Glass ceramic ZERODUR®: Even closer to zero thermal expansion: a review, part 2

Abstract

Observational astronomy has sought better telescopes with higher resolution from its beginning. This needs ever-larger mirrors with stable, high-precision surfaces. The extremely low-expansion glass ceramic ZERODUR ® has enabled such mirrors for more than 50 years with significant improvements in size and quality since then. We provide a survey of the progress achieved in the last 15 years. Equally important as the thermal expansion coefficient CTE is its homogeneity. The CTE variation in 4-m mirror blanks lies below 5 ppb∕K in radial and axial directions on large and short scales. Improved measurement capabilities allow reduced bias, which in the past made variations look greater than they were. Isotropy and uniformity of ZERODUR are outstanding. A method for lifetime calculation increases reliability considerably with respect to mechanical loads. The production and metrology capability and capacity are greatly extended. Surface figure and texture of large blanks allow starting directly with polishing. Filigree structures with up to 90% weight reduction are well-suited for space mirrors. The progress with low thermal expansion and its measurement, the insensitivity of ZERODUR against ionizing radiation in space, and outstanding application examples are presented in the first part of our review. © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Comprehensive review of the effects of ionizing radiations on the ZERODUR® glass ceramic

“…Macroscopically the interplay between these two coexisting phases results in material having a net macroscopic CTE close to 0 ppm.K−1 over temperature range that may be centered on T=20°C and spanning tens of degrees 1 . This supports form stability under thermal perturbations, but furthermore provides the critical features of high homogeneity, high isotropy and availability in large monolithic lightweigthed forms made of ZERODUR ® the material of choice for sensitive telescope mirrors, typically in orbiting missions 5 , 6 . For these reasons ZERODUR ® continues to be a widely employed material in space where high stability is needed.…”

“…1 This supports form stability under thermal perturbations, but furthermore provides the critical features of high homogeneity, high isotropy and availability in large monolithic lightweigthed forms made of ZERODUR ® the material of choice for sensitive telescope mirrors, typically in orbiting missions. 5,6 For these reasons ZERODUR ® continues to be a widely employed material in space where high stability is needed. It is a critical part of more than 40 orbiting missions 2,3 among these two NASA great observatories, Hubble-M2 and all Wolter mirrors on Chandra.…”

Ultralow Expansion Glass as Material for Advanced Micromechanical Systems

“…Different kinds of deep etched cantilevers before and after the releasing step are shown in Figure 3. Here, scanning electron Temperature dependence of the coefficient of thermal expansion (CTE) of different materials for micromechanical systems (left; fused silica, [36] Zerodur, [6] ULE glass, [20] and undoped Si [37,38] ). Temperature dependence of Young's modulus of different materials for micromechanical systems (right; fused silica, [39] Zerodur, [40] ULE glass, [41] and Si <100> to <111> [42] ).…”

“…Notably, most applications of ultralow expansion (ULE) materials are so far still linked to macroscopic systems such as mirror substrates for astronomical applications, components of nanopositioning and nanomeasuring tools or as glass scales. [4][5][6] ULE materials, such as ULE glasses from Corning as well as the glass-ceramic Zerodur from Schott are hitherto rarely used for microscale applications, which originates mainly from a lack of capable manufacturing techniques that provide a superior patterning accuracy and reproducibility at the microscale. With our preliminary work on the reactive ion etching (RIE) of complex glasses and glass-ceramics, we established a technology for the fabrication of deep etched microstructures in glass with lithographic precision up to released micromechanical elements in the glass-ceramic Zerodur, with some restrictions in structure geometry and quality.…”

Towards new zero-thermal-expansion materials: Li-free quartz solid solutions stuffed with transition metal cations

“…This combination rapidly led to the development of zero-thermal-expansion glass-ceramic materials, which are still today essential for applications spanning from transparent fire-viewing windows to telescope mirror substrates [11,12]. Between 0 °C and 50 °C, commercially available materials based on Qss such as Zerodur ® can achieve a coefficient of thermal expansion (CTE) as low as (0 ± 0.007) x 10 -6 K -1 , with spatial variations that are less than 0.005 x 10 -6 K -1 over the whole volume of a telescope mirror blank with a diameter of 4 meters [13,14]. Indeed, the thermal expansion of Qss can be finely tuned, taking advantage of their variable Al/Si ratio and of the full solid solution [8,9,15,16] existing between: (i) Mgbearing Qss, exhibiting trigonal symmetry and positive thermal expansion [17][18][19][20][21]; (ii) Li-and Znbearing Qss, which were reported to be fully hexagonal (at a sufficiently high level of chemical stuffing) and to contract upon heating [8,12,[22][23][24][25][26][27].…”