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Nanomaterials 2020, 10, 892 8 of 14 but also the overall temperature of the layer and substrate remains lower as compared to oven sintering (as there is local heating and direct sintering), implying no (or less) spread of the heat. Even though the required sintering time for the JS-B40G ink is significantly reduced by switching from thermal to NIR curing, the process is still not roll-to-roll compatible for printable electronics. In addition, thermal degradation of conventional plastic substrates (such as Polyethylene terephthalate (PET)) cannot be eliminated since temperatures of 200 ◦C are still required to successfully sinter the silver patterns. Nevertheless, the degradation effects will be reduced due to the significant shorter sintering time, but optimization of the ink is clearly required. A fundamental insight regarding the different mechanisms such as particle migration (coalescence), aggregation and Ostwald ripening, during sintering is necessary to optimize the ink [35–40]. Ostwald ripening can occur at the early stage of sintering when particles still have their original size and are driven by the Gibbs–Thompson phenomenon. If the ink consists of a widespread nanoparticle size distribution, then smaller particles agglomerate into bigger particles, as smaller particles are less stable due to high surface energy [35,36]. As sintering progresses, the surface free energy of all particles will gradually reduce, resulting in less Ostwald ripening. Meanwhile, particle migration (coalescence) becomes the main sintering mechanism. The bigger particles will coalesce and aggregate together randomly, forming necks and eventually completely merge together. Besides these main mechanisms, there are still some particle and ink formulation related properties which could influence the sintering progress. One of these is the effect of a polymer capping agent used to stop nanoparticles from agglomerating before sintering. As the thickness of the capping layer is less than a few nm and decompose in the primary stage of the sintering procedure, its effect on sintering is negligible [26,36]. However, the thermal stability of the capping plays a significant role on the sintering temperature required to form a conductive path. Another condition which affects the sintering is the type of solvent used in the ink. A low boiling point solvent evaporates much faster than a high boiling point solvent, resulting in the particles coming closer to each other faster and therefore help the two earlier described mechanisms (Ostwald ripening and coalescence). This has a significant influence on the sintering time. Finally, the last described condition is the particle size. Since the size of these nanoparticles is in the lower nm range (between 20 and 100 nm), they are subjected to scattering when being photonically sintered. Upon photo incidence, the localized surface plasmon resonance (LSPR) of the silver nanoparticles prevails if the incident light resonantly matches with the localized field of AgNPs and hence, alters the absorbance by scattering the radiation. LSPR primarily depends on the size of the AgNp, dielectric constant of solvents used and the wavelength of incident radiation. Figure 6 shows both ink absorbance and the lamp spectral radiation as a function of the wavelength. The UV-VIS measurement shows that the JS-B40G ink has its maximum absorbance peak at 486 nm. Dynamic light scattering measurement of the ink batch undertaken by the supplier shows that the JS-B40G ink has an average particle size of 69 nm. When calculating the maximum extinction based on the Mie theory, 69 nm diameter particles have their maximum absorbance at 482 nm, which is in accordance with our experiment [41–45]. Increasing the particle size shifts the maximum absorbance peak towards a higher wavelength and matching better with the wavelength of our NIR light source. However, a bigger particle size will also reduce the maximum absorbance of the ink and reduce the effect of Ostwald ripening because these particles are more stable. This results in a trade-off system between the sintering mechanism speed and the amount of light, and thus heat absorbance. Based on these parameters, a second commercially available ink was studied, namely, JS-A102A. This ink is stated to be optimized for photonic sintering. JS-A102A has an average particle size of 36 nm, based on a dynamic light scattering measurement (DLS) of the ink batch performed by the supplier. However, the size distribution in this ink is different from JS-B40G ink. DLS data show two Gaussian curves, instead of one for JS-B40G, with maxima at a particle diameter of ±12 nm and ±76 nm. The particle size distribution consists of both small and bigger particles, the maximum absorbance wavelength of bigger sized particles matches with the NIR radiation, increasing the absorption andPDF Image | Nanoparticle Inkjet Inks for Near-Infrared Sintering
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