Synthesis of fine particles of hydrated Al 2O 3 and Al 2O 3-coated Cr 2O 3 composite powders by homogeneous precipitation method

Abstract

The homogeneous precipitation method using urea was applied to the synthesis of fine particles of hydrated Al2O3 and Al2O3-coated Cr2O3 composite powders from aqueous solutions of Al2(SO4)3 as well as Al(NO3)3. Hydrated Al2O3 particles from Al2(SO4)3 solution were spherical having average particle size of 0.6-2.7μm for the initial Al3+ concentrations of 0.006-0.072M/l. Change of these products by heat-treatment was examined by powder X-ray diffraction measurement and DTA. It was found that those from the solutions with [Al3+]≤0.036M/l were the coprecipitate of amorphous hydrated Al2O3 and hydrated aluminum sulfate, and those from [Al3+]≥0.036M/l contained basic aluminum sulfate too. By heating at 1200°C for 1h, hydrated Al2O3 from Al2(SO4)3 was transformed to α-Al2O3 without changing their original shape. Hydrated Al2O3 particles from Al(NO3)3 solution could be prepared by adding (NH4)2SO4, and their average particle size decreased down to about 0.3μm. Hydrated Al2O3-coated Cr2O3 composite powders were prepared from both solutions of Al2(SO4)3 and Al(NO3)3, and heat-treatment at 1200°C for 1h converted them to α-Al2O3-coated Cr2O3 composite powders.