Space charge characterisation by EDS microanalysis in spinel MgAl2O4

It was already showed that spinel presents a grain boundary sliding deformation accommodated by diffusion during creep at high temperature [Béclin, F., Duclos, R., Crampon, J. and Valin, F., Microstructural superplastic deformation in MgO·Al2O3 spinel. Acta Metall. Mater. 1995, 43, 2753–2760; Addad, A., Etude de la plasticité haute température de matériaux céramiques spinelle-zircone. Ph.D. thesis, Laboratoire de Structure et Propriétés de l’Etat solide, Villeneuve d’Ascq, 1995]. A space charge layer at grain boundary in ionic ceramics can explain the observed interface reaction controlling diffusion at low stress (less than 60 MPa). The nonstoichiometric area due to ionic defects should create an electrostatic potential between the grain surface and inside grain. The purpose of this work 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 stresses, is studied by TEM nano-scale microanalysis. The cation ratio Al/Mg variation is well described. It varies from 2.34 at grain boundary to 2.10 at 100 nm inside the grain. The ratio Al/O shows no variation across the grain boundary and suggests that the stoichiometry defect observed is due to an excess of magnesium vacancies located at the grain boundary.