Failure of tungsten ribbon filaments by formation and growth of hot spots

Abstract

The mechanism of formation and growth of hot spots was studied using filaments made of doped tungsten foil which were burned at constant voltage to incipient failure in vacuo and in half an atmosphere of nitrogen. The grain size and morphology of the filaments were set by controlling the heating rate to the initial operating temperature, 2300 °C. The nucleation and growth of hot spots were observed and measured by optical pyrometry. The rate of growth of the hot spot was initially linear and directly related to the grain size of the filament. Filament life was greater the smaller the grain size except for filaments with grain size approaching that of a single crystal. The position of the hot spot was fixed along the length of the filament and centered on a particular grain boundary or group of boundaries. From the linear rate of growth of the hot spot and from other evidence, it was concluded that the hot spot was generated by migration of vacancies to form voids at the potassium bubbles near a grain boundary. Growth and coalescence of these voids decreased the cross-section area of the filament, thus nucleating a hot spot. This mechanism explained all of the experimental observations.