GB640528A - Improvements in and relating to glass-to-metal seals - Google Patents

Abstract

640,528. Glass-to-metal seals. BRITISH THOMSON-HOUSTON CO., Ltd. Oct. 9, 1947, No. 27164. Convention date, Oct. 11, 1946. Drawings to Specification. [Class 56] The glass of a glass to metal seal for XRay generators contains cerium oxide as a discolouration ingredient and the glass and metal have a coefficient of thermal expansion of (4.7Œ0.3) Î 10<SP>-6</SP> cms. per ‹C. Desired properties of glass for the envelopes of X-Ray tubes are as follows, (a) a low discolouration rate, i.e., so that it does not darken on exposure to X-Rays, (b) coefficient of expansion of (4.7Œ0.3) Î 10<SP>-6</SP> cms. per ‹C. in the range 1-300‹C., (c) dielectric strength about 100 volts per mil at 200‹ C. and 500 volts per mil at normal atmospheric temperature 25‹ C., (d) dielectric constant 4-8 at 60 cycles and 110‹C., (e) electrical resistivity of the order of 10(<SP>6</SP>Œ<SP>0</SP>-<SP>5</SP>) ohms cm<SP>3</SP>. (f) power factor 0.01-0.05 at 60 cycles and 100‹C. Glass with these properties is formed from a batch containing SiO 2 about 64 per cent by weight, B 2 O 3 about 23 per cent, Na 2 O about 6 per cent, Fe 2 O 3 and Al 2 O 3 about 5 per cent, Li 2 O about 1 per cent, CaO and MgO about 0.35 per cent, K 2 O about 0.2 per cent, Sb 2 O 3 about 0.12 per cent AS 2 O 3 about 0.015 per cent, and CeO about 1 per cent and not to exceed 5 per cent, but these properties may be varied and ingredients other than CeO may be replaced or omitted. The glass has a softening point of 700‹C. or less, and the coefficient of expansion is substantially the same as that of Fe/Ni alloys, e.g., steel containing 42 per cent Ni and makes a good seal with ferrous metal alloys consisting of Fe with about 29 per cent Ni, 17 per cent Co, and 0.3 per cent Mn.