Processes during liquation of Vycor glass studied by positron annihilation lifetime spectroscopy

Abstract

This study concerns Vycor glass subjected to liquation in the temperature range 725–950 K and leached in hot acid in order to produce pores. Three stages of pore creation were observed with increasing liquation temperature: The initial structure of nanopores with a radius of about 0.5 nm (like in the nonliquated glass) disappears, giving way to the next group, dominant at 875 K, whose average radius is 1.15 nm, independently of the liquation time. The spread of these radii is narrow, 0.15 nm. At the highest temperatures, the intensity of this component decreases rapidly (down to 1%), and large pores begin to grow; the specific surface area diminishes. About 900 K, the total positronium formation probability decreases significantly, which can be ascribed to the partial crystallization of silica.