Low-Temperature Synthesis of MgAl<sub>2</sub>O<sub>4</sub> by Capsule HIP Using Hydroxides as Starting Materials

Abstract

Low-temperature synthesis of spinel (MgAl2O4) was investigated by capsule hot isostatic press (HIP) using hydroxides as starting materials; Mg(OH)2, MgO, Al(OH)3, g-AlOOH, g-Al2O3. MgAl2O4 has been conventionally prepared above 1400 °C, whereas the mixtures containing the hydroxides reacted at 400-500 °C for 1 h under 200 MPa of isostatic pressure to form MgAl2O4. The hydroxides showed high reactivity to form MgAl2O4 with high crystallinity. However, the products more or less contained Mg(OH)2. The single phase of MgAl2O4 was obtained by adding 20 mass% of water to the starting mixtures of Mg(OH)2 - Al(OH)3, Mg(OH)2 - g-Al2O3 and MgO - Al(OH)3. The average grain size of the MgAl2O4 was 70 – 100 nm. It was concluded that the transformation of AlOOH or g-Al2O3 was promoted by a small amount of water and that Mg(OH)2 rapidly reacted with the active intermediate compound between AlOOH or g-Al2O3 and a-Al2O3.