Graphite-less FeCo nanoparticles by optimizing reductive annealing using carbon monoxide gas

A. Azizi, N.S. Pesika, S.M. Mostafavi, S.K. Sadrnezhaad
Tulane University, US

Keywords: FeCo nanoparticles, reductive annealing, carbon deposition, magnetic properties

Summary:

Graphite phase formation during reductive annealing by a carbon-bearing gas is a probable mechanism which strongly depends on the condition and temperature of the reduction reaction. While a high enough temperature is required for both the completion of the reduction reaction and the prevention of the graphite phase formation, large increase in annealing temperature might cause a severe grain growth resulting in the destruction of nanostructures. Attempt is made in this work to find the optimum condition for production of single-phase FeCo nanoparticles with high saturation magnetization and no graphite phase formation using a controllable mechanochemical procedure. Cobalt-ferrite particles synthesized by mechanical milling were then reduced under an atmosphere of carbon monoxide gas at different circumstances. TEM images revealed that graphite-less FeCo nanoparticles with relatively cube shape and a mean edge length of ~100 nm were synthesized by annealing at 850°C, while the encapsulation of agglomerated FeCo particles into graphite was occurred by decreasing the annealing temperature down to 800°C. The absence and presence of graphite in the samples were confirmed by XRD and Raman spectroscopy results. A decrease in the saturation magnetization of the sample reduced at 800°C was also detected, showing the presence of graphite in the sample.