S. Alleg

Magnetic properties of nanostructured ball-milled Fe and Fe50Co50 alloy

Nanostructured Fe and Fe50Co50 powders were prepared by high-energy ball milling. Microstructural and magnetic properties changes with milling time were followed by x-ray diffraction, differential scanning calorimetry and vibrating sample magnetometry. The nonequilibrium microstructure originates from a grain size reduction to about 12 nm and the introduction of internal strain up to 1.5% (root-mean-square strain). The occurrence of disorder in the ball-milled powders is evidenced by the broad exothermic reaction during the heating of ball-milled samples, the variation of lattice parameters and the increase of the saturation magnetization during the first 3 h of milling for Fe and continuously for the Fe50Co50 powder mixture. According to both the reduction of Fe Curie temperature, Tc, and the increase of the phase transformation , the paramagnetic temperature domain of nanostructured bcc α-Fe is extended by about 50 °C. The Fe50Co50 nanostructured powder behaves as a soft ferromagnet with low values of both the coercive field and the squareness ratio Mr/Ms.

Synthesis and morphological characterization of nanocrystalline powders obtained by a gas condensation method

Abstract The preparation of nanocrystalline powders of Fe and Fe50Ni50 has been performed by a gas-condensation method under pure helium atmosphere. The characterization of the prepared materials which was carried out by means of Transmission Electron Microscopy, X-rays diffraction and Mossbauer Spectrometry, evidences for the presence of oxide phases. Fe and FeNi based ultrafine particles are observed with a size comprised within the range 10–70 nm and they occur as clusters or chains.

Study of the local environment during the phase decomposition of Fe-30.8Cr-12.2Co alloy by Mössbauer spectrometry

Abstract The local environment issued from the phase decomposition in the Fe–30.8Cr–12.2Co alloy has been investigated by 57 Fe Mossbauer spectrometry as a function of the ageing time and temperatures. In order to enable a precise determination of the different configurations, the analysis of Mossbauer spectra is improved using several different fitting models. It has been concluded that the phase decomposition in the Fe–30.8Cr–12.2Co alloy is spinodal and gives rise to the formation of a paramagnetic (α′) phase and a magnetic (α) phase, with relative areas of ∼5% and ∼95%, respectively. The magnetic phase results from three main domains: an Fe-rich, a Cr-rich and a Co-rich with relative proportions of 9, 42 and 44%, respectively. The Cr content in the Fe-rich (α) phase is about 9 at.% Cr while that of the Cr-rich (α′) phase is ranged between 71 and 80 at.% as a function of time and ageing temperature, which can be attributed to the kinetics of the phase decomposition. The (α′) volume fraction estimated at about 28% is consistent with an isotropic decomposition where both (α) and (α′) phases are strongly interconnected.

Structural study of the mechanically alloyed Fe-P

Elemental Fe and red phosphorus powders with a composition close to Fe-xP (x = 10, 15 and 20 wt. %) were mechanically alloyed in a planetary ball mill under an argon atmosphere. Structural changes were studied by X-ray diffraction. The complete dissolution of the elemental powders is achieved within 3 h of milling. Detailed analysis of the X-ray diffraction patterns reveal the formation of a Fe(P) solid solution with two structures (α-Fe1 and α-Fe2) having different lattice parameters, crystallite size and microstrains in addition to FeP, Fe2P and Fe3P phosphides. The structural parameters and phase percentages are P content dependent.

Mossbauer study of mechanically alloyed Fe57Cr31Co12

Nanostructured powders of Fe 57 Cr 31 Co 12 were prepared by mechanical alloying from elemental Fe, Cr and Co powders, using a planetary ball mill type Fritsch Pulverisette 7. The powders were characterized by X-ray diffraction and 57 Fe Mossbauer spectrometry. A detailed analysis of the diffraction patterns reveals a decrease of the crystalline grain size as a function of milling time. For the first hours of milling, Mossbauer spectra are composed of a magnetic contribution and a single line. The paramagnetic component, whose relative area increases with milling time, can be attributed to paramagnetic Cr-rich clusters. The results are compared to those obtained on the same alloy prepared by conventional melting technique.

Thermal Stability of the Nanostructured Powder Mixtures Prepared by Mechanical Alloying

Nanocrystalline materials present an attractive potential for technological applications and provide an excellent opportunity to study the nature of solid interfaces and to extend knowledge of the structure-property relationship in solid materials down to the nanometer regime. Nanocrystalline materials can be produced by various methods such as mechanical alloying, inert gas condensation, sol–gel process, electrodeposition, chemical vapour deposition, heat treatment of amorphous ribbons, high speed deformation, etc. Mechanical alloying is a non-equilibrium process resulting in solid state alloying beyond the equilibrium solubility limit. During the milling process, mixtures of elemental or prealloyed powders are subjected to heavy plastic deformation through high-energy collision from the balls. The processes of fracturing and cold welding, as well as their kinetics and predominance at any stage, depend mostly on the deformation characteristics of the starting powders. As a result of the induced heavy plastic deformation into the powder particles during the milling process, nanostructured materials are produced by the structural decomposition of coarser-grained structure. This leads to a continuous refinement of the internal structure of the powder particles to nanometer scales.

Structural characterisation of the mechanically alloyed Fe57Co21Nb7B15 powders

X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were used to investigate the phase identification and the thermal behaviour of the mechanically alloyed Fe57Co21Nb7B15 powders. The diffusion of B into the Nb lattice leads, after 1 h of milling, to the formation of a bcc Nb(B) solid solution with a lattice parameter close to a = 0.3425 nm. The solid state reaction between Fe and B gives rise to the formation of Fe23B6 and Fe2B boride phases after 3 and 6 h of milling, respectively. On further milling (96 h), an amorphous matrix (∼80%), where nanocrystalline bcc α-Fe, bcc Nb(B), Fe2B and Fe3B phases were embedded, is obtained. The broad exothermic reaction in the DSC scans consists of several exothermic peaks and spreads over the entire temperature range 100-700°C. The enthalpy release at temperatures below 300°C can be attributed to recovery and strain relaxation. Crystallisation and grain growth are the dominating processes at high temperatures.

Solid state amorphisation of mechanically alloyed Fe-Co-Nb-B alloys

Fe 61 Co 21 Nb 3 B 15 powder mixture was prepared by mechanical alloying process in a high energy planetary ball mill. Structural and thermal changes of the milled powders were followed by X-ray diffraction (XRD), Mossbauer spectrometry and differential scanning calorimetry (DSC). Both XRD and Mossbauer spectrometry results reveal the formation, after 48 h of milling, of a highly disordered amorphous-like structure where nanometer-sized iron borides were embedded. A mechanical recrystallisation process gives rise to the formation of α-Fe and α-FeCo nanograins on further milling. The occurrence of structural disorder in the milled powders might be confirmed by broad exothermic reaction in the DSC scans which consists of several overlapping exothermic peaks. Such behaviour originates from recovery, strain relaxation, grain growth and crystallisation.

Microstructural properties of Fe-doped ZnO thin films and first-principals calculations

Microstructural, and morphological properties of Fe-doped nanostructured ZnO semiconducting thin films have been investigated by optical, SEM, XRD, and first-principals computing. ZnO thin films grown on glass and Si substrates by the spray pyrolysis method at 300°C, and under ambient atmosphere, have initial preferred (002) orientation. XRD peak intensity changed rapidly as the Fe-concentration is increased from 1 to 5 mol.%, despite the fact that lattice parameters changed monotonously. Fe ions occupied the Zn sites without changing the original hexagonal wurtzite structure. All Fe-doped ZnO films are polycrystalline with an average grain size of 23 nm. Band structure and density of states of the possible phases of crystal ZnO computed using first principal methods, confirmed that pure ZnO is a direct band gap semiconductor when obtained in the B3 or B4 type structure phase. However, the B1 phase turned out to be an indirect band gap semiconductor.

Thermal stability of the nanocrystalline Fe-8P (wt.%) powder produced by ball milling

GRAPHICAL ABSTRACT ABSTRACT The thermal stability of Fe-8P (wt.%) ball milled powders was investigated by differential scanning calorimetry X-ray diffraction and 57Fe Mössbauer spectrometry. The effect of structural disorder is evidenced in the DSC thermogram by the presence of a large exothermic reaction consists of several overlapping peaks and spread over the temperature range (150-700)°C. The result of the Rietveld refinement of the XRD patterns indicates that during the annealing of the powders up to 210°C, three phases are observed: α-Fe(P), solid solution Fe3P and FeP phosphides. The Mössbauer spectra analyses show that the paramagnetic FeP phosphide phase is the only product after the annealing (∼ 2%). The annealing at 450°C leads to a mixture of α-Fe(P) solid solution, Fe3P nanophase and a small amount of a paramagnetic FexP (1 < x < 2) phosphide phase (∼ 3%) in addition to iron oxides.