Group led by the University of Tokyo creates wood biomass material with a thermal conductivity 100 times higher than paper

A research group led by Professor Junichiro Shiomi of the School of Engineering, the University of Tokyo, Associate Professor Masaki Kudo of the Tokyo Metropolitan College of Industrial Technology, Professor Tsuguyuki Saito of the Graduate School of Agricultural and Life Sciences, the University of Tokyo, and Professors Fredrik Lundell and L. Daniel Söderberg of the KTH Royal Institute of Technology, Sweden, has discovered a filament material made from cellulose nanofibers (CNF), a bio-based nanomaterial obtained from wood biomass. When the material was oriented on a molecular scale by a fluidic process and solidified with acid, it was found to be five times more thermally conductive than advanced wood biomass such as cellulose nanopaper and 100 times more thermally conductive than conventional wood biomass such as paper.

The research group wondered if it would be possible to develop higher value-added applications of CNFs for practical use, and conducted research leading to heat dissipation technology, for which there is a growing need in electronic devices and other applications. They fabricated CNF filaments by injecting an aqueous dispersion of CNF into a flow-focusing channel system, orienting the CNF to a high degree, and solidifying it using an acid separately injected into the channel. In addition, the thermal conductivity of naturally dried CNF filaments was measured using the T-type thermal conductivity measurement method.

As a result, they found that the thermal conductivity reached 14.5 (W/m-K) for filaments made with a specific acid, which is more than five times higher than advanced woody biomass with high thermal conductivity, such as cellulose nanopaper and cellulose nanocrystals, and more than 100 times higher than conventional wood biomass, such as paper. The researchers also found that the finer the CNF filament, the higher the thermal conductivity.

In order to identify these mechanisms, they evaluated the chemical structure of the CNF filaments, as well as their internal orientation, bonding state, crystallinity, and residual stress using Raman and infrared spectroscopy. It became clear that under conditions where the CNF orientation reaches a certain level, the thermal conductivity is higher when there are more hydrogen bonds connecting the CNFs and less structural disorder within the CNFs caused by residual stress.

Vibrations from atoms, called phonons, are responsible for conducting heat in solids, and the more hydrogen bonds between CNFs, the easier it is for phonons to penetrate the interface between CNFs, and the smaller the structural disorder inside CNFs, the easier it is for phonons to propagate and the higher the thermal conductivity. In the flow-focusing channel, hydrogen bonds are formed as the acid diffuses from the surface to the interior of the CNF filament, and smaller diameter filaments exhibit higher thermal conductivity because the hydrogen bonds form more uniformly throughout.

"We are looking to reduce environmental impacts by applying these findings to electronic base substrates such as flexible printed circuit boards, which require thermal conductivity," commented Professor Shiomi. "Biocompatibility may also be an advantage for devices in biological applications. Another well-known application is the idea of incorporating filaments into clothing to make ribbons and fabrics with them. Using them to absorb moisture and condensation is also attractive."

■ Flow-focusing channel: A two-dimensional microfluidic channel in which polymers dispersed in a liquid are oriented by a condensed flow.

Journal Information
Publication: Nano Letters
Title: Enhanced High Thermal Conductivity Cellulose Filaments via Hydrodynamic Focusing
DOI: 10.1021/acs.nanolett.2c02057