Al2O3 Characteristics of the colorless synthetic
sapphire, i.e. corundum single crystal
Physical crystal system hexagonal
Properties chemical formula Al2O3
purity 99.99%
cleavage conchoidal
specific gravity 3.99 ÷ 3.98
dislocation density 10 ÷ 10 /cm

Thermal melting point 2050°C
Properties softening point 1800°C
specific heat at 25°C 0.18cal/g
thermal conductivity at 25°C 0.1 cal/sec . cm . °C
thermal expansion
face to c axis 5.4 . 10/°C
face = to c axis 6.2 . 10/°C

Mechanical Hardness Mohs 9
Properties Knoop scale face to c axis 1800
Knoop scale face = to c axis 2200
modulus of elasticity 4.4 x 10 kg/cm
1% Young E
modulus of rupture at 25°C 4000 kg/cm
compressive strength at 25°C 21000 kg/cm
ultimate tensile strength 1900 kg/cm
chemical resistance unattacked by
common acids
or NaOH

Chemical acids and alkalis attack at 300°C 0
Properties porosity 0

Electrical dielectric constant (permittivity) 7.5 at 10.5
Properties electrical resistance at 500°C 10 ohm/cm
electrical resistance at 1000°C 10 ohm/cm
electrical resistance at 2000°C 10 ohm/cm