The nanosized powders formed on the steel surface in contact with aqueous solutions of zinc and nickel inorganic salts have been studied
using an X-ray diffraction method, thermal analytical measurements (TG / DTG, DTA), scanning electron microscopy and magnetometry.
It has been shown that a single mineral phase of a non-stoichiometric spinel ferrite is formed when zinc and nickel nitrate solutions were
used as the dispersion medium, but in the presence of chloride-containing salts three mineral phases (spinel ferrite, lepidocrocite, and
goethite) are formed in the phase composition of the surface powders. When sulfate solutions were chosen as the dispersion medium the
mixed hydroxysulfate layered double hydroxides (LDHs) simultaneously appeared among other components of the powders. Due to cation
exchange between Fe2+ and Zn2+ or Ni2+ the mixed LDH structures achieved stability against further oxidation and thermal transformation.
Hence, the presence of mixed LDHs in the phase composition of the powders significantly complicates the obtaining of homogenous
superparamagnetic 3d-metal doped spinel ferrite and iron oxide powders.
Key words: Steel surface, nanoparticles, spinel ferrites, thermal transformation, superparamagnetics.
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