CVD SILICON CARBIDE shells have been fabricated by a scalable chemical vapor deposition vapor deposition process to demonstrate the feasibility of producing thin, lightweight optics for x-ray applications. These shells were produced by depositing SiC on shaped graphite mandrels in a CVD chamber, removing the graphite mandrels from SiC deposit followed with controlled machining to produce the required double cone surface. the size of these shells is as follows: diameter equals 250-600 mm, length equals 240-400 mm and thickness equals 1.5-2.7 mm. The roundness of the inside surface of these shells was the inside surfaces of a few shells by the epoxy replication process. Testing of two SiC shells for x-ray applications yielded a half energy width of < 15 arcsecs over a wide field of view. Important issues involved in near-net-shaping and machining of these shells are discussed.
The 1-m diameter lightweight secondary mirrors (M2) for the Gemini 8-m telescopes will be the largest CVD-SiC mirrors ever produced. The design and manufacture of these mirrors is a very challenging task. In this paper we will discuss the mirror design, structural and mechanical analysis, and the CVD manufacturing process used to produce the mirror blanks. The lightweight design consist of a thin faceplate (4-mm) and triangular backstructure cells with ribs of varying heights. The main drivers in the design were weight (40 kg) and manufacturing limitations imposed on the backstructure cells and mirror mounts. Finite element modeling predicts that the mirror design will meet all of the Gemini M2 requirements for weight, mechanical integrity, resonances, and optical performance. Special design considerations were necessary to avoid stress concentration in the mounting areas and to meet the requirement that the mirror survive an 8-g earthquake. The highest risk step in the mirror blank manufacturing process is the near-net-shape CVD deposition of the thin, curved faceplate. Special tooling and procedures had to be developed to produce faceplates free of fractures, cracks, and stress during the cool-down from deposition temperature (1350 C) to room temperature. Due to time delay with the CVD manufacturing process in the meantime a backup solution from Zerodur has been started. This mirror is now in the advanced polishing process. Because the design of both mirrors is very similar an excellent comparison of both solutions is possible.
The fabrication process, properties and optics applications of transparent and opaque chemical vapor deposited (CVD) (3-SiC are reviewed. CVD-SiC is produced by the pyrolysis of methyltrichlorosilane, in excess H<sub>2</sub>, in a low-pressure CVD reactor. The CVD process has been successfully scaled to produce monolithic SiC parts of diameter upto 1.5-m and thickness 2.5-cm. The characterization of CVD-SiC for important physical, optical, mechanical and thermal properties indicates that it is a superior material for optics applications. Important properties of CVD-SiC are compared with those of the other candidate mirror and window materials. The applications of CVDSiC for lightweight optics, x-ray telescopes, optical baffles, lens molds, optical standards and windows and domes are discussed in detail.
Important properties and applications of CVD SILICON CARBIDE<SUP>TM</SUP> with particular emphasis on high heat loads are reviewed. Data on the mechanical and thermal properties of CVD-SiC as function of temperature are presented. Further, the effect of different high temperature treatments on flexural strength of CVD-SiC is discussed and the thermal shock resistance of CVD-SiC is compared to other competing materials. Finally, several high heat flux applications in the areas of optics, semiconductor processing and wear parts are discussed.
Bidirectional reflectance distribution function has been measured on highly polished uncoated and silver coated samples of CVD SiC in the wavelength range 0.325-10.6 microns to determine the dependence of scatter as a function of wavelength. From these data, total integrated scatter, the power spectral density as function of spatial frequency, and the root mean square surface roughness were calculated. The results indicate that the uncoated CVD-SiC scatter topographically (i.e., follow the lambda exp -4 scaling law) in the wavelength region, 0.325-1.06 micron but not in the region, 1.06-10.6 microns. At 10.6 microns, CVD-SiC exhibits unusually large surface scatter which can be significantly improved by coating CVD-SiC with a thin layer of silver.
In this paper properties of chemical vapor deposited (CVD) SiC and Si optical substrates for use in severe environments are presented. Important data on CVD SiC concerning the elastic modulus polishability scattering measurement thermal and cryogenic stability degradation due to atomic oxygen and electron beam are included. Further scattering measurement data and atomic oxygen degradation effects on CVD Si are also presented. These measurements show that CVD SiC substrates exhibit excellent polishability 1 A RMS) with low scatter good retention of mechanical properties up to 1500 C superior thermal and cryogenic stability (-190 C to 1350 C) and high resistance to atomic oxygen and electron beam degradation. VD Si substrates exhibit excellent polishability 2 A RNS) with low scatter and good resistance to atomic oxygen degradation. These preliminary results suggest that CVD SiC and Si are good optical substrates for severe environment such as outer space lasers combustion and synchrotron x-rays. 1 . 0