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Fully printed, functionalized multi-walled carbon nanotube (FMWCNT)/hydroxyethyl cellulose (HEC) composite-based humidity sensor.

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In recent years, traditional printing methods have been integrated to print flexible electronic devices and circuits. Since process requirements for electronics differ from those for graphic printing, the fundamentals require rediscovery mainly to optimize manufacturing techniques and to find cost reduction methods without compromising functional performance. In addition, alternative inks need to be formulated to increase the variety of functional inks and to pioneer new product developments. In this report, we investigate a thermoplastic-based nickel ink prototype to print electrodes using a screen-printing process. Process fundamentals are explored, and cost reduction methods are addressed by studying the effect of substrate roughness, print direction, and number of ink layers on the electrical performance of printed nickel. Multilayered electrodes are printed on paper and heat stabilized engineered film. A novel fundamental mechanism is found that explains the effect of substrate roughness on ink film roughness in screen printing, including the roughness measurement of the screen mesh wire that is reported for the first time. Results demonstrated that (i) surface roughness of substrates does not have significant effect on printed ink film roughness in screen printing; (ii) ink film thickness is higher on nonabsorbent materials, while line gain is higher on absorbent materials; (iii) the effect of electrode orientation on electrical performance is insignificant; and (iv) the effect of substrate roughness on the electrical performance for the first print layer can be eliminated by printing multiple layers. The results significantly affect substrate choice and number of ink layers, which are considered the major cost factors in the manufacturing of printed electronics.

An interpretation of solid surfaces is generated based on physical considerations and the laws of thermodynamics. Like the widely used Owens–Wendt (OW) method, the proposed method uses liquids for characterization. Each liquid provides an absolute lower bound on the surface energy with some uncertainty from measurement variations. If multiple liquids are employed, the largest lower bound is taken as the most accurate, with uncertainty due to measurement errors. The more liquids used, the more accurate is the greatest lower bound. This method links generalizations of the Good–Girifalco equation with a general thermodynamic inequality relating the three‐interfacial tensions in a three‐phase equilibrium system. The method always satisfies this inequality with better than a 65% certainty. However, the OW seldom, if ever, conforms to this inequality and even then, the degree of satisfaction is insignificant. A reconciliation of the two methods is proposed based on rescaling the OW surface energies to conform to the inequality. This enables interpretations of dispersion and polar components of the surface energy, which are thermodynamically self‐consistent. The proposed method is also capable of dealing with material exchange between liquid and solid phases, when the surface tension and contact angle of the saturated liquids can be measured.

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