Electrical and elastic properties of new monolithic wood-based carbon materials

Carbonaceous monolithic materials were prepared from especially designed wood-based composites consisting of wood fibres and phenolic resin binder. By compressing more or less the starting materials, the monoliths were obtained with densities ranging from 0.3 to 1.2 g cm^-3. After carbonisation, electrical conductivity and elastic moduli of a number of samples were investigated, and typical percolation behaviours were evidenced for both properties close to their respective critical points. Careful study of the apparent density and pore texture of the uncompacted carbonised fibres allowed the determination of the conductivity threshold φ_c. The morphologies of both the constitutive carbon particles and the interparticle voids were derived from application of effective-medium theory; the calculated aspect ratio of the fibres was found to be in good agreement with both SEM characterisations and other calculations based on percolation theory. Observation of the universal 3D value of the critical conductivity exponent supported the accuracy of the estimated value of φ_c- The rigidity threshold φ_r was also determined, and the relevance of the Kirkwood-Keating model accounting for the observed relationship between φ_c and φ_r was established. The value of the elasticity critical exponent suggested central forces between the fibres, further supporting the suitability of the Kirkwood-Keating model. To the knowledge of the authors, such a model was shown to apply to only one other material so far: expanded graphite. Hence, the present work shows the relevance of the classical concepts of disordered matter physics for describing heterogeneous random carbonaceous materials.