Effect of thiourea on grain refinement and defect structure of the pulsed electrodeposited nanocrystalline copper

Abstract

The present investigation reports an extensive study on the grain refinement and the defect structure in the pulsed electrodeposited nanocrystalline copper. Copper is deposited on the stainless steel substrate using aqueous acidic copper sulfate solution prepared from 0.25 M CuSO4.5H2O and 0.5 M H2SO4 with addition of varying amounts of thiourea. Copper deposits are made using 0, 9, 18, 27 and 36 mg/L addition of thiourea in the electrolyte. The results indicate formation of nanocrystalline copper grains by even small addition of thiourea with three orders of magnitude reduction in the grain size as compared to the sample deposited without thiourea. The defects formed in the copper deposits are found to change predominantly from dislocations to the twins with addition of thiourea. The grain refinement in the thiourea containing samples is deemed to be due to change in the nucleation kinetics, growth as well as the kind of defect structure developed in the samples. The transient studies at the initial stage of deposition using chronoamperometry indicate the change in nucleation mode from instantaneous to progressive by addition of thiourea. EFTEM investigation conclusively proves the preferential segregation of thiourea at the grain boundaries of copper during pulsed electrodeposition. Detailed microscopic analysis reveals the grain refinement affected by the defect structure.