High Temperature Vaporization of Trace Impurity Elements in Tungsten Filament and the W-Br Chemical Cycle in Bromine Lamp

Abstract

Changes inithe contents of trace impurity elements in tungsten filaments (doped K, Al and Si) during lighting time (0, 3, 25 h) of a bromine lamp was determined by the spark source mass spectrometry. The vaporized elements at 2750°C were Fe (32.3 mole ppm/W) Al (24.5), Ba (14.5), Ca (13.3), Na (8.8) and Sr (5.7) after the lighting for 25 h. Potassium was hardly evaporated despite of the high initial content. It was probably due to the formation of potassium aluminosilicate. The composition of a deposit containing Fe, W, Al, Br and 0from the gas phase below 400°C (lamp wall temperature: 500-600°C) was in agreement with the decrease in the content of the impurity elements in the tungsten filament. A thermo chemical calculation supported that bromine and tungsten oxides were formed at a high tem perature near the filament, while tungsten bromides and tungsten bromide oxides were formed near the bulb wall. Their partial pressures were independent of the existence the trace Fe, Al and K. The partial pressures of FeBr₂ and KBr were nearly constant from 800 K to 3000K, and that of AlBrs was constant below 2300 K but lowered steeply above the temperature.