Importance of dislocations in ultrasound-assisted sintering of silver nanoparticles

For acoustic softening of solid materials, it has been suggested that the enhancement of dislocation motion by ultrasonic irradiation plays some role. In order to study the role of dislocations in ultrasound-assisted sintering of silver nanoparticles, numerical simulations are performed using a model developed from the solid-state sintering model by Kraft and Riedel coupled with the models of evolution of mobile and immobile dislocations and that of vacancies. It has been clarified that the most important effect of ultrasound is the reduction in the pore size, which results in the lager saturated density. In addition, the time-averaged densification rate increases by ultrasound irradiation because the increase in the magnitude of the plastic strain rate at the compression phase of ultrasound overwhelms the decrease at the rarefaction phase due to a much larger bulk viscosity at the rarefaction phase caused by the nonlinear effect. The relative density is saturated after about 3 min under ultrasound irradiation, which agrees with the experimental result. The total dislocation density does not increase by ultrasound irradiation under the present condition as the mobile dislocations are immediately immobilized due to the small initial grain size of 20 nm. For the increase in dislocation density, initial grain size should be larger than about 10 μm irrespective of the presence or absence of ultrasound.