Influence of Morphology of the Press Powder Particles and Thermal Annealing on the Structure and Microhardness of Palladium Obtained by Solid-Phase Sintering

Abstract

The article is devoted to the investigation of the structure of pressed palladium-barium cathodes. These cathodes are made from a polycrystalline palladium matrix with inclusions of the particles of the activator phase Pd5Ba. The high efficiency and durability of pressed palladium-barium cathodes is ensured by the formation of an active-emission BaO layer on the working surface. The active metal Ba comes from the volume to the surface by diffusion of atoms on the defects of the crystal structure of the palladium matrix. When comparing the electrical parameters of the cathodes, the matrices of which were made from the same fractions but different batches of palladium powder, a considerable spread of electrical parameters was established. There were also revealed significant differences in the roughness of the cathode working surface. These differences affect the uniformity and stability of the emission current. This indicates the need for a detailed study of the characteristics of the initial palladium powder and their effect on the structure of the sintered material, and, consequently, on the physicomechanical properties of its surface. In this paper, the morphological features of eight batches of palladium powder in the initial state and after the purification annealing were studied by the method of electron microscopy. Significant differences in the form of particles and agglomerates of powder from different batches have been revealed. A metallographic analysis of microsections of the palladium samples prepared by solid-phase sintering was carried out. The influence of the morphological features of the particles of the initial powder on the grain size and the mechanical properties of the sintered compact is determined. Studies have shown that to obtain a compact palladium with reproducible and predictable properties, stability of the morphological characteristics of the original powder is necessary.