Permeability of fibrous carbon materials

The air permeability of a number of commercial fibrous carbon materials: soft non-woven felts, rigidised felts and rigid boards, based on either PAN- or Rayon-derived fibres presenting various diameters, graphitised or not, and consolidated by different methods, was measured and investigated. Consistent behaviours were found within families of closely related materials, but the diversity of porous structures prevented any model, including the very popular Tomadakis–Sotirchos equation, to fit all results. The Archie’s coefficient and the tortuosity factor for viscous flow were thus calculated. Not only all data were perfectly aligned on one single master curve, but the analysis was extended to many other fibrous materials and the same master curve was found to be relevant. The Archie’s coefficient thus appears to be an intrinsic property, purely defined by the material geometry, as it does not depend on the 1D, 2D or 3D type of flow. A fitting equation was proposed, encompassing all fibrous materials in very broad ranges of porosities and porous structures

Effect of the porosity and microstructure on the mechanical properties of organic xerogels

The synthesis of resorcinol–formaldehyde (RF) xerogels is versatile enough to provide materials with custom pore size distributions in the meso-/macroporous range. Specifically, seven xerogels were synthesised by changing the pH of the same RF solution, from pH 3 to pH 6. The resulting materials presented meso-/macroporous size distributions with average pore sizes from  < 5 to 510 nm, as determined by Hg intrusion and N2 adsorption. Most of the RF xerogels had very similar geometric densities, except for the gels obtained at the two highest pH values, which were more dense. Both flexural and uniaxial compression tests were carried out to determine the dependence of strength and stiffness on the porosity of the xerogels. The results followed a power-law relationship between the mechanical properties and the density of the materials. However, two series of RF xerogels were found to fit such law independently, with the gels obtained at the three most acidic pH values (pH 3–4) showing unexpectedly high compression moduli. Further characterisation of the xerogels microstructure revealed the existence of rod-like microstructures that would bear most loads during the compression tests. These microstructures would act as struts that, once broken, would cause the catastrophic failure (bursting) of the xerogel.The authors gratefully acknowledge the financial support received from the Consejo Superior de Investigaciones Científicas (Project I-LINK1200), Ministerio de Economía, Industria y Competitividad (Project CTQ2017-87820-R) and Principado de Asturias– FICYT-FEDER (Project PCTI-Asturias IDI/2018/000118).Peer reviewe