Development and dispensing of a nickel nanoparticle ink for the diffusion brazing of a microchannel array

A process was developed for producing nickel nanoparticle (NiNP) films for use in diffusion-brazing stainless steel 316L microchannel laminae at 800 °C and 1 MPa of bonding pressure. NiNPs were synthesized in 45 s at 80 °C using a NiCl2·6H2O salt solution, a combination of NaBH4 and N2H5OH as reducing agents and PVP-40K as a stabilizing agent. A minimum molar ratio of 8:1 [NaOH]:[NaBH4] was required to obtain pure fcc-Ni with an average particle size of 4.2 ± 0.6 nm. Using TGA and DSC, phase change behavior was observed at temperatures as low as 720 °C. A continuous and uniform NiNP film with a thickness of 18.1 ± 2.3 μm and a roughness of 3.1 ± 0.5 μm was dispensed using a fluid pressure of 0.6 psi, a dispense gap of 1.5 mm, and a head speed of 0.5 mm/s. A microchannel array was bonded and hermetically tested up to a pressure of 120 psi with no leakage. The ultimate lap shear strength of the joint was found to be 341 ± 29 MPa. Migration of Ni into the stainless steel 316L laminae was confirmed using SEM and EDS.