1. Field of the Invention
The invention relates generally to thermionic emission cathodes and more particularly to an expendable dispenser cathode and method for making the same.
2. Description of Related Art
A dispenser cathode serves as the primary source of electrons for microwave tubes or cathode ray tubes (CRT). Electrons provided by a dispenser cathode are accelerated to form an electron beam. Typically, dispenser cathodes include a refractory metal body such as tungsten or molybdenum which is porous. A electron emitting material such as a barium-calcium aluminate is impregnated into the pores of the porous metal body. When the cathode is heated, emissive material migrates through the pores to the emission surface. The dispenser cathode emits electrons as long as there is emitting material left in the body.
There are several methods known in the art to manufacture dispenser cathodes. In a typical example the manufacturing process may include pressing tungsten powder into a billet and sintering the billet in a furnace to diffuse the particles together. Copper is melted into the billet, in a high temperature furnace, to hold the billet together and aid machining. Thereafter, the billet is placed in a lathe and machined to the desired size forming a pellet. The sized pellet is cleaned and the copper removed therefrom by evaporation in a high temperature vacuum furnace. The pellet is then brazed into a metallic cathode sleeve. Thereafter, the pellet is impregnated with an aluminate mixture which is melted into the pores of a pellet in a high temperature furnace. Next, the emitting surface of the pellet may be sputter coated with osmium/ruthenium. In the end, a highly reliable dispenser cathode results. However, the process for manufacturing this cathode involves highly time-consuming, labor intensive steps, including several machining steps, several high temperature processing steps, brazing and sintering. Manufacturing a batch of ten such cathodes can easily take more than a week. In many applications, high reliability dispenser cathodes are needed, such as in space. However, in other applications such as radar systems for aircraft and missiles and cathode ray tubes for televisions or oscilloscopes, for example, the method steps involved in manufacturing such high reliability dispenser cathodes undesirably and unnecessarily increase the expense of the finished product.
Accordingly, a much simpler and cost effective method of manufacturing a dispenser cathode is needed.