Improper management, on the other hand, can be very costly. For a better understanding of this sensitive aging mechanism, the filament itself must be understood. A filament made of this wire is not a suitable electron emitter for extended life applications until it is processed. Once the filament is formed into the desired shaped and mounted, it is heated to approximately 2100 C, in tbe presence of a hydrocarbon. The resulting thermochemical reaction forms di-tungsten carbide on the filament's surface.
Life is proportional to the degree of carburization.
If the filament is over-carburized, however, it will be brittle and easily broken during handling and transporting. As the tube is used, the filament slowly de-carburizes. At some point in life, all of the di-tungsten carbide layer is depleted, and the reduction of thoria to free thorium stops. The filament is now de-carburized and is no longer an effective electron emitter. The key to extending the life of a thoriated tungsten filament emitter is to control operating temperature.