Sapphire Properties: Thermal Conductivity, Crystal Structure & Data
Typical properties of sapphire wafers and substrates, plus Valley Design's sapphire finishing services
Sapphire is single-crystal aluminum oxide (α-Al₂O₃) with a trigonal crystal structure, usually indexed on hexagonal axes. Sapphire wafers and substrates are available in C, R, A and M plane orientations.
Sapphire is single-crystal aluminum oxide (α-Al2O3), prized for extreme hardness (Mohs 9), a melting point above 2,000°C, strong electrical insulation, and optical transparency from the ultraviolet to the mid-infrared. This page collects typical sapphire properties for wafers, substrates, and windows. Valley Design does not grow sapphire — we provide the lapping, polishing, thinning, and dicing services that finish sapphire to your specification.
Values below are typical figures. Sapphire properties vary with growth method, purity, crystal orientation, and temperature, and suppliers publish slightly different numbers. Confirm critical values with your crystal supplier’s datasheet.
Sapphire at a glance
| Chemical formula |
Al2O3 (single crystal, α-phase) |
| Crystal system |
Trigonal (rhombohedral lattice), space group R-3c |
| Density (20°C) |
3.98 g/cm3 |
| Melting point |
≈ 2,040–2,054°C (3,700–3,730°F) |
| Hardness |
Mohs 9; Knoop ≈ 1,800–2,200 depending on orientation |
| Thermal conductivity (room temperature) |
≈ 25–35 W/(m·K) |
| Transmission range |
≈ 0.15–5.5 µm |
Crystal structure and wafer orientations
Sapphire has a trigonal crystal structure with a rhombohedral lattice. It is normally described on hexagonal axes, with lattice constants of approximately a = 4.758 Å and c = 12.991 Å. Because the structure is anisotropic, many properties differ parallel to and perpendicular to the c-axis. Sapphire wafers and substrates are cut in four common orientations:
| Plane |
Miller index |
Typical use |
| C-plane (0°) |
(0001) |
Most common orientation; GaN epitaxy for LEDs and laser diodes; optical windows |
| R-plane |
(1-102) |
Silicon-on-sapphire (SOS) for RF integrated circuits and pressure sensors |
| A-plane (⊥ to C) |
(11-20) |
Hybrid microelectronics; windows where low birefringence is needed |
| M-plane (⊥ to C and A) |
(10-10) |
Specialty epitaxy and research |
See our sapphire wafers and substrates and C-plane sapphire windows, plates, and disks pages.
Mechanical properties of sapphire
| Density (20°C) |
3.98 g/cm3 |
| Hardness |
Mohs 9; Knoop ≈ 1,800–2,200 |
| Tensile strength |
275–400 MPa (40,000–58,000 psi) |
| Flexural strength |
480–895 MPa (70,000–130,000 psi) |
| Compressive strength (ultimate) |
2.0 GPa (300,000 psi) |
| Young’s modulus |
≈ 335–435 GPa (~345 GPa typical) |
| Bulk modulus |
250 GPa (36 × 106 psi) |
| Shear modulus |
145 GPa (21 × 106 psi) |
| Modulus of rupture |
350–690 MPa (50,000–100,000 psi) |
| Poisson’s ratio |
0.25–0.30 |
| Melting point |
≈ 2,040–2,054°C |
Thermal properties of sapphire
| Thermal conductivity, room temperature |
≈ 25–35 W/(m·K); higher parallel to the c-axis |
| Thermal conductivity, ⊥ to c (23°C / 77°C) |
23.0 / 16.8 W/(m·K) |
| Thermal conductivity, ‖ to c (24°C / 70°C) |
25.8 / 17.35 W/(m·K) |
| Specific heat (18°C) |
≈ 756 J/(kg·K) |
| Specific heat (−182°C) |
104 J/(kg·K) |
| Thermal expansion (~20°C), ⊥ to c |
≈ 5.0 × 10-6 /K |
| Thermal expansion (~20°C), ‖ to c |
≈ 5.6 × 10-6 /K |
Thermal conductivity falls steeply as temperature rises. Thermal-expansion figures vary by supplier and temperature range.
Electrical properties of sapphire
| Volume resistivity (25°C) |
> 1014 to 1016 Ω·cm, depending on purity |
| Dielectric strength |
≈ 4.8 × 105 V/cm (48 kV/mm) |
| Dielectric constant, ⊥ to c |
9.4 |
| Dielectric constant, ‖ to c |
11.5 |
| Dissipation factor (tan δ) |
≈ 10-4 |
Need sapphire wafers or windows finished to spec?
Reach out to our Technical Team Request a Quote
Valley Design’s sapphire finishing services
Valley Design does not grow sapphire. We finish sapphire that is customer-supplied or sourced elsewhere, including:
- Sapphire wafer back-thinning and ultra-thin sapphire wafers and windows
- Precision lapping and polishing
- Dicing and edge grinding, edge rounding, chamfering, and diameter reduction
- Hole drilling, slots and steps, v-grooves, and surface pyramid structures
- Blending and polishing, and laser slag removal
Have a sapphire part in mind? Request a quote and our technical team will help scope the work.
Frequently asked questions about sapphire
What is the thermal conductivity of sapphire?
At room temperature, sapphire’s thermal conductivity is roughly 25–35 W/(m·K), slightly higher along the c-axis than perpendicular to it. It drops steeply as temperature rises, so published values depend on the test temperature, purity, and orientation.
What is the melting point of sapphire?
Sapphire melts at approximately 2,040–2,054°C (about 3,700–3,730°F). Suppliers and references quote slightly different values within this range.
What is the crystal structure of sapphire?
Sapphire is single-crystal α-Al2O3 with a trigonal structure (space group R-3c). It is usually indexed on hexagonal axes with lattice constants of about a = 4.758 Å and c = 12.991 Å.
What is the difference between C-plane, A-plane, R-plane and M-plane sapphire?
They are wafers cut at different angles to the crystal’s c-axis. C-plane (0001) is the most common and is used for GaN LEDs and optical windows. R-plane (1-102) is used for silicon-on-sapphire, A-plane (11-20) for hybrid microelectronics and low-birefringence windows, and M-plane (10-10) for specialty epitaxy.
How hard is sapphire?
Sapphire is Mohs 9, second only to diamond among common materials, with Knoop hardness of roughly 1,800–2,200 depending on orientation. That hardness is why it needs diamond-based lapping and polishing to finish.
Can Valley Design thin, dice, lap or polish sapphire wafers?
Yes. Valley Design finishes sapphire wafers and substrates supplied by the customer or sourced elsewhere, including back-thinning, edge grinding and rounding, dicing, drilling, lapping, and polishing. Request a quote for your specification.
Does Valley Design manufacture sapphire?
No. Valley Design does not grow sapphire crystals. We are a finishing partner for sapphire that is already grown and cut.
Related sapphire resources
Ready to get started with your next project?
Reach out to our Technical Team Request a Quote