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Title:Synthesis and patterning of reactive silver inks
Author(s):Walker, Stephen
Director of Research:Lewis, Jennifer A.
Doctoral Committee Chair(s):Lewis, Jennifer A.
Doctoral Committee Member(s):Braun, Paul V.; Rogers, John A.; Nuzzo, Ralph G.
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):silver ink
reactive silver ink
tollens' reagent
conductive ink
printed electronics
Abstract:Silver inks are ubiquitous in printed electronic devices due to their high conductivity and oxidation resistance. However, many inks developed to date have numerous drawbacks such as difficult synthesis, high annealing temperatures, and electrical conductivities far below bulk silver. The objectives of this dissertation are to (1) design and synthesize reactive silver inks, (2) optimize these inks for various printing processes, and (3) produce patterned features with high conductivities at modest annealing temperatures. Towards these objectives, three reactive inks have been created based on a modified Tollens’ reagent process using silver acetate and either ammonium hydroxide, ammonium hydroxide with alkyl amines, or primary amines. The first ink is prepared simply by dissolution of silver acetate in ammonium hydroxide. This ink is particle-free and optically clear, yet undergoes rapid particle formation upon patterning as evaporation promotes silver reduction. This ink exhibits a low viscosity (2 mPa•s) and high surface tension (63 mN/m). It can be patterned by direct-write assembly using nozzles as small as 100 nm to create features with line widths of approximately 5 μm. In addition, it can be deposited conformally over large areas using aerosol spray deposition. The patterned features exhibit an electrical conductivity of 90% of bulk silver upon annealing at 100°C. The conductivity of these features can vary based on heating rates and relatively humidity. Also, this ink cannot be readily patterned by inkjet printing due to its low viscosity and high surface tension and exhibits only moderate adhesion on both plastic and inorganic substrates. To address these limitations, we created a hybrid reactive silver ink in which alkylamines are incorporated with ammonium hydroxide. This ink is particle-free and optically clear, yet undergoes rapid particle formation upon patterning as evaporation promotes silver reduction. This ink exhibits both a higher viscosity (7.3 mPa•s) and lower surface tension (22 – 24 mN/m) than the original ink design, enabling patterning by inkjet printing over a wide frequency range (2 kHz to 80 kHz). However, the labile nature of this ligand chemistry results in fast drying that ultimately yields printed discrete features that are porous and exhibit inconsistent electrical properties (≤1% of bulk silver) when annealed at temperatures ≤ 100°C. Improvements in their electrical performance can be obtained by photonic annealing, which induces rapid coalescence of the printed features minimizing their porosity and yielding 85 – 90% of the conductivity of bulk silver. Finally, we modified our reactive silver ink chemistry by solely using primary amines rather than ammonia, which allows both the ink viscosity and surface tension to be tuned over a wider range of values, from 10 - 12 mPa•s and 25 – 27 mN/m, respectively to enable facile patterning by inkjet printing and spin-coating techniques. This ink exhibits significantly improved adhesion relative the ammonia-based and hybrid reactive silver inks, while retaining similar electrical properties. Inkjet printed traces annealed at 100°C for 5 minutes exhibit an electrical conductivity of 5 – 10% of bulk silver after a single-pass and nearly 90% of bulk silver after five passes. This ink chemistry is suitable for printed electronics and can be further modified for conductive textile applications.
Issue Date:2013-08-22
Rights Information:Copyright 2013 Steven Walker
Date Available in IDEALS:2013-08-22
Date Deposited:2013-08

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