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PELCO® Sapphire Substrate Discs, Technical Information

Crystal Structure:
Sapphire is an anistrophic crystal, hexagonal system, composed of unicrystalline alpha aluminum oxide, essentially 100% pure. Various properties are a function of crystallographic direction (related to the optic axis of the crystal). Sapphire discs are made from sapphire rods, the c-axis of the hexagonal unit cell is in the longitudinal direction. In the tables below, if no orientation is shown, this indicates that the property listed does not vary appreciably in relation to orientation or the variation is less than the experimental error of measurement.

Hexagonal Unit Cell of Sapphire
Hexagonal Unit Cell of Sapphire

Transmission:
Transmission of synthetic sapphire is shown in the following curve. Data in UV region is approximate, as transmission depends on surface finish, internal quality and purity of individual specimen. The following curve shows transmission of sapphire uncorrected for Fresnel losses.

Percent transmission

Crystaline Structure: Rhombohedral Single Crystal Young's Modulus: 50 to 55,000,000 PSI
Hexagonal System: A=4.763Å   C=13.003Å

Melting Point: 2040°C

Density: 3.97 g/cm³ 		Bending Modulus (Minimum):
20° C 60,000 PSI
500° C 40,000 PSI
1000° C 60,000 PSI
Refractive Index: 20° C
300nm 1.814
400nm 1.785
700nm 1.763
1000nm 1.757
2000nm 1.740
3000nm 1.713
4000nm 1.677
5000nm 1.623
Thermal Conductivity:
12K (-261° C) = 8.0 cal/cm2/sec/C/cm
300K (23° C) = 0.9 cal/cm2/sec/C/cm
50° C = .07 cal/cm2/sec/C/cm

Coefficient of Expansion
(Mean between 20° C and T) per° C
  Perpendicular to C-axis Parallel to C-axis
50° C .0000050 .0000067
500° C .0000077 .0000083
1000° C .0000083 .0000090
Electrical Resitivity:
20° C 1014 ohm-cm
500° C 1011 ohm-cm
1000° C 109 ohm-cm 		Loss Tangent: < 1 x 10-4 at 1MHz

Hardness: Moh 9, Knoop 1525 to 2000
Dielectric Constant: 11.5 at 1MHz (parallel to c-axis)
9.3 at 1MHz (perpendicular to c-axis)
 
Chemical Resistance:
Inert to virtually all reagents at room temperatures and many at high temperatures. Essentially inert to all acids including HF, and resistant to alkalis but becoming soluble at higher temperatures.
Sealing Characteristics:
Sapphire can be wetted by glass, titanium, zirconium or moly-manganese mixtures. It can be matched to titanium, molybdenium, the high nickel-iron allows such as Carpenter 49, Kovar and the Corning glass 7520. With the good technique, bonds can be made directly to Corning 7052.

As can be seen from the list of properties, sapphire is unique when compared to optical materials useful within its transmission range in that it is by far the strongest, toughest, thermal shock and chemically resistant material available, and it can be used at far higher temperatures than most optical materials. Also, its thermal conductivity is relatively high despite its extreme electrical non-conductivity. Moderate refractive index, transparency in visible region, good transmission and relatively low emission at high temperatures plus unusual stability combine to make it valuable as a component on military optics.