A mild aqueous synthesis of ligand-free copper nanoparticles for low temperature sintering nanopastes with nickel salt assistance

Abstract

An organic ligand-free aqueous-phase synthesis of copper (Cu) nanoparticles (NPs) under an air atmosphere was successfully achieved by reducing copper(II) oxide particles with a leaf-like shape in the presence of Ni salts at room temperature. The resulting Cu NPs with a mean particle diameter of ca. 150 nm exhibited low-temperature sintering properties due to their polycrystalline internal structure and ligand-free surface. These Cu NPs were applied to obtain Cu NP-based nanopastes with low-temperature sintering properties, and the resistivities of the obtained Cu electrodes after annealing at 150 °C and 200 °C for 30 min were 64 μΩ∙cm and 27 μΩ∙cm, respectively. The bonding strength between oxygen-free Cu plates prepared using the Cu NP-based nanopastes reached 32 MPa after pressure-less sintering at 260 °C for 30 min under a nitrogen atmosphere. The developed manufacturing processes using the developed Cu nanopastes could provide sustainable and low-CO2-emission approaches to obtain Cu electrodes on flexible films and high-strength bonding between metal plates as die-attach materials for power devices under energy- and resource-saving conditions.

Low-Temperature Sintering of l-Alanine-Functionalized Metallic Copper Particles Affording Conductive Films with Excellent Oxidative Stability

“…The resistivities of the copper films are higher than that of bulk metallic copper (1.7 × 10 –6 Ω cm), and this observation is expected as a sintering temperature of ≤120 °C is inadequate to achieve complete fusion of the copper particles. However, it is important to note that the copper films fabricated using the Cu-Ala20 particles show lower resistivity compared to other copper structures sintered at this temperature range (≤120 °C). ,,− Furthermore, some reports use a mix of either copper micro and nanoparticles or copper salts and particles to achieve conductivity at a lower temperature, ,− whereas our work utilizes only the synthesized submicron copper particles and annealing has been performed at significantly lower temperatures than other studies that report low-temperature sintering. ,, The observed conductivity at this low temperature could be attributed to the fact that the submicron copper particles synthesized in this work are capped with a compact layer of l -alanine ligands and, as proven by XPS studies, the l -alanine ligands interact with the surface atoms of copper by a Cu–N bond, which leaves the carboxylate end of the ligand free to facilitate solvation. The ink formulation uses water as part of the solvent system; therefore, the surface-anchored l -alanine ligands can also interact with water through favorable non-covalent interactions of the carboxylate group.…”

“…However, it is important to note that the copper films fabricated using the Cu-Ala20 particles show lower resistivity compared to other copper structures sintered at this temperature range (≤120 °C). 18 , 21 , 54 − 57 Furthermore, some reports use a mix of either copper micro and nanoparticles or copper salts and particles to achieve conductivity at a lower temperature, 21 , 58 − 60 whereas our work utilizes only the synthesized submicron copper particles and annealing has been performed at significantly lower temperatures than other studies that report low-temperature sintering. 20 , 59 , 61 The observed conductivity at this low temperature could be attributed to the fact that the submicron copper particles synthesized in this work are capped with a compact layer of l -alanine ligands and, as proven by XPS studies, the l -alanine ligands interact with the surface atoms of copper by a Cu–N bond, which leaves the carboxylate end of the ligand free to facilitate solvation.…”

In situ electron tomography for the thermally activated solid reaction of anaerobic nanoparticles

“…It is well known that thermally activated solid-state reaction of materials, especially sintering, is an important process relevant to various industrial products, e.g., catalysts, magnets, and electronic devices. 1 Metal nanoparticles (NPs), such as Ag and Cu, have attracted great attention in printed electronics [2][3][4][5] because they can be sintered at relatively low temperatures compared to the bulk state, meaning the NPs are environmentally friendly and enable low-cost manufacturing processes. 6 The electrical conductivity of sintered NPs is affected by the degree of neck growth between adjacent particles.…”