Stoichiometric default in MgAl2O4 spinel:: Space charge in grain boundary investigation

It was already showed that spinel present a grain boundary sliding deformation accommodated by diffusion during creep at high temperature [1,2]. The observed interface reaction controlling diffusion at low stress (under 60 MPa) can be explained by a space charge layer at grain boundary in ionic ceramics, The nonstoichiometric area due to ionic defect should create an electrostatic potential between the grain surface and inside grain inside. This surface dipole region suppose an excess free energy necessary to form cation and anion vacancies [3, 4]. The purpose of this presentation is to study the grain boundary region stoichiometry to confirm this theory. Fine-grained spinel (average grain size under micron), which presents an interface reaction at low stress, is studied by TEM microanalysis, The cation ratio Al/Mg variation is well described in undeformed sample. It varies from 2.5 at grain boundary to 2.15 at 100nm inside the grain.