Structural defect and activated alumina of spheric α-Al2O3 improving alumina ceramics density from the industrial coarse gibbsite
The quality of alumina ceramics generally depends on alumina powder. High-density alumina ceramics were prepared from industrial coarse gibbsite. Near-spheric α-Al 2 O 3 with particles smaller than 500 nm was produced via the synergy of B and F from NH 4 BF 4 . The near-spheric α-Al 2 O 3 calcined at low temperature generated alumina ceramics with high density and low porosity. Meanwhile, the wet pretreatment yielded smaller particles with structural distortion and high microstrain and aluminum hydroxide, notably increasing the relative density of alumina ceramics. 99.18 % relative density, 0.017 % porosity, and 27.54 GPa Vickers hardness of alumina ceramics were achieved at 1650 °C for 2 h without additives and pressed sintering. In addition, the early stage of sintering of alumina green body followed particle boundary diffusion with reduced activation energy in the sintering process. The close packing of the spheric α-Al 2 O 3 , high microstrain of α-Al 2 O 3 , structural defects with considerable AlO 4 units calcined at low temperature, structural distortion in the alumina, and activated alumina on the α-Al 2 O 3 surface collectively promoted the densification of alumina ceramics, achieving remarkable hardness, high density, and tiny porosity.
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