Submicron Ag-coated Cu particles and characterization methods to evaluate their quality

Three different types of submicron particles, pure Cu, Cu@Ag prepared without a reductant, and Cu@Ag prepared with a reductant (L-ascorbic acid), were fabricated, and two characterization methods, direct pelletization and curing of paste containing the particles, were used to evaluate the average quality of the submicron particles for use as a conductive filler. The fabricated Cu particles exhibited a polygonal shape and a smooth surface; the Cu@Ag particles prepared without a reductant showed tiny bumps lightly formed on the surfaces; and the Cu@Ag particles fabricated with an L-ascorbic acid indicated very rough surfaces. The Ag shell in the particles fabricated without a reductant was thinner than that in the particles fabricated with the reductant. In the as-pelletized samples, the sample containing the particles fabricated with the reductant exhibited a slightly high electrical resistivity; the slight change in the resistivity was interpreted in terms of difference in the contact area between the particles after pelletization. The as-pelletized samples after oxidation showed a striking difference in the resistivities of pure Cu particles and the Cu@Ag particles fabricated without a reductant. However, the Cu@Ag particles fabricated with the reductant presented a nearly consistent resistivity with a superior antioxidation property. When the resistivities of the films was measured after curing the paste co-containing micron-sized Cu@Ag flakes and submicron particles, it was confirmed that the pure Cu particles and Cu@Ag particles prepared without a reductant are oxidized in the paste during curing. In contrast, the paste containing Cu@Ag particles fabricated with the reductant presented the lowest resistivities, which were 2.85 × 10⁻³and 8.90 × 10⁻⁴ Ω cm in air and nitrogen, respectively, indicating the strongest antioxidation behavior than the other pastes.