Localstructural orders in nanostructured Al2O3 preparedby high-energy ball milling

Abstract

Nanostructured Al2O3 powders were prepared byhigh-energy ball milling of corundum. Both the solid statenuclear magnetic resonancespectra of the Al3+ ions and the solid state electronparamagnetic resonance spectraof incorporated Fe3+ ions are governed by noticeablespectral changes dependent on the duration of the mechanicaltreatment. The quadrupolar parameters of the 27Al nucleiand the zero-field splitting parameters of the Fe3+ ionsas well as their statistical distributions were determined asfunctions of the milling time. Structural changes of theAl2O3 matrix were also followed by powder x-raydiffraction and transmission electron microscopy measurements.Direct relations between the structural disorder as obtained by x-ray data and the spinHamiltonian parameters of both ions could be established.These results suggest that the milled powders consist ofnanocrystalline grains embedded in amorphous grain boundarieseven for the longest milling time. The grains can be describedin terms of ordered AlO6-octahedra as in the startingcrystalline material exhibiting a slight rhombic distortion.The grain boundaries look like random arrangements of these octahedral units. The specific behaviour of the environment ofthe Fe3+ paramagnetic probe points out that such a pointdefect acts as an activation centre of the amorphizationprocess.