Abstract
The increasing development of flexible and printed electronics has fueled substantial advancements in selective laser sintering, which has been attracting interest over the past decade. Laser sintering of metal nanoparticle dispersions in particular (from low viscous inks to high viscous pastes) offers significant advantages with respect to more conventional thermal sintering or curing techniques. Apart from the obvious lateral selectivity, the use of short-pulsed and high repetition rate lasers minimizes the heat affected zone and offers unparalleled control over a digital process, enabling the processing of stacked and pre-structured layers on very sensitive polymeric substrates. In this work, the authors have conducted a systematic investigation of the laser sintering of micro-patterns comprising Ag nanoparticle high viscous inks: The effect of laser pulse width within the range of 20–200 nanoseconds (ns), a regime which many commercially available, high repetition rate lasers operate in, has been thoroughly investigated experimentally in order to define the optimal processing parameters for the fabrication of highly conductive Ag patterns on polymeric substrates. The in-depth temperature profiles resulting from the effect of laser pulses of varying pulse widths have been calculated using a numerical model relying on the finite element method, which has been fed with physical parameters extracted from optical and structural characterization. Electrical characterization of the resulting sintered micro-patterns has been benchmarked against the calculated temperature profiles, so that the resistivity can be associated with the maximal temperature value. This quantitative correlation offers the possibility to predict the optimal process window in future laser sintering experiments. The reported computational and experimental findings will foster the wider adoption of laser micro-sintering technology for laboratory and industrial use.
Highlights
The increasing advancement of flexible/stretchable and large area electronics has broadened the spectrum of the applications which benefit from their unique characteristics: Flexible and stretchable sensors [1], low cost and low weight non pervasive wearable systems [2], optically transparent tactileMaterials 2018, 11, 2142; doi:10.3390/ma11112142 www.mdpi.com/journal/materialsMaterials 2018, 11, 2142 devices [3], are a few examples of the application potential offered by this rapidly evolving field.At the same time, the increasingly demanding requirements of such applications have highlighted the technological value of digital and high resolution additive manufacturing processes
We have demonstrated for the first time in literature, that despite the complex non ideal geometry of the laser printed patterns, laser sintering can be effectively applied over a wide process window, which has been clarified by numerous experimental and computational results
By conducting simulation for varying pulse width with multiple laser pulses, the former show that for longer pulse widths the heat affected zone increases, and the latter have shown that the resulting temperature values increase for increasing repetition rate
Summary
The increasing advancement of flexible/stretchable and large area electronics has broadened the spectrum of the applications which benefit from their unique characteristics: Flexible and stretchable sensors [1], low cost and low weight non pervasive wearable systems [2], optically transparent tactileMaterials 2018, 11, 2142; doi:10.3390/ma11112142 www.mdpi.com/journal/materialsMaterials 2018, 11, 2142 devices [3], are a few examples of the application potential offered by this rapidly evolving field.At the same time, the increasingly demanding requirements of such applications have highlighted the technological value of digital and high resolution additive manufacturing processes. Among a plethora of additive manufacturing technologies, laser based additive processing, i.e., laser printing and laser sintering, stands out owing to the unique attributes it offers: Processing of materials in liquid and solid phase, high resolution, and minimal damage to the printed or sintered material and underlying substrate. Post-printing laser sintering can be efficiently employed for the selective transformation of the printed patterns to solid tracks with electrical behavior comparable to bulk metals. Numerous papers have reported on inkjet printed metal nanoparticle micropatterns which showed high conductivity after post-printing laser sintering. In References [5,6] the authors report on laser sintered, inkjet printed patterns comprising Ag nanoparticles, with resistivity down to 5× bulk using 60 s irradiation of continuous wave laser on glass substrates. Other selective sintering methods have been reported over the past decade, e.g., Öhlund et al have shown that inkjet printed Ag nanoparticles with an average size of
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