Thermal stability of nanocrystalline iron

Nanocrystalline iron was obtained by reduction of magnetite doped with structural promoters at 773 K and characterized by various methods i.e. thermal desorption of gases (BET), X-ray diffraction (XRD) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Crystallite size distribution was determined using a novel method based on a phenomenon unique to nanomaterials, i.e. the dependence of the crystallite phase transition on the size of the crystallites. Thermal treatment of the nanocrystalline iron in a hydrogen atmosphere at 1073 K revealed that it is thermally unstable. The parameters of the log-normal crystallite size distribution were d0 = 15.3 nm, σ = 0.35 and d0 = 23.5 nm, σ = 0.17 for iron treated at 773 K and 1073 K, respectively. The corresponding average crystallite sizes determined from the Scherrer formula were 18 nm and 24 nm, respectively. The size distribution of the sintered materials clearly shows that the thermal stability is a function of the size of the crystallites, i.e. the smallest crystals are the least thermally stable. However, no increase in the contribution of crystallites above 35 nm has been observed. Application of this phenomenon combined with the determination of crystallite size distribution enables fine-tuning of the crystallite size distribution.