Dielectric Properties of Wood-Derived Cellulose Nanofiber Films in 10–40 GHz Band

Wood-derived cellulose nanofiber (CNF) offers excellent electrical insulation, mechanical strength, flexibility, thermal stability, and low thermal expansion, meeting the growing demand for sustainable materials in electronic devices. However, the dielectric properties of CNF-based material crucial for beyond-fifth-generation (B5G) applications at frequencies above 10 GHz remain underexplored. This study investigates dielectric behaviors of CNF films with varying surface functional groups and porosities over 10-40 GHz under controlled humidity. Carboxylated CNF films exhibited distinct dielectric characteristics compared to noncarboxylated CNFs, with moisture sensitivity significantly reduced by substituting proton counterions with hydrophobic tetra-<i>n</i>-butylammonium. Increased porosity led to linear decrease in relative permittivity but increase in loss tangent. The intrinsic permittivity of CNFs also decreased with larger Scherrer crystal size. These findings demonstrate that controlling interfacial polarization and interfibrillar interactions effectively lowers the dielectric response of CNF films, highlighting their potential in B5G and high-frequency electronic applications.