In large mirrors used in astronomical applications and mirrors used in low-temperature environments, low-thermal expansion glass (ULE®) or glass ceramic (ZERODUR® or CLEACERAMTM-Z) is generally used because the mirror shape is not deformed due to changes in environmental temperature. In recent years, low-thermal expansion ceramic cordierites have been attracting attention as a new material. This is because it exhibits high thermal conductivity and rigidity while having the same linear expansion coefficient (CTE) as low-expansion glass and low-expansion glassceramic. CTE is the most important thermal property that indicates the usefulness of low-expansion mirror materials, but uniformity of CTE within the material is also an important property, especially in large glasses. This is because the inhomogeneity of CTE generates internal thermal stress of the material, which appears as an error in the surface shape on the mirror surface. Therefore, we investigated the inhomogeneity of the CTE of Fine cordierite (CO220O, CO720O from Kyocera Corporation), which has not been clarified so far. After producing a Fine Cordierite mirror and dividing it into sample pieces, the CTE value of each sample was measured using a laser thermal dilatometer. The structure function at 22 °C was calculated and the inhomogeneity of CTE was evaluated.
Cordierite is a ceramic with low thermal expansion properties similar to Zerodure and ULE glasses in large-mirror telescopes used in Astronomy. Coefficient of thermal expansion of Cordierite is not only comparable to low thermal expansion glasses, but also has additional advantages with minimal dimensional deformation with time (~3.8nm/m/year), high Young's modulus (~145GPa), and high thermal conductivity (~4W/mK). However, Cordierite mirror is susceptible to voids generated by crushed grains leading to tetrahedral gaps from sphere packing of raw powder material during CIP (Cold Isostatic Pressing) process. These voids often appear with mirror polishing of Cordierite and may become a source of light scattering. To solve this problem, we propose Flexible Molding (Fmolding) as our new fabrication method for Cordierite. With F-molding, raw material powder is converted to liquid slurry which is then poured into a resin mold and solidified directly into near net shape of a mirror. Liquid slurry of Fmolding are thus cured in forming without pressurized sphere packing which generates void defects. In addition, the mold prepared from F-molding is an effective tool for processing multiple volumes of identical structures and is an ideal process for segmented mirror designs. To confirm the feasibility of F-molding for large mirrors used in Astronomy, we prepared a light-weighted flat mirror (270 mm in diameter) using the method with our Cordierite. In this paper, we introduce details of our F-molding mirror fabrication method, and report on measurement results of its optical performances.
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