Local structural orders in nanostructured Al₂O₃prepared by high-energy ball milling

Abstract

Nanostructured Al 2 O 3 powders were prepared by high-energy ball milling of corundum. Both the solid state nuclear magnetic resonance spectra of the Al 3+ ions and the solid state electron paramagnetic resonance spectra of incorporated Fe 3+ ions are governed by noticeable spectral changes dependent on the duration of the mechanical treatment. The quadrupolar parameters of the 27 Al nuclei and the zero-field splitting parameters of the Fe 3+ ions as well as their statistical distributions were determined as functions of the milling time. Structural changes of the Al 2 O 3 matrix were also followed by powder x-ray diffraction and transmission electron microscopy measurements. Direct relations between the structural disorder as obtained by x-ray data and the spin Hamiltonian parameters of both ions could be established.These results suggest that the milled powders consist of nanocrystalline grains embedded in amorphous grain boundaries even for the longest milling time. The grains can be described in terms of ordered AlO 6 -octahedra as in the starting crystalline material exhibiting a slight rhombic distortion. The grain boundaries look like random arrangements of these octahedral units. The specific behaviour of the environment of the Fe 3+ paramagnetic probe points out that such a point defect acts as an activation centre of the amorphization process.

Characterisation of Nanocrystalline Magnesium Oxide by X‐Ray Absorption Spectroscopy

“…These results indicate that the reduction in the intensity of the ball milled samples is due to an increase in the disorder of the samples with increased milling time. This is in agreement with results for other ball milled nanocrystalline oxides such as ZrO 2 , [47] LiNbO 3 [44] and Al 2 O 3 , [43] where the disorder has been shown to increase with the fraction of amorphous material.…”

“…[32][33][34] Generally, for nanocrystalline metal oxides, carefully prepared by sol-gel routes, there is now reliable EXAFS evidence for the nanocrystals having similar grain boundaries to those in the bulk materials, with similar levels of static and dynamic disorder. [36][37][38][39][40][41][42] High-energy ball milling is widely used for the preparation of nanocrystalline samples, [17,[42][43][44][45][46][47] however, it is emerging that the microstructure obtained is very different to that obtained using other methods. For example, a study of 13 nm corundum, Al 2 O 3 , prepared by ball milling, revealed a significant fraction of amorphous material in the sample.…”

“…For example, a study of 13 nm corundum, Al 2 O 3 , prepared by ball milling, revealed a significant fraction of amorphous material in the sample. [43] Studies comparing ball milled and sol-gel prepared samples, performed for a range of nanocrystalline oxides, [42,44,47] clearly show the presence of amorphous material. It has been concluded from these experiments that the samples consist of nanocrystalline grains embedded in the amorphous fraction.…”

Structure and dynamics in nanoionic materials

“…At this measurement temperature the differences can only be due to differences in the level of static disorder in the samples. We interpret this as the formation of significant levels of amorphous material in the ball-milled samples, a feature of this method of sample preparation [44,73,75].…”

“…This has to be minimized by choosing appropriate materials for the milling vial and balls, respectively. A further disadvantage is that the milling can produce amorphous debris, to the extent that recent work on ball milled Al 2 O 3 indicated that the sample consisted of nanocrystalline grains embedded in amorphous material [44].…”