Production of cylindrical carbon xerogel monoliths: effect of sample diameter on drying-induced cracks
Advancing the chemical engineering fundamentals
Transport Phenomena in Porous/Granular Media - II (T2-7b)
Keywords: resorcinol-formaldehyde xerogel, convective drying, shrinkage, stresses, modeling
It has been showed recently that it was possible to produce carbon xerogels by convective drying of resorcinol(R)-formaldehyde(F) hydrogels, followed by pyrolysis [1]. The carbon material obtained using subcritical drying conditions keep large mesopore volumes and BET specific surface areas, provided that synthesis conditions are adequate. These properties lead to a wide range of potential applications (adsorbents, catalysts supports, electrode material, energy storage device, column packing materials, …). Drying constitutes a particularly critical step when carbon monoliths are required, e.g. for electrodes or supercapacitors. When the drying conditions are too severe, the mechanical stresses caused by differential shrinkage may lead to the cracking of sample. In this work, we focus on the effect of the size of the gels on the mechanical stresses induced during drying, both from an experimental and modelling point of view.
Hydrogels were prepared by sol-gel polycondensation of resorcinol solubilised in deionised water with formaldehyde, in the presence of Na2CO3, usually called catalyst (C). Since the carbonate concentration controls the gel texture and its shrinkage behaviour, the R/C ratio was fixed at 300 in order to get a volume reduction after drying close to 65%. The molar ratio R/F and the dilution ratio were fixed at 0.5 and 5.7, respectively. Cylindrical samples with diameters of 23 and 46 mm were obtained by casting 5 ml or 50 ml solution into glass moulds. After gelation under saturated atmosphere at 70°C during 24h, the gels were dried in a classical convective drying rig controlled in air relative humidity, temperature and velocity. A camera was used in order to follow the development of cracks during the drying process. The velocity was kept constant at 2 m/s for all experiments while the temperature and relative humidity were adjusted in order to produce crack-free monoliths.
A series of experiments showed that the drying conditions have to be softened to avoid cracking when the size of sample increases. For the large samples, the temperature has to be set to 40°C, together with 60% relative humidity, to obtain crack-free monoliths. Small samples withstand more severe drying conditions, i.e. 70°C and 15% RH. As the cracks are induced by mechanical stresses due to differential shrinkage, the evaluation, and the control, of the state of stress within the material is essential. This required the use of a coupled thermo-hygro-mechanical model of the drying process [2]. Simulations results confirmed the effect of the sample size on the maximal Von Mises stress, i.e. the state of stress within the sample.
In order to validate these results and to develop a relevant cracking criterion, Brazilian tests will be carried out to determine the failure strength at various water contents. Other synthesis conditions, i.e. R/C ratios, will be also considered
[1] Léonard A., Job N., Blacher S., Pirard J.P., Crine M., Jomaa W. Suitability of convective air drying for the production of porous resorcinol-formaldehyde and carbon xerogels. Carbon 2005; 43(8):1808-11.
[2] Léonard A., Crine M., Jomaa W. Modelling of the convective drying of resorcinol-formaldehyde resins: influence of the drying conditions on the induced stress tensor. In: Farkas I (editor). Drying 2006, 2006; 273-8.
Presented Tuesday 18, 15:20 to 15:40, in session Transport Phenomena in Porous/Granular Media - II (T2-7b).
