Effect of machine variables in the mechanosynthesis of NiFe2O4: A microstructural and kinematical study

Abstract

We have carefully investigated the effect of some machine variables in the formation of nickel ferrite when it is obtained via high-energy planetary ball milling of a stoichiometric mixture of nickel oxide and hematite, by combining both microstructural and kinematical studies. The angular speed of disc (Ω), ball to powder mass ratio (mb:mp), number of balls (Nb), and milling time (t) were explored. These variables were grouped into two sets according to their values as follows: the first set was for mb:mp=30:1, Ω=330rpm, Nb=7 and 0h≤t≤20h, whereas the second set was for mb:mp=40:1, Ω=390rpm, Nb=9 and 0h≤t≤50h. The products of the mechanosysnthesis were microstructurally characterized by quantitative X-ray diffraction (XRD) using the Rietveld method and by 57Fe Mössbauer spectrometry (MS). For both sets of machine conditions, as the milling time increased, the intensities of the Bragg peaks of nickel oxide and hematite decreased differently, whereas those of the spinel phase increased. The production rate of NiFe2O4 by the first set of conditions was very low, however by using the second set and after 50h of milling, about 86wt.% of this phase was formed. The MS of the samples generally consisted of sextets and doublets, the sextets were associated to micrometric- or nanometric-sized hematite and/or magnetically ordered nano-sized NiFe2O4, whereas the doublet was assigned to superparamagnetic nano-sized NiFe2O4. Finally, the energy transferred to the powders by the balls during milling in a planetary ball mill was calculated by using a kinematical model. According to the model, a cumulative kinetic energy normalized to the powder mass (Ecum) of the order of 1.08×109J/kg was required to produce about 86wt.% of nickel ferrite when the second set of machine conditions for 50h was employed. Ecum was used as a criterion of comparison for the machine variables investigated.