Transparent magnesium aluminate spinel (MgAl2O4) has been developed as an optical ceramic for a variety of applications, including as windows. As a broadband, ultraviolet (UV) thru midwave infrared (MIR) material, it has been developed for windows and has many desirable properties compared with alternative infrared glasses and other transparent ceramics. Current efforts to advance high strength spinel manufacturing processes have demonstrated progress toward large format windows. Although low-absorption spinel, specifically in the near-infrared (NIR) has been demonstrated previously, additional processing is required for new, large-scale spinel manufacturing processes to decrease the effects of impurities near one-micron wavelengths. In this work we present recent results that show measured absorption near 1 μm is reduced by annealing, which reduces effects of trace impurities. Experimental results from photo-thermal common-path interferometer measurements are reported.
A new reactive chemical mechanical polishing process has been developed and optimized for polishing CVD SiC mirror samples. The studies show that the abrasives, chemical nature of the slurry, and other additives play an important role in the material removal rate and surface finish of the SiC mirror. The use of different abrasive types and sizes resulted in differing roughness and removal rates. The smaller abrasives created surface defectivity or higher roughness. This can be explained by different polishing rates of different orientations of SiC grains, resulting in the grain enhancement. Under optimal conditions with appropriate abrasive particles, roughness RMS as low as 0.2 nm was achieved on CVD SiC samples. The process also did not show any scratch-like features in the optical interferometry measurements.
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