Positron Annihilation in MnFe2O4/MCM-41 Nanocomposite

In the paper results of studies of MnFe2O4/MCM-41 nanocomposites have been presented. The influence of manganese ferrite loading on changes of porous properties of mesoporous MCM-41 structure was studied by means of N-2 sorption/desorption method and positron annihilation lifetime spectroscopy. Disappearance of the longest-lived ortho-positronium component (tau(5)) of pure MCM-41 mesoporous material in the positron annihilation lifetime spectra of MnFe2O4/MCM-41 measured in vacuum is a result of either o-Ps quenching or the Ps inhibition mechanism. Filling of pores in the studied nanocomposites by air at ambient pressure causes partial reappearance of the (tau(5)) component except for the sample with maximum ferrite content. Both the (tau(5)) component lifetime and intensity are suppressed together with increasing MnFe2O4 content by chemical quenching and inhibition of Ps formation occur. Observed anti-quenching effect of air is a result of two processes: neutralization of some surface active centres acting as inhibitors and considerably weaker paramagnetic quenching by O-2 molecules.