No abstract

Solid solution magnesium aluminate spinel with the alumina‐rich composition MgO·1.2 Al2O3 has been prepared as a transparent polycrystalline ceramic with average in‐line transmission at 550 nm of 84.8 ± 2.7% and >82% throughout the visible spectrum. Starting powders are prepared from mixtures of high purity Mg(OH)2 and γ‐Al2O3 thoroughly mixed in an aqueous slurry. Water is removed by rotary evaporation. The solids are collected, dried, calcined, mixed with LiF (as a sintering aid), and sieved. The powders are sintered into dense ceramics by hot pressing at 1600°C under vacuum and 20 MPa uniaxial load followed by hot isostatic pressing at 1850°C under 200 MPa Ar. Final grain sizes ranged between 300 and 1000 μm. Samples exhibited flexural strength of 176.8 ± 46.2 MPa; hardness of 12.3 ± 0.2 GPa; and elastic modulus of 292.9 ± 7.5 GPa. Control samples of stoichiometric magnesium aluminate spinel (MgO·Al2O3) were prepared with the same procedure and exhibited comparable values for transmission and physical properties.

The effect of different hot pressing and hot isostatic pressing (HIP) temperatures and pressures on the optical properties of spinel was studied. Extinction coefficients of spinel samples were estimated by comparing the measured transmittance with the theoretical transmittance as calculated via a Sellmeier model. Results showed that the relative size of the scattering sites was large compared with the wavelengths of light (0.35-5.5 lm). Overall, increasing HIP temperature and pressure resulted in decreasing the optical extinction. The lower of two hot pressing temperatures (16201 vs 16501C) prior to HIPing resulted in lower scatter coefficients after HIPing; this effect was most significant in the infrared.

Recently, the U.S. Army Research Laboratory (ARt) has focused increased attention on the development of transparent armor material systems for a variety of applications. Future combat and non-combat environments will require lightweight, threat adjustable, multifunctional, and affordable armor. Current glass/polycarbonate technologies are not expected to meet the increased requirements. Results over the past few years indicate that the use oftransparent crystalline ceramics greatly improve the performance of a system. These results coupled with recent processing and manufacturing advances have revitalized the interest in using transparent ceramics for armor systems. The materials currently under investigation at ARL are magnesium aluminate spinel (MgAI2O4), aluminum oxynitride spinel (A1ON), single crystal sapphire (A1203), glasses, and glass-ceramics. The polymers under investigation are polycarbonate (PC) and polyurethane (PU). An overview of current ARL efforts in these areas, including the motivation for using transparent ceramics, the requirements, the potential applications, and the ongoing processing research will be reviewed.

Magnesium aluminate spinel with the alumina rich composition MgO · 1.5 Al2O3 has been prepared as a transparent polycrystalline ceramic with average in‐line transmission at 550 nm of 83.3 ± 0.9% and >80% throughout the visible spectrum. This finding significantly increases the compositional range over which polycrystalline magnesium aluminates can be prepared as fully dense ceramics with high transparency to visible light. Starting powders are prepared from combinations of high purity Mg(OH)2 and γ‐Al2O3 thoroughly mixed in an aqueous slurry, and the solids are collected, dried, calcined, mixed with LiF sintering aid, and sieved. The powders are sintered into dense ceramics by hot pressing at 1600°C under vacuum and 20 MPa uniaxial load, followed by hot isostatic pressing at 1850°C under 200 MPa Ar. The crucial parameter for forming highly transparent MgO · 1.5 Al2O3 ceramic from this procedure is to hold the amount of LiF to 0.25 wt%.