Low temperature and pressureless Cu-to-Cu direct bonding by green synthesized Cu nanoparticles

Study on the preparation of ascorbic acid reduced ultrafine copper powders in the presence of different protectants and the properties of copper powders based on methionine protection

High-purity, monodisperse, and low-oxygen submicron copper powder particles with particle sizes in the range of 100-600 nm were synthesized under alkaline conditions using ascorbic acid (C6H8O6) as a reductant and copper chloride (CuCl2·2H2O) as a copper source. The redox potential of the Cu-Cl-H2O system was obtained by calculations and plotted on pH-E diagrams, and a one-step secondary reduction process (Cu(ii) → CuCl(i) → Cu2O(i) → Cu(0)) was proposed to slow down the reaction rate. The commonalities and differences in the nucleation and growth process of copper powders under methionine (Met), hexadecyl trimethyl ammonium bromide (CTAB), and sodium citrate dihydrate (SSC) as protectants and without the addition of protectants are compared, and the reaction mechanism is discussed. Among them, methionine (Met) showed excellent properties and the Cu2O(i) → Cu(0) process was further observed by in situ XRD. The synthesized copper powder particles have higher particle size controllability, dispersibility, antioxidant properties, and stability, and can be decomposed at lower temperatures (<280 °C). The resistivity can reach 21.4 μΩ cm when sintered at a temperature of 325 °C for 30 min. This green and simple synthesis process facilitates industrialization and storage, and the performance meets the requirements of electronic pastes.