Drying step optimization to obtain large size transparent ... · Drying step optimization to obtain...
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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 MgAl2O4 Optically isotropic
high thermomechanical resistance
General process
Slurry D50~110 nm ~50wt% into
water
Green body processing through a
nanoporous filter
Drying: temperature
and hygrometry control
HIP 1500 C 2000 bar
3h
Natural sintering 10-6 mbar
1500 C – 2h
Annealing under air or O2 1200 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
tvolC ,max
Definition of the water activity: Pv = aw * Psat
1
1
B
w XAa 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
Ther
mal
con
duct
ivity
(W/K
m)
Grain size (micrometers)
High transparency
200 300 400 500 600 700 800 900 10000
102030405060708090
100
Real Transmission (7.6 mm) Transmission adjusted for 1 mmTr
ansm
issi
on %
(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