GB924996A - Improvements in or relating to devitrified glass ceramies and their production - Google Patents

Abstract

924,996. Ceramic glasses. ENGLISH ELECTRIC CO. Ltd. Sept. 3, 1959 [June 3, 1958; Feb. 26, 1959], Nos. 17688/58 and 6764/59. Drawings to Specification. Class 56. In the manufacture of ceramic glass articles from glass capable of controlled devitrification, glass batch is used containing a phosphate corresponding to 0.5 to 6.0% by weight of phosphorus pentoxide as a nucleating agent and the article is heat treated to devitrify the glass to a predominantly crystalline ceramic glass material. Preferably a metal phosphate is used, of a metal whose oxide forms a major constituent of the glass batch. Major constituents are given as within the following ranges by weight: SiO2 45 to 88 ; Li2O 0 to 27; MgO 0 to 32; Al2O3 0 to 36. Other constituents are given as follows:-Na2O and/or K2O up to 5; ZnO up to 10; SrO, BaO and/or PbO up to 5; CaO up to 10; B2O3 up to 10. For the nucleating agent, orthophosphates of Li, Mg, Al are preferred but orthophosphates of Ca, Zn, Na or K may be used or meta- or pyro-phosphates of the metals. A proportion of the metallic ion, Li or Mg, may be replaced by ammonium or hydrogen ions. Examples of glasses are given. The melted glass batches are shaped as by casting, pressing, drawing or blowing and they may be annealed if required to store before processing. To form the ceramic glass, the articles are heated at 4-5 C. per minute to the softening point and this temperature is maintained for at least one hour to initiate crystallization. In refractory glasses containing Al2O3, the temperature is then raised at 4-5‹ C. per minute to about 40-50‹ C. above the softening temperature and maintained for at least one hour. In all glasses, the heating is then continued at the same rate to the final crystallization temperature and maintained for at least one hour. Final temperatures are given, for certain compositions, from 850‹ to 1200‹ C. Specification 752,243 is referred to.