Gerard Le Caër

Some recent developments in mechanical activation and mechanosynthesis

After outlining the general characteristics of high-energy ball-milling, mechanochemical synthesis is argued to be an attractive method for the synthesis and transformation of materials. Phase transformations induced by milling, annealing of mechanically activated iron silicides, mechanically activated self-heat sustaining reactions (MASHS), for instance of FeAl, are first discussed. The route, which starts from mechanosynthesized powders to reach consolidated alumina-(Fe,Ti) composites, yields materials whose original morphologies and some mechanical properties are then described.

Reduction of haematite with carbon by room temperature ball milling

Abstract The solid state reduction of haematite (α-Fe2O3) to mainly nanocrystalline wustite by room temperature dry ball milling of haematite and carbon, in an inert atmosphere, is described. In addition to wustite, a smaller amount of nanocrystalline magnetite is also formed, possibly by a disproportionation reaction of wustite. The reaction is completed in about 70 h in a Spex vibratory ball mill. Haematite alone is converted by dry milling for 70 h into nanocrystalline magnetite with an average crystallite size of 10 nm as shown by X-ray diffraction and Mossbauer spectroscopy.

Synthesis of nanostructured materials by mechanical alloying

Materials can be processed by grinding not only for comminution, but also to obtain a variety of structures (amorphous, nanophased), fine mixing of phases, alloys and to directly synthesize compounds such as carbides. Nanocomposites can also be synthesized by reduction, exchange and mixing reactions driven by grinding, as well as by combining the above processes. Large scale economical production of such materials is feasible by grinding in mills designed for that purpose. The mechanical properties of materials are enhanced by the nanophase structure.

Mössbauer study of Fe1 − xBx amorphous alloys: A model of the atomic structure

Abstract Fe1 − xBx (0.12 ⩽ x ⩽ 0.25) glasses have been studied by Mossbauer spectroscopy. The hyperfine field distributions show that the structure of boron rich glasses (x ⩾ 0.20) may be described as a packing of iron trigonal prisms centered by boron atoms. Below x = 0.20, a description with a mixture of such environments and of iron sites without boron neighbours is appropriate. A model grounded on the structural operations which generate the crystalline counterparts from close packed structures is then proposed. The decomposition of the structure in two kinds of environments follows from this model and the corresponding critical concentrations are discussed.

Mechanically activated room temperature reduction of sulphides

Abstract Solid state room temperature reduction and exchange reactions of metal (M) sulphides are described. Reactions are driven by ball mining powder mixtures of the sulphide and the reductant (R) or exchange compound (CaO). Reduction reactions show a complete reduction of the metal sulphide with formation of M-R sulphide nanocomposites. Exchange reactions with CaO show the formation of M oxide -CaS nanocomposites. Reactions having positive enthalpy or even positive free-energy changes in a wide temperature range are shown to occur. The effect of milling iron sulphide alone is also reported. Milling was performed for 24 h to assess the feasibility of the various reactions involved. X-ray diffraction and Mossbauer spectroscopy for iron-containing systems were used to characterize the powders before and after processing.

A microstructural study of mechanical alloying of Fe and Sn powders

Elemental Fe and Sn powders in the ratio of 1:2 were ball-milled for various times at room temperature, and phase transformations in the powders were studied by 57 Fe and 119 Sn Mossbauer spectrometries, transmission electron microscopy, and x-ray diffractometry. Although Fe-Sn alloys are not obvious candidates for solid-state amorphization reactions, an amorphous phase formed after only a few hours of ball milling. Nanocrystalline intermetallic FeSn 2 particles nucleated and grew within the amorphous phase, and FeSn 2 became the major phase after 10–20 h of ball milling. These small particles were superparamagnetic, and were above the blocking temperature at room temperature.

Kinetic effects in the mechanically activated solid-state reduction of haematite

Nanometer-sized α-Al2O3-Fe composites were obtained by solid-state reduction in ball mills of Fe2O3 and Al in nitrogen and air. Together with α-Al2O3 and Fe, the formation of Hercynite and clusters of Fe in alumina are observed in both cases.

Exchange reaction milling in iron nitrides, fluorides and carbides

Abstract Solid state room temperature exchange reactions of iron nitride, fluoride and cementite with silicon or aluminium, carbon and chromium respectively are described. Reactions are driven by ball milling powder mixtures of the compound and the exchange element at room temperature. In the different systems the formation of Si3N4, AIN, carbon fluorides and an amorphous (Fe,Cr)C alloy is observed. Nanocomposite materials are synthesized in this way. X-ray diffraction and Mossbauer spectroscopy were used to characterize the powders before and after processing.

The determination of θ=(Vzz, H) from 12→32 Mössbauer spectra

Abstract A determination of θ =( V zz , H ) is made from 1 2 → 3 2 Mossbauer spectra with a quadrupole interaction which can be treated as a first order perturbation upon the magnetic interaction.

Mechanical Alloying of Fe-Cu Alloys from As-received and Premilled Elemental Powder Mixtures

We have investigated the effect of premilling elemental powders of Fe and Cu on the mixture mechanism of a Fe50Cu50 alloy. The alloying process was compared and studied using electron probe microanalysis, X-ray diffraction, scanning electron microscopy and Mossbauer spectroscopy. In both processing routes (with and without premilling), a broad distribution of different local environments of the iron atoms was observed in the Mossbauer spectra of the fcc-FeCu phase. Mossbauer spectra show further that a fcc non magnetic Fe-rich phase is formed transiently besides the bcc-Fe phase. The reactions are slowed down when premilled powders are used as starting powders but similar fcc solid solutions are formed after 16 h and 32h in our milling conditions when as-received and premilled powders are used respectively as starting powders.