Method for applying reacted boron oxide layer to vitreous silica substrate

摘要

Mercury metal-halide HID lamp having a discharge-sustaining filling comprising sodium halide and scandium halide is improved by providing a thin transparent layer comprising boron oxide adhered to and reacted with the exterior surface portion of the vitreous silica arc tube. Such a coating substantially inhibits migration of sodium ions through the arc tube. The coating also imparts devitrification resistance to any vibreous silica surface to which it is applied. The layer comprising boron oxide preferably is applied to the vitreous silica substrate by forming a clear, partially hydrolyzed solution principally comprising boron alkoxide and silicon alkoxide which is applied to the vitreous silica substrate. The substrate and applied solution are then heated to react the glass-forming constituents of the applied solution with the arc tube.