The application of silicon carbide (SiC) in large-aperture optical mirrors primarily benefits from its high specific stiffness, excellent thermal stability.
Advantages:
High Hardness: Silicon carbide (SiC) has a hardness of 20-30 GPa (Vickers 6-7), making it highly resistant to wear and mechanical damage.
Excellent Thermal Conductivity: With a thermal conductivity of 370 W/(m·K), SiC efficiently dissipates heat, which is crucial for high-power optical applications.
Low Thermal Expansion: The thermal expansion coefficient of SiC is 4.3x10^-6/°C, which minimizes thermal distortion and maintains optical precision under varying temperatures.
High Maximum Operating Temperature: SiC can withstand temperatures up to 1600°C, making it suitable for high-temperature environments.
Chemical Stability: SiC is highly resistant to corrosion from high temperatures, acids, and alkalis (except hydrofluoric acid), ensuring durability in harsh conditions.
Lightweight: SiC is approximately 70% lighter than traditional optical materials, which is beneficial for the weight-sensitive applications.
Optical Transparency: SiC has good optical transparency, particularly in the ultraviolet (UV) range, making it suitable for UV optical applications.
Precision Polishing: SiC can be polished to a high degree of precision, which is essential for high-quality optical surfaces.
Characteristic | Silicon Carbide (SiC) | Glass Ceramics | Fused Silica |
Material Type | Ceramic/Semiconductor | Glass Ceramic | Amorphous Silica |
Hardness | 20-30 GPa (Vickers) | Mohs 6-7 | Mohs 7 |
Thermal Conductivity | 370 W/(m·K) | 1.5 W/(m·K) | 1.4 W/(m·K) |
Thermal Expansion Coefficient | 4.3×10-6/℃ | ≈0 (zero expansion) | 5.4×10-7/℃ |
Maximum Operating Temperature | 1600℃ | 800℃ | 1100℃ |
Chemical Stability | Acid and alkali resistant (except HF) | Resistant to high-temperature corrosion | Acid resistant (except HF), may soften at high temperatures |
Lightweight Capability | High (70% lighter) | Weak | Weak |
Optical Performance | Requires coating | Good light transmission | Best in UV range |
Processing Difficulty | Complex | Can be precision polished | Easy to process but brittle |
Typical Applications | semiconductors | Telescopes, gyroscopes | UV optics, laboratory ware |
Cost | High | Medium | Low |
Technological Breakthroughs in Manufacturing
Mirror Blank Preparation: Using a colloidal forming process akin to "making tofu," micrometer-sized silicon carbide powder is shaped into mirror blanks, supporting complex lightweight structures. The three preparation process routes are illustrated in the figure below:
Process Route | Reaction Bonded Sintering (RB-SiC) | Chemical Vapor Deposition (CVD-SiC) | 3D Printing + CVD Composite Forming |
Raw Materials | Silicon Carbide Powder + Carbon Powder + Binder | Gaseous Precursor (CH₃SiCl₃/H₂) | Silicon Carbide Slurry / Photosensitive Resin + SiC Nanopowder |
Forming Method | Compression / Injection Molding → High-Temperature Sintering (1600-2000°C) | Gas Phase Deposition on Graphite Substrate (1200-1400°C) | Photocuring / Laser Sintering Forming → CVD Densification |
Lightweight Structure | Mechanical Machining of Honeycomb Holes (Weight Reduction: 50%-60%) | Direct Deposition of Honeycomb / Foam Structure (Weight Reduction: 70%-80%) | Topology Optimization Design + Hollow 3D Printing (Weight Reduction: >80%) |
| 92%-95% (Residual Pores Need Impregnation Filling) | >99.9% (Fully Dense) | 85% (Printed Parts) + CVD Densification to 99% |
| After Sintering: Ra ~1 μm (Requires Mechanical Polishing to Ra <5 nm) | Deposited Layer: Ra ~10 nm (Requires Ion Beam Polishing to Ra <0.5 nm | Printed Layer: Ra ~20 μm (Requires CVD Layer Polishing to Ra <1 nm) |
| 4.5×10⁻⁶/°C (Slightly Higher than CVD-SiC) | 4.3×10⁻⁶/°C (Isotropic) | Comparable to CVD-SiC |
Typical Dimensions | Diameter ≤2 meters (Limited by Sintering Furnace) | Diameter ≤4 meters (Sectional Deposition and Splicing) | Theoretically Unlimited (Modular Printing and Splicing) |
