Transformations in oxides induced by high-energy ball-milling

Abstract

This paper, by no means exhaustive, focuses on high-energy ball-milling of oxides, on their mechanically induced changes and on the consequences of such changes on their physical and chemical properties. High-energy ball-milling offers a fortunate combination of technical simplicity and of complexity both of physical mechanisms which act during milling and of mechanosynthesized materials. Its basic interest, which stems from the large diversity of routes it offers to prepare oxides either directly or indirectly, is illustrated with various families of oxides. The direct path is to be favoured when as-milled oxides are of interest per se because of their nanocrystalline characteristics, their defects or their modified structures which result from mechanically driven phase transformations. The indirect path consists of a sequence of steps starting with mechanically activated oxides which may be subsequently just annealed or submitted to a combination of thermal treatments, with the possible occurrence of various chemical reactions, to prepare the sought-after materials with potential gains in processing temperatures and times. High energy ball-milling of oxides is more and more currently used to activate powders and to prepare nano-oxides at moderate temperatures. The interest of an activation step is well illustrated by the broad development of doped titania powders, synthesized by heat treatment of pre-ground reactants, for photocatalytic applications or to develop antibacterial materials. Another important class of applications of high-energy ball-milling is the formation of composites. It is exemplified here with the case of oxide-dispersed strengthened alloys whose properties are considerably improved by a dispersion of ultra-stable nanosized oxides whose formation mechanisms were recently described. The basic understanding of the mechanisms by which oxides or oxide mixtures evolve by high-energy ball-milling appears to be less advanced than it is for metallic materials essentially because of the overall complexity of the oxide structures, of their surfaces, of their defects and of their mechanical behavior.

The mechanically induced structural disorder in barium hexaferrite, BaFe12O19, and its impact on magnetism

“…Note that the atomic arrangement in internal interfaces/external surfaces of the mechanically treated materials may lack any long-or short-range order. 3,4 Because of the sensitivity to medium-and long-range structural order, the applied XRD technique loses much of its resolving power in such nanoscale and disordered systems. Therefore, the nature of the mechanically induced structural disorder in BaFe 12 O 19 will be analyzed concurrently with the discussion of Mössbauer data (see next paragraph).…”

“…We note that, in general, up to 2 nm is a typical thickness of the grain boundary/surface shell regions in nanostructured mechanochemically prepared oxides. 3,4 Assuming a spherical shape of BaFe 12 O 19 nanoparticles and taking both their average diameter (D ¼ 14 nm) and the thickness of their surface shell (t ¼ 2 nm) as determined experimentally by XRD and TEM, respectively, one can easily deduce quantitative information on the volume fraction of disordered surface shell regions, w, to the volume of whole particles (w ¼ V shell /(V core + V shell )) in the nanomaterial. ‡ The estimated value of w z 0.636 indicates that about 64% of atoms in the milled ferrite are in a structurally disordered state located in the surface shell of nanoparticles.…”

“…In this context, it should be noted that nanomaterials, in general, and especially those prepared by mechanochemical processing exhibit a high volume fraction of structurally disordered regions. 3,4 The far-from-equilibrium structural state, conned to the particle's nearsurface layers of the mechanically treated ferrite, has signicant implications for its magnetism. Fig.…”

“…These involve, for example, (i) the mechanically induced redistribution of cations over non-equivalent cation sublattices provided by a complex oxide structure, (ii) the formation of canted spin arrangements in the case of magnetic compounds, (iii) the changes in the geometry of constitutive polyhedra, and (iv) the mechanically triggered formation of defective cation centers with an unsaturated oxygen coordination. 3,4 Although there is a surge of investigations in the eld of mechanochemistry of oxides, studies on the mechanically induced response of complex oxides possessing more than two cation sublattices, such as M-type barium hexaferrite, BaFe 12 O 19 (with ve cation sublattices), are very scarce in the literature. 5,6 As a result of its specic magnetic properties, BaFe 12 O 19 is widely used in permanent magnets, magnetic recording media and microwave applications.…”

Effects of mechanical processing and annealing on optical coherence properties ofEr3+:LiNbO3 powders

“…In particular, high-energy planetary ball milling has been extensively used to produce monodisperse nano-powders with crystallite sizes down to less than 10 nm [39,40]. However, it is known that high-energy ball milling can cause significant damage to the crystal structure, with the resulting disorder leading to amorphous-like behavior or even transformations to different crystal phases after extended milling [40,41].…”