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Ahmed Addad Laboratoire de Structure et Proprie´te´s de l’Etat Solide, UMR CNRS 8008, Universite´ des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq Cedex, France

Mats Nygren Arrhenius Laboratory, University of Stockholm, 10691 Stockholm, Sweden

Christian Guizard and Sylvain Deville Laboratoire de Synthe`se et Fonctionnalisation des Ce´ramiques, UMR CNRS/Saint-Gobain 3080, Saint-Gobain C.R.E.E., 84306 Cavaillon Cedex, France (Received 5 October 2008; accepted 20 March 2009)

Spark plasma sintering (SPS) of MgAl2O4 powder was investigated at temperatures between 1200 and 1300
C. A significant grain growth was observed during densification. The densification rate always exhibits at least one strong minimum, and resumes after an incubation period. Transmission electron microscopy investigations performed on sintered samples never revealed extensive dislocation activity in the elemental grains. The densification mechanism involved during SPS was determined by anisothermal (investigation of the heating stage of a SPS run) and isothermal methods (investigation at given soak temperatures). Grain-boundary sliding, accommodated by an in-series {interface-reaction/lattice diffusion of the O2 anions} mechanism controlled by the interface-reaction step, governs densification. The zero-densification-rate period, detected for all soak temperatures, arise from the difficulty of annealing vacancies, necessary for the densification to proceed. The detection of atomic ledges at grain boundaries and the modification of the stoichiometry of spinel during SPS could be related to the difficulty to anneal vacancies at temperature soaks.

I. INTRODUCTION

Fully dense polycrystalline alumina-magnesia spinel (referred to as spinel hereafter), MgAl2O4, is an attractive material for its excellent optical properties (in-line transmittance) in the visible to mid-infrared ranges.1,2 It is currently considered as a cost effective alternative to monocrystalline sapphire for the manufacturing of infrared-domes, intended to be mounted on the new generation of high speed air-to-air/ground-to-air missiles coming onto the market. However, most of the polycrystalline materials developed up to now exhibit a grain size in the 10–150 micrometers range, explaining the disappointing mechanical/thermomechanical properties reported.3,4 A different approach has recently proved possible to obtain fully dense polycrystalline spinel with a grain size well below the micrometer,5,6 using a sinter/ hot-isostatic pressing (HIP) strategy. a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0243 J. Mater. Res., Vol. 24, No. 6, Jun 2009

http://journals.cambridge.org

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To simultaneously limit grain growth and obtain nearly maximum densification, spark plasma sintering (SPS) has been successfully applied to other materials, such as TiN,7 Al2O3,8,9 Si3N4,10 3, and 8 mol% yttria-stabilized ZrO211,12 and b-SiC.13 Dense polycrystalline spinel with acceptable optical pro