Structural Characterization of Porous Materials Using SAS

Abstract

This chapter begins with an overview of various engineered and naturally occurring porous solids and their applications in modern technologies. Different modes of confinement, such as topological, chemical, biological, and interfacial, can affect structural and kinetic aspects of the fluid phase equilibria, mechanical and viscoelastic properties, diffusion, and flow in confined spaces. Such dramatic modification of the fluid properties under confinement can be efficiently exploited in a variety of technologies, provided there are ways of controlling the internal structures and surface chemistry of the target porous material at the macroscopic, mesoscopic, microscopic, and molecular levels. SAS represents a nonintrusive technique that can provide information on the structure of porous media that is inaccessible to other methods of structural characterization. The rest of the chapter is devoted to the description of SAS structural analysis of porous silicas, carbons, membranes, polymer monoliths, sedimentary rocks and other common materials.

Thick Transparent Nanoparticle-Based Mesoporous Silica Monolithic Slabs for Thermally Insulating Window Materials

“…Figure b shows Porod plots and their slopes for both slabs shown in Figure a, obtained after subtracting correlation contributions from disordered worm-like mesopores (see the Supporting Information for details of the analysis). In the Porod region, the scattering intensity I relates to the scattering vector modulus Q according to the power law Here, a power law exponent D < 3 indicates a mass fractal structure with a mass fractal dimension D m = D while 3 < D < 4 indicates a surface fractal structure with a surface fractal dimension D s = 6 – D , and D > 4 indicates a nonfractal structure . The mass fractal dimension D m quantifies how the mass of a fractal structure increases with its increasing size where m is the mass contained in a sphere of mesoporous material of radius r .…”

“…SAXS patterns of both slabs consist of (a) Guinier region for Q < 0.4 nm –1 , (b) weak correlation peaks in the range of Q = 0.4–1.0 nm –1 , and (c) a Porod region for Q > 1.0 nm –1 . The presence of Guinier and Porod regions indicates a fractal structure made of aggregated nanoparticles . The weak correlation peaks indicate limited pore-to-pore correlation due to the narrow pore size distribution and the uniform pore wall thickness .…”

“…(a) Representative small-angle X-ray scattering patterns of nanoparticle-based mesoporous silica monolithic slabs synthesized on PFC liquid (SiO 2 -PFC-rt) and on PTFE (SiO 2 -PTFE-rt) and (b) corresponding Porod plots obtained after subtracting contributions from disordered worm-like mesopores. The black dashed line with a slope of −4 corresponds to the behavior of nonfractal materials obeying the Porod’s law . Intensity scales in both panels are in arbitrary units.…”

Coherent pore-scale imaging: phase contrast and ptychography

“…As expected, the source flux needed for a clear pore imaging is proportional to the inverse square of the pore size. The flux can be decreased by an appropriate choice of the probing photon energy to reduce the sample thickness down to l  2/ (see (6)). On the other hand, the smallest value of the thickness in ( 7) is limited by the mentioned condition >> and restrictions of a sample manufacturing.…”

“…On the contrary, a production of laser ceramics needs a careful removal of the micropores 2 . A study of the pores is known to be both fundamental and practical .X-ray porosimetry as a part of general porosimetry [3][4][5] is a fast developing technique based on X-ray small angle scattering 6 and X-ray imaging 7 . At present it profits from current advances in X-ray source development (new generation of synchrotron radiation and laboratory high brightness X-ray sources) as well as of the state-of-the-art X-ray imaging methods, such as phase contrast tomography 8 and ptychography 9,10 .…”