Phase composition and structure of high-disperse iron powders and Fe-Co melt from Moessbauer and X-ray methods

Influence of thermal treatment (470-870 K) in the hydrogen medium is studied on structure and phase composition of high-disperse powders (HDP) of Fe and its melt with Co obtained by electrolysis of aqueous solutions of corresponding metals. Particles size is determined by Hg porometry method to be ∼1-3 μm. It has been shown recently by conversion Moessbauer spectroscopy that the main phase changes during thermal treatment take place in the surface layer. Up to 40% of oxides are α-Fe2O3; maghemite in 470-570 K and α-Fe2O3 particles in supermagnetic state. Increase of Ttr up to 570 K causes growth of oxidation areas on the particle surface and disappearing of superparamagnetic phase. At Ttr > 570 K the quantity of oxides starts decreasing abruptly and at 770 K iron contains only α-Fe phase. With Ttr increasing the size of coherent dispersion areas gets larger (23 - 190 nm) and the magnetic properties of HDP improve. No iron oxides have been found in the particles of Fe-Co melt (40% Co) at all treatment temperatures. Composition and structure of particles get established at Ttr > 600 K and they present a solid solution of Co-substitution in Fe. Moessbauer measurements show significant (up to 20%) increase of magnetic field in Fe nuclei as compared to the pure α-Fe phase. The absence of oxides and the increased magnetic value explain the higher magnetic characteristics of HDP in the Fe-Co melt. (author)