Preparation of mullite from alumina/aluminum nitrate and kaolin clay through spark plasma sintering process

Abstract

In the present study, the in-situ synthesized mullite has been prepared successfully by mixing kaolinite with alumina and aluminum nitrate nonahydrate (ANN) powders through high energy milling followed by spark plasma sintering (SPS). Using a high-energy ball-mill, the stoichiometric compositions of the starting powders, considering their final transformation to Al2O3 and SiO2, have been mixed. The SPS process has been performed at 1400 and 1375 °C for the specimens containing Al2O3 and ANN, respectively. XRD patterns of the milled powders after 30 h showed the formation of quartz from kaolinite for both starting batches. The displacement-temperature-time (DTT) curves and the corresponded vacuum changes indicated the dehydration and phase transformation of ANN and kaolinite at different stages of the sintering process. The XRD patterns of the sintered samples revealed the formation of mullite alongside un-reacted Al2O3 and crystobalite for the batches containing Al2O3 and ANN, respectively. The results of the physical and mechanical properties tests showed higher amounts of bending strength (397 ± 18 MPa), Vickers hardness (16.32 ± 0.21 GPa) and fracture toughness (3.81 ± 0.24 MPa m−1/2) alongside a lower porosity (0.070 ± 0.02%) for the prepared sample containing Al2O3, than those of the sample containing ANN.