Scandate dispenser cathodes have demonstrated an immense improvement in current densities for thermionic cathodes. However, the emission properties are inconsistent from cathode to cathode. The cathodes also lack the lifetime desired for space based applications. Further scientific investigation of the reason for the increased electron emission and limited life is needed to solve the manufacturing inconsistencies and limited lifetimes.
Radio frequency magnetron sputter deposited thin film squares (25 x 25 μm and 100 x 100 μm) of barium, scandium and the oxides of both were prepared in a variety of configurations on tungsten and scandium foils for study and characterization of electron yield in a photoelectron emission and thermionic electron emission microscope (PEEM/ThEEM) fitted with a Faraday cup for current density measurements. The samples were studied from a room temperature to brightness temperatures in excess of 1600 K.
It was determined that sub-monolayer oxide coverage is not necessary for increased current densities. It was discovered that application of a 200 nm thin film of scandium oxide increases the electron yield of tungsten, and the increased yield is dependent on the interface between these two materials. Barium oxide on top of scandium oxide also increased the electron yield. Both barium and scandium metal wet the surface of tungsten, and thus their physical position cannot be controlled. Barium oxide and scandium oxide, however, presented physical stability to brightness temperatures of 1600 K and above.
A model is presented, using data acquired from ThEEM, UPS and TES, explaining the increased electron yield and transport through thick oxide layers. The model proposes electron injection from tungsten into a gap state in scandium oxide. This gap state is above the lowest occupied orbital, and is proposed to be in the 3d electron levels of scandium. Electrons in the gap state are then free to move to the surface, where they have an effectively lower work function from the gap state energy level which is 2.54 eV below the vacuum level.