Molten salt synthesis and formation mechanism of magnesium aluminate spinel using different shape Al2O3 as the templets

In order to verify the “Al2O3-template-formation” mechanism of magnesium aluminate (MA) spinel proposed previously using Al2O3 and MgO micro-powders as raw materials, in this work, MA spinel was synthesized by nanograined, plate-like, and fibrous Al2O3 in LiCl molten salt at 1150 °C for 3 h, respectively. The products were characterized by XRD, SEM, TEM, and EDS techniques, and the grain size of the products and raw materials were analyzed. The results showed clearly that the MA spinel was initially nucleated and subsequently developed to octahedral crystal. When the reaction further took place, when using nanograined Al2O3, the newly formed MA spinel seeds initially moved and attached to the surface of the large octahedral MA spinel crystal, and they were subsequently engulfed by the large MA spinel crystal, which further grew via layer-by-layer to become micro-sized crystal. Using plate-like or fibrous Al2O3 raw material, MgO diffused continuously into the interior of Al2O3 to form MA spinel with a gradient growth from surface to depth. These revealed that whatever shape of Al2O3 was used, the synthesis of MA spinel was governed by “Al2O3-template formation mechanism”, i.e., by the reaction of MgO diffusion into the Al2O3 templet in the molten salt.