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

Materials involved in the catalytic formation of carbon nanotubes are for the first time systematically studied by Mossbauer spectroscopy between 11 K and room temperature. Mg1-xFexAl2O4 (x = 0.1, 0.2, 0.3, 0.4) solid solutions are transformed into carbon nanotubes-Fe/Fe3C-MgAl2O4 composite powders by reduction in a H-2-CH4 gas mixture. The oxides are defective spinels of general formulae (Mg(1-x)(2+)Fe(x-3 alpha)(2+)Fe(x-3 alpha)(2+)Fe(2 alpha)(3+)rectangle(alpha)Al(2)(3+))O-4(2-). Ferromagnetic alpha-Fe, ferromagnetic Fe3C and a gamma-Fe form, the latter possibly corresponding to a gamma-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 Fe3C in the post-reaction Mossbauer spectroscopy analysis. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.