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Optical Sapphire is:

A synthetic sapphire
A duplication of natural sapphire chemically, physically and optically, but without nature's contaminants or inclusions
Water clear
A triagonal, (hexagonal) crystal system
A negative, uniaxial crystal

Major Advantages:

Hardest natural substance next to diamond

Much stronger than other optical materials

Extreme surface hardness

Highly resistant to scratching and abrasion

Very wide optical transmission band from UV to near-IR

Extremely high melt temperature

High electrical resistance

Chemically inert

Totally unaffected by all chemicals except some very hot caustics

High thermal conductivity for a non-conductor, even better than copper at cryogenic temperatures

High dielectric constant

Optical Properties:

Refractive Indices:

Ordinary Ray (N

_o) (C-axis direction): 1.768

Extraordinary Ray (N

_e) (perpendicular to C-axis direction): 1.760

Birefringence (N

_e - N

_o): 0.008

Temperature Coefficient of Refractive Index: 13x10

^-6/°C (@ 0.57 µm, 20°C)

Transmittance: >85% 0.3-4.0 µm (@ 0.1 mm thick) uncorrected

Emissivity @ 3,4,5 µm: 16%, 25%, 70% (@500°C)

Absorption @ 0.66 mm @ 1600°C: 0.1 - 0.2 cm

^-1

Mechanical Properties:

Hardness (9 Mohs std): 2000 kg/mm

², Knoop

Coefficient of friction: 0.14 (on steel)

Young's Modulus: 400 GPa @ 20°C

Poisson's Ratio: 0.29

Compressive Strength: 2.0 GPa

Creep @ 100 Mpa, 1600°C: 1.5x10

^-4/hr

Fracture Toughness: 2.0 MPa (m

^½)

Flexural Strength: 900 MPa

Bulk Modulus: 2.4 GPa

Shear Modulus: 175 GPa (Rigidity Modulus)

Tensile Strength: 300 to 400 MPa

Rupture Modulus: 65 - 100,000 psi

Electrical Properties:

Bulk Resistivity: 10

¹⁶ ohm-cm @ 25°C, 10

¹¹ ohm-cm @ 500°C

Dielectric Strength: 48 kv/mm, (1.2kv/mil)

Dielectric Constant 25°C: 9.4 perpendicular to the c-axis, 11.6 parallel to the c-axis between 10Hz and 3x10

⁹ Hz

Loss Tangent 25°C:
3.0 - 8.6x10

^-5; between 10 Hz and 3x10

⁹ Hz

Magnetic Susceptibility:
-0.21x10

^-6 to -0.25x10

^-6

Physical Properties:

Density: 3.98 g/cm

³, (0.143 lb/in

³)

Hardness: Knoop microindenter: 1800 FACE perpendicular to c-axis, 2200 FACE parallel to c-axis

Young's Modulus: 400 GPa @ 20°C

Tensile Strength: 300 to 400 MPa

Point Group; Symmetry:3 2/m; [C,1A3, 3A2, 3P]

Lattice Dimensions: a = 4.748 Angstroms, c = 12.957 Angstroms

Sound Speed: ~10 km/s

Chemical Properties:

Non-porous, unlike alumina ceramics
Unaffected by weathering
Unaffected by hydration
Virtually unaffected by any solvents or acids at room temperature. (Some etching by hot phosphoric acid and strong caustics at temperatures exceeding 600°C - 800°C)

Thermal Properties:

Melting Point: ~2053°C (3727°F)
Most properties useful to (maximum): ~1800°C (3272°F)
Conductivity: 40 W/M°K @ 298°K
Expansion @ 25°C: 4.5x10^-6/K^-1 and @ 1000°C: 9.0x10^-6/K^-1 (90° orientation)
Specific Heat Capacity: 750 J/K at 300°K
Viscosity @ 2053°C: 0.0584 Pa·s

Crystal Facts:
The angular relationship between the inherent optical axis of the crystal and the required part is known as orientation. Typical choices for part orientation are:

Zero Degree: The direction of view is parallel to the optical axis of the crystal
90 Degree: The direction of view is perpendicular to the optical axis of the crystal
C-Axis: In a rod, the direction along its length. In a window, the direction perpendicular to the face
M-Plane: The plane containing the optic axis (C) and inclined 30 degrees to the A-axis
A-Plane: The plane that is perpendicular to the A-axis, containing the C-axis
R-Plane: A plane inclined 57.5667 degrees to the optic axis and in the same zone as the M-plane
Random: There is no specified relationship between the part and the crystalline orientation. The part is manufactured without concern about orientation

Sapphire Grades:
Sapphire has an infinite number of grades:
Grades are entirely arbitrary and are decided upon after inspection of each synthetic sapphire batch which has been grown.
Synthetic sapphire is graded by what is important for a particular application, either optical or mechanical.
A high grade of sapphire would have little or no light scatter or lattice distortion and be used mainly for the most demanding optical applications.
A lower grade of sapphire may have extensive light scatter or lattice distortion, being used mainly for mechanical and structural uses such as bearings, fixtures, and less demanding optical applications.
An ultraviolet (UV) grade sapphire or non-browning sapphire will not solarize on exposure to UV light.
An infinite number of grades fall between the high and lower synthetic sapphire grades, with each sapphire manufacturer giving a name to their own grades.

* NOTE: These sapphire properties may be dependent upon relative crystal orientation, form and surface quality.

The information on this page has been compiled from numerous sources and is intended only as a reference, not a recommendation, for the use of sapphire. Runlan Sapphire Crystals Mfg. Co. does not guarantee the accuracy or appropriateness of these data for any application.