission means a higher signal-to-noise ratio and higher-resolution imaging, which improve awareness for the warfighter in low-light situations. Mechanical properties Transparency alone is not enough for warfare situations. These materials must endure stresses encountered in manufacturing, transport to theater, deployment in service, or ultimately, under ballistic conditions. Ballistic properties and environmental Wavelength (micrometers) durability are critically important properties in the military context. Although γ-AlON and magnesiumspinel are cubic spinel lattices, their bonding and bond strength differences make γ-AlON mechanically superior to magnesium-spinel, giving it a higher hardness and elastic modulus than magnesium-spinel. In fact, the hardness of γ-AlON approaches that of singlecrystal sapphire, making it the hardest polycrystalline transparent material currently available commercially. The combination of high hardness and elastic modulus makes γ-AlON a leading candidate material for transparent armor applications, followed by magnesium-spinel and single-crystal sapphire. Table 2 compares the important mechanical properties of the two transparent polycrystalline materials as well as some of their thermal properties. Although lower elastic modulus and American Ceramic Society Bulletin, Vol. 92, No. 2 | www.ceramics.org 21 (Credit: Surmet.) Table 1. Important optical properties of transparent polycrystalline spinels Property γ-AlON Mg-spinel Unit Refractive index (at wavelength 0.5 μm)6 1.80 1.723 dn/dT (in 3–5 μm wavelength range)6 3 3 10–6 K–1 Absorption coefficient (at 3.39 μm wavelength)6 0.1 0.018 cm–1 Total integrated optical scatter (at 0.64 μm; ~5 mm thick 2.1 7.2 % sample)6 Transmission wavelength range* 0.22–6 0.25–6.5 μm Optical homogeneity achieved in 15 in. 3 25 in. part with ~5 N/A ppm 3.4 in. aperture Typical transmittance without AR coatings (in the visible >84 75–80 % range)* Typical haze (in the visible range)* <2 <10 % Typical clarity (in the visible range)* >98 >95 % *Varies depending on thickness and processing conditions.
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