Chemically and thermally highly durable, can be fused with tungsten borosilicate glass and its use

摘要

A highly chemically resistant borosilicate glass which is capable of forming a seal with tungsten and has a glass transition temperature Tg570 DEG C., a coefficient of thermal expansion of between 3.95 and 4.5x10-6 K-1, a hydrolytic resistance in accordance with DIN 12111 in class one and a TK100 value in accordance with DIN 52326 of at least 240 DEG C. is described which has the composition (in % by weight, based on oxide) of SiO2 70 to 78, B2O3 9 to 12, Al2O3 1.5 to 4, Li2O 0 to 4, Na2O 1 to 5, K2O 1 to 5, total of alkali metal oxides 5 to 7, MgO 0 to 3, CaO 1 to 3, BaO 0 to 2, ZnO 0 to 2, ZrO2 0.5 to 3, total of MgO+CaO+BaO+ZnO+ZrO2 6 to 10, and optionally conventional refining agents. It is preferred that the K2O:Na2O ratio be greater than 1 and that alkaline earth metal oxides, except for CaO, be omitted. In order to reduce the UV transparency, the glass is particularly suitable for use as a lamp glass in lamps which are subjected to very high temperatures, and as a flame-resistant safety glass.