Kinetics of physico-chemical processes during intensive mechanical processing of ZnO–MnO 2 powder mixture

Abstract

Experimental results of electron paramagnetic resonance spectra, X-ray diffraction, scanning electron microscopy and infrared spectroscopy demonstrate that the kinetic of the physical and chemical processes that takes place during prolonged intensive mechanical processing (MP, 0 < t MP > 3120 min ) of powder mixtures of 50%wt ZnO+50%wt MnO 2 can be described as a three stage process. (1) 0 < t MP > 30 min , particles destruction, formation of superficial defects, fast increment of sample average temperature (from 290 to ∼ 600 K ) and annealing of defects with the lowest energy of activation E ac . (2) 30 < t MP > 390 min , further particle destruction, slow increment of sample average temperature (from ∼ 600 to ∼ 700 K ), formation and growth of a very disordered layer of β ‐ MnO 2 around ZnO particles, dehydration of MnO 2, formation of solid solution of Mn 2+ ions in ZnO, formation of nano-quasiamorphous states in the ZnO–MnO 2 mixture and onset of the formation of the ZnMnO 3 phase. (3) 390 < t MP > 3120 min , the sample average temperature remains constant ( ∼ 700 K ), the reaction is completed and the spinel ZnMnO 3 phase with a unit cell a=8.431(1) Å and space group Fd 3 ¯ m is the only phase present in the sample. No ferromagnetism at room temperature was detected in this study.