Advanced Ceramics: A View From Space
Key PropertiesPrecision optical mirrors must have extremely fine surface finishes to avoid image distortion. It has been reported that chemically vapor deposited (CVD) SiC can be polished to an ~3 A RMS finish and hot pressed SiC to an ~5 A RMS finish. These are both high quality finishes. While the native reflectivity of SiC may be adequate for some applications, SiC mirrors are most often used as substrates with a thin reflective coating of gold or chrome/nickel bond coat, which is then coated with gold. These coatings are frequently sputtered on and range from 500 to 2500 A thick. CVD SiC has yielded a reflectivity of >98% with a gold coating.
The weight of anything that goes into space is of critical importance to minimize cost and fuel requirements. Similarly, the minimization of distortion due to temperature changes, temperature gradients or inertial loads is critical in optical systems for spacecraft. Key properties include density (r), Young's Modulus (E), and a low value of thermal expansion (CTE, or a). Additionally, several "figures of merit" used in space optics design relate fundamental materials properties to behavior. The most important of these are: the thermal distortion parameter (k/a [thermal conductivity/a]), which identifies materials that will minimize thermal stress and thermal distortion of the optical train; and the specific modulus, also known as the inertia loading parameter (E/r), which helps to identify materials that will minimize deflection under inertial loading (acceleration ).
While beryllium is significantly lighter than SiC, the specific moduli of the two materials are very close. It is the higher value of SiC's thermal distortion parameter that is a major factor in its expanding use.
Athermal Optical SystemsIf an optical structure is composed of several different materials, temperature fluctuations will lead to misalignment of the optics, degrading performance. However, if the same material is used for both the optics and the optical structure, CTE matching results and alignment can be maintained. Further, with the high thermal stability of SiC, distortions due to thermal gradients or transients will be minimal. Such systems can be aligned at room temperature and maintain alignment down to the low temperatures encountered in space. NASA's Miniature Integrated Camera and Spectrometer (MICAS), which provides a suite of optical remote sensing instruments, is an example of such an athermal design. Both the optics and the structure are made of SiC.
DS-1, launched in early 1999, integrates four optical instruments covering the spectral range from 80 nm in the ultraviolet (UV) to 2.4 m in the infrared (IR). In addition to two cameras operating in the visible wavelengths, the DS-1 features two spectrometers (one UV and one IR), all of which share common SiC mirrors and structure. Use of this common optical train is facilitated by the athermal behavior provided by the SiC components.
The payoff of such integrated optics for future spacecraft can be seen by comparing the power and weight consumed by the essentially equivalent optical systems on the DS-1 and the Cassini (launched in late 1997) spacecraft, which used more conventional optical designs and materials. The four separate instruments used on the Cassini craft collectively have a weight of 110 kg. By contrast, MICAS weighs only 12 kg. The instruments on the Cassini require ~100 W of power, while the MICAS requires only 8 W.