Rheological fingerprints of time-evolving polymer-particle interaction and sol–gel transition in silver pastes
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The performances of a conducting paste and the thick film fabricated from it depend critically on the dispersion state of the functional powders. Detailed mechanisms dictating the interactions between functional powder, surfactant, and, in particular, polymer binder that lead to optimum particle dispersion remain elusive in general. For a series of practical micrometer-sized silver pastes (with a powder content of ~47 wt% or 7 vol%, in a commonly used solvent α-terpineol), we reported rheological fingerprints (i.e., significantly promoted sample elasticity) suggesting that while the surfactant (stearic acid, SA) aids the initial, local (~1 nm) particle dispersion, the polymer binder (ethyl cellulose, EC) then becomes effective to help achieve larger-scale (~30 nm) particle dispersion, permitting the development of colloidal fractals and time-evolving microphase transition closely mimicking typical sol–gel transition. The enhanced particle dispersion is evidenced also by the scanning electronic microscope morphologies of dried printing thick films. Overall, similar phase transition had rarely been reported for metal pastes, yet the underlying particle dispersion seems crucial to achieve as (environmentally required) decreasing powder content makes a paste and the fabricated thick film increasingly unlikely to fulfill the percolated state desirable for efficient electron conduction.
KeywordsRheology Polymer binder Silver paste Particle network Sol–gel transition
This research was supported in part by China Steel Corporation of ROC and in part by the Ministry of Science and Technology of ROC.
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