When a high-energy laser (HEL) beam transmits through a window material, a part of the laser energy is absorbed by the material and causes optical aberrations. This absorbed energy results in material heating in the local exposed region, changing its refractive index based on the material’s thermo-optic coefficient, thermal expansion coefficient, and stress optic coefficient. These changes result in beam distortion and loss of output power, measured as optical path distortion (OPD), which has a severe impact on system performance.

The ceramics made from the Spinel Powder exhibit excellent visible and infrared transparency. (a) An uncoated 5-cm diameter spinel window, and (b) a 10
For HEL systems operating in the shortwave infrared (SWIR) wavelength region, the materials of choice are limited to just a few, including fused silica and oxyfluoride (OFG) glasses. Some of the HEL systems are expected to operate in harsh environmental conditions where fused silica and OFG glasses will not survive.

Transparent magnesium aluminate spinel (MgAl2O4) ceramic was developed as a rugged window and dome material for protecting sensors operating from the ultraviolet (UV) to the mid-IR. The transparent spinel ceramic was made from high-purity powder synthesized using aqueous chemical methods. The 5 × 9’s pure Al and Mg chloride were mixed together to form a homogeneous solution at 80 ºC. Ammonium hydroxide was added to the solution to form a precipitate that was subsequently filtered, washed with water and then acetone, and baked to dry. The powder was then calcined at 600 ºC to convert to magnesium aluminate spinel.

Ceramic spinel was made by hot-pressing ball-milled spinel powders at 1400-1650 ºC for 2-4 hours using a uniform coating of a small amount of LiF sintering aid that was eliminated by evaporation prior to full densification. The hot-pressed samples were transparent, with densities greater than 99% of theoretical. The samples were subsequently hot isostatically pressed (HIP) at 1600 ºC for 2 hours under an Ar gas pressure of 30,000 psi to produce fully dense and transparent ceramics. The samples ranged in size from 5 cm in diameter and 6 mm thick, to larger samples (15 cm diameter) from which smaller samples were cut, ground, and polished for many of the measurements. High-damage-threshold anti-reflective (AR) coatings (SiO2/ZrO2) were applied to the surface of polished 5-cmdiameter samples using dc-magnetron sputtering.

Spinel powder synthesized by the aqueous process produced approximately 100-200 nm crystallites with excellent phase purity as highlighted by X-ray diffraction analysis and chemical analysis. Compared to commercial powder, the impurity content is several orders of magnitude lower. The surface area of the synthesized powder was 25 m2/g compared with a range of 10-30 m2/g for commercial powder.

Rain and sand erosion tests performed on spinel ceramic were successful. The spinel samples were able to withstand impact from rain droplets at speeds up to 600 mph and sand particles at speeds up to 460 mph without damage, without surface pitting, and with no change in transmission, unlike glass, which exhibits considerable damage.

Spinel is a rugged ceramic material that transmits from the UV to 5 μm and could be used as an exit aperture for HEL systems. However, spinel made from commercial powder exhibits high absorption losses due to extrinsic impurities. Ceramic samples made from spinel powders demonstrated a record low absorption loss of 6 ppm/cm at 1.06 μm.

This work was done by Jas Sanghera, Shyam Bayya, Guillermo Villalobos, Woohong Kim, Jesse Frantz, Brandon Shaw, Colin Baker, and Ishwar Aggarwal of the Naval Research Laboratory; Bryan Sadowski, R. Miklos, and Fred Kung of GTEC Inc.; Michael Hunt of University Research Foundation; David Reicher and Stan Peplinski of the Air Force Research Laboratory; Al Ogloza and Peter Langston of NAWC; Chuck Lamar of the Army Space & Missile Defense Command; Peter Varmette of SAIC; Mark Dubinskiy of the Army Research Laboratory; and Lewis DeSandre of ONR Global. NRL-0048

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Transparent ceramics for high-energy laser systems

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This article first appeared in the December, 2011 issue of Defense Tech Briefs Magazine.

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