Study On The Nanocrystalline Mgal₂o₄ Spinel Powder Prepared By Microwave-assisted Low-temperature Combustion Synthesis Technology

The nanocrystalline MgAl₂O₄ spinel powder has been widely applied in refractory materials, anode materials, electronic materials and catalyst because of its high-melting point (2135℃), high mechanical strength, high-thermal conductivity, low-expansion coefficient. Presently, the main methods to prepare nanocrystalline MgAl₂O₄ spinel powders include metal alkoxide method, co-precipitation, solution combustion synthesis, gel-combustion synthesis, high temperature solid-state reaction method. However, the above methods have shortages of complicated process, agglomeration, high energy consumption, long production period and introduction of impurity, thus, it is important to develop new techniques to prepare nanocrystalline MgAl₂O₄ spinel powders.In this paper, nanocrystalline MgAl₂O₄ spinel powders were prepared by a novel microwave-assisted combustion synthesis (MWCS) method which combines microwave heating technology with combustion synthesis technology. In comparison, MgAl₂O₄ powders were also prepared by conventional combustion synthesis (CCS) route. Morphology of particle, high temperature stability, crystallinity and grain size distribution are characterized by the means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Thermo-Gravimetric/Differential Thermal Analyzer (TG/DTA) and BET analysis, respectively.It was shown that, when urea was used as fuel and microwave power was 800W, the MgAl₂O₄ powders prepared by MWCS had high crystallinity, better high temperature stability and less impurities, and the average grain size was 41.29nm. However, the powder prepared by CCS at 500℃had poor high temperature stability, more impurities, and the average grain size was 39.62nm. It was also found that as the weight of the raw materials increase, average grain sizes of powders prepared by both routes increased accordingly.When composite fuel (urea : glycine = 9:2) was used as fuel, it was found that residual organic in the product can be reduced and the crystallinity can be improved because the composite fuel can release more heat and increase the combustion temperature. By using composite fuel, the powders prepared by MWCS have spherical morphology and uniform particle size distribution. However, agglomeration was observed in powders prepared by CCS, and the obtained powders have irregular shape. BET result shows that the specific surface area of the former is 1.4 times of the latter, which demonstrates that microwave irradiation has advantages over conventional heating.