Characterization of porosity over many length scales: Application to colloidal gels

Processing-microstructure relationships in a silica gel system, based on mi xtures of colloidal sol and soluble potassium silicate, have been studied. Quantitative microstructural information regarding colloidal cluster sizes, size distributions, surface areas, and pore-size distribution from the nan opore range to the macropore range was determined via small-angle scatterin g and transmission electron microscopy. The colloid cluster size distributi on varies systematically, with gels fabricated with the least colloidal fra ction possessing the most polydisperse microstructure. It is shown that the porosity over the entire range can be tailored by selecting the appropriat e starting chemistry; under the same pH conditions, the ratio of the two si licate ingredients controls the average size, the polydispersity of sizes, and the connectivity of the pores. A population of fine (2 nm) uniformly di spersed nanopores, which result from leaching, is responsible for large inc reases in surface area. The leaching process can be controlled by the surro unding macropore void size, which determines alkali transport. The product material consists of 85% large, open pores, with fine pores within the gel skeleton, making this gel an ideal candidate for controlled-porosity applic ations such as catalyst supports and magnetic composites.