The effect of Ir and Sc on the emission capacity of W–Ir matrix scandate cathodes prepared via a novel in situ method

Abstract

The size and distribution of Sc2O3 in the composite powder precursor has great effect on the emission property of scandate cathode. By one-step in-situ synthesis method, the composite powder exhibits the structure of quasi-spherical Sc2O3 nanoparticles grown in situ on the surface of W/Ir grains, and the obtained W–Ir cathode with 3wt % Sc2O3 could provide the emission current density of 83.43 A/cm2 at 900 °Cb, which is higher than other W–Ir mixed matrix scandate cathodes and W matrix scandate cathode. The XRD results showed that the high emission ability depended on the high content of active phase α-Ba2ScAlO5 and low content of non-active phase BaAl2O4. In-situ TEM has been applied to observe the microstructure evolution of the cathode surface at high temperature for the first time. A serial change in the microstructural features of cathode surface and the migration of Ba, Sc and O on the substrate surface has been found, confirming the formation of Ba-Sc-O active layer. The first-principles calculations indicate that the active substances (Sc) are more stable (low adsorption energy of − 8.58 eV) on the IrW matrix cathode surface than on the tungsten matrix cathode surface, which is consistent with the low evaporation rate of the active substance detected by TFMS technique. Besides, the interaction between the active substance and the matrix was strengthened due to more electron transfer of Sc (1.88 e) and Ba (1.45 e), therefore lowering the work function. The feasible preparation technique makes such a cathode promising for applications in high-power microwave devices.