Understanding the surface structure evolution and electron emission behaviors during the activation of Ir-coated dispenser cathodes

Abstract

Metal coated dispenser cathode is of great interest due to its low operation temperature, high current density and long lifetime, however, the emission mechanism is still far from being fully understood. In this work, the tungsten dispenser cathodes were coated with iridium film of 300 nm in thickness. The phase transition and surface structure evolution during cathode activation was systematically investigated. The as-deposited iridium coating shows a columnar grain morphology with ultrafine size. Upon heating for 1 h at 1150 °C, a multiple-layer of Ir/Ir3W/IrW was detected and it fully converted to IrW layer after 4 h heat treatment. Such a structure evolution can be well explained based on thermodynamics. The Ir → Ir3W → IrW phase transition induces obvious film thickening and grain coarsening. The full activation could increase the emission current density from 17.35 A/cm2 to 30.92 A/cm2 at 1050 °C. Density function theory (DFT) simulation results demonstrate the enhanced emission of IrW-coated cathode may be due to its balanced adsorption behaviors with Ba and O atoms. Our findings will provide deeper insights into the design and development of metal coated cathodes with better performance.