Xiaoxia Wang

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.

The backward wave oscillator (BWO) is an important vacuum electron source that utilizes backward waves to interact with electron beam, resulting in high-frequency self-excited oscillations. Due to the negative dispersion property, self-excited oscillations, tunability, room-temperature operation, and relativistic effects, the BWO has significant application prospects in fields such as terahertz (THz) imaging and nuclear fusion. The development direction of BWO is high frequency and high power. This article jointly reviews the research progress of THz BWOs in the last decade from many aspects, such as the theory of BWO, slow wave structure (SWS) design, and common fabrication methods used in the THz band, and introduces the latest methods and mechanisms for improving the performance of BWOs. In addition, the latest progress of relativistic BWOs (RBWOs) and the applications of BWOs was also introduced. The above contents will help to understand the theoretical and experimental studies of BWOs and RBWOs and promote their practical applications in the fields of identification imaging and energy.

The application of scandate cathode with high emission current density in microwave vacuum electronic devices is constrained by issues of poor emission uniformity and reproducibility. An effective strategy to enhance the emission performance of scandate cathodes is through the doping of additional elements. This article prepared yttrium-doped scandate cathodes and investigated the mechanism of yttrium affecting cathode emission performance via impregnant composition analysis, cathode emission property testing, surface element analysis, and density functional theory (DFT) calculations of surface work function and adsorption energy. As the yttrium content increases from 0 to 6 atom%, the total amount of aluminate compounds remains above 85%. The content of Ba₂ScAlO₅ in impregnant increased first and then decreased, while cathode emission property test results also increased first and then declined. The cathode with yttrium content of 3 atom% in impregnant has the best emission property, with an emission current density of 466.4 A/cm² at 2000 V pulse voltage under 1100 ^∘C_b, the practical work function of 1.64–1.68 eV. The theoretical surface work function of the x = 3 cathode is 1.67 eV, and the theoretical adsorption energy indicates a low evaporation rate on the surface of yttrium atoms during cathode operation. It can be inferred that the doping of yttrium has a visible impact on the composition of the impregnant. An appropriate amount of yttrium doping facilitates the formation of Ba₂ ScAlO₅ and enhances cathode emission properties, while superfluous yttrium inhibits Ba₂ScAlO₅ formation and results in an increased presence of BaY₂O₄ in impregnant, reducing the coverage of Ba atoms on the cathode surface, suppressing the enhancement of cathode emission performance.

Scandate cathodes have received widespread attention due to the high emission current density, existing results have shown that the content of strontium in impregnant has a certain degree of impact on the emission property of scandate cathodes. In this work, the mechanism by how strontium affects the emission property was investigated through experiments and theoretical analysis. Five precursor powders with different strontium element contents were prepared by freeze-drying method, and the impregnant was obtained after sintering. Then composition analysis was conducted on the impregnant and prepared experimental cathodes for emission performance testing, also the surface emission structure of cathode was established for density functional theory (DFT) calculations. The experimental results indicated that the variation in strontium content can alter the material composition of the impregnant and have impact on cathode emission performance. During the process of increasing strontium content from 0 to 12 atom%, the emission performance of the experimental cathodes increased first and then decreased. Under a testing temperature of 1100 $^{\circ}$ C $_\text{b}$ , the cathodes with 6 atom% Sr have the highest emission current density of 34.9 A/cm $^{\text{2}}$ at 260 V dc and 523.2 A/cm $^{\text{2}}$ at 2000 V pulse voltage among the five experimental cathodes, and this cathode exhibits good emission uniformity. DFT results indicate that strontium does not directly participate in reducing the work function of cathode surface. It can be considered that strontium has a promoting effect on the formation of active substance, an appropriate amount of strontium promotes the formation of emitting active substance Ba $_{\text{2}}$ ScAl $_{\text{5}}$ during the preparation process of the impregnant, thus promoting the improvement of cathode emission performance; while excessive strontium can lead to the generation of impurities that lower the emission performance of cathodes.

In order to prolong the service life and high-temperature stability of direct-heated cathode, a novel cathode is proposed and prepared, in which La2O3, Gd2O3, and HfO2 are used as raw materials for the first time. The X-ray diffraction (XRD) structure characterization shows that emission material consists of La2O3 and La2Hf2O7 two compounds. The thermionic emission results indicate that this kind cathode can supply a current density of about 5.5 A/cm2 at 1600 °C. The energy dispersive spectroscopy (EDS) detection manifests the emission material distributes homogeneously around the W–Re base. The cathode has been working stably for 10 920 h without obvious fall at 1550 °C with the loading of 0.65 A/cm2 in life test process.