Shixian Ding

Vacuum microwave devices require cathode materials with high emission current density ( $\ge$ 50 A/cm²) and long operational lifetime. To address issues of poor emission uniformity and limited reproducibility in Sc₂O₃-W mixed-matrix cathodes, this study introduces a mechanical fusion coating process. Using classified tungsten powder (average size: $3.8~\mu $ m; span $\le 1.2$ ), a uniform W@Sc₂O₃ structure nano-Sc₂O₃ coating (100–290nm) was applied. This method eliminated Sc₂O₃ agglomeration and yielded a sintered matrix with closed porosity $\le 0.5$ %, a pore size of 0.6– $1~\mu $ m, and carbon content $\le 400$ ppm. At $1100~^{\circ }\text {C}_{\text {b}}$ , the cathode achieved pulsed emission current densities of 149.86 A/cm² (1% duty cycle) and 81.36 A/cm² (5% duty cycle), representing a 108.1% improvement over planetary milled cathodes. The W@Sc₂O₃ increases specific surface area, supplying more active sites, while Sc₂O₃ migration to sintering necks enhances matrix stability. The uniform Sc₂O₃ distribution ensures consistent sintering shrinkage, enabling batch-suitable cathodes with high structural stability and machinability.

Dispenser cathodes for vacuum electron devices require higher current density and longer lifetime. To identify promising dopant elements that could enhance the cathode emission capability, first-principles calculations were performed to systematically evaluate the work functions of Ba_0.25M_0.25-O-W(001) (M = dopant). The results show that doping with Zr, Sc, Hf, Nb, and Ta reduces the work function by 0.404, 0.376, 0.289, 0.120, and 0.011 eV, respectively, compared with Ba_0.25-O-W(001) configuration. Furthermore, Zr doping on the W(112) and W(110) surfaces reduces the work function by 0.603 and 0.095 eV, respectively. Based on the result, ZrO₂-doped aluminate dispenser cathodes were fabricated. Experimental evaluation shows that the 4 wt.% ZrO₂-doped cathode achieves 17% and 29% improvements in direct current (DC) and pulsed emission current densities at $1050~^{\circ }$ C over the undoped cathode, respectively. Its evaporation rate at $1050~^{\circ }$ C is only 71% of that of the undoped cathode. Lifetime test results show that the emission current of the 4 wt.% ZrO₂-doped cathode decreased by 4.7% with an initial current density of 3 A/cm² at $1000~^{\circ }$ C after 1008 h, compared to a 10.7% decrease for the undoped cathode. Based on X-ray photoelectron spectroscopy (XPS) and first-principles calculations, Zr doping optimizes the Ba/O ratio on the cathode surface, thereby indirectly reducing the work function.

The stringent high-vacuum requirement for scandate cathodes, leading to high costs and operational complexities, remains a critical bottleneck limiting their broader application in modern microwave vacuum electronic devices. This work systematically investigates the degradation mechanisms of scandate cathode emission performance in nonideal vacuum environments. Based on the test results of cathode emission current density decreasing with vacuum degree and conducting comparative assessments before and after atmosphere exposure, we separated the degradation mechanism into two distinct pathways: physical damage from ion bombardment and chemical property alteration from surface poisoning. The mechanisms were elucidated through a combination of surface morphology characterization in a scanning electron microscope (SEM), ion bombardment energy calculation via COMSOL/MATLAB simulation, and depth-profiling elemental analysis by argon ion sputtering-assisted X-ray photoelectron spectroscopy (XPS) of the cathode, combined with DFT calculations of the surface work function. The results show that the two damage pathways fundamentally disrupt the formation and stability of the surface dipole layer, impairing its ability to maintain low work function, which ultimately reduces the emission performance of the cathode. These findings provide valuable insights and a theoretical foundation for designing effective protection of scandate cathode emission performance.