MgO-Y₂O₃ is an oxide/oxide optical nanocomposite ceramic that attracts great interest to be used for high temperature infrared (IR) applications, due to its good transmittance at 3–5 µm wavelengths, low emissivity and low light scattering. It presents better performance than pure Y₂O₃ or MgO ceramics, owing to its better optical transmittance, thermal shock resistance and mechanical strength, that are crucial factors for these applications. This is mainly because the mixing of the two ceramic compounds inhibits grain growth, leading to the production of transparent ceramic with small grains. The synthesis of the initial powder used for the fabrication of these nanocomposites, is a critical step, since its purity, particle size and particle size distribution determine the sinterability, the final microstructure and thus the properties of the final material [1].
In the present work, Y₂O₃-MgO nanopowders are synthesize by the sol-gel method, which is a low cost and low-processing temperature technique, that under optimized conditions, allows the precise control of composition, morphology, particle size and phase distribution. The purpose is to use high temperature in-situ X-ray diffraction experiments to investigate the evolution of the crystal structure, lattice parameters and crystallite size, during the calcination treatment, up to 1300°C. The impact of calcination time and temperature on the crystallization process are investigated by collecting sequentially patterns, while performing isothermal and step-by-step heating. This allows the prevention of compression strain developed during cooling down and therefore the better comprehension of the involved mechanisms lacking the hydrostatic pressure effect [2]. The in-situ results are complemented with ex-situ investigations of the nanopowder calcined at different temperatures (650°C – 1300°C) by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The different steps of the sol-gel process are investigated by thermogravimetric analysis.
At the explored temperature range, the two compounds do not interact, the nanopowder is a two-phase mixture of cubic MgO (Fm-3m) and cubic Y₂O₃ (Ia-3). The crystallite size increases as temperature increases, in the range of 9 to 65nm, however, MgO has a bigger crystallite size than Y₂O₃, at calcination temperatures up to 1000°C and this trend is reversed for further temperature increase. This reveals their different crystallization behavior and performing Arhenius plot for the crystallite size versus temperature, it is found that Y₂O₃ has a 10% higher activation energy for crystal growth process than MgO. During isothermal holding for 10hrs, the crystallite size increases rapidly, during the first 2hrs (higher rate for higher calcination temperature), and slows down over time following a logarithmic trend. The effect of the crystallite size on the lattice parameter is also investigated and discussed, since the latter is a key physical dimension influencing the functional properties of the material. The reduction in the particle size leads to an increase in the lattice parameter denoting that the lattice expansion in nanoparticles is primarily dependent on the particle size.

[1] S.Yang etal. Materials 16 (2023) 126
[2] C. Barad etal. J. Alloys Compd 885 (2021) 161199