Operating model for scandia doped matrix scandate cathodes

Abstract

In this paper, scandate cathodes with scandia doped tungsten matrices are studied. Attempts are made to clarify some interesting phenomena presented in the whole process. The sub-micron porous tungsten matrix with uniformly distributed Sc 2O 3 was formed by quasi-spherical scandia doped W particles. During impregnation the reaction between Sc 2O 3 and barium aluminates impregnant results in the formation of compounds, preferentially of Ba 2ScAlO 5. But after the subsequent ultrasonic water cleaning, Ba 2ScAlO 5 is dissolved and mostly removed from the surface region. In the activation process, Ba and Sc re-aggregate to the surface. A uniformly distributed Sc and Ba layer on surface of tungsten substrate after proper activation has been verified by the results of high resolution scanning Auger microscopy (SAM) and Auger electron spectroscopy (AES) depth profile. For such cathodes typical emission of more than 30 A/cm 2 space charge limited current density at 850 °C b has been achieved. However, the uniform distribution of Sc and Ba on surface still seems not be able to remedy the abnormal Schottky effect. We propose that a surface multi-layer containing Ba, Sc and O, formed after proper activation on the W base, is the reason for copious emission and abnormal Schottky effect of such cathodes.