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Drying step optimization to obtain large size transparent magnesium-aluminate spinel samples Lucile Lallemant, Johan Petit ONERA, The French Aerospace Lab, 29 avenue de la Division Leclerc, 92320 Chatillon, France. Introduction Applications Armor windows Aerospace windows missile IRdomes High power lasers Transparency in the visible & IR ranges High mechanical resistance: toughness, hardness sand, hail, bullet, birds,… High temperature resistance: Supersonic missiles Chemical resistance Rain erosion Glass complex shape processing weak thermomechanical resistance Single crystal Difficult processing into complex shapes High thermomechanical resistance Transparent ceramics complex shape processing high thermomechanical resistance Spinel MgAl 2 O 4 Optically isotropic high thermomechanical resistance General process Slurry D 50 ~110 nm ~50wt% into water Green body processing through a nanoporous filter Drying: temperature and hygrometry control HIP 1500㼻C 2000 bar 3h Natural sintering 10 -6 mbar 1500㼻C – 2h Annealing under air or O 2 1200㼻C – 10h Drying optimization Theory Water concentration gradient internal stresses microcracks during drying large cracks during sintering Finite element method Minimization of the water concentration gradient Minimization of the final water content Minimization of the process duration t vol C , max Definition of the water activity: P v = a w * P sat 1 1 B w X A a Adjustment of A and B parameters Fitting experimental curves Optimization of the drying program Other materials Results and Characterizations 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 14.0 14.5 15.0 15.5 16.0 16.5 17.0 Low transparency Thermal conductivity (W/Km) Grain size (micrometers) High transparency 200 300 400 500 600 700 800 900 1000 0 10 20 30 40 50 60 70 80 90 100 Real Transmission (7.6 mm) Transmission adjusted for 1 mm Transmission % (nm) 75xh10 mm Influence of the grain size and residual porosity on the thermal conductivity residual porosity effect on the transparency Alumina Yttria Excerpt from the Proceedings of the 2016 COMSOL Conference in Munich
