Characterization of Porosity Over Many Length Scales: Application to Colloidal Gels
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MATERIALS RESEARCH
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Characterization of porosity over many length scales: Application to colloidal gels Helen M. Kerch United States Department of Energy, Germantown, Maryland 20874
Gabrielle G. Long, Susan Krueger, and Andrew J. Allen National Institute of Standards and Technology, Gaithersburg, Maryland 20899
Rosario Gerhardt Georgia Institute of Technology, Atlanta, Georgia 30332
Frederick Cosandey Rutgers University, Piscataway, New Jersey 08855 (Received 10 April 1998; accepted 28 September 1998)
Processing-microstructure relationships in a silica gel system, based on mixtures 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 nanopore range to the macropore range was determined via small-angle scattering and transmission electron microscopy. The colloid cluster size distribution varies systematically, with gels fabricated with the least colloidal fraction possessing the most polydisperse microstructure. It is shown that the porosity over the entire range can be tailored by selecting the appropriate starting chemistry; under the same pH conditions, the ratio of the two silicate ingredients controls the average size, the polydispersity of sizes, and the connectivity of the pores. A population of fine (2 nm) uniformly dispersed nanopores, which result from leaching, is responsible for large increases in surface area. The leaching process can be controlled by the surrounding 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 applications such as catalyst supports and magnetic composites.
I. INTRODUCTION
Porous materials are used in a myriad of engineering applications, including super insulators, membranes, catalyst supports, and porous hosts for second-phase filtration both in bulk and thin-film form.1,2 Because microstructure and performance are strongly related in these materials, the realization of different applications depends on our ability to produce porous media with specific pore sizes and pore-size distributions, pore shapes, and pore connectivities. Often, more than one microstructural parameter needs to be engineered for the product to function in a satisfactory manner. In catalysis devices, for example, high surface areas ensure ample active sites, but the void volumes simultaneously must allow adequate transport of chemical species. Additional properties, such as mechanical and/or chemical stability, may also be essential. In view of these challenging design requirements, the importance of understanding and controlling the microstructure-processing link cannot be overemphasized. The sol-gel fabrication route has been utilized for many polymer-based porous materials schemes because 1444
http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 4,
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