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Li X., Zhu L., Kasuga T., et al. Frequency-tunable and absorption/transmission-switchable microwave absorber based on a chitin-nanofiber-derived elastic carbon aerogel. Chemical Engineering Journal 469, 144010 (2023); https://doi.org/10.1016/j.cej.2023.144010., The increasing use of microwaves in wireless communications has caused severe electromagnetic pollution. As the frequency range for wireless communication is expanding, it is highly desirable to develop a microwave absorber that can smartly and reversibly tune its absorption and transmission properties on demand to transmit required frequencies and absorb unwanted frequencies. Herein, an absorption-frequency-tunable and absorption/transmission-switchable microwave absorber is developed based on the controlled compression of a chitin-derived elastic carbon aerogel. The maximum absorption frequency is tuned from 10.4 to 11.0, 11.5, and 12.1 GHz by varying the compression strain from 0 to 20, 40, and 60%, respectively, while the maximum absorption intensity is maintained at approximately −40 dB. This frequency-tunable absorption is achieved by reducing the thickness of the carbon aerogel while retaining its moderate dielectric loss tangent. Further compression from 60 to 80% switches the carbon aerogel from being a microwave absorber to transmitter while causing impedance mismatch and changing its dielectric loss tangent from moderate to low levels. The frequency tunability and microwave absorption/transmission switching capability are reversible and repeatable for at least 60,000 cycles of compression and recovery. This study provides insights into the smart and reversible control of microwave absorbing properties and paves the way for multifunctional and robust absorbers.

Thanakorn Yeamsuksawat, Yoshitaka Morishita, Jun Shirahama, Yintong Huang, Takaaki Kasuga, Masaya Nogi, and Hirotaka Koga, Semicarbonized Subwavelength-Nanopore-Structured Nanocellulose Paper for Applications in Solar Thermal Heating, Chem. Mater. 2022, 34, 16, 7379–7388. ©2022 American Chemical Society., Recently, there has been remarkable progress in solar thermal heating by applying biomass-derived carbons, which can absorb and convert solar light into thermal energy. The design of subwavelength nanoporous and molecular structures of biomass-derived carbons is required for suppressed reflection and enhanced absorption of solar light. However, such designs are difficult because conventional biomass-derived carbons exhibit intrinsic microstructures and are prepared under specific carbonization conditions. In this study, a wood cellulose nanofiber-derived carbon is proposed to tailor both subwavelength nanoporous and molecular structures. Cellulose nanofibers are first constructed into a paper, denoted as "nanopaper", exhibiting subwavelength nanoporous structures by tailoring the pore spaces between cellulose nanofibers. The as-prepared nanopaper is then carbonized at various controlled temperatures to tailor the cellulose molecular structure, i.e., grow graphitic carbon domains. The graphitic carbon domains grown by semicarbonization at 500 °C adequately balance solar absorption and reflection, while the subwavelength nanoporous structures suppress solar reflection. Thus, the semicarbonized nanopaper with tailored nanoporous and molecular structures exhibits superior solar thermal heating to competitive nanocarbons, also affording thermoelectric power generation. This study can guide the structural and functional design of bionanocarbons for solar thermal heating.

Li, C.; Kasuga, T.; Uetani, K.; Koga, H.; Nogi, M. High-Speed Fabrication of Clear Transparent Cellulose Nanopaper by Applying Humidity-Controlled Multi-Stage Drying Method. Nanomaterials 2020, 10, 2194. https://doi.org/10.3390/nano10112194., As a renewable nanomaterial, transparent nanopaper is one of the promising materials for electronic devices. Although conventional evaporation drying method endows nanopaper with superior optical properties, the long fabrication time limits its widely use. In this work, we propose a multi-stage drying method to achieve high-speed fabrication of clear transparent nanopaper. Drying experiments reveal that nanopaper’s drying process can be separated into two periods. For the conventional single-stage evaporation drying, the drying condition is kept the same. In our newly proposed multi-stage drying, the relative humidity (RH), which is the key parameter for both drying time and haze, is set differently during these two periods. Applying this method in a humidity-controllable environmental chamber, the drying time can be shortened by 35% (from 11.7 h to 7.6 h) while maintaining the same haze level as that from single-stage drying. For a conventional humidity-uncontrollable oven, a special air flow system is added. The air flow system enables decrease of RH by removing water vapor at the water/air interface during the earlier period, thus fabricating clear transparent nanopaper in a relatively short time. Therefore, this humidity-controlled multi-stage drying method will help reduce the manufacturing time and encourage the widespread use of future nanopaper-based flexible electronics.