Grain-Boundary Characterization in a Nonstoichiometric Fine-Grained Magnesium Aluminate Spinel: Effects of Defect Segregation at the Space-Charge Layers Nicolas Nuns, Franck Be´ clin, w and Jacques Crampon Laboratoire de Structure et Proprie´ te´ s de l’Etat Solide, UMR-CNRS 8008, Bat. C6, Universite´ des Sciences et Technologies de Lille, F-59655 Villeneuve d’Ascq, Cedex, France The grain-boundary chemistry of fine-grained spinel MgO . nAl 2 O 3 (mean grain size below micron) has been inves- tigated by STEM microanalysis. We have quantified the con- centration of each element across the grain boundaries. Stoichiometry variations are observed from the grain-boundary region to the bulk. The Al/Mg ratio increases from 2.1 in the bulk to 2.35 at the grain-boundary regions. X-ray quantification allows us to reveal and to characterize the space-charge layer in the subgrain boundary. The grain-boundary cores are negatively charged due to V 00 Mg vacancies in excess, and in the subgrain- boundary region, an opposite, positive space-charge layer is ob- tained. The point defect composition and the characteristic (sign, space-charge potential F N ) of the space-charge layer are discussed. I. Introduction M ANY macroscopic properties of ceramics (plasticity, grain growth, sintering, conductivity, etc) directly depend on grain boundary or subgrain-boundary solutes and defect segregation. 1–3 Most of the results on the superplastic creep of ceramics suggest that their ductility may be related to their non- stoichiometry. Among the ceramics that are intrinsically non- stoichiometric, fine-grained magnesium aluminate spinel has been observed to be easily superplastic. The nonstoichiometry of magnesium aluminate spinel derives from its crystalline struc- ture, which consists of a cubic cell containing a close-packed array of 32 oxygen ions with cations in tetrahedral and oct- ahedral interstices. In the normal structure, Mg 21 ions occupy eight tetrahedral sites (out of 64 in a unit cell) and Al 31 ions occupy 16 octahedral sites (out of 32 in a unit cell). Because the number of available cationic sites is greater than the number of cations, many of these cationic sites are empty. In an equimolar synthetically grown spinel, the predominant intrinsic defects are those resulting from the site exchange between a Mg 21 ion and an Al 31 ion, often referred to as an antisite defect pair. Besides this, magnesium aluminate spinel presents stoichiometric devi- ations, i.e. MgO  nAl 2 O 3 with n41, the excess Al 31 in solution solid being charge compensated by cation vacancies. The super- plastic properties of spinel have been related to the possible seg- regation of these defects in space-charge layers adjacent to the grain-boundary interfaces of the polycrystals. 4,5 From this idea, the concept of space charge has been introduced into the diffu- sion creep mechanism to derive a new equation for Nabarro– Herring creep in such nonstoichiometric materials. 6 The possi- bility of space charge in the subgrain-boundary regions in spinel can be investigated by measuring the Al/Mg ratios, across the grain boundaries, by analytical transmission electron micros- copy (ATEM). In order to understand the grain-boundaries’ mobility in spinel MgO  nAl 2 O 3 , Chiang and Kingery 7,8 have studied their chemical composition by energy dispersive spec- troscopy (EDS) microanalysis. They found an excess of Al and O relative to Mg in the grain-boundary regions in spinel poly- crystals with different nonstoichiometries. Their results lead to the inference that the grain-boundary regions in spinel harbor a space charge. However, because oxygen X-ray absorption is se- vere in MgAl 2 O 4 , they could not establish the oxygen concen- tration. The purpose of this work is to verify experimentally, by the quantification of the concentration of each element in the bulk and at the grain-boundary region, the existence and the sign of the space-charge layer in the subgrain-boundary regions in a spinel. The experimental results obtained are consistent with the measurements of Chiang and Kingery. 7,8 Their interpretation is made, in the present work, on the basis of a space-charge theory analysis, and the physical properties (sign, composition) of the space-charge layer will also be discussed. Moreover, this paper presents a numerical estimation of the space-charge potentials F N : about 1.15 V for the stoichiometric spinel, and between 1.5 and 2 V for the MgO  1.05Al 2 O 3 spinel. II. Experimental Procedures (1) Sample The material was obtained from a commercial powder sintered by hot isostatic pressing (HIP) at 13801C under 190 MPa. These sintered conditions allow to obtain a near-dense spinel with fine grains. 9 The nearly stoichiometric spinel in this study was syn- thesized with cation excess (molar ratio n 5 Al 2 O 3 /MgO41). (2) Thin Foil Preparation for Analytical Transmission Electron Microscopy Specimens for ATEM examinations have been prepared using the technology tripod polisher. 10 They are thinned mechanically to electron transparency by polishing both the sides. In a first stage, polishing takes place with a series of plastic diamond lapping films, with grains decreasing size (15, 9, 6, 3, 1, and 0.5 mm). The final polishing is done on a soft felt covered disc impregnated with a silica colloidal solution. The two faces are not polished in a parallel way, so that the edge of the sample is transparent for the electron beam. This thinning method pro- duces high-quality transmission electron micrographs, which il- lustrate the microstructure accurately. Samples are free of artifacts and unaltered in any way. In addition, this technique offers a large observation area that is free from contamination. H. M. Chan—contributing editor This work was financially supported by the ‘‘Re´ gion Nord-Pasde Calais’’ and the ‘‘Fonds Europe´ en de De´ veloppement Re´ gional (Feder)’’ for supporting this work. w Author to whom correspondence should be addressed. franck.beclin@univ-lille1.fr Manuscript No. 24967. Received July 24, 2008; approved November 22, 2008. J ournal J. Am. Ceram. Soc., 92 [4] 870–875 (2009) DOI: 10.1111/j.1551-2916.2008.02901.x r 2009 The American Ceramic Society 870