Electrostatic flocking is applied to create an array of aligned carbon fibers from which an elastomeric thermal interface material (TIM) can be fabricated with a high through-plane thermal conductivity of 23.3 W/mK. A high thermal conductivity can be achieved with a significantly low filler level (13.2 wt%). As a result, this material retains the intrinsic properties of the matrix, i.e., elastomeric behavior.

The thermal conductive properties, including the thermal diffusivity and resultant thermal conductivity, of nonwoven nanocellulose sheets were investigated by separately measuring the thermal diffusivity of the sheets in the in-plane and thickness directions with a periodic heating method. The cross-sectional area (or width) of the cellulose crystallites was the main determinant of the thermal conductive properties. Thus, the results strongly indicate that there is a crystallite size effect on phonon conduction within the nanocellulose sheets. The results also indicated that there is a large interfacial thermal resistance between the nanocellulose surfaces. The phonon propagation velocity (i.e., the sound velocity) within the nanocellulose sheets was estimated to be ∼800 m/s based on the relationship between the thermal diffusivities and crystallite widths. The resulting in-plane thermal conductivity of the tunicate nanocellulose sheet was calculated to be ∼2.5 W/mK, markedly higher than other plastic films available for flexible electronic devices.

We report the successful use of a high-speed blender in nanofibrillating never-dried pulp to cellulose nanofibers (CNFs) with a uniform diameter of 15-20 nm. Pulp treated for 30 min in a blender showed the same degree of fibrillation with less damage to the CNF compared with that treated in a grinder. Observing the process of nanofibrillation clarified that the straw-like pulp was fibrillated in a very characteristic way, by forming many "balloon-like structures". As the balloons extended to the edges, the fibrils were rapidly individualized. However, the pulp fragments with ripped cell walls were split into finer fragments and gradually disintegrated into nanofibers. Changing the agitation speed and pulp concentration during the treatment revealed that the pulp concentration of 0.7 wt % at 37,000 rpm was the optimum fibrillation condition in this blender method. Through treatments in various NaCl solutions, the effect of the surface charge of CNF on the fibrillation was studied from the viewpoint of colloidal surface physics. It was found that repulsion due to the electric double layer of the CNF surface may play a critical role in the occurrence of fibrillation.

We developed flexible polymeric “heat-guiding materials” by simply drawing bacterial cellulose (BC) hydrogels to align the cellulose nanofibers and form “nanopapers” with anisotropic thermal conductivity. The in-plane anisotropy of thermal conductivity between the drawn and transverse directions increased as the draw ratio increased. For the drawn BC nanopapers, the coefficient of thermal expansion was found to be inversely correlated with the thermal diffusivity. We fabricated a planar spiral sheet by assembling the drawn BC strips to visualize the “heat flux controllability”. The coexistence of heat-diffusing and heat-insulating capacities within the single nanopaper plane could help to cool future thin electronics.

In this review, we summarize the recent progress in thermal conductivity analysis of nanocellulose materials called cellulose nanopapers, and compare them with polymeric materials, including neat polymers, composites, and traditional paper. It is important to individually measure the in-plane and through-plane heat-conducting properties of two-dimensional planar materials, so steady-state and non-equilibrium methods, in particular the laser spot periodic heating radiation thermometry method, are reviewed. The structural dependency of cellulose nanopaper on thermal conduction is described in terms of the crystallite size effect, fibre orientation, and interfacial thermal resistance between fibres and small pores. The novel applications of cellulose as thermally conductive transparent materials and thermal-guiding materials are also discussed.