Microwave Hydrothermal Synthesis And Sintering Properties Of Magnesia-alumina Spinel

Magnesium-aluminum spinel(MAS)not only has the characteristics of high melting point,low thermal expansion coefficient and low thermal conductivity,but also has good thermal shock resistance and alkali slag erosion resistance.Therefore,MAS is widely used in cement rotary kilns,ladle linings and aluminum electrolytic cells.The use of MAS as a refractory material will be affected by the sintering properties of the block such as the degree of densification,grain size distribution and grain size of MAS.The sintering performance of MAS is mainly affected by the activity of raw materials,sintering methods and sintering aids.For this reason,this paper mainly studies the activity of raw materials,sintering methods and sintering aids in order to obtain MAS refractories with excellent sintering performance.(1)A new synthesis method(microwave hydrothermal methods)is explored to synthesis MAS precursor powder.The optimal microwave hydrothermal synthesis process conditions of MAS precursor are 150°C and 200 min.The MAS precursor obtained under the optimal process conditions contains magnesium-aluminum composite hydroxy hydrate,boehmite and magnesium carbonate.And the median particle size is smallest(5.958?m)and uniform distribution.It revealed that the mechanism of microwave hydrothermal synthesis of MAS precursor is the dipole mechanism.(2)The MAS material is prepared by a two-step sintering method.The best two-step sintering process condition is that the first step is 1650°C/3 min and the second step is 1550°C/20 h.The relative density of MAS obtained under this process condition is 95.75%,the average grain size is 1.98?m,and the hardness is 1199 HV.Compared with traditional sintering,two-step sintering can inhibit abnormal growth of grains,and the microstructure of MAS is more uniforme and denser.(3)The influence of gadolinium oxide(Gd₂O₃)on the densification of MAS sintering is studied.The optimal addition amount of Gd₂O₃ is 2 wt.%,and the relative density and hardness of the obtained MAS are 97.60%and 1373 HV,respectively.The addition of Gd₂O₃ enhances mass transfer,reduces the activation energy required for grain boundary diffusion and migration,and promotes the increase of densification.