Mössbauer and x-ray investigations of oxidized ultrafine iron particles treated with high-pressure and electrodischarge pulses

Abstract

Ultrafine iron particles with an average size of 30 nm were obtained by the levitation method in an inert gas flow. Electrodischarge experiments were carried out under a pressure of 3 GPa. The values of the parameters of electrodischarge pulses were sufficient to move defects (ions, atoms, dislocations) by conductivity electrons to destroy the metastable state of nanocrystals, but not sufficient to generate heat wave. This effect had threshold features. After the first pulse the relative resistance drop was more than five times. Next, electrical pulses slightly reduced resistance up to its saturation. Properties of ultrafine iron particles have been studied using Mössbauer absorption spectra and x-ray diffraction for three types of samples: (a) primary powder, (b) powder treated to 3 GPa in a ‘‘toroid’’ device, (c) powder treated by electrical pulses at 3 GPa. An x-ray diffraction pattern revealed seven slightly broadened iron reflexes as well as more broadened oxide film reflexes. Oxide film reflexes correspond to the most intense reflexes of a spinel structure. The integrated intensities of 220,311 spinel reflexes divided by integrated intensity of 110 iron reflex are equal to 0.12, 0.14, and 0.19 for samples (a), (b), and (c) respectively. Thus, the relative content of oxide iron phase increases from samples (a) to (c). The increase of relative content of the oxide iron phase was confirmed by a Mössbauer absorption experiment. This effect can be explained by the transformation of the metastable solution of oxygen in iron lattice to iron oxide with a spinel structure, under the action of high-density current pulses.