Mechanical performance optimisation of tannin-based carbon foams

PI : Alain Celzard (UMR 7198  Institute Jean Lamour)

Co-applicants : M. Khelifa (LERMAB)

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Context — The mechanical properties of vitreous carbon foams have been extremely poorly investigated despite their huge applicative potential : porous electrodes in electrochemical devices for energy conversion, scaffolds for tissue engineering, lightweight high-frequency electromagnetic shields, filters for hot or corrosive fluids, catalyst supports, high temperature insulators, precursors of cellular ceramics, etc. Newly developed synthesis techniques have, for the first time, made it possible to separate density and pore size, two parameters inversely proportional to each other in conventional foams. The present flavonoid tannin-derived vitreous carbon foams will be used as model materials for understanding the mechanics of such highly porous media, and the results will allow for their optimization.

Objectives — Optimizing the mechanical performances of cellular vitreous carbons derived from vegetable resources through the  preparation of samples of strictly controlled structure, the experimental measurement of their mechanical properties and the modeling of the results.

Approach — We prepared and thoroughly studied cellular vitreous carbon samples of fixed chemical composition but with structural parameters (mainly porosity and pore size) controlled independently from each other, which has never been carried out before.

Key results —

• Unprecedented analysis of the compressive properties of several hundreds of samples, split into 9 families of structures and 6 classes of densities
• In-depth analysis of the impact of the measurement conditions, including with and without rigid plates glued to their faces, and depending on the speed of solicitation, and determination of the relevant conditions to measure moduli and compressive strengths
• No effect of the pore size, all else equal, was noticed
• The modulus must be determined with rigid plates glued to the samples’ faces, whereas the compressive strength can be determined either with or without plates
• All materials, cellular or reticulated, or even in-between, obey a same refinement of the law of Gibson & Ashby, and with the same critical exponent
• From this refined law, the fraction of solid only present in the struts of the foams could be calculated and found in good agreement with calculations based on independent measurements of thermal and acoustic properties
• The definition of a class of semi-open foams was suggested, i.e., which changes compressive stress as a function of density similar to open foam, although presenting higher values, but which modulus behaves as that of closed-cell foams
• The demonstration was done that the relative density and the fraction of solid contained in the foams’ struts are the only two key parameters to be tuned for getting the desired mechanical behavior.

Main findings — In practical terms, integrating our test results with real-world system modeling for simulation calculations allowed our investigation to go further than previous works. These material models proved to be essential experimental references for understanding the effects of individual structural parameters. In technical terms, these results will lead to optimized use of these materials for which each proposed application requires a minimum of mechanical properties for use in a given context.

Future perspectives — Work for this project has opened up new possibilities to further our understanding of organic foams, whose various compositions have certain effects (enhanced elasticity, ductility) in addition to those with porous structure. Tests for shear strength and flex fatigue, both difficult to carry out given the intrinsically fragile nature of these materials, would be worth pursuing to determine the Weibull modulus for different structures.