Adsorption of H2O, H2S, and N2 on MnZn ferrite

MnZn ferrites show extensive subcritical crack growth. In this process water plays an important role. The fracture surfaces of MnZn ferrites with three different stoichiometries were investigated with adsorption isotherm measurements, temperature-programmed desorption, diffusive reflection infrared Fourier transform spectroscopy (DRIFT), low-energy ion spectroscopy (LEIS), and x-ray photoelectron spectroscopy (XPS). The XPS results confirmed the presence of Mn, Zn, Fe, and O atoms in the surface region, but LEIS indicated that the outer layer of the fracture surfaces contained no Zn atoms. DRIFT confirmed that hydroxyl groups were present on the fracture surfaces. Experiments with H2S, for which the XPS is more sensitive, indicated that at low coverage pairs of S atoms bridge a pair of Fe atoms. With increasing coverage the S bridge shifts to one Fe atom while at the highest coverage each Fe atom is bonded to one S atom. In experiments with N2 the highest nitrogen absorption is found for the lowest oxygen concentration in the ferrite, which is probably related to the polarity of the surface. Both the adsorption and desorption tests indicated that N2 had a larger adsorption affinity to MnZn ferrite surfaces than H2O.