The Woodruff School

SUMMARY

As electronics become increasingly prevalent in everyday life, demand for flexible applications necessitate the development of compliant conductors which maintain performance while subjected to high levels of strain. This has led to the emergence of flexible hybrid electronics, whereby conductive materials compatible with printing technologies are deposited onto flexible substrates. Conductors are commonly found to be composites of conductive metal particles in a polymer matrix. Their electrical conductivity is traditionally characterized by percolation theory that predicts superior results for composites with a polymer matrix of high Poisson’s ratio, v. While qualitatively valid, this model fails to accurately predict the measured electrical response to applied strains. This suggests other factors are at play. The goal of this work is to understand the strain-induced change in microstructure and its role behind electrical performance of two silver-based inks screen-printed with a single pass (10um thick) onto polymer substrates. 5025 ink, with an acrylic binder (v ~0.3), and PE874 ink, with a polyurethane binder (v ~0.45), contain similar Ag flake volume fractions of ~50%, and are the subject of study.

In situ characterization of these inks reveals normalized resistance increases with applied strain at a faster rate for 5025 ink than that of PE874. At 35% applied uniaxial strain, normalized resistance differs by a factor of three. While qualitatively consistent with percolation theory, strain map analysis and post-mortem fractography reveal differences in the root causes of the inks’ electrical behavior. Both inks manifest strain localization bands with similar spacing. However, for the 5025 ink, strain localized regions are associated with significant thickness reduction. Instead, for the PE874 ink, strain localization is associated with cracking more likely triggered by pre-existing voids. These two strain localization phenomena alter the local particle volume fractions differently, and necessitate higher order approximations of the percolation model for better predictions of electrical conductivity. Future characterization will explore the advancement of observed failure mechanisms due to strain cycling. Capture of viscous properties through relaxation testing will contribute to a better understanding of the strain-time aggregate rearrangement of conductive filler, and its ultimate effect on conductor performance.

SUBJECT:	Ph.D. Proposal Presentation

BY:	Gabriel Cahn

TIME:	Wednesday, December 18, 2019, 10:00 a.m.

PLACE:	Love Building, 109

TITLE:	Understanding the electrical response to applied strain of polymer supported screen printed inks

COMMITTEE:	Dr. Olivier Pierron, Co-Chair (ME) Dr. Antonia Antoniou, Co-Chair (ME) Dr. Samuel Graham (ME) Dr. Suresh Sitaraman (ME) Dr. Meisha Shofner (MSE)

SUMMARY As electronics become increasingly prevalent in everyday life, demand for flexible applications necessitate the development of compliant conductors which maintain performance while subjected to high levels of strain. This has led to the emergence of flexible hybrid electronics, whereby conductive materials compatible with printing technologies are deposited onto flexible substrates. Conductors are commonly found to be composites of conductive metal particles in a polymer matrix. Their electrical conductivity is traditionally characterized by percolation theory that predicts superior results for composites with a polymer matrix of high Poisson’s ratio, v. While qualitatively valid, this model fails to accurately predict the measured electrical response to applied strains. This suggests other factors are at play. The goal of this work is to understand the strain-induced change in microstructure and its role behind electrical performance of two silver-based inks screen-printed with a single pass (10um thick) onto polymer substrates. 5025 ink, with an acrylic binder (v ~0.3), and PE874 ink, with a polyurethane binder (v ~0.45), contain similar Ag flake volume fractions of ~50%, and are the subject of study. In situ characterization of these inks reveals normalized resistance increases with applied strain at a faster rate for 5025 ink than that of PE874. At 35% applied uniaxial strain, normalized resistance differs by a factor of three. While qualitatively consistent with percolation theory, strain map analysis and post-mortem fractography reveal differences in the root causes of the inks’ electrical behavior. Both inks manifest strain localization bands with similar spacing. However, for the 5025 ink, strain localized regions are associated with significant thickness reduction. Instead, for the PE874 ink, strain localization is associated with cracking more likely triggered by pre-existing voids. These two strain localization phenomena alter the local particle volume fractions differently, and necessitate higher order approximations of the percolation model for better predictions of electrical conductivity. Future characterization will explore the advancement of observed failure mechanisms due to strain cycling. Capture of viscous properties through relaxation testing will contribute to a better understanding of the strain-time aggregate rearrangement of conductive filler, and its ultimate effect on conductor performance.