Powders And Oxidation Behavior Of The The Synthesis Mgalon Of Different Raw Materials

This work first investigated the influences of different the raw materials (alumina-based raw materials and bauxite-based raw materials), different heating methods (the ordinary furnace and microwave oven) and different firing temperatures on synthesis of MgAlON powder by reduction nitridation. Then phase composition of the synthesized powders was examined by XRD and microstructure observed by SEM and EDS. The oxidation behavior of the oxide-based and bauxite-based MgAlON powders was studied by TG-DSC and XRD. The influence of impurities Fe₂O₃,TiO₂ and SiO₂ in bauxite on oxidation behavior of MgAlON powder was discussed. Finally, in-situ MgAlON bonded Al2O3-C composite materials were prepared and the influences of alumina-based MgAlON and bauxite-based MgAlON on the properties of the composite materials were compared.According to the XRD analysis, XRF analysis and the study of oxidation behavior of the powder, it was confirmed that single phase MgAlON powder was synthesized by both oxide-based and bauxite-based raw materials with carbon embedding in the ordinary furnace and microwave oven at 1550℃and 1400℃respectively. The nitridation synthesis temperature in microwave oven is 150℃lower than that in ordinary furnace. The powders obtained in both furnaces were granular, though the powder synthesized from alumina-based had smaller grain size than that from bauxite-based, which can be ascribed to the presence of impurities Fe₂O₃,TiO₂ and SiO₂ in the bauxite.On the basis of the study of oxidation behavior of MgAlON powder by TG-DSC and XRD patterns of samples sintered at different temperature, the differences of oxidation behaviors of MgAlON powders based on oxides and bauxite was observed as follows:①MgAlON powders based on oxides by the nitridation synthesis in ordinary furnace began to oxidize at about 850℃, and got fully oxidation decomposition at 1500℃, while that based on bauxite began to oxidize at about 720℃and completely decomposed at 1300℃. The beginning oxidation temperature and full decomposition temperature of bauxite-based MgAlON powder were 130℃and 200℃lower than that based on alumina-based MgAlON powder respectively.②MgAlON powders based on oxides by the nitridation synthesis in microwave oven began to oxidize at about 900℃, and got fully oxidation decomposition at 1300℃, while that based on bauxite began to oxidize at about 550℃and totally decomposed at 1200℃. The beginning oxidation temperature and full decomposition temperature of bauxite-based MgAlON powder were 350℃and 100℃lower than that based on oxides respectively. By calculation and comparison of the lattice constant of oxide-based and bauxite-based MgAlON powders, it was proposed that the impurities SiO₂ and TiO₂ in the bauxite may enter into the MgAlON lattice through forming solid solution, and thus lead to the distortion of crystal lattice of bauxite-based MgAlON powders, which can be explained why the oxidation resistance of bauxite-based MgAlON powders was rather worse than that of oxide-based MgAlON powders.With carbon-embedding at 1550℃, both bauxite-based in-situ MgAlON bonded Al₂O₃-C composites and oxide-based in-situ MgAlON bonded composites by reduction nitridation were successfully prepared. The latter materials possess superior properties though both composites are characterized with high MOR and goodthermal shock resistance.