Analyses of Palladium Cathodes Used for Heavy Water Electrolysis

Abstract

The sporadic nature of the excess heat reported from heavy water electrolysis has been widely attributed to variability among the different palladium cathodes used. Experimental repeatability should, therefore, be enhanced if the microstructure of the palladium can be controlled. Toward this end, palladium rod samples from two heavy water electrolysis experiments were compared to a sample representative of the as-installed condition. The samples examined showed equiaxed grains and significant abnormal grain growth. The rod axes had strong textures, which were attributed to their prior thermomechanical history. The postelectrolysis palladium rods were sampled at two locations that were suspected to have operated at different average current densities. The suspected higher current density regions consisted of single-phase Pd-D_0.7 microstructures. Surface-originated cracks were seen along the grain boundaries in one of two such specimens. Cracks were absent in samples from the suspected lower current density regions, which showed two-phase microstructures with Pd-D_0.7 as the dominant phase. The minor phase, indexed as palladium in the X-ray pattern, was dispersed nonuniformly, mostly in the form of stringers, across the grain boundaries. It is concluded that high current densities resulted in high deuterium loadings in palladium. Smoothing effects from the electrolytic process, resulting in preferential material removal from the grain boundaries, were seen on the cathode surface. A number of high-mass impurities were seen to have deposited on the exposed surface. An initial secondary ion mass spectrometry examination of the specimen interior indicated a significant presence of mass 2 species and considerably lower concentrations of mass 3 and 4 species. Repeat analyses failed to confirm the presence of the mass 3 and 4 species. This work shows that the Pd-D_0.7 phase is reasonably stable at room temperature and that metallography could be a powerful tool for studying the deuteriding process in palladium at high concentrations.