Synthesis of impregnants for dispenser cathodes via homogeneous-lyophilization: Phase structure, optical response and emission property of four aluminates

Dispenser cathodes are the most widely used electron emission source in vacuum electron devices. The impregnant is the source for producing active substances in the dispenser cathodes, which play an irreplaceable role. However, impregnants’ phase structure and optical response properties have yet to be systematically investigated. Herein, four aluminates were prepared as impregnants via homogeneous-lyophilization followed by calcination. Due to the unique preparation process, the elemental distribution of aluminates is exceptionally uniform. Based on the first-principles DFT calculations, the primary phases of the two aluminates 532 and 411 were confirmed to be (Ba0.61Ca0.39)3Al2O6 (P213) and (Ba0.75Ca0.25)4Al2O7 (Cmca). At the same time, α-Ba2ScAlO5 and β-Ba2ScAlO5 are the only phases of aluminates 411-0.75Sc and 411-1.25Sc, respectively. XPS confirmed two chemical states of Ba and Sc in the aluminates. FT-IR and 27Al NMR indicate that the aluminum-oxygen skeleton in all aluminates exists mainly as [AlO4] tetrahedrons, accompanied by a small amount of distorted [AlOx] (x = 5, 6) polyhedrons. The UV–visible light absorption capacity, band gap and intra-structural electron transfer capacity of all aluminates are determined by UV–Vis DRS and PL spectra. The dispenser cathodes impregnated with aluminates 532 and 411 have zero-field current densities of 5.3 A/cm2 and 14.3 A/cm2 at 1100°Cb, respectively. The dispenser cathodes impregnated with aluminate containing scandium have different space-charge-limited current densities, i.e., 29.48 A/cm2 for 411-0.75Sc and 13.66 A/cm2 for 411-1.25Sc at 1000°Cb. This work will provide more options for synthesizing and applying impregnants.