Silicon Carbide Mirrors

14 Feb 2022

Silicon Carbide Mirrors Overview

Silicon carbide (SiC) mirrors are light weight, high strength, high stiffness, high thermal conductivity and thermal stability. Ideal for larger telescopes, high-speed scanning systems and space applications and where instrument weight and thermal stability is important. High thermal stability and good mechanical properties enable silicon carbide (SiC) mirrors to be used in high temperature environments.

Front & back of silicon carbide mirror

At Shanghai Optics we produce high quality custom lightweight silicon carbide mirrors with coating. Our precision machining process results in the highest quality product, and our state of the art metrology equipment ensures that every mirror is well suited to meet even the most stringent requirements.

Understanding Silicon Carbide

A crystal compound in which silicon and carbon are bonded to each other in a tetrahedral structure, silicon carbide (SiC) is an advanced ceramic material that can be used at temperatures up to 1400 C, and boasts a flexural strength of 59465 to 93549 PSI.  Important properties of highly performing silicon carbide include:

• Thermal shock resistance
• Oxidation resistant
• Chemical resistance
• Low thermal expansion
• Low density (lightweight)
• High mechanical strength (Note: it can be brittle)

Manufacturing of Silicon Carbide (SiC)

There are three primary methods used to produced SiC for silicon carbide optics: reaction bonding, sintering, and vapor deposition.

To form reaction bonded SiC a mixture of alpha SiC, a mixture of SiC and C, is cast, dried, and fired. This porous mixture is is infiltrated with liquid silicon. The silicon reacts with the carbon, forming SiC, which fills the empty spaces to form a dense product. Reaction bonded SiC is fairly economical to produce and lends itself well to lightweight optical design.

Sintered SiC is produced when SiC powder is sintered at a temperature above 2000 C with non oxide sintering additives. The procedure is carried out in an inert atmosphere and high pressure of 1000-2000 atmospheres, and produces a very hard ceramic that is suitable for use in optical mirrors and other SiC optics. Sintered SiC is considered a better grade than reaction bonded SiC, although only relatively small and simple mirror shapes can be produced and extensive machining is necessary to eliminate surface roughness.

A third method of producing SiC involves using chemical vapor deposition to produce very pure Silicon Carbide in a face center cubic crystal form. This type of SiC has especially high thermal conductivity, but it is also much more expensive to produce than either of the types of Silicon Carbide.

Physical properties of the compound do depend on the procedure used in manufacturing. For instance, reaction bonded silicon carbide has a Young’s modulus of 330 Gpa and a density of 2.89 g/cm3, while chemical vapor deposited SiC has a Young’s modulus of 465 Gpa and a density of 3.21.

Silicon Carbide Mirrors at Shanghai Optics

Shanghai Optics is able to produce custom silicon carbide mirrors to fit your exact specification requirements for applications such as larger telescopes, high-speed scanning system and space applications. Please contact us for design assistance or to discuss our manufacturing limitations.