In situ formation of transparent spinel with the spark plasma sintering of magnesia-alumina nanocomposite granules without the help of sintering aid

In this project, magnesia-alumina composite granules were prepared via the spray drying method. Next, the synthesized powder was sintered at 1400 °C–1500 °C by the spark plasma sintering method under the pressure of 100 MPa without using any sintering aids. The effects of two sintering temperatures (1400 °C and 1500 °C) on the phase evolution, density, fracture toughness, and light transmission of the samples in the visible and IR range were investigated. SEM results indicated that the magnesia-alumina composite granules had spherical morphology with a mean particle size of 7-8 μm. The XRD pattern showed that after spark plasma sintering at 1400 °C and 1500 °C, magnesium aluminate phase spinel was obtained from the penetration and reaction of alumina and magnesia nanoparticles. The disc sintered at 1400 °C had more transparency than the sample sintered at 1500 °C within the UV–Vis and middle IR region because of the lower porosity of the sample. The magnesium aluminate spinel sintered at 1400 °C had a density of 99.98% theoretical density, hardness of 18 GPa, and fracture toughness of 1.6 MPam^1/2.