  Previous efforts to directly write conductive metals have been narrowly
focused on nanoparticle ink suspensions that require aggressive sintering (>200
{\deg}C) and result in low-density, small-grained agglomerates with electrical
conductivities <25% of bulk metal. Here, we demonstrate aerosol jet printing of
a reactive ink solution and characterize high-density (93%) printed silver
traces having near-bulk conductivity and grain sizes greater than the electron
mean free path, while only requiring a low-temperature (80 {\deg}C) treatment.
We have developed a predictive electronic transport model which correlates the
microstructure to the measured conductivity and identifies a strategy to
approach the practical conductivity limit for printed metals. Our analysis of
how grain boundaries and tortuosity contribute to electrical resistivity
provides insight into the basic materials science that governs how an ink
formulator or process developer might approach improving the conductivity.
Transmission line measurements validate that electrical properties are
preserved up to 20 GHz, which demonstrates the utility of this technique for
printed RF components. This work reveals a new method of producing robust
printed electronics that retain the advantages of rapid prototyping and
three-dimensional fabrication while achieving the performance necessary for
success within the aerospace and communications industries.
