N. Randrianantoandro

Direct phase transformation from hematite to maghemite during high energy ball milling

Abstract Hematite α-Fe 2 O 3 powder was milled with a dispersing liquid (ethanol) for different times using a conventional planetary ball mill equipped with a steel vial. Both high resolution transmission electron microscopy (HRTEM), X-ray diffraction and zero- and in-field Mossbauer spectrometry measurements reveal that the nanostructured powders contain maghemite, suggesting that our milling conditions favour the direct transformation from α-Fe 2 O 3 to γ-Fe 2 O 3 . Both the origin and the kinetics of the transformation are discussed in terms of mechanical aspects.

Electrochemical intercalation of lithium into the perovskite-type NbO2F: influence of the NbO2F particle size

Abstract Niobium(V) oxyfluoride, NbO2F, has a perovskite structure and presents the property of lithium intercalation by topotactic chemical reaction either with n-butyllithium dissolved in n-hexane or by electrochemical reaction. The intercalation leads to the reduction of the transition metal from the oxidation state Nb(V) to the oxidation state Nb(III). This allows a theoretical Li/NbO2F intercalation ratio of 2. In this paper we will show that this theoretical value can be approached by using micron the sized active material particles. Moreover, the electrical properties of the cathode studied by the galvanostatic intermittent titration technique and a.c. impedance spectroscopy are explained in terms of structural and grain size considerations. Results of cycling experiments are also described.

Surface and bulk crystallization mechanisms in the Metglas Fe78B13Si9 investigated by Mössbauer spectroscopy

Abstract The amorphous-to-crystalline transformation of the as-recieved Metgals ∗ 2605S2) was studied by transmission and reflection Mossbauer spectroscopy in order to compare shiny and dull surfaces and bulk crystallization mechanisms. In agreement with previous results, the onset of crystallization occurs on both surfaces, simultaneously to the out-of-plane anisotropy. Higher kinetics of crystallization is observed at dull surface. Then, primary and eutectic crystallization processes seem to be evidenced for the shiny and dull surfaces respectively. It is proposed that these different phenomena originate from the presence of structural and chemical inhomogeneities induced during the solidification process.

Magnetic interactions in γ-Fe2O3@SiO2 nanocomposites

Structural and magnetic properties of γ-Fe2O3 nanoparticles of 4 nm diameter, dispersed into silica SiO2 matrix with a wide range value of volume fraction (0.05 to 1), were investigated. Produced γ-Fe2O3@SiO2 nanocomposites consist of an assembly of very small single domain magnetic object (<10 nm) with a random distribution of both inter-particle distance and direction of particle magnetic moment. We focused on the determination of a magnetic percolation threshold, defined as the magnetic particles concentration value above which the magnetic properties of isolated particles vanished in favor of a magnetic collective behavior induced by magnetic interactions. A percolation threshold value of 0.63 was obtained by a local probe technique such as 57Fe Mossbauer spectrometry and confirmed by global magnetic measurements through zero-field cooled, field-cooled, and ac susceptibility data. Below this threshold, dynamic ac magnetization measurements show a thermally activated Arrhenius dependence of the blockin...

Magnetic properties of Fe - Cr-based nanocrystalline alloys

FeCrCuNbSiB nanocrystalline alloys, with low fractions of the crystalline phase (10 - 25%), have been studied by means of static magnetic measurements and Mossbauer spectroscopy over a wide temperature range (4.2 - 800 K). As a reference the temperature dependences of the hyperfine parameters measured in ordered Fe - Si alloys were used. It has been shown that at temperatures close to the Curie point of the amorphous matrix the coercivity exhibits a maximum that corresponds to a kink in the thermal evolution of the hyperfine field of the crystalline phase. These effects originate from the lessening of the interphase exchange interactions at the ferro - paramagnetic phase transition of the amorphous matrix and superparamagnetic fluctuations of the magnetization in single-domain grains. A mechanism considering different energy contributions as well as their changes with temperature in such a mesoscopic system is discussed.

Structural behavior of laser-irradiated γ-Fe2O3 nanocrystals dispersed in porous silica matrix : γ-Fe2O3 to α-Fe2O3 phase transition and formation of ε-Fe2O3

Abstract The effects of laser irradiation on γ-Fe2O3 4 ± 1 nm diameter maghemite nanocrystals synthesized by co-precipitation and dispersed into an amorphous silica matrix by sol-gel methods have been investigated as function of iron oxide mass fraction. The structural properties of γ-Fe2O3 phase were carefully examined by X-ray diffraction and transmission electron microscopy. It has been shown that γ-Fe2O3 nanocrystals are isolated from each other and uniformly dispersed in silica matrix. The phase stability of maghemite nanocrystals was examined in situ under laser irradiation by Raman spectroscopy and compared with that resulting from heat treatment by X-ray diffraction. It was concluded that ε-Fe2O3 is an intermediate phase between γ-Fe2O3 and α-Fe2O3 and a series of distinct Raman vibrational bands were identified with the ε-Fe2O3 phase. The structural transformation of γ-Fe2O3 into α-Fe2O3 occurs either directly or via ε-Fe2O3, depending on the rate of nanocrystal agglomeration, the concentration of iron oxide in the nanocomposite and the properties of silica matrix. A phase diagram is established as a function of laser power density and concentration.

Mössbauer study of a nanocrystalline FeCr-based metallic glass

Abstract Mossbauer spectroscopy has been performed over the temperature range 4.2–833 K in the nanocrystalline Fe 66 Cr 8 CuNb 3 Si 13 B 9 metallic glass. The sample was found to consist of α-(FeSi) crystalline phase with 14 at% Si embedded in a chromium enriched amorphous matrix. The Curie temperature of the matrix is found to be 403 K.

Numerical Analysis of Time Dependent Effects in Bulk Nanocrystalline Hard Magnets

The presented work concerns numerical analysis of t-dependent magnetization curves. We propose the approach consisting in application of the so-called simulated annealing algorithm (SA) supplemented with local maximum entropy condition. Efficiency of the SA procedure and its parameters was tested with the use of computer generated curves derived from known distributions of relaxation time τ. Based on the determined SA parameters the analysis of empirical M(t) dependences measured for (Fe80Nb6B14)1-xTbx (x=0.08, 0.16) bulk nanocrystalline alloys were also carried out. As it was shown the SA procedure allows determining distributions of relaxation time without preliminary assumption about its shape and number of components.