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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