GB1419068A - Glass ceramics - Google Patents

Abstract

1419068 Glass ceramics TRADE & INDUSTRY SECRETARY OF STATE FOR 15 Dec 1972 [17 Dec 1971] 58629/71 Heading C1M A glass ceramic material having a lifetime at 1100‹ C. of at least 20 hours to 0À2% strain and/ or at least 60 hours to 0À3% strain under a bend stress of 77 MN/m.<SP>2</SP> is formed from a composition comprising, by weight, 40-55% SiO 2 , 25- 37% Al 2 O 3 , 8-15% MgO, 0-5% ZnO, 4-14% TiO 2 , 0-10% ZrO 2 , not greater than 0À15% CaO, not greater than 0À2% Na 2 O, not greater than 0À3% K 2 O, and not greater than 0À2% Li 2 O, wherein the total amount of CaO, Na 2 O, K 2 O and Li 2 O is not greater than 0À3%, the balance, if any, consisting of other compatible ingredients. These levels of alkali metal and calcium are low compared with what is normal in the constituent raw materials and accordingly it is necessary in the production of the glass ceramics to use raw materials which permit the overall concentration of alkali metal and calcium (in the products) to fall within the stated limits. The glass ceramic may optionally contain Fe 2 O 3 , Cr 2 O 3 , WO 3 , CeO 2 or ThO 2 . A typical heat treatment used for producing the glass ceramic is to heat at 800‹ C. for 4 hours, raise the temperature at 4‹ C./min. to 1040‹ C., then raise the temperature at 1À5‹ C./min. to 1280‹ C., and finally to hold at that temperature for 1 hour. Numerous examples of specific glass ceramic materials are disclosed. Comparative examples disclose compositions containing, in per cent by weight, 0À4% Na 2 O, 0À4% K 2 O, 0À2% CaO and 0À35% CaO.