Mössbauer spectroscopy study of MgAl2O4-matrix nanocomposite powders containing carbon nanotubes and iron-based nanoparticles

Abstract

ÐMaterials involved in the catalytic formation of carbon nanotubes are for the ®rst time systematically studied by MoÈ ssbauer spectroscopy between 11 K and room temperature. Mg 1Àx Fe x Al 2 O 4 x 0:1, 0.2, 0.3, 0.4) solid solutions are transformed into carbon nanotubes±Fe/Fe 3 C±MgAl 2 O 4 composite powders by reduction in a H 2 ±CH 4 gas mixture. The oxides are defective spinels of general formulae Mg 2 1Àx Fe 2 xÀ3a Fe 3 2a q a Al 3 2 O 2À 4: Ferromagnetic a-Fe, ferromagnetic Fe 3 C and a g-Fe form, the latter possibly corresponding to a g-Fe±C alloy, are detected in the composite powders. An attempt is made to correlate these results with the microstructure of the powder. It seems that the nanoparticles, which catalyze the formation of the carbon nanotubes, are detected as Fe 3 C in the post-reaction MoÈ ssbauer spectroscopy analysis. Re sumeÂÐDes mate riaux implique s dans la formation catalytique de nanotubes de carbone sont pour la premieÁ re fois syste matiquement e tudie s par spectroscopie MoÈ ssbauer entre 11 K et la tempe rature ambiante. Des solutions solides Mg 1Àx Fe x Al 2 O 4 x 0:1, 0.2, 0.3, 0.4) sont transforme es en poudres composites nanotubes de carbone±Fe/Fe 3 C±MgAl 2 O 4 par re duction dans un me lange gazeux de H 2 et de CH 4 . Les oxydes sont des spinelles lacunaires de formule ge ne rale Mg 2 1Àx Fe 2 xÀ3a Fe 3 2a q a Al 3 2 O 2À 4 : Du Fe-a ferro-magne tique, du Fe 3 C ferromagne tique et une forme de Fe-g, cette dernieÁ re correspondant probablement aÁ un alliage Fe±C-g, sont de tecte s dans les poudres composites. Des corre lations sont faites entre ces re sultats et la microstructure de la poudre. Il semble que les nanoparticules qui catalysent la formation des nanotubes de carbone sont de tecte es comme du Fe 3 C dans l'analyse par spectroscopie MoÈ ssbauer des produits obtenus apreÁ s la re action.

Integral low-energy electron Mössbauer spectroscopic studies of the surfaces of carbon nanotube-nanocomposite powders

“…Four components were used in the fits of the spectra: one doublet, two sextets and one singlet. No significant differences in parameter values between the presently measured transmission data and those obtained earlier by Coquay et al [4]were observed. According to these authors, the doublet is attributable to Fe 2+ substituting for Mg 2+ in the spinel structure.…”

“…The synthesis of the CNTs-Fe-Al 2 O 3 and CNTs-Fe-MgAl 2 O 4 and their characterization by X-ray diffraction, transmission Mössbauer spectroscopy, transmission-and scanning electron microscopy are described in detail elsewhere [3,4]. Briefly, the CNTs-Fe-Al 2 O 3 nanocomposite powders were prepared by a catalytic chemical vapor deposition (CCVD) route involving the reduction of α-( Mössbauer spectra at room temperature were collected with a spectrometer operating in constant acceleration mode with triangular reference signal.…”

“…In this contribution ILEEMS results will be presented for different selected CNTs-nanocomposite systems. These nanocomposite systems were synthesized and characterized by several techniques earlier [3,4]. It is intended to demonstrate that the characteristics of the top surface layers of these systems are The values of isomer shifts are with reference to metallic iron B hf,m hyperfine field at maximum of the distribution (T ), 2ε Q quadrupole shifts (mm/s), E Q,m quadrupole splitting (mm/s),δ isomer shifts (mm/s) a Fixed parameters Results for transmission spectra are from reference [3] very often significantly different as compared to what is observed for the bulk by transmission Mössbauer spectroscopy.…”

Carbon Nanotubes by a CVD Method. Part II: Formation of Nanotubes from (Mg, Fe)O Catalysts

“…The increase in the amount of Fe 2+ upon reduction of the Fe/MgO oxide powders and the evolution of the amount of Fe 2+ in the CNTsFe-MgO nanocomposite powders versus the urea ratio are consistent with the values of the MgO unit-cell parameters presented in Table 2 46 have indeed shown that a 1-h reduction in H 2 at 1300°C is necessary to reduce a Mg 0.9 Fe 0.1 O solid solution fully. Previous works 4,14 indicated that the Fe 3 C phase detected in the post-reduction analysis corresponds to particles on the surface of the matrix grains that are involved in the formation of the CNTs and the thick carbon nanofibers. The amount of Fe 3 C is high in Fe10U1R, decreases steeply to reach a minimum in Fe10U4R, and then increases in Fe10U8R, where it is similar to the amount in Fe10U1R (Figure 12b).…”

Evolution of catalyst particle size during carbon single walled nanotube growth and its effect on the tube characteristics

“…Formation of iron carbide phases during carbon SWNT growth has been discussed previously in the literature based on in situ TEM, Mössbauer spectra analysis, and magnetic properties. [30][31][32][33][34] Those studies concluded that the Fe-C alloy particles are possibly responsible for the nucleation and growth of the nanotubes. A detailed analysis of carbon nanotube formation mechanism using the Fe-C phase diagram was described in Ref.…”