Characterization of nickel-chromium-iron spinel-type oxides

The authors report studies on compounds in the two series NixFe1-xFe2O4 (0≤x≤1) and NiCrxFe2-xO4 (0≤x≤2), prepared by forming solid solutions of the appropriate single oxides at a temperature of 9500C in atmospheres of CO2 or CO2 containing 2% CO. For each spinel series prepared a calibration curve has been generated which relates lattice parameter, as determined by X-ray diffraction, to spinel composition. These calibration curves facilitate the identification of the spinels in oxide layers formed on stainless power reactor steels. Energy dispersive X-ray analysis has been carried out on each standard spinel prepared to determine elemental composition. It was found that variations in the oxidizing of reducing nature of the reaction atmosphere affected the stability of spinels containing Fe(II) or Fe(III). Small quantities of nickel metal were identified in nickel rich spinels formed in CO2/CO atmospheres. This observation could be an important factor in the deposition behavior of such surfaces under reactor conditions. (orig./MM)