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The recent trend toward the Internet of Things (IoT) has increased the importance of technologies for the mass production of high-sensitivity flexible antennas for efficient wireless communication. Here, we describe the rapid low-temperature fabrication of high-sensitivity flexible antennas based on hot-water sintering of silver (Ag) precursor-based ink. Ag precursor-based ink was printed on various flexible substrates such as plastic films and cellulose nanofiber paper, followed by pre-drying the ink solvent and then treatment at 80 °C and 95% relative humidity for 2 min. The resulting printed Ag lines achieved a volume resistivity of approximately 6 μΩ cm, providing excellent sensitivity as dipole antennas for wireless communication. This technique enables the mass production of printed high-sensitivity antennas, which will likely be required for a future IoT society.

Transparent electrodes based on silver nanowire (AgNW) films have attracted considerable attention owing to their high electrical and thermal conductivity, optical transparency, and flexibility. However, the long-term reliability of AgNW electrodes, has seldom been studied. In this work, the effects of storage environment, such as, natural light and humidity, on the long-term reliability have been investigated in detail. The increase of electrical resistance with storage time greatly depended on the natural light illumination, especially ultraviolet (UV) illumination, and high humidity seriously accelerated the failure of AgNW electrodes. All the results were dependent on the storage temperature. Some over-coating layers have been used to protect the AgNW electrodes from light and humidity in environment and the results showed that an epoxy resin protecting layer exhibited the longest lifetime: over 40 days at 85 °C and 85% relative humidity.

Multi-layer graphene oxide (MGO) has attracted considerable interest in conductive polymer composites. However, in most cases the optimal MGO content is determined using a complex procedure. Avoiding the complicated work of processing polymerized samples followed by testing various properties, here a facile strategy is proposed to directly identify the critical MGO content among formulations for epoxy composites with optimal thermal properties, simply by monitoring the “unusual” nonlinear MGO content-dependent cure behaviors, as well as the “unusual” pattern of double-peak curing curves. The formation mechanism of the double-peak pattern was explored, with emphasis on studying the epoxy/MGO reaction using a modified Shrinking Core Model. The optimal content determined in this work (2 wt% MGO) was verified by the thermal properties (thermal conductivity, structural thermal stability and coefficient of thermal expansion) of MGO/epoxy composites. Based on the inherent relationship between the effect of MGO percolating chains on the thermal polymerization behavior and the resulting thermal properties, this strategy can be easily extended to different kinds of conductive MGO/polymer composites.

N. Isobe, T. Kasuga and M. Nogi. Clear transparent cellulose nanopaper prepared from a concentrated dispersion by high-humidity drying. RSC Adv., 2018, 8, 1833. https://doi.org/10.1039/C7RA12672G., Optically transparent cellulose nanopaper is a promising candidate for flexible device substrates because of its light weight, surface smoothness, and high dimensional stability with respect to temperature. Conventionally, clear transparent nanopaper has been fabricated from cellulose nanofiber dispersions with quite low concentration: less than 0.5 wt%. However, this diluteness leads to several problems, such as huge energy consumption and long operation time for drying. Therefore, nanopaper should be fabricated from a concentrated dispersion to mitigate these problems. In this study, transparent nanopaper was fabricated from cellulose nanofiber dispersions with various concentrations (0.24-1.81 wt%). Optical experiments revealed that the haze of the transparent nanopaper increased monotonically with cellulose nanofiber dispersion concentration, when the cellulose nanofiber dispersion was prepared from holocellulose pulp and conventional over-drying was applied. Based on our insight into the origin of this increase in the haze of transparent nanopaper, we developed high-humidity drying, which successfully produced clear transparent nanopaper from a concentrated dispersion without prolonged drying time.

Low concentrations of metallic nanoparticle inks often produce a coffee-ring effect, thereby resulting in high electrical resistance in inkjet-printed lines. The coffee-ring effect is due to a convection flow of ink droplets and can be overcome by reducing the flow. Here we report the formation of absorption layers of ink vehicles on pristine polyimide films and fabrication of convex-shaped lines without the coffee-ring effect even if a low concentration of commercially available ink is used. The coated layers increased the ink concentration and prevented a convection flow of ink droplets. As a result, the electrical resistance of the inkjet-printed lines on polymer-coated polyimide films (8 Ω) was improved threefold over that of lines printed on pristine polyimide films (24 Ω). This is a similar improvement to inkjet-printed lines that were heated gradually (7 Ω), which is one of the methods that can reduce convection flow. Durability tests were conducted and electrical resistance was measured on inkjet-printed lines on polymer-coated polyimide films. Even under harsh environments, the lines showed excellent electrical performance, and they can easily be integrated into practical applications.