Evaporative Dry Powders Derived from Cellulose Nanofiber Organogels to Fully Recover Inherent High Viscosity and High Transparency of Water Dispersion.

Water containing low amounts of cellulose nanofiber (CNF) is widely used as a thickening agent owing to its three unique properties: high transparency, viscosity, and controllable viscosity based on the shear rate. CNF dry powders are used to reduce the transportation and storage costs or expand applications as a thickening agent. Herein, the preparation of CNF dry powders that can be used to obtain redispersions while maintaining the aforementioned properties is reported. In this regard, the dehydration and vaporization procedures for a CNF water dispersion without using additives are discussed. When dry powders are prepared by removing water by boiling, their redispersions do not exhibit all their unique properties because of dense aggregations. However, when their redispersions are vigorously stirred to break the dense aggregations, they become transparent, although they do not recover their initial viscosity. Freeze-dried powders recover all their initial properties after redispersion. Nevertheless, their large volume does not reduce the transportation and storage costs. When the liquid is evaporated from the solvent-exchanged CNF organogels, their redispersions also fully recover all their properties. Furthermore, the evaporative dry powders with dense small volumes and good handling contribute to reducing the transportation and storage costs.

"Return to the Soil" Nanopaper Sensor Device for Hyperdense Sensor Networks.

A nanopaper sensor device that combines humidity sensing, wireless information transmission, and degradability has been fabricated using wood-derived nanopaper as the substrate and dielectric layers. The nanopaper shows excellent suitability for capacitor dielectric layers because of its high dielectric constant, insulating properties suitable for thin-film formation, and lamination properties. A wireless transmission circuit containing the nanopaper capacitor can transmit radio signals in the megahertz band, and the relative humidity change can be output as a change in the radio signal owing to the humidity sensitivity of the nanopaper capacitor. More than 95% of the total volume of the nanopaper sensor device decomposes in soil after 40 days. Because the nanopaper sensor device does not need to be recovered, it is expected to greatly contribute to a sustainable society through realization of hyperdense observation networks by mass installation of sensor devices.

Clearly Transparent Nanopaper from Highly Concentrated Cellulose Nanofiber Dispersion Using Dilution and Sonication.

Nanopaper prepared from holocellulose pulp is one of the best substrates for flexible electronics because of its high thermal resistance and high clear transparency. However, the clearness of nanopaper decreases with increasing concentration of the starting cellulose nanofiber dispersion-with the use of a 2.2 wt % dispersion, for example-resulting in translucent nanopaper with a high haze of 44%. To overcome this problem, we show that the dilution of this high-concentration dispersion with water followed by sonication for 10 s reduces the haze to less than 10% while maintaining the high thermal resistance of the nanopaper. Furthermore, the combination of water dilution and a short sonication treatment improves the clearness of the nanopaper, which would translate into cost savings for the transportation and storage of this highly concentrated cellulose nanofiber dispersion. Finally, we demonstrate the improvement of the electrical conductivity of clear transparent nanopaper prepared from an initially high-concentration dispersion by dropping and heating silver nanowire ink on the nanopaper. These achievements will pave the way toward the realization of the mass production of nanofiber-based flexible devices.

Transparent Conductive Nanofiber Paper for Foldable Solar Cells.

Optically transparent nanofiber paper containing silver nanowires showed high electrical conductivity and maintained the high transparency, and low weight of the original transparent nanofiber paper. We demonstrated some procedures of optically transparent and electrically conductive cellulose nanofiber paper for lightweight and portable electronic devices. The nanofiber paper enhanced high conductivity without any post treatments such as heating or mechanical pressing, when cellulose nanofiber dispersions were dropped on a silver nanowire thin layer. The transparent conductive nanofiber paper showed high electrical durability in repeated folding tests, due to dual advantages of the hydrophilic affinity between cellulose and silver nanowires, and the entanglement between cellulose nanofibers and silver nanowires. Their optical transparency and electrical conductivity were as high as those of ITO glass. Therefore, using this conductive transparent paper, organic solar cells were produced that achieved a power conversion of 3.2%, which was as high as that of ITO-based solar cells.

Chemical Modification of Cellulose Nanofibers for the Production of Highly Thermal Resistant and Optically Transparent Nanopaper for Paper Devices.

Optically transparent cellulose nanopaper is one of the best candidate substrates for flexible electronics. Some types of cellulose nanopaper are made of mechanically or chemically modified cellulose nanofibers. Among these, nanopapers produced from chemically modified cellulose nanofibers are the most promising substrate because of their lower power consumption during fabrication and higher optical transparency (lower haze). However, because their thermal durability is as low as plastics, paper devices using chemically modified nanopaper often do not have sufficiently high performance. In this study, by decreasing the carboxylate content in the cellulose nanofibers, the thermal durability of chemically modified nanopaper was drastically improved while maintaining high optical transparency, low coefficient of thermal expansion, and low power consumption during fabrication. As a result, light-emitting diode lights illuminated on the chemically modified nanopaper via highly conductive lines, which were obtained by printing silver nanoparticle inks and high-temperature heating.