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Nanomaterials 2020, 10, 892 2 of 14 In recent literature, numerous methods to cure thin silver layers have been investigated and described for industrial scale applications. The most commonly used curing method is thermal sintering, where heat is applied with an oven or a hotplate. Depending on the ink composition, this method requires high temperatures of 140 to 400 ◦C for 10 to 60 min to achieve a conductive printed silver layer [10,15–17]. However, these high temperatures and long curing times make it impossible to sinter silver patterns on heat-sensitive substrates without causing thermal degradation [18]. Nevertheless, this sintering method results in very smooth surfaces (Ra: 10–20 nm) with a low sheet resistance, ranging between 0.04 and 0.13 Ω/ for commercially available JS-B40G silver nanoparticle ink, bought from Novacentrix (Austin, TX, USA), as one of the inks studied in this paper [10]. To overcome the thermal degradation effects of substrates during curing, many selective sintering approaches are investigated in recent literature [19–22]. Electrical, chemical and radiation-induced sintering methods are considered as promising alternatives to remediate the abovementioned detrimental thermal effects. The onset of electrical sintering is achieved by preheating the sample until it is slightly conductive, followed by the application of a fixed current. Due to the Joule heating, the applied current generates heat and, hence, sintering. As the sintering proceeds, the conductivity of the samples increases, reducing the Joule-effect inside the ink layer and thus less heat is generated, which naturally halts the sintering further. However, the need for physical contact between the probes and the sample and the possibility of local overcurrent damages the printed structure and therefore seems to be inadequate for roll-to-roll printing applications. In case of chemical sintering, a sintering agent is added to the ink, and a reactive decomposition of the dispersing and/or capping agent will take place. Hence, the nanoparticles will be forced to fuse into a conductive path. Finally, radiation-induced sintering can be segmented into laser-, microwave-, infrared (IR)-, ultraviolet (UV) and near-infrared (NIR) sintering. Laser sintering uses precisely targeted high-power light pulses of 8 picoseconds to 1 s. These short pulses demand expensive equipment and complicated handling, therefore this method is difficult and less attractive to use in roll-to-roll printing [19–22]. Compared to thermal sintering and laser sintering, microwave sintering achieves better sintering speeds. Although microwaves ensure rapid heating and sintering, with lower energy consumption than oven sintering and its compatibility with other sintering techniques, it is slightly less selective compared to laser sintering because the microwaves could heat the whole sample depending on its absorption coefficient of the substrate [23–25]. The inception of UV sintering started with an ingenious combination of radiation-induced and chemical sintering where a photo initiator added to the ink will decompose under UV exposure resulting in a reaction. Finally, a short thermal treatment is performed to form a conductive pathway [26]. Infrared (IR) sintering is a relatively new sintering technique. It combines less process complexity, with shorter processing times [27,28]. Since it heats up the whole sample, this technique is less selective than laser sintering for most substrates. Due to the high absorption coefficient of silver nanoparticle inks in the NIR spectrum, NIR radiation will interact efficiently with the silver nanoparticles resulting in very local and fast heating. Meanwhile, most of the substrates remain relatively unaffected since it has a much lower absorption coefficient in the NIR spectrum [29]. Cherrington et al. proved that NIR sintering could be a fast alternative for thermal sintering and that it can also be applied for flexible polymer substrates in roll-to-roll applications [16]. It has been shown before that for NIR sintering, there is a promising synergy of short sintering durations, compatibility with heat-sensitive substrates, process simplicity and selectivity. Therefore, this study looks deeper into this sintering method, focusing on roll-to-roll industrialization without having to cut down on important properties such as conductivity and surface roughness. At first, a comparison of the direct influence of NIR sintering relative to oven sintering is made. In addition, a more optimized ink towards NIR sintering is selected and investigated, while trying to reduce the maximum processing temperature, without losing the focus on roll-to-roll industrialization. An innovative way to monitor the moment the structure becomes conductive, based on the emissivity of the printed layer, is applied and compared to standard four-probe sheet resistance measurements.PDF Image | Nanoparticle Inkjet Inks for Near-Infrared Sintering
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