Positron Annihilation Lifetime Spectroscopy (PALS) Study of the Influence of Aluminosilicate Gel Ageing on Its Microstructure

The solid phases of: (i) freshly prepared aluminosilicate gel, (ii) the same gel aged for 48 h at either room temperature or at 40 °C and (iii) products of hydrothermal treatments (at 80 °C for 240 min) of gels (i) and (ii) were analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and positron annihilation lifetime spectroscopy (PALS). Additionally, crystal size distributions (CSD) of the crystalline end products obtained by hydrothermal treatment of (i) and (ii) were measured. By analysis of the o-Ps annihilation lifetimes τ3 and τ4, it was concluded that in all amorphous (gel) and crystalline (zeolite A) samples annihilation takes place in voids characteristic for the crystal structure of zeolite A. The number of such structural elements increases with ageing of the hydrogel, as evidenced by the increase of the o-Ps lifetime intensities I3 and I4 of the aged hydrogels. Based on the combined analysis of PALS (measuring of intensities I3 and I4, associated to lifetimes τ3 and τ4) and CSD (measuring of average crystal size and specific number of crystals) data, a correlation between the amount of (quasi)crystalline phase and the intensities I3 and I4 is assumed.