A novel Y2O3-Gd2O3-HfO2 impregnated W base direct-heated cathode in magnetron tube

Abstract

As the heart of a magnetron, cathode plays an important role in the device. At present, the pure W cathode is mainly used in high-power continuous wave magnetron tube. However, the pure W cathode has low thermionic emission capability and secondary electron emission yield (1.25-1.50), which result in the cathode operating at a high temperature (2450-2700 K). The higher the operating temperature of the cathode, the faster the evaporation of its surface is, which can shorten the cathode lifetime. In order to enhance the emission current, reduce the operation temperature and prolong the lifetime of the pure W cathode, a novel refractory Y2O3-Gd2O3-HfO2 impregnated W base direct-heated cathode (Y-Gd-Hf-O impregnated cathode) is developed in this paper. The present investigation mainly focuses on the thermionic emission, work function, lifetime, emission mechanism, and anti-bombing property. The direct current (dc) emission properties of the Y-Gd-Hf-O impregnated cathode are investigated, showing that it can provide more than 0.4, 1, 4.0, 7.74, 10.5 A/cm2 current density for the space charge limitation (SCL) at 1300, 1400, 1500, 1600, 1700 ℃ respectively. Absolute zero work function for the cathode is only 1.68 eV obtained by the Richardson line method. The effective work function for the cathode is in a range of 2.6-3.1 eV obtained by the Richardson-Dushman formula. The lifetime for the cathode is more than 3600 h with an initial load of 1.5 A/cm2 at 1600 ℃. The surface microstructure, element composition and content of the Y-Gd-Hf-O impregnated cathode are analyzed by the scanning electron microscope (SEM), Auger electron spectroscopy (AES), and energy disperse spectroscopy (EDS). The analysis results show that the surface of the cathode contains the Y2O3-x semiconductor layer, which causes an improvement of the electro-conductivity during the activation. The work function of the cathode can also be reduced due to the improvement of the electro-conductivity. Besides, the addition of the transition-metal oxide HfO2 changes the internal lattice energy level, which can further reduce the work function. Therefore, the Y-Gd-Hf-O impregnated cathode has good thermionic emission capability. In addition, the anti-bombing performance of the cathode is also studied, which shows that the dc emission current density decreases linearly from the initial current density of 1.5 A/cm2 to 0.4 A/cm2 after 150-h continuous electron bombing at 10 W/cm2. In the future research, we will focus on enhancing the anti-bombing property for the Y-Gd-Hf-O impregnated cathode by using Y-Gd-Hf-O doped W base direct-heated cathode.