Sintering behavior and microstructure of (1-x)MgAl2O4-xMg2TiO4 spinel solid solutions prepared by isostructural heterogeneous nucleation

Abstract

Magnesium aluminate (MgAl2O4) spinel has grasped considerable attention in high-temperature application by right of its excellent properties. However, the poor sintering behavior of MgAl2O4 is detrimental to its further development. In the present work, the application of isostructural heterogeneous nucleation method provides a novel idea for optimizing the sintering behavior of refractory materials. A series of (1-x)MgAl2O4-xMg2TiO4 (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) spinel solid solutions with a present ration of components were fabricated from light calcined magnesia, reactive alumina and pre-preparation Mg2TiO4. The effect of Mg2TiO4 heterogeneous nucleating agent on the crystalline phase, densification, and microstructure evolution of MgAl2O4–Mg2TiO4 spinel solid solutions was studied. The XRD, XPS, and EDS results showed that Mg2TiO4 entered the lattice of MgAl2O4 to form a spinel solid solution, and the heterovalent substitution process was identified, where Ti4+ and Mg2+ ions of larger radius in the Mg2TiO4 replaced the Al3+ of smaller radius in the MgAl2O4. For the sample at x = 0.08, the spinel solid solutions exhibited the optimized densification with a relative density of 93.3%, an apparent porosity of 1.2%, and a compressive strength of 84.5 MPa. A significant increase in densification was related to the lattice distortion induced by ion size mismatch during the heterovalent substitution, thus accelerating the diffusion rate of Mg2+ and Al3+ ions in the spinelisation state. Moreover, the solid solubility content of Ti4+ in the MgAl2O4–Mg2TiO4 spinel solid solutions had a significant effect on the grain morphologies. The Mg2TiO4 heterogeneous nucleating agent significantly increased the spinelisation rate of MgAl2O4 spinel with negligible effect on densification.