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Nanomaterials 2020, 10, 892 6 of 14 particle migration and coalescence for this specific ink, because the maximum lamp power will be reached, again resulting in a rough surface and a correlated higher sheet resistance. Furthermore, the coffee ring effect was observed during sintering with both methods. As mentioned before, the JSB-40G ink is not optimized for NIR sintering. As such, the coffee-ring effect is expected to manifest itself due to the non-optimized gradient of evaporation rates over the ink layer surface. At the edge of the ink surface, the evaporation rate of liquid is higher due to the bigger surface area versus bulk material, hence creating an outward convective flow (capillary flow) in the liquid ink state upon heating. This stream of liquid carries nanoparticles to the edge [32]. The opposing flow (Marangoni flow), in this case, is less prominent, resulting in an imbalance of particle migration. Consequently, silver nanoparticles build up at the edge and are immobilized after solvent evaporation, resulting in a crater-like deposition pattern. These coffee rings reduce layer quality and can potentially lower the resolution of printed applications. JSB-B40G is an ink optimized for oven sintering and therefore it’s engineered to have a low tendency to create these aforementioned flows in a thermal convection oven. This leads to a coffee ring which is 130% to 140% of the regular layer thickness. However, in NIR sintering, heat is transferred through radiation instead of thermal convection. This heating process has a different influence on these convective flows in the ink, creating an imbalance. In Figure 3B, the average layer and coffee ring thickness are presented. NIR-continuous sintering at 1 cm creates a coffee ring which has a thickness of about 240% of the regular layer thickness. At 3 cm, the coffee ring is very significant, raised towards 350% of the normal layer thickness 3.2. Silver Nanoparticle Ink’s Emissivity and NIR Absorption To justify the decrease in cut-off sheet-resistance upon increasing sintering duration, the emissivity and absorbance of the inks are investigated in detail. Cherrington et al. [16] have shown that there is a transition in light absorbance before and after curing, Tobjörk et al. also stated that the decrease of absorbance during sintering acts as natural protection against overheating during photonic sintering [33]. However, one has never visualized this transition directly. This transition also corresponds with the abovementioned phenomenon of gradually increasing NIR radiation output as the PID controller was set to maintain a constant temperature on the ink-covered surface. Before curing, the JS-B40G ink has an emissivity (absorptivity) of ±0.96. It can be seen from Figure 4A that there is a rapid decrease in emissivity for the NIR sintered JS-B40G between 0 and 120 s and a more gradual decrease after 120 s, eventually attaining a value of 0.02. The rapid decrease in average emissivity is in accordance with the results shown by Cherrington et al. and it indicates the occurrence of a series of events associated with the curing process like; solvent evaporation, the release of capping agent and particles size growth [16]. The latter process is clearly noticeable in the SEM images presented in Figure 2. As the size of the particles increases, the optical density, as well as the emissivity, will decrease [34]. The small residual decrease after 120 s is related to the fact that the pattern edges sinter slower and therefore still have a higher emissivity than the center, which influences the average emissivity measurement. This effect is visible in Figure 5. Adding the sheet resistance to Figure 4A, after different sintering durations, clarifies that conductivity remains constant when emissivity is low. When finally considering the spot emissivity measurement in Figure 4A, which was located at the contact points for sheet resistance measurements to exclude the edge effect, an even stronger relation between emissivity and sheet resistance is observed. To provide additional support to the correlation between emissivity and sheet resistance, the same test was performed for oven sintered JS-B40G samples which needed a much longer curing time, as demonstrated before (Figure 2). Again, Figure 4B shows a strong relation between average/spot emissivity and sheet resistance. It is clear that the emissivity transition is a result of the sintering step during the curing process. The only difference in emissivity progression of NIR versus oven sintered samples is the emissivity transition pattern. For NIR sintered samples, emissivity starts of lower in the center. This is caused by the heat sink effect of the substrate. The glass substrate absorbs less NIR radiation and thus stays relatively unaffected and cool, meanwhile the ink layer absorbs aPDF Image | Nanoparticle Inkjet Inks for Near-Infrared Sintering
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