Synthesis of Spinel Powder by the Homogeneous Precipitation Method from Inorganic Salts and Their Sintering.

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Spinel (MgAl_2O_4) powders were prepared by the homogeneous precipitation method from the aqueous solutions of nitrates or sulfates mixture of aluminum and magnesium using urea, and their sinterability was studied.Precipitates with spinel composition obtained from the nitrate solution consisted of Al_2O_3 gel and Mg-Al double hydroxide. From the sulfate solution. MgO hydrate deposited on Al_2O_3 hydrate particles. On heating above 800^ﾟC for 1h in air, both precipitates became spinel phase. Indicating the higher reactivity of both components in the present powders than that in the powder mixture of MgO and Al_2O_3.The powder prepared from the solution of nitrates mixture became solid coagulation on drying. When powder ground and calcined at 800^ﾟC was sintered at 1500^ﾟC for 4h, a sintered body with 95% of theoretical density was obtained. In sintering at 1600^ﾟC, the same density was obtained.On the other hand, in the powder prepared from the solution of sulfates mixture, a sintered body with 96.7% of theoretical density was obtained when the synthesized powder was calcined at 1000^ﾟC and sintered at 1500^ﾟC for 4h. A sintered body with 99.2% of theoretical density was obtained by sintering at 1600^ﾟC for 4h from the powder calcined at 800^ﾟC and ground.As a fundamental study on synthesis of spinel powder, the effect of synthesis conditions, especially the effect of additives, on the morphology of Al_2O_3 hydrate was investigated in the precipitation from aluminum salt solution by the homogeneous precipitation using urea.