Positron Lifetime Spectroscopy on Controlled Pore Glass Porosimetry and Pore Size Distribution

Abstract

Positron annihilation lifetime spectroscopy (PALS) is used to study a series of controlled pore glasses (CPG). The lifetime spectra were decomposed into four components using the routine LifeTime, version 9.0 (LT9). The largest lifetime τ4, which is attributed to the annihilation of ortho-positronium (o-Ps) localized at mesopores, varied at 300 K between 21 and 131 ns. The size of mesopores (mean free path), D, was determined by N2 adsorption and Hg intrusion techniques to vary between 1.8 and 56 nm. It is shown that the Tao-Eldrup model extended to cylinders of infinite length and diameter d = D describes well the experiment for an overlap parameter δ fitted to be δ = 0.193 nm. The spectra were also analyzed allowing a distribution of o-Ps lifetimes. A method is developed to calculate the pore size distribution n(d) from the τ4 distribution. This method is of particular interest since PALS is very sensitive to pores being too small to be exactly analyzed by conventional porosimetry.