Porosity characterization of aerogels using positron annihilation lifetime spectroscopy

We utilized positron annihilation lifetime spectroscopy (PALS) for non-destructive characterization of porosity in aerogels, using positronium (Ps; an electron–positron bound state) as a probe. PALS measures Ps lifetime in pores and uses models to translate that into a spatial measure (mean free path), which can then be converted to pore size for known shape. For these feasibility studies, aerogel samples with different densities were compared and distinguished on the basis of the PALS results alone. Discrete component comparative analysis was carried out for all samples and a continuous distribution was derived for one sample. All samples exhibited Ps signals ascribed to a bimodal porosity structure comprising a dominant (80–85%) component, spanning nearly the entire meso- and macro-pore size ranges, and a smaller (10–15%) microporous contribution with a spherical equivalent diameter of ∼2 nm. To describe the irregularly shaped pores in aerogels, the diameter of a sphere and the cross-section of a long narrow channel were considered, respectively, as the upper and lower bound of the actual pore sizes. The maximum of the mesoporous distribution was thus between 8 nm (channels) and 12 nm (spheres), with the lower bound arguably being more representative of the aerogels porosity. A comparison between the results of PALS and these of other commonly used techniques is discussed.