Preparation and optical properties of highly transparent MgAl1.9Ga0.1O4 ceramics via aqueous gel-casting method

Abstract

The aqueous gel-casting technology has been widely used to prepare high-quality green body for various transparent ceramics with large dimension and complex shape. However, owing to the severe hydrolysis of MgAl2O4 powder, it is challenging to obtain thick aqueous slurry with high homogeneity and flowability. In this paper, the surface chemical state of MgAl2O4 powder was modified by introducing Ga3+, and stable MgAl1.9Ga0.1O4 aqueous slurry with high solid-phase loading (52 vol. %) and low viscosity (136 mPa·s, at a shear rate of 50 s-1) was successfully prepared. After pressureless presintering and hot isostatic pressing, the gel-cast sample exhibited much higher optical transmittance and more homogeneous microstructure than the dry-pressed sample, which is mainly derived from the improved homogeneity and densification of the green bodies and ceramics. The optical band gap, infrared cutoff wavelength, static refractive index and dispersion of both MgAl1.9Ga0.1O4 and MgAl2O4 transparent ceramics were systematically compared. It is indicated that the transparent MgAl1.9Ga0.1O4 ceramic has the increscent static refractive index of 1.695, the decrescent direct band gap energy of 6.15 eV and absorption coefficient of 0.49 cm-1 at 5 µm, which could be ascribable to the fact that Ga3+ has different electronic structure, higher electronic polarizability and larger ionic radius in comparison with Al3+. This work provides a dependable solution for preparation of spinel oxide ceramics with superior optical properties and large dimension.