Section 5. Characterization Porosity characterization of aerogels using positron annihilation lifetime spectroscopy I. Mincov a , M.P. Petkov b, * , P. Tsou b , T. Troev a a Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, Sofia BG-1784, Bulgaria b Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA Available online 6 November 2004 Abstract 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 compo- nent 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 chan- nel were considered, respectively, as the upper and lower bound of the actual pore sizes. The maximum of the mesoporous distri- bution 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. Ó 2004 Elsevier B.V. All rights reserved. PACS: 70.70.Bj; 61.18.j; 61.43.Er 1. Introduction Accurate characterization of the microstructure of aerogels enable the understanding and prediction of material behavior. The configuration of the solid network governs the thermal–mechanical properties, whereas the pore morphology determines the permeabil- ity of molecular or atomic species. Porosity characteri- zation is important for many areas of fundamental and applied physics, chemistry, microelectronics, bio- logy and medicine. For aerogels, whose porous phase comprises one continuous open porosity network, the use of the actual term Ôpore sizeÕ has a rather intuitive meaning. Research in this field has utilized various experimental techniques (neutron and X-ray scattering, electron microscopy, gas sorption, mercury intrusion porosimetry, etc.); however, the interpretation of the resultant data with regard to pore-size derivation has not been free of ambiguities. In this work, we demonstrate the use of positron annihilation spectroscopy (PAS) in a comparative study of the porosity of aerogel materials. PAS is a non- destructive technique for the characterization of open- volume defects in materials (from single vacancies to mesopores) [1,2]. The unique interactions of the anti- matter probe – the positron – with material electrons give PAS extreme sensitivity limits with respect to small defect size at low density, surpassing that of other tech- niques [3]. The positron lifetime, number of annihila- tion photons, and electron momentum are measured 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.06.024 * Corresponding author. Tel.: +1 818 354 1715; fax: +1 818 393 4559. E-mail address: mihail.p.petkov@jpl.nasa.gov (M.P. Petkov). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 350 (2004) 253–258