Glass-ceramic

Abstract

The invention relates to a glass and a glass-ceramic comprising beta-quartz and/or keatite solid solutions, and to a process for their production, and to their use as substrate material for coating. Glass-ceramic comprising beta-quartz and/or keatite solid solutions with a surface roughness without polishing of Ra<50 nm, a thermal expansion in the temperature range between 20° C. and 300° C. of <1.2·10 −6 /K, a transmission in the near infrared region at 1050 nm of >85% for a 4 mm thickness, and a composition in % by weight, based on the total composition, containing: Li 2 O 3.0-5.5   Na 2 O 0-2.5 K 2 O 0-2.0 Σ Na 2 O + K 2 O 0.5-3.0   Σ MgO + ZnO <0.3 SrO 0-2.0 BaO 0-3.5 B 2 O 3 0-4.0 Al 2 O 3 19.0-27.0   SiO 2 55.0-66.0   TiO 2 1.0-5.5   ZrO 2 0-2.5 Σ TiO 2 + ZrO 2 3.0-6.0   P 2 O 5 0-8.0 Fe 2 O 3 <200 ppm F 0-0.6 as substitute for O and, if appropriate, at least one refining agent, such as As 2 O 3 , Sb 2 O 3 , SnO 2 , CeO 2 , sulphate and chloride compounds.

1 - 21 . (canceled)  
     
     
         22 . A process for producing a vitreous substrate material, which can be converted into a glass-ceramic comprising beta-quartz and/or keatite solid solutions, for coating with a mirror coating, the shaping taking place via a feeder, in which a molten drop of defined weight is added to a pressing die, and the parabolic contour of the substrate material is pressed using a ram with a smoothed surface.  
     
     
         23 . The process according to  claim 22 , in which the conversion of the pressed vitreous substrate material into the glass-ceramic containing beta-quartz solid solution as the main crystal phase takes place at nucleation temperatures of 63° C. to 750° C., for a duration of >15 min and the crystallization takes place at temperatures of 700° C. to 850° C. for a duration of >30 min.  
     
     
         24 . The process according to  claim 22 , in which the pressed vitreous substrate material is converted into a glass-ceramic with keatite solid solution as the main crystal phase at temperatures of from 780° C. to 1000° C.