New Glasses for the Construction of Sodium-Vapour Discharge Lamps

JSGT 1951 V35 T185-T192

Phosphate glasses can be made with low thermal expansion coefficients and excellent resistance to attack by sodium vapour. Alumino-borophosphate glasses, containing approximately 20% of divalent oxides, were made which matched the borosilicate glasses of thermal expansion coefficient approximately 50×10^-7. Cased tubing was drawn satisfactorily from one such glass and made into sodium-vapour discharge lamps having excellent lumen maintenance. Finally, trial melts on a 50 lb scale were made of a glass of percentage composition: P₂O₅ 15·1, Al₂O₃ 22·5, CaO 5·5, MgO 13·0, SiO₂ 7·3, B₂O₃ 36·6, Fe₂O₃ 0·03. This had a thermal expansion coefficient (0°-400°) of 52×10^-7. Mention is made of some of the difficulties encountered in producing this glass on a larger scale and how these difficulties were overcome.

A. E. Dale & J. E. Stanworth