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Nanomaterials 2020, 10, 892 11 of 14 roughness in nm for JS-A102A sintered samples as a function of source-substrate distance. In contrast to the NIR-continuous sintering of JS-B40G, increasing the sintering distance when NIR flash sintering of JS-A102A will lead to an exponential decrease of Ra roughness. Flash sintering at 3 cm creates very smooth, mirror-like silver layers with an average Ra surface roughness of 5 nm. At shorter distances (e.g., 0.5 and 1 cm) the NIR light becomes too intense given the sudden weight loss in the TGA profile upon heating, related to the sudden evolution of volatiles. For sintering distances greater than 2 cm, Ra roughness is less than 10 nm. However, an improvement in surface morphology is achieved, increasing the sintering distance is at the expense of the conductivity (Figure 8A), suggesting an incomplete decomposition of residual organic components in the ink. Thus, a trade-off between both figures of merit has been demonstrated. Finally, in Figure 8C, the average layer and coffee ring thickness is presented. It can be observed that during NIR flash sintering, the average layer thickness and coffee ring peak thickness decreases linearly as a function of increasing source–substrate distance. This decrease is probably due to the speed of evaporation of solvents and the change of particle distribution. Sintering at longer distances results in less radiation density at fixed lamp intensity, therefore sintering is slower, and the particles have more time to migrate and settle. Flash sintering at 0.5 cm results in a coffee ring with a thickness of 137% as compared to the overall layer thickness, while flash sintering at 3 cm results in a coffee ring with a thickness of 150% as compared to the overall layer thickness. Besides the influences of sintering conditions (such as distance and intensity) on the coffee ring effect, it is finally elucidated that ink formulation towards a specific sintering technique is crucial. As earlier described in the final part of Section 3.1, NIR-continuous sintering of an oven optimized ink leads to the formation of a prominent coffee ring due to the lack of balance in the convective flow mechanisms (capillary flow and Marangoni flow). The optimization towards this balance for a specific sintering technique becomes visible in the TGA analysis: one can observe that inks optimized for oven sintering consists of a complex solvent matrix ranging from low to high boiling solvents. This matrix ensures the above-mentioned balance during the relative slow sintering process. However, upon thermal analysis of the Flash NIR optimized ink, the solvent matrix proves to be more simplified since an optimized NIR ink is meant to fully sinter in matter of seconds, limiting the possibility of convective flows. Table 2 represents the coffee ring effect for different combinations of inks and sintering techniques. It can be concluded that it is very important to design the ink composition towards a preferred sintering method or vice versa. Table 2. Overview of the appearance of a coffee ring during different ink and sintering configurations. Using the appropriate sintering technique according to the ink type leads to a significant lower coffee ring effect (130–150% instead of 240–350% of the average layer thickness). Ink Type and Sintering Method JS-B40G/OVEN JS-B40G/NIR (cont.) JS-A102A/OVEN JS-A102A/NIR (flash) 4. Conclusions Coffee Ring Effect (% of Average Layer Thickness) 130–140 240–350 260–330 137–150 Ink Optimization OVEN OVEN NIR NIR For the JSB40G ink, a direct improvement of NIR-continuous sintering compared with oven sintering in terms of processing time is achieved. The sintering duration was reduced from 30–60 min to 2–5 min with no significant reduction in sheet resistance, which only slightly increases from 0.027–0.050 Ω/ for oven sintering to 0.055–0.085 Ω/ for NIR sintering. Furthermore, no significant increase of surface roughness, which slightly raises from ±13.5 nm for oven sintering to ±25 nm for NIR-continuous sintering, was found. However, the heating mechanism during NIR sinteringPDF Image | Nanoparticle Inkjet Inks for Near-Infrared Sintering
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