Low-cost Preparation And Structure Control Of Phenolic-novolac Based Organic Aerogels And Carbon Aerogels

Carbon aerogels are a kind of non-crystalline and porous solids with a unique three-dimensional network. Due to their high conductivity, large surface areas, biocompatibility and chemical inert, they have wide potential applications in the fields of energy storage, environment protection, catalysis, etc. Up to now, carbon aerogels are generally prepared by the sol-gel polymerization of phenolics and aldehydes, followed by supercritical fluid drying and carbonization. The supercritical drying which needs high pressure and elevated temperature, is most complex, time-and-energy consuming procedure, making the material more difficult for scalable preparation and commercial applications.In this paper, we develop a facile and low-cost method to prepare high-strength carbon aerogel monoliths via an alcohol sol-gel reaction coupled with ambient pressure drying. The microstructure control and mechanical property were systematically studied. The major concludes are summarized as follows.1. Organic and carbon aerogel monoliths can be prepared by sol-gel polymerization of linear phenolic novolac resin (P) and hexamethylenetetramine (HMTA) in a wide condition, followed by the ambient drying and carbonization. During the sol-gel process, the HMTA plays important roles as not only a catalyst but also a cross-linked reagent, which could affect the growth and crosslinking of initial polymer clusters. Changing the reactant concentration and ratio of P/H can affect the process of spinodal decomposition, which could control the network of aerogels and resulting pore structure. Moreover, for preparing high-strength carbon aerogel monoliths, the optimum conditions of reactant concentration should be in the range of0.05g/ml to0.06g/ml, for different concentrations, the relevant mass ratios of P/H should be from4:1to10:1.2. The phenolic novolac-HMTA carbon aerogels are typical macroporous materials with controllable porous structure. By adjusting the reaction concentration and the ratio of P/H, the macropore size and pore volume can be controlled in the range of100～4000nm and0.2～3.2cm3/g respectively. A certain amount of micropore and mesoopore also co-exist in the carbon aerogels with the BET surface areas changing from400m2/g to700m2/g.3. The carbon aerogel monoliths have very excellent mechanical properties. While the carbon aerogel monoliths are under vertical stress, the maximum compressive strength could be3.2Mpa. There is a proportionality relationship between compressive strength and density of carbon aerogel. The relationship between modulus of compression and density is also in the same way.4. The surface areas and micropores volume of carbon aerogel monoliths could be improved effectively by CO2and water vapour activation. After2h CO2activation at1000℃the micropore volume increased two-fold to0.39cm3/g, while the BET surface areas increased to1400m2/g. The apparent bulk morphology and the microscopic network of carbon aerogel monoliths did not change much after CO2activation, but changed drastically after water vapor activation.