Wednesday, November 11, 2015: 10:35 AM
253B (Salt Palace Convention Center)
Screen printing of metal nanostructure inks is a promising technology for the fabrication of current collectors on solar voltaic panels. Among metal nanostructures, silver nanowires (AgNWs) have high potential as conductive fillers in screen-printable ink. Inks suitable for screen printing should exhibit thixotropic rheological behavior with both time dependence and shear thinning. This rheological behavior permits charging of the ink at low shear onto the screen, followed by printing through the screen at high shear. Rapid recovery of the low-shear viscosity after printing is necessary to obtain sharp line definition. The purposes of this work are investigation of the rheological behavior of silver nanowire inks, evaluation of the effect of silver nanowire content on the rheological behavior of the ink, and modeling of the build-up of the ink structure after screen printing. The critical parameter here is the characteristic time for viscosity recovery and its dependence on temperature. Aqueous inks consisting of silver nanowire with dispersant and binder were formulated. The rheological behavior of the inks was characterized by Stress Sweep Step (SSS), Steady State Flow Step (SSFS), Frequency Sweep Step (FSS) and Peak Hold Step (PHS) experiments with a parallel plate rheometer. Rheological measurements under conditions that mimic the screen printing process were done on the conductive inks through the PHS test to assess viscoelastic properties induced by flow alignment of the wires. The stretched exponential model (SEM) was used to model the recovery process after screen printing in order to obtain the characteristic time of recovery for various ink formulations. The characteristic time was determined at several temperatures to assess the temperature dependence of the characteristic time. The recovery mechanism could be due to either Brownian motion or non-Brownian motion. The domination of Brownian motion or non-Brownian motion behavior can be characterized by a Peclet number, which is the ratio of shear rate to the rotational diffusion coefficient. The Peclet number was calculated for different inks at recovery process to illustrate the mechanisms of the build-up after screen printing process.