Morphology prediction of reactive silver ink systems

Abstract

The increase in demand for additive manufacturing technologies necessitates from its potential applications in various fields that could benefit tremendously. Some of these include, the medical field, where doctors can build a model of any damaged body part and analyze it for pre-treatment planning, aerospace industry, where they use rapid prototyping to model structures with a goal to reduce the weight, and even artists use it to explore their creativity, and more recently, printed electronics. Over the last few decades, what originated as rapid prototyping, diverged and evolved to accommodate more specific manufacturing needs. Some of these new technologies such as Stereolithography, Vacuum deposition, Electroless plating, etc. have further evolved to satisfy the time and cost demands of current systems. Drop-on-Demand (DoD) or Inkjet printing is one such technology that can print high resolution features precisely in a timely and inexpensive manner. As mentioned earlier, one area of application that could benefit vastly from DoD printing is printed electronics, specifically optoelectronics such as solar cells which is a $2 billion/ year industry. However, printed electronics have advanced to a point where the demand for a new class of materials with unique properties that target future technologies is extremely. For example, future Silicon Heterojunction (SHJ) solar cells have top-layer Transparent conducting oxides (TCO's) that are extremely temperature sensitive. Therefore, the demand for newer conductive inks that are compatible with these future technologies that require them to be stable at lower temperatures has increased. Recent exploration into silver precursor inks has yielded promising results. For example, silver compounds with carbamate or other relatively low molecular weight ligands (compared to polymer stabilizers) have been synthesized that decompose at temperatures near 150 °C, yielding electrical conductivities approaching that of bulk silver. Unfortunately, even these temperatures render the ink incompatible with many plastic and paper substrates used in flexible electronic and biomedical devices along with the above mentioned Silicon Heterojunction (SHJ) solar cells. Reactive inks are a viable alternative to the current technology that involves screen printing of metallic pastes, offering a low-cost, higher performance alternative to these traditional, particle-based inks. However, one of the major challenges when using reactive inks is their high sensitivity of print morphologies to processing parameters such as substrate temperature, ink composition, thermo-physical properties of solvents, etc. In order to control morphologies of reactive inks, the underlying mechanisms that drive the reaction kinetics must be understood. Currently, very little is know about the mass transport kinetics of printed reactive inks or their impact on morphology and material properties. While numerous models exist to guide the design of colloidal/particle based inks and solute/precipitating inks (e.g., salts, polymers, proteins) so that a desired morphology can be targeted, no models exist for reactive inks that include the kinetics of the reduction reaction, particle nucleation, and particle growth. This work will develop the understanding necessary to control the morphology of printed reactive inks. This outcome will be accomplished by experimentally measuring the chemical and mass flow kinetics of self-reducing silver and copper reactive inks and then correlating those kinetics to the mass and thermal transport phenomena involved in droplet evaporation/evolution so that particle nucleation, growth, aggregation, and chemical sintering can be properly modeled and morphology predicted. The heat and mass transport of both reactants and particles within the droplet will be modeled in multiphysics simulations and combined with kinetic models of the reduction process to predict particle formation rate as a function of fluid properties (surface tension, viscosity) and reduction kinetics (activation energy, concentration, temperature). Overall, this work will improve the quality of materials printed using reactive inks by understanding the mechanisms that contribute to overall morphology and using this insight to design inks, print processes, and post-print processes for broad classes of reactive ink materials (low cost metals, magnetics, oxides). This study will also help develop new knowledge on how ligand selection, reduction activation energy, solvent properties, contact angle, and substrate temperature dictate the physical phenomena that control physical structure.