GB441603A - Improvements in and relating to alkali metal vapour electric discharge devices - Google Patents

Abstract

441,603. Glass manufacture. BRITISH THOMSON - HOUSTON CO., Ltd., Crown House, Aldwych, London. Aug. 21., 1935, No. 23492. Convention date, Aug. 23, 1934. [Class 56] [See also Group XL] Glass parts of electric discharge devices are protected against attack by hot alkali metal by means of a thin transparent film of a boric oxide glaze fused thereon, after the operation of shaping the device has been performed. In addition to boric oxide (90- 35 per cent) the glaze may contain 4-50 per cent of oxides stabilizing it against weathering or vaporization, such as alkaline earths (barium oxide up to 10 per cent) or preferably alumina. Silica may be present in amounts small enough to be protected from alkali metal attack by the other ingredients. Alkali metal oxides (3-15 per cent) may be added to act as fluxes, to improve transparency and to control the coefficient of expansion. Acid oxides such as phosphorus pentoxide (4 per cent), and fluorine may be present, the latter if cryolite is used as a source of alumina. Compounds of lead, zinc, cadmium, thallium and iron should be avoided. Examples of glaze compositions are given, a prepared formula containing B2O3, 70 per cent; Al2O3, 7 per cent ; BaO, 3 per cent; SrO, 3 per cent ; Na2O, 4 per cent ; K2O, 8 per cent ; and SiO2' 5 per cent. When the boric oxide is used alone, this is preferably applied in the form of boric acid in solution or suspension in methyl alcohol or diethyl carbonate. The coated glass is then heated to about 600‹ C. to convert to boric oxide. A subsequent additional heating to the working temperature of the glass reduces the hygroscopic properties of the boric oxide, possibly by producing combination with the glass. As boric acid is not hygroscopic, the coating is preferably kept in this form, until the glass part is required.