In situ high-temperature Mössbauer spectroscopic study of carbon nanotube-Fe-Al2O3 nanocomposite powder

The oxidation of a carbon nanotube–Fe–Al2O3 nanocomposite powder was investigated using notably thermogravimetric analysis, room temperature transmission and emission Mössbauer spectroscopy and, for the first time, in situ high-temperature transmission Mössbauer spectroscopy. The first weight gain (150–300 °C) was attributed to the oxidation into hematite of the α-Fe and Fe3C particles located at the surface and in the open porosity of the alumina grains. The 25 nm hematite particles are superparamagnetic at 250 °C or above. A weight loss (300–540 °C) corresponds to the oxidation of carbon nanotubes and graphene layers surrounding the nanoparticles. The graphene layers surrounding γ-Fe–C particles are progressively oxidized and a very thin hematite layer is formed at the surface of the particles, preventing their complete oxidation while helping to retain the face-centered cubic structure. Finally, two weight gains (670 and 1120 °C) correspond to the oxidation of the intragranular α-Fe particles and the γ-Fe–C particles

Synthesis of Fe-ZrO2 nanocomposite powders by reduction in H2 of a nanocrystalline (Zr, Fe)O2 solid solution

The formation of Fe-ZrO2 nanocomposite powders by reduction in hydrogen of a nanocrystalline totally stabilized Zr0.9Fe0.1O1.95 solid solution was investigated by X-ray diffraction (XRD), field-emission-gun scanning electron microscopy (FEG-SEM) and Mössbauer spectroscopy. The reduction of the stabilized Zr0.9Fe0.1O1.95 solid solution and the formation of metallic particles precedes the transformation of zirconia into the monoclinic phase, which becomes the major zirconia phase upon reduction at 950 °C. α-Fe particles with a size distribution slightly increasing from 10–50 to 20–70 nm upon the increase in reduction temperature are observed and a second population of smaller (<5 nm) γ-Fe nanoparticles is also noticed when the reduction is performed at 1000 °C. Another metallic phase with a hyperfine field of not, vert, similar200 kOe at RT (not, vert, similar250 kOe at 80 K) is detected, which could account for an Fe/Zr phase. It could be formed by the reduction on an Fe2+-rich transient phase incorporating a small fraction of the Zr4+ ions, formed by a phase partitioning process superimposed to the reduciton processes

Catalytic chemical vapor deposition synthesis of single- and double-walled carbon nanotubes from α-(Al1−xFex)2O3 powders and self-supported foams

An investigation of the potential interest of α-alumina–hematite foams, as opposed to powders, as starting materials for the synthesis of carbon nanotubes (CNTs) by catalytic chemical vapor deposition method was performed. The oxide powders and foams as well as the corresponding CNT–Fe–Al2O3 composite powders and foams are studied by X-ray diffraction, specific surface area measurements, electron microscopy, Raman spectroscopy and Mössbauer spectroscopy. The latter technique revealed that four components (corresponding to α-Fe, Fe3C, γ-Fe-C and Fe3+) were present in the Mössbauer spectra of the composite powders, and that an additional sextet, possibly due to an Fe1−yCy alloy, is also present in the Mössbauer spectra of the composite foams. Contrary to some expectations, using foams do not lead to an easier reduction and thus to the formation of more α-Fe, Fe3C and/or γ-Fe–C potentially active particles for the formation of CNTs, and hence to no gain in the quantity of CNTs. However, using foams as starting materials strongly favors the selectivity of the method towards SWCNTs (60% SWCNTs and 40% DWCNTs) compared to what is obtained using powders (5% SWCNTs, 65% DWCNTs and 30% MWCNTs)

Mössbauer characterisations and magnetic properties of iron cobaltites CoxFe3−xO4 (1 ≤ x ≤ 2.46) before and after spinodal decomposition

Iron cobaltite powders CoxFe3_xO4 (1 ≤ x ≤ 2.46) were synthesized with compositions in between the cobalt errite CoFe2 O4 and Co2.46Fe0.54O4. The cationic distribution of pure spinel phases was determined by Mossbauer spectroscopy: as Co content increases in the spinel oxide, Co3+ cations replace Fe3+ cations in the octahedral sites and Co2+ cations migrate from octahedral to tetrahedral sites. Saturation magnetizations MS measured at 5 K by a SQUID magnetometer were consistent with the values calculated from the cationic distribution. MS decreases as diamagnetic Co3+ cations replace strongly magnetic Fe3+ cations. Two spinel phases were formed by spinodal decomposition of Co1.73Fe1.27O4 phase submitted to a subsequent thermal treatment, one with a high amount of iron Co1.16Fe1.84O4 and one other containing mostly cobalt Co2.69Fe0.31O4. Increase of the experimental MS value obtained after the spinodal decomposition is in accordance with the calculated value deduced from the cationic distribution of the two phases

Synthesis, characterization and thermal behaviour of Fe0.65Co0.35-MgAl2O4 and Fe0.65Ni0.35-MgAl2O4 nanocomposite powders

Fe0.65Co0.35-MgAl2O4 and Fe0.65Ni0.35-MgAl2O4 nanocomposite powders are prepared by selective hydrogen reduction of monophasic oxide solid solutions synthesized by combustion in urea. The alloy particles, which are dispersed in the spinel matrix, are about 10 nm in size. An increase in reduction temperature from 700 to 1000 °C produces a narrowing of the particles’ composition range and an average composition closer to the target one. The magnetic nature of the alloy nanoparticles is discussed. The nanoparticles dispersed inside the oxide grains, which account for more than two thirds of the total metallic phase, are stable in air up to ca. 800 °C

Fe/Co Alloys for the Catalytic Chemical Vapor Deposition Synthesis of Single- and Double-Walled Carbon Nanotubes (CNTs). 2. The CNT−Fe/Co−MgAl2O4 System

A detailed 57Fe Mössbauer study of the Mg(0.8)Fe(0.2-y)Co(y)Al2O4 (y = 0, 0.05, 0.1, 0.15, 0.2) solid solutions and of the CNT-Fe/Co-MgAl2O4 nanocomposite powders prepared by reduction in H2-CH4 has allowed characterization of the different iron phases involved in the catalytic process of carbon nanotube (CNT) formation and to correlate these results with the carbon and CNT contents. The oxide precursors consist of defective spinels of general formulas (Mg(1-x-y)(2+)Fe(x-3alpha)(2+)Fe(2alpha)(3+)[symbol: see text](alpha)Co(y)(2+)Al2(3+))O4(2-) . The metallic phase in the CNT-Fe/Co-MgAl2O4 nanocomposite powders is mostly in the form of the ferromagnetic alpha-Fe/Co alloy with the desired composition. For high iron initial proportions, the additional formation of Fe3C and gamma-Fe-C is observed while for high cobalt initial proportions, the additional formation of a gamma-Fe/Co-C phase is favored. The higher yield of CNTs is observed for postreaction alpha-Fe(0.50)Co(0.50) catalytic particles, which form no carbide and have a narrow size distribution. Alloying is beneficial for this system with respect to the formation of CNTs

Iron-stabilized nanocrystalline ZrO2 solid solutions: Synthesis by combustion and thermal stability

The synthesis of Fe3+-stabilized zirconia by the nitrate/urea combustion route was investigated. Using several characterization techniques, including X-ray diffraction, field-emission-gun scanning electron microscopy and notably Mo¨ ssbauer spectroscopy, it was possible to determine the appropriate amount of urea that allows to obtain a totally stabilized Zr0.9Fe0.1O1.95 solid solution. The nanocrystalline zirconia solid solution is mostly tetragonal, but the presence of the cubic phase could not be ruled out. An indepth study of the thermal stability in air showed that the Fe3+ solubility in the stabilized solid solution starts to decrease at about 875 8C which results in the formation of hematite (possibly containing some Zr4+) at the surface of the zirconia grains and further provokes the progressive transformation into the monoclinic zirconia phase

Influence of the borohydride concentration on the composition of the amorphous Fe-B alloy produced by chemical reduction of synthetic, nano-sized iron-oxide particles Part I: Hematite

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