Transparent ceramics are materials of choice for high temperature IR window applications: they are a compromise between transparency, thermal shock resistance and processing costs. Alumina, AlON and spinel-type ceramics have been developed for the 3-5 μm atmospheric transparency band. Nevertheless, these compounds show a degradation of their optical properties (transparency and emissivity) at high temperature (above 500°C) limiting their use in that wavelength range. MgO and Y₂O₃ have a broader transparency window up to 9 µm and they are transparent enough in the 3-5 µm range even at high temperature but their thermomechanical resistance is weak. Authors (Kear et al., Proc. Of SPIE, 2005; D. C. Harris et al., J. Am. Ceram. Soc., 2013; L. Liu et al., J. Eur. Ceram. Soc., 2020) have combined these compounds into a nanocomposite ceramic to improve this while keeping good IR transparency. To reach such properties, porosity ratio must be close to zero and the average grain size must stay as small as possible (< 200 nm).
Through this study, Pechini’s esterification sol-gel route was chosen in order to prepare the Y₂O₃-MgO nanocomposite powder. Then, the classical one-step sintering is compared with an original two-step sintering (with a quick cooling and pressure release when reaching the sintering temperature before a classical stage of 10 min) and also an original low temperature sintering profile (under 800°C) using the the Spark Plasma Sintering technique. With conventional one step sintering, the more the sintering temperature, the less the density is. The hygroscopic power of the nanopowder seems to be at the source of this porosity creation. Denser samples are obtained with our two-step sintering which promotes densification over granular growth. Our low temperature profiles allow to obtain grain sizes below 100 nm, which improves transparency at shorter wavelengths. Afterwards, post treatments as air annealing and Hot Isostatic Pressing at 400 MPa improved the quality of the ceramics. Finally, structural, microstructural, optical (up to 1000°C) and thermomechanical characterisations have been carried out. Samples with different nanostructures were obtained. The best samples have average grain diameters around 100- 200 nm with no porosity. Good mid-IR transparency, up to 80% for a thickness of 1 mm, is obtained in band II. The material shows a loss of transparency at 5 µm less than 10% at 1000°C compared with sapphire which loses up to 50% in the same conditions (D. C. Harris et al., Proc. Of SPIE, 1999).