What is Sapphire?
Sapphire is a form of Aluminum Oxide, one of nature's most abundant compounds. In its natural state, Aluminum Oxide (Al2 O3) is a white powdery material used extensively as an industrial abrasive. When heated to about 2050°C (almost 4000°F) the powder melts and can then be formed into a single crystal using any of several crystal growth methods. Because it is a single crystal, sapphire can not be molded, drawn or cast. It must be "grown" into a specific shape as dictated by the selected growth process. Synthetic, or man-made sapphire has the same single crystal rhombohedral structure as the natural gemstone, however, it is of a much higher purity and is water clear. The beauty and color of sapphire gemstones are a result of the impurities they received when the earth's crust was solidified.
While some crystal growth processes yield near net shapes, almost all sapphire components have to be fabricated from these shapes by various cutting, grinding and polishing operations. Guild Optical does not produce the raw sapphire. Our expertise is in the precision fabrication of the material as supplied by our selected worldwide growers. We select the most cost-effective material for the particular application or we work with customer furnished sapphire when requested.
Benefits Of Sapphire Optics
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 Sapphire is:
- • 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
Sapphire Properties
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
Optical Properties:
- • Refractive Indices:
- • Ordinary Ray (No) (C-axis direction): 1.768
- • Extraordinary Ray (Ne) (perpendicular to C-axis direction): 1.760
- • Birefringence (Ne - No): 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/mm2, 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 (m½)
- • 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: 1016 ohm-cm @ 25°C, 1011 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 3x109 Hz
- • Loss Tangent 25°C: 3.0 - 8.6x10-5; between 10 Hz and 3x109 Hz
- • Magnetic Susceptibility: -0.21x10-6 to -0.25x10-6
Physical Properties:
- • Density: 3.98 g/cm3, (0.143 lb/in3)
- • 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
NOTE: These sapphire properties may be dependent upon relative crystal orientation, form and surface quality
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
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. Guild Optical Associates, Inc. does not guarantee the accuracy or appropriateness of this data for any application.
