Development of porous ceramic moulds for hot slumping of astronomical glass mirror substrates

The key goal of this project was to develop a porous ceramic that could be used as a mould material for the heat bending of glass. Heat–bent —- slumped —- glass has a wide range of uses, from miniature lenses for smartphone cameras or elegantly shaped touch displays to precisely shaped mirrors. The project PräBieD (Präzisionsbiegen von Dünnglas, “precise bending of thin glass”) brought together different stakeholders from the fields of science and industry. The outcome of their cooperation was an exemplary mould for shaping glass segments for the experimental Wolter I X-ray telescope. Compared, for example, to nickel (XMM) or Zerodur (Chandra), telescope segments of this kind could have a much lower weight and thickness, and therefore also a higher collecting area. The most important requirements in terms of the material are a coefficient of thermal expansion close to that of the glass so as not to disturb the required shape during the hot slumping process, chemical and heat stability to prevent mirror contamination by outgassing during slumping, no adhesivity between the glass and the mould, porosity so that glass bending can be supported by pumping through the mould’s surface, and the possibility to machine the surface with sufficient accuracy. This paper briefly describes the process of manufacturing ceramics and the features of the material developed as they are revealed during its evolution.